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Virtual Private Network the Solution of Networking

2007-09-04T11:55:00.000+08:00

Tunneling's technological conjugate and enkripsi makes VPN( Virtual Private Network) as technology which admirably and helps user work myriad it.
Both of its technology non-negotiable and discomfited sue again in forms one VPN'S communication. Both of technology it shall be fused to get perfect result, which is safe data communication and efficient. Safe meaning Your data secrecies awake regular and its perfection. Are not bungling side get to catch and reads Your data, even that data then grass at public communication band. Constant perfection awake fathoms a meaning not bungling person get to confound content and Your data path. It needs to be looked after since if was passing public band, a great many fad person
one that maybe just destroy Your midway data. To that is, why technology second this really gets essential role in be formed VPN'S communication solution.

Any kind Technology Tunneling?
To make one tunnel, necessary one its regulator protocol so tunnel logically it gets to walk with every consideration such as connection point to point actually. Now, in store maker protocol myriad tunnel who can be utilized. But, tunneling protocol that commonest and at most is utilized consisting of three types hereunder:

• Layer 2 Tunneling Protocol (L2TP)

L2TP is one tunneling protocol who fuses and compounding two numbers tunneling protocol who gets proprietary's character, which is L2F (Layer 2 Forwarding) Cisco Systems's belonging with PPTP (Point to Point Tunneling Protocol) Microsoft's belonging.

Initially, all product Cisco utilizes L2F to manage that tunneling, meanwhile operating system is antecedent Microsoft just utilizes PPTP to service its user that wants to play by tunnel. But currently, Windows NT's Microsoft / 2000 got utilizes PPTP or L2TP in technological that VPN.

L2TP usually being utilized deep makes Virtual Private Dial Network (VPDN) one that gets to work takes in all communications protocol type at in it. Besides, L2TP also gets independent media character because get to above work any media. L2TP enables its user for regular can connected with local network theirs with policy same security and of whichever they lie, via VPN'S connection or VPDN. This connection oft is looked on as medium lengthen local network belongs to its user, but passes through public media.

But, this tunneling's technology have no mechanism to provide enkripsi's facility because really quite a pure just form tunnel's network. Besides, what last grass in tunnel this can be a prey to and be monitored by use of protocol analizer.

• Generic Routing Encapsulation (GRE)
tunneling's protocol this the one has ability to take in more than one addressing protocol type communication. Are not just package get even one internet protocol address get to be taken in it, but a lot of other protocol package as CNLP, IPX, and a lot of again. But, all that was packaged or to enkapsulation becomes one package which get IP addressing system. Then that package is distributed system thru also tunnel working above IP communications protocol.

By use of tunneling GRE, router is aught at the end tunnel does enkapsulation other protocol packages in header of IP protocol. It will make package previously get to be taken in to whichever by and method that exist on IP technology. With marks sense this ability, therefore protocol which took in by that IP package gets free more move to go to whichever location which is wended, provided that achievable IP addressing ala.

Quite a lot application utilize tunneling's protocol help this is merge local network that separatedly distance ala is back get to get communication. Or in other words, GRP there are many is utilized to lengthen and mengekspansi is proprietary local network the its user. Even quite a lot is utilized, GRE not also provide enkripsi's system grasses last data at its tunnel, so all its data activity can monitor to utilize protocol analyzer so-so.

• security Protocol's IP (IPSec)
IPSec is one for felicitous tunneling protocol option to be utilized deep VPN korporat's level. IPSec constitutes protocol that gets open's character default who can provide data security, data perfection, and autentikation is data among peer second that participates in it.

IPSec provides data security system as it by use of one peacemaker method that named Internet Goes To y. Exchange (IKE). IKE this on call to handle negotiation problem of protocol and security algorithm that is created bases from policy which is applied on network the user. IKE on eventually will result one enkripsi's system and its peacemaker key that will be utilized for autentikasi on this IPSec's system.

How with Enkripsinya's Technology?
Besides technology tunneling, enkripsi's technology in VPN also highly varied. Actually technological enkripsi is not just belonging VPN just, but far-flung its purpose. Enkripsi on call to look after privasi and that data secrecy can't with easy to read by side that don't deserve. Marginally tech enkripsi is divided up two types, which is:

Symmetric Encryption
Symmetric Encryption is known even with nickname one diarrhoea goes to y. encryption. Enkripsi is this type a lot of is utilized deep enkripsi's process data in volumed one outgrows. Up to data communication term, network peripheral that have enkripsi's ability this type will change data that as text of purification (cleartext) as gets text form already at random or its terminology is ciphertext. This random text obviously been made by use of algorithm. This random text really is not easily to be read, so Your data security awakes.

Succeeding question, how is that random data opened by really party be wended? To open this random data, seeker's algorithm previously also makes one key which can open all content in origin. This key proprietary by the consigner and also data consignee. Key this is that will be utilized deep enkripsi's process and dekripsi ciphertext it.

Digital Encryption Default (DES) constitute one standard algorithm that is utilized to make this symmetric encryption's process. This algorithm at claim as one commonest being utilized currently. DES'S algorithm operating deep measure 64 bit obstructs data. Fathom a meaning, this algorithm will carry on one series of randomization process 64 incoming data bits for then is issued as 64 random data bits. That process utilizes 64 bit key whereabouts 56 its bit be chosen at random, 8 its bit comes from parity bits of Your data. Bit eighth that was slipped bit 56th betwixt previous.

Resulting key then is sent to data consignee.
With enkripsi's system such, DES is not easy to be conquered But along with technology developing, DES can be uncovered by use of supercomputer in the period of few days only. Alternative for DES is triple DES (3DES) one that do process in DES as much thrice. So key which is resulted and is needed to open enkripsi is as much three numbers.

Asymmetric Encryption
Enkripsi is this type is frequent at conceive of system public goes to y. encryption. enkripsi's process this type can utilize algorithm any kind, but enkripsi's result of this algorithm will function as complement in to seeker and data collation. In enkripsi this type is needful two peacemaker keys that variably, but mutually gets bearing in processes its algorithm. Both of this peacemaker key is frequent so-called with Public's terminology Goes To y. and Private Goes To y..
For example it, Andi and Kindness wants to get safe communication by use of system enkripsi this. To it, both has to have public goes to y. and private goes to y. beforehand. Andi shall have public and private goes to y., so even with Kindness. While processes communication be begun, they will utilize keys that variably to enkrip and dekrip is data. Key may variably, but data get flawlessly been delivered same algorithm blessing.

Public's makings mechanism and private goes to y. this complex enough. Usually goes to y. to y. this at generate utilizes going generator RSA'S algorithm (Ron Rivest, Adi Shamir, Leonard Adleman) or EL Gamal. Result of this generator usually is two random numeral formations huge ones. One random number functions as public goes to y. and one again for private to go to y.. This random numbers really have as much been made and as random as maybe to strengthen uniquenesses of go to y. to y. You.
To genberationi goes to y. to y. this really need tall CPU process. Therefore that, this process can't be done every time You do data transactions. In other words, enkripsi is this type never been utilized to secure data truthfully because its complex character it. Even so, enkripsi this will so effective in autentikasi's process data and its application that involve signature's digital system and goes to y. management.

How Choose VPN'S Technology in point?
VPN'S technology so a lot of its option for You to utilize. How choose the best one for You? VPN'S technology the best one for Your really clings to traffic's requirement data that wants then grass at its settle.
IPSec's technology constitute main option and the most complete to give solution for VPN'S network enterprise's level. But unhappily, IPSec just backs up traffic that berbasiskan IP and package technology that gets unicast's characteristic only. So if characteristic Your data that wants to be overlooked by appropriate VPN with competence IPSec, therefore not necessarily again utilizes it because IPSec easier relative at configuration and at troubleshoot. But if traffic You consisting of protocols besides IP or IP communication get multicast's characteristics, therefore utilizes GRE or L2TP.
Well-matched GRE is utilized if You want to make site to site VPN's communication that will be passed by various communications protocol kind. Besides, GRE also well-matched being utilized deep through multicast's IP package as one a lot of is utilized deep routing protocol. So match is utilized as band of communication among router. GRE that to enkapsulation will all traffic without source and aim care it.
For network what do a lot of impassable by traffic for stationary networking Microsoft, L2TP really close-fitting to be utilized in here. Since its relationship that hand in glove with PPP protocol, L2TP also well-matched being utilized deep build access VPN's remote that need multiprotokol's support.
But one becomes constraint be good GRE and also L2TP no that have enkripsi's system and data perfection keeper. Therefore of that, usually deep implementation both of this VPN'S technology merged by its purpose with IPSec to get enkripsi's facility and integrity keeper mechanism its data.

Safe and Comfortable
VPN really molded of second conjugate technological already been enlightened in broad outline upon. There is one principle which amends among data communication practitioner that says that “ safe data communication will never cozy ”. That principle maybe available its scorpion is right, whereabouts You shall make policy policy that dazes to carry the wind, tunneling's teches and enkripsi what do You will utilize, and rule rule what do so tights and play by play to stop all rioter that don't deserve to access Your data. But, technology VPN may can be counted out deep that principle.

Really correctness, performa is network VPN won't can as good as tissue personal truthfully. Big latensi time must espouse to whichever VPN goes. Besides, this network so sensitif to happening trouble midway entah whereabouts. But, all that risk still maybe accepted since if was connected, tremendous convenience You can enjoy. Moreover, to You practitioner carry on business, a great many business applications which can be made by use of VPN.

Proxy For Sharing Internet

2007-07-26T13:34:00.000+08:00

Tech proxy is tech standard one for ala internet access goes together by severally computer at a swoop in one Local Network's Area (LAN) via one modem or one communication channel. Proxy's terminology own a lot of recognised / is utilized especially at the world / diplomatic circle. Classically proxy is someone / acting institute as intermediate or on behalf from other people / institute / other state.

This tech is recognised with severally name which is at marketing, e.g.:

• Connection Sharing's internet (ICS) – this terminology utilized by Microsoft on its Windows 2000.
• Proxy is server – it usually as software of affix that is assembled at acting computer as intermediate.
• Sharing's internet server (ISS) – usually as selfsupporting hardware furnished with modem, hub and proxy's software in it.
• Network Address Translation (NAT) – other terminology that is utilized for proxy's software server.
• IP Masquerade – tech that is utilized at NAT / Proxy's software server to do proxy's process.

Why is proxy's tech becomes to be of important for share internet access from one LAN ala goes together? As picture of common, in one computer network – included Internets, all network component at identifies by one number (at Internet is known as Protocol internet address, internet protocol address, IP address). Why utilized by number? Since IP's number purpose will make easy route's process & forwarding data – than if utilize no name its order. Approximately kindred concept ala by patterns that is used at phone number.

Nah is its hoodoo, (1 ) this IP's number its circumscribed amount and (2 ) we oftentimes not want persons to know from computer which / network which we access Internet in order not to be opened for attack cracker from Internet network that its public character.

Base two (2 ) main reasons upon, therefore developed private network's concept, private's network or then recognised with IntraNet (as foe from Internet). This IntraNet's network is next become basis for network at complex about office, plant wide, campus, Internet booth (WARNET) etcetera. Technologically no difference it among IntraNet & Internet, difference that significant is internet protocol address that is utilized. In Internet deal, one Intanet (private's network) can utilize internet protocol address in 192.168.x.x's region or 10.x.x.x. IP 192.168 & 10 not at all utilized by Internets because really is allocated for IntraNet's need only.

pengkaitan's process tissue typing second that variably it is done in a simple via one computer or going tool proxy's software upon. So on functioning computer as intermediate this, will ever have two (2 ) interface (among face), usually one as modem to tack on to Internet network, and one Ethernet Card to tack on to IntraNet's network that private's cipher.

To link network second that variably it, which is Internet & IntraNet, need to be done by translasi address / IP Address. proxy / Network Address Translation's tech own for that matter simple by use of table eight (8 ) columns, one that meaty information:

• workstation's internet protocol address that asks for relationship.
• workstation's application port that asks for relationship.
• proxy's internet protocol address server that accepts to see dammed hell first proxy.
• proxy's application port server that accepts to see dammed hell first proxy.
• proxy's internet protocol address server that keeps on to see dammed hell first proxy
• proxy's application port server that keeps on to see dammed hell first proxy.
• Intent server internet protocol address.
• Intent server application port.

In this way, package with internet protocol address couple information:port from workstation user what does ask for internet protocol address couple service:intent server port can be substituted that server intenting to suspect that service requisition its coming from internet protocol address couple:proxy's port server that keeps on to see dammed hell first proxy. Intent server will send all requested data to internet protocol address couple:proxy's port server that keeps on to see dammed hell first proxy – is next keep on it again to internet protocol address couple:workstation's port user that utilize 192.168.x.x's internet protocol address.

If we see blur ala, for that matter tech proxy this constitute simplest tech from one firewall. Why? With proxy's tech, intent server doesn't know that computer address that ask for that data for that matter is at turn back proxy server & utilize private's internet protocol address 192.168.x.x.

The concept of the IMS (IP Multimedia Service)Procedure

2007-04-09T15:34:00.000+08:00

The IMS foundation four safety the main specification.

One safety
Delivery the service of multimedia communication characteristically real time and person to person with the IP basis (like voice or videotelepony), likewise his matter with communication person to mechine (like the service gabling).

Two safety
Integrated the service of multimedia communication real time (like the video streaming and live chatting).

Three safety
Could serve and interact with the service and the application that were varied like combined presence and instant messaging.

Four safety
The Ease in melkukan the set up the multi-service in one sesion single or multi sesion simultaneously.

IMS could in toimplementasion wth GPR/EDGE, CDMA EV-DO, UMTS, xDSL or WLAN. To use IMS, the operator carried him out parallel from the available network.
When the operator wanted to place the service voice with communication IP, then IMS increased the application VoIP parallel.

As signaling protocol him, IMS used SIP (session initiation protocol) that was standardised by IETF (the internet engineering task force).
However, because initially standarisai this only was focussed to fixed the internet, the standard that was used by the network selullar will be different.
The theory SIP proxy server this that was worn 3GPP as the concept of the IMS foundation.

SIP that was used by IMS was as protocol application servers and softswitch that could interact with IAD or Access Gateway as well as developed communication between caller and that was called.

SIP could be used constructive caller, nerworking, as well as constructive the session of multimedia communication.
At this time was developed SIP especially to telepony that was mentioned SIP-T. the numbering System SIP-T fully pointed system cash that was used ITU-T, as well as to akomododir the mechanism interegrasi the service telepony with web like: UMS, the internet call waiting, click to dial and instant messanging

Wireless Communication technology for the Multimedia

2007-03-22T09:26:00.000+08:00

Bluetooth is Service was a communication technology wireless (without the cable) that operated in the frequency tape 2.4 of GHz unlicensed ISM (industrial, Scientific and Medical) by using one frequency hopping tranceiver that could provide the service and the voice of data communication in a manner real-time between host-host bluetooth and the distance of the range of the limited service (around 10 metre).
Bluetooth personally could take the form of card that the form and his function almost be the same as card that was used to wireless local the area network (WLAN) where using the frequency of IEEE standard radio 802.
11, only in bluetooth had the range of the distance of the shorter service and the transfer capacity of the lower data.

Basically bluetooth was created not only to replace or eliminated the use of the cable inside carried out the exchange of information, but also could offer fitur that was good for technology mobile wireless at a cost of that was relatively low, consumption of the low power, interoperability that was promising, was easy in the operation and could provide the service that various things.

To give the picture that was sharper concerning technology bluetooth that was relatively new this to the reader, along with was untangled about the history of the emergence bluetooth and his development, technology that was used in the system bluetooth and the aspect of the service that could be provided, as well as few analyses about the comparison of the modulation method spread spectrum FHSS (Frequency Hopping Spread Spectrum) that was used by bluetooth compared with the method spread spectrum DSSS (Direct Sequence Spread Spectrum).
In May 1998, the champion's 5 companies that is Ericsson, IBM, Intelligence, Nokia and Toshiba formed one of Special Interest Group (SIG) and began to make the specification that was named by them ‘bluetooth’.

In July 1999 the specification document bluetooth the version 1.
0 began to be launched.
In December 1999 was begun again by the production of the specification document bluetooth the version 2.
0 with the addition 4 new champions that is 3Com, Lucent Technologies, Microsoft and Motorola.
At this time, more than 1800 companies in various fields in part in the field semiconductor manufacture, PC manufacture, mobile network carrier, companies automobile and water lines bergambung in a consortium as adopter technology bluetooth.
These foremost companies in part like Compaq, Xircom, Phillips, Texas instruments, Sony, BMW, the Puma, NEC, Casio, Boeing, etc..
Although the Bluetooth SIG standard at this time ‘dimiliki’ by the group of the champion but he will it was hoped become a IEEE standard (802.15).

Protocol bluetooth used a combination between circuit switching and packet switching.
Bluetooth could support a data canal asinkron, three synchronous voice canals simultaneous or a canal where simultaneously supported the data service asinkron and the synchronous voice.
Each voice canal supported a synchronous voice canal 64 kb/s.
The canal asinkron could support the maximal speed 723.2 kb/s asymmetric, where for the direction was the reverse could support up to the speed 57.6 family planning/s.
Whereas to mode symmetrical could support up to the speed 433.9 kb/s.

An equipment that had technology wireless bluetooth will have the capacity to carry out the exchange of information with the distance of the range up to 10 metre (~30 feet).
The system bluetooth provided the communication service point to point and communication point to multipoint.
The product bluetooth could take the form of PC card or USB adapter that was put into equipment.
Equipment that could diintegerasikan with technology bluetooth in part: mobile PC, mobile phone, PDA (Personal Digital Assistant), headset, the camera, printer, router et cetera.
Applications that could be provided by the service bluetooth this in part: PC to PC file the transfer, PC to PC file synch (notebook to desktop), PC to mobile phone, PC to PDA, wireless headset, LAN connection via ethernet access point and etc.

Bluetooth FHSS vs WLAN DSSS
In fact why bluetooth more chose the FHSS method (Frequency Hopping Spread Spectrum) compared with DSSS (Direct Sequence Spread Spectrum).
The reason that made why bluetooth did not use DSSS in part as follows:

1.FHSS needed consumption of the power and the complexity that were lower compared with DSSS this was caused because DSSS used the speed chip (chip rate) compared with the speed of the symbol (symbol rate) that was used by FHSS, so as cost that was needed to use DSSS will be higher.

2.FHSS used FSK where endurance of the disturbance noise relative better compared with DSSS that usually uses QPSK (for IEEE 802.
11 2 Mbps) or CCK (IEEE 802.
11b 11 Mbps).

Although FHSS had the distance of the range and the transfer of the data that was lower compared with DSSS but for the service was supervised 2 Mbps FHSS could give the solution cost-effective that was better.

Seamless Mobility

2007-03-15T12:20:00.000+08:00

During him to no longer distinguished the function of the house telephone, the office telephone, personal enamel, and phonsel you.
The existence seamless mobility enabled anyone to terkoneksi by any, including communicating with plasama the house TV.

For the last three year you have enjoyed many benefits of cellular technology latest, from that was simplest sepertti voice calling, smsed, GPRS, MMS, the video streaming, the speed of data access via EDGE and CDMA-1X, to now that just just emerged like technology 3G and convergence between cellular and access broadband.

Further possibly you still do not think about many matters to terkoneksi with the digital thing thing in and around us.
The concept seamless mobility that was introduced by Avaya Technology and Motorola became one of the new solutions to speed up the process kovergensi between you, technology wired and wireless, and any available in and around us.

Alcatel and Ericsson mentioned him as mobile triple play tried to be visited by you http://www.freescape.com/seamless, there was shown that this concept enabled you to continue terkoneksi and synchronisation with the digital device that has disesuikan with your personal data.
For example when you were being in the carriage sembari saw the presentation of football from live the TV in the screen phonsel, and you too could continue him when arriving in the station and following the trip by the car.

You could listen to his direct report from car stereo radio or the screen plasmaTV in the car.
When until in home, then the presentation live this TV will continue to the personal TV in the house without did setting the manual to the channel TV. And like that henceforth.

This all was enabled by the blessing konfergensi the network that was high between wired and wireless, used internet technology protocol (IP), and interoperability and kontabilitythat was high.
The concept seamless mobility made the cellular phone the main key from all the activities, good in the house, in the office, and anywhere.
And as means to connect with the directory database in the office and most synchronous with digital equipment in the house as re-beads.

All was done in a real manner time by giving priority to the concept seamless that connected between humankind and equipment, humankind with humankind, equipment with humankind, and equipment with equipment.
Seamless mobility introduced in user a new convergence technology that connected between the function wired and wireless, voice and the data, as well as local and wide the area.
Wired voice and the network of the data in the function enterprise him normally went with fitur voip, in his convergence between keduan him, the data went together on the network of the cellular area, and in implemetasikan inside voice-enabled WLANs.
The intensity in this very convergent environment, the user only needed one kind handheld then.
To connect the two funsi this, then was created one single handheld
that very multimode, the multi-band, and multimedia.
One of them with made dual-network phone that could terkoneksi with IEEE 802.
11 standards of Wi-Fi to connect with FANTASTIC-based IP-PBX, and in the environment of the cellular phone industry could do seamless handoff between WLAN access point in one industrial environment, and outside between voice enabled-WLAN and the network selular.
This solution was very useful for enterprise market, because handheld the user apart from terkoneksi with the cellular channel, also could enjoy facilities that were given by the company, including the conference call, call hold and access voice inside database and the company's directory.

Basically with the existence of the solution seamless mobility, was hoped the profit that was obtained from convergence of several networks was to reduce the management cost from several networks.
With the technological emergence 3G this end end, was hoped for by all sitem multimedia as well as the implementation seamless mobility could real was felt in and around us, his matter with the existence 3G, technology seamless mobility can be realised.

Lightweight Directory Access Protocol

2007-02-13T15:43:00.000+08:00

The Lightweight Directory Access Protocol, or LDAP in computer networking,is a networking protocol for querying and modifying directory services running over TCP/IP.

A directory is a set of information with similar attributes organized in a logical and hierarchical manner. The most common example is the telephone directory, which consists of a series of names (either of a person or organization) organized alphabetically, with an address and phone number attached.

An LDAP directory often reflects various political, geographic, and/or organizational boundaries, depending on the model chosen. LDAP deployments today tend to use Domain Name System (DNS) names for structuring the topmost levels of the hierarchy. Deeper inside the directory might appear entries representing people, organizational units, printers, documents, groups of people or anything else which represents a given tree entry (or multiple entries).

Telecommunication companies introduced the concept of directory services to information technology and computer networking, as their understanding of directory requirements was well-developed after some 70 years of producing and managing telephone directories. The culmination of this input was the comprehensive X.500 specification, a suite of protocols produced by the International Telecommunication Union (ITU) in the 1980s.

X.500 directory services were traditionally accessed via the X.500 Directory Access Protocol (DAP), which required the Open Systems Interconnection (OSI) protocol stack. LDAP was originally intended to be a "lightweight" alternative protocol for accessing X.500 directory services through the simpler (and now widespread) TCP/IP protocol stack. This model of directory access was borrowed from the DIXIE and Directory Assistance Service protocols.

Standalone LDAP directory servers soon followed, as did directory servers supporting both DAP and LDAP. The latter has become popular in enterprises, as LDAP removed any need to deploy an OSI network. Today, X.500 directory protocols including DAP can also be used directly over TCP/IP.

The protocol was originally created by Tim Howes of the University of Michigan, Steve Kille of ISODE and Wengyik Yeong of Performance Systems International, circa 1993. Further development has been done via the Internet Engineering Task Force (IETF).

In the early engineering stages of LDAP, it was known as Lightweight Directory Browsing Protocol, or LDBP. It was renamed as the scope of the protocol was expanded to include not only directory browsing and searching functions, but also directory update functions.

LDAP has influenced subsequent Internet protocols, including later versions of X.500, XML Enabled Directory (XED), Directory Service Markup Language (DSML), Service Provisioning Markup Language (SPML), and the Service Location Protocol (SLP).

Protocol LDAP
A client starts an LDAP session by connecting to an LDAP server, by default on TCP port 389. The client then sends operation requests to the server, and the server sends responses in turn. With some exceptions the client need not wait for a response before sending the next request, and the server may send the responses in any order.

The basic operations are:

* Start TLS - optionally protect the connection with Transport Layer Security (TLS), to have a more secure connection
* Bind - authenticate and specify LDAP protocol version
* Search - search for and/or retrieve directory entries
* Compare - test if a named entry contains a given attribute value
* Add a new entry
* Delete an entry
* Modify an entry
* Modify DN - move or rename an entry
* Abandon - abort a previous request
* Extended Operation - generic operation used to define other operations
* Unbind - close the connection (not the inverse of Bind)

In addition the server may send "Unsolicited Notifications" that are not responses to any request, e.g. before it times out a connection.

A common alternate method of securing LDAP communication is using an SSL tunnel. This is denoted in LDAP URLs by using the URL scheme "ldaps". The default port for LDAP over SSL is 636. The use of LDAP over SSL was common in LDAP Version 2 (LDAPv2) but it was never standardized in any formal specification. This usage has been deprecated along with LDAPv2, which was officially retired in 2003.

LDAP is defined in terms of ASN.1, and protocol messages are encoded in the binary format BER. It uses textual representations for a number of ASN.1 fields/types, however.

The protocol accesses LDAP directories, which follow the 1993 edition of the X.500 model:

* A directory is a tree of directory entries.
* An entry consists of a set of attributes.
* An attribute has a name (an attribute type or attribute description) and one or more values. The attributes are defined in a schema (see below).
* Each entry has a unique identifier: its Distinguished Name (DN). This consists of its Relative Distinguished Name (RDN) constructed from some attribute(s) in the entry, followed by the parent entry's DN. Think of the DN as a full filename and the RDN as a relative filename in a folder.

Be aware that a DN may change over the lifetime of the entry, for instance, when entries are moved within a tree. To reliably and unambiguously identify entries, a UUID may be provided in the set of the entry's operational attributes.

An entry can look like this when represented in LDIF format (LDAP itself is a binary protocol):

dn: cn=John Doe,dc=example,dc=com
cn: John Doe
givenName: John
sn: Doe
telephoneNumber: +1 888 555 6789
telephoneNumber: +1 888 555 1234
mail: john@example.com
manager: cn=Barbara Doe,dc=example,dc=com
objectClass: inetOrgPerson
objectClass: organizationalPerson
objectClass: person
objectClass: top

dn is the name of the entry; it's not an attribute nor part of the entry. "cn=John Doe" is the entry's RDN, and "dc=example,dc=com" is the DN of the parent entry. The other lines show the attributes in the entry. Attribute names are typically mnemonic strings, like "cn" for common name, "dc" for domain component, and "mail" for e-mail address.

A server holds a subtree starting from a specific entry, e.g. "dc=example,dc=com" and its children. Servers may also hold references to other servers, so an attempt to access "ou=department,dc=example,dc=com" could return a referral or continuation reference to a server which holds that part of the directory tree. The client can then contact the other server. Some servers also support chaining, which means the server contacts the other server and returns the results to the client.

LDAP rarely defines any ordering: The server may return the values in an attribute, the attributes in an entry, and the entries found by a search operation in any order. This follows from the formal definitions - an entry is defined as a set of attributes, and an attribute is a set of values, and sets are inherently unordered.

GPS of Navigation Signals

2007-02-05T15:13:00.000+08:00

The user's GPS receiver is the user segment of the GPS system. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years such that, as of 2006, receivers typically have between twelve and twenty channels.

GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of a RS-232 port at 4,800 bps speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. As of 2006, even low-cost units commonly include WAAS receivers.

Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. NMEA 2000[9] is a newer and less widely adopted protocol. Both are proprietary and controlled by the US-based National Marine Electronics Association. References to the NMEA protocols have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the SiRF protocol. Receivers can interface with other devices using methods including a serial connection, USB or Bluetooth.

Navigation signals
GPS satellites broadcast three different types of data in the primary navigation signal. The first is the almanac which sends coarse time information along with status information about the satellites. The second is the ephemeris, which contains orbital information that allows the receiver to calculate the position of the satellite. This data is included in the 37,500 bit Navigation Message, which takes 12.5 minutes to send at 50 bps.

GPS satellites broadcast three different types of data in the primary navigation signal. The first is the almanac which sends coarse time information along with status information about the satellites. The second is the ephemeris, which contains orbital information that allows the receiver to calculate the position of the satellite. This data is included in the 37,500 bit Navigation Message, which takes 12.5 minutes to send at 50 bps.

The satellites also broadcast two forms of clock information, the Coarse / Acquisition code, or C/A which is freely available to the public, and the restricted Precise code, or P-code, usually reserved for military applications. The C/A code is a 1,023 bit long pseudo-random code broadcast at 1.023 MHz, repeating every millisecond. Each satellite sends a distinct C/A code, which allows it to be uniquely identified.

The P-code is a similar code broadcast at 10.23 MHz, but it repeats only once a week. In normal operation, the so-called "anti-spoofing mode", the P code is first encrypted into the Y-code, or P(Y), which can only be decrypted by units with a valid decryption key. Frequencies used by GPS include:
• L1 (1575.42 MHz) - Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted precision P(Y) code.
• L2 (1227.60 MHz) - P(Y) code, plus the new L2C code on the Block IIR-M and newer satellites.
• L3 (1381.05 MHz) - Used by the Defense Support Program to signal detection of missile launches, nuclear detonations, and other high-energy infrared events.
• L4 (1379.913 MHz) - Being studied for additional ionospheric correction.
• L5 (1176.45 MHz) - Proposed for use as a civilian safety-of-life (SoL) signal (see GPS Modernization). This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2008.

Calculating positions
The coordinates are calculated according to the World Geodetic System WGS84 coordinates system. To calculate its position, a receiver needs to know the precise time. The satellites are equipped with extremely accurate atomic clocks, and the receiver uses an internal crystal oscillator-based clock that is continually updated using the signals from the satellites.

The receiver identifies each satellite's signal by its distinct C/A code pattern, then measures the time delay for each satellite. To do this, the receiver produces an identical C/A sequence using the same seed number as the satellite. By lining up the two sequences, the receiver can measure the delay and calculate the distance to the satellite, called the pseudorange.

The orbital position data from the Navigation Message is then used to calculate the satellite's precise position. Knowing the position and the distance of a satellite indicates that the receiver is located somewhere on the surface of an imaginary sphere centered on that satellite and whose radius is the distance to it. When four satellites are measured simultaneously, the intersection of the four imaginary spheres reveals the location of the receiver. Earth-based users can substitute the sphere of the planet for one satellite by using their altitude. Often, these spheres will overlap slightly instead of meeting at one point, so the receiver will yield a mathematically most-probable position (and often indicate the uncertainty).

Calculating a position with the P(Y) signal is generally similar in concept, assuming one can decrypt it. The encryption is essentially a safety mechanism; if a signal can be successfully decrypted, it is reasonable to assume it is a real signal being sent by a GPS satellite. In comparison, civil receivers are highly vulnerable to spoofing since correctly formated C/A signals can be generated using readily available signal generators. RAIM features will not help, since RAIM only checks the signals from a navigational perspective.

Accuracy and Error Sources
The position calculated by a GPS receiver requires the current time, the position of the satellite and the measured delay of the received signal. The position accuracy is primarily dependent on the satellite position and signal delay.
To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version. By comparing the rising and trailing edges of the bit transitions, modern electronics can measure signal offset to within about 1% of a bit time, or approximately 10 nanoseconds for the C/A code. Since GPS signals propagate nearly at the speed of light, this represents an error of about 3 meters. This is the minimum error possible using only the GPS C/A signal.

Position accuracy can be improved by using the higher-speed P(Y) signal. Assuming the same 1% accuracy, the faster P(Y) signal results in an accuracy of about 30 centimeters.
Electronics errors are one of several accuracy-degrading effects outlined in the table below. When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (50 ft). These effects also reduce the more precise P(Y) code's accuracy.

Atmospheric effects
Changing atmospheric conditions change the speed of the GPS signals as they pass through the Earth's atmosphere and ionosphere. Correcting these errors is a significant challenge to improving GPS position accuracy. These effects are minimized when the satellite is directly overhead, and become greater for satellites nearer the horizon, since the signal is affected for a longer time. Once the receiver's approximate location is known, a mathematical model can be used to estimate and compensate for these errors.

Because ionospheric delay affects the speed of radio waves differently based on frequency, a characteristic known as dispersion, both frequency bands can be used to help reduce this error. Some military and expensive survey-grade civilian receivers compare the different delays in the L1 and L2 frequencies to measure atmospheric dispersion, and apply a more precise correction. This can be done in civilian receivers without decrypting the P(Y) signal carried on L2, by tracking the carrier wave instead of the modulated code. To facilitate this on lower cost receivers, a new civilian code signal on L2, called L2C, was added to the Block IIR-M satellites, first launched in 2005. It allows a direct comparison of the L1 and L2 signals using the coded signal instead of the carrier wave.

The effects of the ionosphere are generally slow-moving, and can be averaged over time. The effects for any particular geographical area can be easily calculated by comparing the GPS-measured position to a known surveyed location. This correction is also valid for other receivers in the same general location. Several systems send this information over radio or other links to allow L1 only receivers to make ionospheric corrections. The ionospheric data are transmitted via satellite in Satellite Based Augmentation Systems such as WAAS, which transmits it on the GPS frequency using a special PRN, so only one antenna and receiver are required.

Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the troposphere. This effect is much more localized, and changes more quickly than the ionospheric effects, making precise compensation for humidity more difficult. Altitude also causes a variable delay, as the signal passes through less atmosphere at higher elevations. Since the GPS receiver measures altitude directly, this is much simpler correction to apply.

Multipath effects
GPS signals can also be affected by multipath issues, where the radio signals reflect off surrounding terrain; buildings, canyon walls, hard ground, etc. These delayed signals can cause inaccuracy. A variety of techniques, most notably narrow correlator spacing, have been developed to mitigate multipath errors. For long delay multipath, the receiver itself can recognize the wayward signal and discard it. To address shorter delay multipath from the signal reflecting off the ground, specialized antennas may be used. Short delay reflections are harder to filter out since they are only slightly delayed, causing effects almost indistinguishable from routine fluctuations in atmospheric delay.

Multipath effects are much less severe in moving vehicles. When the GPS antenna is moving, the false solutions using reflected signals quickly fail to converge and only the direct signals result in stable solutions.

Ephemeris and clock errors
The navigation message from a satellite is sent out only every 12.5 minutes. In reality, the data contained in these messages tend to be "out of date" by an even larger amount. Consider the case when a GPS satellite is boosted back into a proper orbit; for some time following the maneuver, the receiver’s calculation of the satellite's position will be incorrect until it receives another ephemeris update. The onboard clocks are extremely accurate, but they do suffer from some clock drift. This problem tends to be very small, but may add up to 2 meters (6 ft) of inaccuracy.
This class of error is more "stable" than ionospheric problems and tends to change over days or weeks rather than minutes. This makes correction fairly simple by sending out a more accurate almanac on a separate channel.

Selective availability
The GPS includes a feature called Selective Availability (SA) that introduces intentional errors between 0 meters and up to a hundred meters (300 ft) into the publicly available navigation signals, making it difficult to use for guiding long range missiles to precise targets. Additional accuracy was available in the signal, but in an encrypted form that was only available to the United States military, its allies and a few others, mostly government users.

SA typically added signal errors of up to about 10 meters (30 ft) horizontally and 30 meters (100 ft) vertically. The inaccuracy of the civilian signal was deliberately encoded so as not to change very quickly, for instance the entire eastern U.S. area might read 30 m off, but 30 m off everywhere and in the same direction. In order to improve the usefulness of GPS for civilian navigation, Differential GPS was used by many civilian GPS receivers to greatly improve accuracy.

During the Gulf War, the shortage of military GPS units and the wide availability of civilian ones among personnel resulted in a decision to disable Selective Availability. This was ironic, as SA had been introduced specifically for these situations, allowing friendly troops to use the signal for accurate navigation, while at the same time denying it to the enemy. But since SA was also denying the same accuracy to thousands of friendly troops, turning it off or setting it to a error of 0 meters (effectively the same thing) presented a clear benefit.

In the 1990s, the FAA started pressuring the military to turn off SA permanently. This would save the FAA millions of dollars every year in maintenance of their own radio navigation systems. The military resisted for most of the 1990s, but SA was eventually "discontinued"; the amount of error added was "set to zero" in 2000 following an announcement by U.S. President Bill Clinton, allowing users access to an undegraded L1 signal. Per the directive, the induced error of SA was changed to add no error to the public signals (C/A code). Selective Availability is still a system capability of GPS, and error could be in theory reintroduced at any time. In practice, in view of the hazards and costs this would induce for US and foreign shipping, it is unlikely to be reintroduced, and various government agencies, including the FAA, have stated that it is not intended to be reintroduced.
The US military has developed the ability to locally deny GPS (and other navigation services) to hostile forces in a specific area of crisis without affecting the rest of the world or its own military systems.

GPS jamming
Jammingof any radio navigation system, including satellite based navigation, is possible. The U.S. Air Force conducted GPS jamming exercises in 2003 and they also have GPS anti-spoofing capabilities. In 2002, a detailed description of how to build a short range GPS L1 C/A jammer was published in Phrack issue 60 by an anonymous author. There has also been at least one well-documented case of unintentional jamming, tracing back to a malfunctioning TV antenna preamplifier. If stronger signals were generated intentionally, they could potentially interfere with aviation

GPS receivers within line of sight. According to John Ruley, of AVweb, "IFR pilots should have a fallback plan in case of a GPS malfunction". Receiver Autonomous Integrity Monitoring(RAIM), a feature of some aviation and marine receivers, is designed to provide a warning to the user if jamming or another problem is detected. GPS signals can also be interfered with by natural geomagnetic storms, predominantly at high latitudes.

The U.S. government believes that such jammers were also used occasionally during the 2001 war in Afghanistan. Some officials believe that jammers could be used to attract the precision-guided munitions towards non-combatant infrastructure; other officials believe that the jammers are completely ineffective. In either case, the jammers may be attractive targets for anti-radiation missiles. During the Iraq War, the U.S. military claimed to destroy a GPS jammer with a GPS-guided bomb.

Relativity
According to Einstein's Theory of relativity, because of their constant movement and height relative to the Earth Centered Inertial reference frame the clocks on the satellites are affected by their speed (special relativity) as well as their gravitational potential (general relativity). Friedwardt Winterberg predicted in 1955 that when observed from the Earth's reference frame, satellite clocks would be perceived as running at a slightly faster rate than clocks on the Earth's surface.

For GPS satellites, this discrepancy is 38 microseconds per day. To account for this, the frequency standard on-board the satellites are given a rate offset prior to launch, making it run slightly slower than its desired frequency on Earth, at 10.22999999543 MHz instead of 10.23 MHz, a difference of -4.465 parts in 1010. The atomic clocks on board the GPS satellites are precisely tuned, making this a practical engineering application of the scientific theory of relativity in a real-world system.

Another relativistic effect to be compensated for in GPS observation processing is the Sagnac effect. The GPS time scale is defined in an inertial system, but observations are processed in an ECEF (co-rotating) system, in which simultaneity is not uniquely defined. The Lorentz transformation between the two systems modifies the signal run time, a correction having opposite algebraic signs for satellites in the Eastern and Western celestial hemispheres. Ignoring this effect will produce an East-West offset in the absolute position solution on the order of tens of metres
Neil Ashby presented in Physics Today (May 2002) an account how these relativistic corrections are applied, and their orders of magnitude. The error introduced by relativistic effects can be as much as 15 meters. The GPS system also makes adjustments for the relativistic drift of the atomic clocks in the satellites. Parts of this correction are carried out in the satellites and parts in the receiver.

Techniques to improve accuracy
Augmentation methods of improving accuracy rely on external information being integrated into the calculation process. There are many such systems in place and they are generally named or described based on how the GPS sensor receives the information. Some systems transmit additional information about sources of error (such as clock drift, ephemeris, or ionospheric delay), others provide direct measurements of how much the signal was off in the past, while a third group provide additional navigational or vehicle information to be integrated in the calculation process.

Examples of augmentation systems include the Wide Area Augmentation System, Differential GPS, and Inertial Navigation Systems

Precise Monitoring
The accuracy of a calculation can also be improved through precise monitoring and measuring of the existing GPS signals in additional or alternate ways.
The first is called Dual Frequency monitoring, and refers to systems that can compare two or more signals, such as the L1 frequency to the L2 frequency. Since these are two different frequencies, they are affected in different, yet predictable ways by the atmosphere and objects around the receiver. After monitoring these signals, it is possible to calculate how much error is being introduced and then nullify that error.
Receivers that have the correct decryption key can relatively easily decode the P(Y)-code transmitted on both L1 and L2 to measure the error. Receivers that do not possess the key can still use a process called codeless to compare the encrypted information on L1 and L2 to gain much of the same error information. However, this technique is currently limited to specialized surveying equipment. In the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies. When these become operational, non-encrypted users will be able to make the same comparison and directly measure some errors.

A second form of precise monitoring is called Carrier-Phase Enhancement (CPGPS). The error, which this corrects, arises because the pulse transition of the PRN is not instantaneous, and thus the correlation (satellite-receiver sequence matching) operation is imperfect. The CPGPS approach utilizes the L1 carrier wave, which has a period 1000 times smaller than that of the C/A bit period, to act as an additional clock signal and resolve the uncertainty. The phase difference error in the normal

GPS amounts to between 2 and 3 meters (6 to 10 ft) of ambiguity. CPGPS working to within 1% of perfect transition reduces this error to 3 millimeters (1 inch) of ambiguity. By eliminating this source of error, CPGPS coupled with DGPS normally realizes between 20 and 30 centimeters (8 to 12 inches) of absolute accuracy.
Relative Kinematic Positioning (RKP) is another approach for a precise GPS-based positioning system. In this approach, determination of range signal can be resolved to an accuracy of less than 10 centimeters (4 in). This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver. This can be accomplished by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests — possibly with processing in real-time (real-time kinematic positioning, RTK).

GPS Time
Atomic clocks on the satellites are set to "GPS time", similar to most time standards, but not corrected to the rotation of the Earth, ignoring leap seconds and other corrections. GPS time was set to match Coordinated Universal Time (UTC) in 1980, but has since diverged as leap seconds were added to UTC.
The current date is expressed in the GPS signal as a week number and a day-of-week number. GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980. The week number is transmitted in a ten-bit field, and so it wraps round every 1,024 weeks, (19.7 years). The transmitted week number rolled over to zero at 00:00:19 TAI on August 22, 1999 (23:59:47 UTC on August 21, 1999). GPS receivers thus need to know the time to within 3,584 days in order to correctly interpret the GPS date signal. A new field is being added to the GPS navigation message that specifies the calendar year number exactly, in a sixteen-bit field.

The GPS navigation message includes the difference between GPS time and UTC, which is 14 seconds as of 2006. Receivers subtract this offset from GPS time to calculate UTC and 'local' time. New GPS units may not show the correct UTC time, or not attempt to show UTC time at all, until after receiving the UTC offset message for the first time. This is usually within 15 minutes after the unit achieves GPS lock. The GPS-UTC offset field is only eight bits, and so it wraps round every 256 leap seconds. At the current rate of change of the earth's rotation, the first wraparound of this field is projected to occur in the year 2330.

GPS Modernization
Having reached Fully Operational Capability on July 17, 1995, the GPS completed its original design goals. However, additional advances in technology and new demands on the existing system led to the effort to "modernize" the GPS system. Announcements from the Vice Presidential and the White House in 1998 heralded the beginning of these changes and in 2000, the U.S. Congress reaffirmed the effort; referred to it as GPS III.
The project aims to improve the accuracy and availability for all users and involves new ground stations, new satellites, and four additional navigation signals. New civilian signals are called L2C', L5 and L1C; The new military code is called M-Code. A goal of 2013 has been established with incentives offered to the contractors if they can complete it by 2011

Basics GPS

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GPS stands for Global Positioning System and it is a system that can provide a position at any point on the Earth's surface to a very high degree of accuracy. The Global Positioning System (GPS) uses 24 active Navstar satellites in orbit 11 000 miles above the surface of the Earth.

Using economic ground based receivers GPS is able to provide position information to within a number of metres. The economic costs have also meant that it is now fitted to many motor vehicles, while separate GPS receivers can be bought for a few hundred pounds or dollars. As a result it is widely used by private individuals, as well as many commercial and professional users. In fact the primary use for GPS is as a military navigation system. The fact that it is used so widely is a by product of its success.

Basic concept
GPS operates by being able to measure the distances from the satellites that are in orbit around the Earth. By knowing the distance from a number of satellites, it is possible to calculate the position on the Earth's surface and the height above it by a process of triangulation. This a great simplification, but this is essentially how it works.

The satellites all send timing information so the receiver knows when the message was sent. As radio signals travel at the speed of light they take a very short but finite time to travel the distance from the satellite to the receiver. The satellites also transmit information about their positions. In this way the receiver is able to calculate the distance from the satellite to the receiver. To obtain a full fix, four satellites are required, and when the receiver is in the clear, more than four satellites are in view all the time.

Satellites
The satellites are orbiting above the Earth. Their orbits are tightly controlled because errors in their orbit will translate to errors in the final positions. The time signals are also tightly controlled. The satellites contain an atomic clock so that the time signals they transmit are very accurate. Even so these clocks will drift slightly and to overcome this, signals from Earth stations are used to correct this.

The satellites themselves have a design life of ten years, but to ensure that there are no holes in service in the case of unexpected failures, spares are held in orbit and these can be brought into service at short notice.

The satellites are provide their own power through their solar panels. These extend to about 17 feet, and provide the 700 watts needed to power the satellite and its batteries when it is in sunlight. Naturally the satellite needs t remain operation when it is on the dark side of the Earth when the solar panels do not provide any power. This means that when in sunlight the solar panels need to provide additional power to charge batteries, beyond just powering the basic satellite circuitry.

Receivers
A large number of GPS receivers are available today. They make widespread use of digital signalling processing techniques. The transmissions from the satellites use spread spectrum technology, and the signal processors correlate the signals received to recover the data. As the signals are very weak it takes some time after the receiver is turned on to gain the first fix. This Time To First Fix (TTFF) may be as long as twelve minutes, although receivers that us a large number of correlators are able to shorten this.

When using a GPS receiver the receiver must be in the open. Buildings, or any structure will mask the signals and it may mean that few satellites can be seen. Thus the receivers will not operate inside buildings, and urban areas may often cause problems.

Applications
The primary use for GPS is as a military navigational aid. Run by the American Department of Defense its primary role is to provide American forces with an accurate means of navigation anywhere on the globe. However its use has been opened up so that commercial and private users have access to the signals and can use the system. Accordingly it is very widely used for many commercial applications from aircraft navigation, ship navigation to surveying, and anywhere where location information is required. For private users very cost effective receivers are available these days and may be used for applications including sailing. Even many motor vehicles have them fitted now to provide SatNav systems enabling them to navigate easily without the need for additional maps.

It can be said that GPS has revolutionised global navigation since it became available. Prior to this navigation systems were comparatively localised, and did not offer anything like the same degrees of accuracy, flexibility and coverage.

NAVSTAR Global Positioning System

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The Global Positioning System (GPS), is currently the only fully-functional satellite navigation system. More than two dozen GPS satellites are in medium Earth orbit, transmitting signals allowing GPS receivers to determine location, speed and direction.

Since the first experimental satellite was launched in 1978, GPS has become indispensable for navigation around the world, and an important tool for map-making and land surveying. GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.

Developed by the United States Department of Defense, it is officially named NAVSTAR GPS (Navigation Signal Timing and Ranging Global Positioning System). The satellite constellation is managed by the United States Air Force 50th Space Wing. Although the cost of maintaining the system is approximately US$400 million per year, including the replacement of aging satellites, GPS is free for civilian use as a public good.

A GPS receiver calculates its position by measuring the distance between itself and three or more GPS satellites. Measuring the time delay between transmission and reception of each GPS radio signal gives the distance to each satellite, since the signal travels at a known speed. The signals also carry information about the satellites' location. By determining the position of, and distance to, at least three satellites, the receiver can compute its location using trilateration. Receivers do not have perfectly accurate clocks, and must track one extra satellite to correct their clock error.

Technical description
System segmentation
The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).

Space segment
The space segment is composed of the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design calls for 24 SVs to be distributed equally among six circular orbital planes. The orbital planes are centered on the Earth, and not rotating with respect to the distant stars. The six planes have approximately 55° inclination (tilt relative to the equator) and are separated by 60° right ascension of the ascending node (angle along the equator).

Orbiting at an altitude of approximately 20,000 kilometers (11,000 nautical miles), each SV makes two complete orbits each sidereal day, so it passes over the same location on Earth once each day. The orbits are arranged so that at least six satellites are always within line of sight from almost anywhere on Earth.

As of January 2007, there are 29 actively broadcasting satellites in the GPS constellation. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail.

The flight paths of the satellites are tracked by monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, along with monitor stations from other agencies. The tracking information is sent to the Air Force Space Command's master control station at Schriever Air Force Base, Colorado Springs, Colorado, which is operated by the 2d Space Operations Squadron (2 SOPS) of the United States Air Force (USAF). 2 SOPS contacts each GPS satellite regularly with a navigational update (using the ground antennas at Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs). These updates synchronize the atomic clocks on board the satellites to within one microsecond and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman Filter which uses inputs from the ground monitoring stations, space weather information, and other various inputs.

System Satellite Orbits

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The variety of different orbits that can be adopted for satellites. The ones that receive the most attention are the geostationary orbit used by many communications and direct broadcast satellites for satellite television and also the low earth orbit ones that travel around the global. Those used in the Navstar or Global Positioning (GPS) system occupy a relatively low earth orbit. There are also many other types of satellite from weather satellites to research satellites and many others.

The actual orbit that is chosen will depend on factors including its function, and the area it is to serve. In some instances the orbit may be as low as 100 miles (160 km) for a low earth orbit (LEO), whereas others may be over 22 000 miles (36000 km) high as in the case of a geostationary orbit. The satellite may even have an elliptical rather than a circular orbit.

Gravity
As satellites orbit the earth they are pulled back in by the force of the gravitational field. If they did not have any motion of their own they would fall back to earth, burning up in the upper reaches of the atmosphere. Instead the motion of the satellite rotating around the earth has a force associated with it pushing it away from the earth. For any given orbit there is a speed for which gravity and the centrifugal force balance each other and the satellite remains in a stable orbit, neither gaining height nor loosing it.

Obviously the lower the orbit, the stronger the gravitational pull, and this means that the satellite must orbit the earth faster to counteract this pull. Further away the gravitational field is less and the satellite velocities are correspondingly less. For a very low orbit of around 100 miles a velocity of about 17500 miles per hour is needed and this means that the satellite will orbit the earth in about 90 minutes. At an altitude of 22 000 miles a velocity of just less than 7000 miles per hour is needed giving an orbit time of about 24 hours.

Circular and elliptical orbits
A satellite can orbit the earth in one of two basic types of orbit. The most obvious is a circular orbit where the distance from the earth remains the same at all times. A second type of satellite orbit is an elliptical one.

When a satellite orbits the earth, either in a circular or elliptical orbit, the satellite orbit forms a plane that passes through the centre of gravity or geocentre of the Earth. The rotation around the earth is also categorised. It may be in the same direction as the earth's rotation when it is said to be posigrade, or it may be in the opposite direction when it is retrograde.

The track of the satellite around the globe is often defined as well. The point on the Earth's surface where the satellite is directly overhead moves around the globe. This is known as the ground track. This forms a circle which has the geocentre at its centre. It is worth noting that geostationary satellites are a special case as they appear directly over the same point of the earth all the time. This means that their ground track consists of a single point on the earth's equator. Also for satellites with equatorial orbits the ground track is along the equator.

Satellites may also be in other orbits. These will cross the equator twice, once in a northerly direction, and once in a southerly direction. The point at which the groundtrack crosses the equator is known as a node. There are two, and the one where the groundtrack passes from the southern hemisphere to the northern hemisphere is called the ascending node. The one where the groundtrack passes from the northern to the southern hemisphere is called the descending node. For these orbits it is usually found that the groundtrack shifts towards the west for each orbit because the earth is rotating towards the east underneath the satellite.

For many orbit calculations it is necessary to consider the height of the satellite above the geocentre. This is the height above the earth plus the radius of the earth. This is generally taken to be 3960 miles or 6370 km.

Velocity is another important factor as already seen. For a circular orbit it is always the same. However in the case of an elliptical one this is not the case as the speed changes dependent upon the position in the orbit. It reaches a maximum when it is closest to the earth and it has to combat the greatest gravitational pull, and it is at its lowest speed when it is furthest away.

Elliptical orbits are often used, particularly for satellites that only need to cover a portion of the Earth's surface. For any ellipse, there are two focal points, and one of these is the geocentre of the Earth. Another feature of an elliptical orbit is that there are two other major points. One is where the satellite is furthest from the Earth. This point is known as the apogee. The point where it is closest to the Earth is known as the perigee.

The plane of a satellite orbit is also important. Some may orbit around the equator, whereas others may have different orbits. The angle of inclination of a satellite orbit is shown in Figure 8.2. It is the angle between a line perpendicular to the plane of the orbit and a line passing through the poles. This means that an orbit directly above the equator will have an inclination of 0 degrees (or 180 degrees), and one passing over the poles will have an angle of 90 degrees. Those orbits above the equator are generally called equatorial obits, whilst those above the poles are called polar orbits.

A further feature of any satellite is the angle of elevation above the Earth's surface at a given position on the Earth and a given time. It is very important because the earth station will only be able to maintain contact with the satellite when it is visible. The angle of elevation is the angle at which the satellite appears above the horizontal. If the angle is too small then signals may be obstructed by nearby objects if the antenna is not very high. For those antennas that have an unobstructed view there are still problems with small angles of elevation. The reason is that signals have to travel through more of the earth's atmosphere and are subjected to higher levels of attenuation as a result. An angle of five degrees is generally accepted as the minimum angle for satisfactory operation.

In order that a satellite can be used for communications purposes the ground station must be able to follow it in order to receive its signal, and transmit back to it. Communications will naturally only be possible when it is visible, and dependent upon the orbit it may only be visible for a short period of time. To ensure that communication is possible for the maximum amount of time there are a number of options that can be employed. The first is to use an elliptical orbit where the apogee is above the planned earth station so that the satellite remains visible for the maximum amount of time. Another option is to launch a number of satellites with the same orbit so that when one disappears from view, and communications are lost, another one appears. Generally three satellites are required to maintain almost uninterrupted communication. However the handover from one satellite to the next introduces additional complexity into the system, as well as having a requirement for at least three satellites.

Circular orbits
Circular orbits are classified in a number of ways. Terms such as Low Earth orbit, Geostationary orbit and the like detail distinctive elements of the orbit:

* Low Earth Orbit (LEO: 200 - 1200km above the Earth's surface)
* Medium Earth Orbit (MEO or ICO: 1200 - 35790 km)
* Geosynchronous Orbit (GEO: 35790 km above Earth's surface)
* Geostationary Orbit (GSO)
* High Earth Orbit (HEO: above 35790 km)

The LEO and MEO are used for many types of satellite. As they are relatively close to the Earth's surface they orbit in times much shorter than those higher up. This is because there is a particular velocity required at any given altitude for the gravitational and centrifugal forces to balance. Also the path loss to and from the satellite is much lower in view of the shorter radio paths involved.

As the height of a satellite increases, so the time for the satellite to orbit increases. At a height of 35790 km, it takes 24 hours for the satellite to orbit. This type of orbit is known as a geosynchronous orbit, i.e. it is synchronized with the Earth.

One particular form of geosynchronous orbit is known as a geostationary orbit. In this type of orbit the satellite rotates in the same direction as the rotation of the earth and has a 24 hour period. This means that it revolves at the same angular velocity as the earth and in the same direction and therefore remains in the same position relative to the earth. Geostationary orbits are very popular because once the earth station is set onto the satellite it can remain in the same position, and no tracking is normally necessary. This considerably simplifies the design and construction of the antenna. For direct broadcast satellites it means that people with dishes outside the home do not need to adjust them once they have been directed towards the satellite.

Once in a geostationary orbit, the satellite needs to be kept in its position and not drift. Small rockets are installed on a satellite to ensure that any deviations can be corrected.

The path length to any geostationary satellite is a minimum of 22300 miles. This gives a small but significant delay of 0.24 seconds. For a communications satellite this must be doubled to account for the uplink and downlink times giving virtually half a second. This delay can make telephone conversations rather difficult when satellite links are used. It can also be seen when news reporters as using satellite links. When asked a question from the broadcasters studio, the reporter appears to take some time to answer. This delay is the reason why may long distance links use cables rather than satellites as the delays incurred are far less.

In some applications high earth orbits may be required. For these applications the satellite will take longer than 24 hours to orbit the Earth, and path lengths may become very long resulting in additional delays for the round trip from the Earth toth e satellite and back as well as increasing the levels of path loss.
The choice of the satellite orbit will depend on its applications. While geostationary orbits are popular for applications such as direct broadcasting and for communications satellites, others such as GPS and even those satellites used for mobile phones are much lower.

Applications of Satellite

2007-01-02T15:43:00.000+08:00

There are many applications for satellites in today's world. Ever since the first satellite, Sputnik 1, was launched in 1957, large numbers of satellites have been launched into space to meet a variety of needs. As satellite technology has developed over the years, so ahs the number of applications to which they can be put. Whatever the type of satellite it is necessary to be able to communicate with them, and in view of the large distances, the only feasible technology is radio. As such radio communication is an integral part of any satellite system, whatever its application.

Satellite applications

Astronomical satellites - these satellites are used for the observation of distant stars and other objects in space. Placing an observation point in space removes the unwanted effects of the atmosphere and enables far greater levels of detail to be seen than would be possible on earth where many observatories are placed on mountain tops that experience low levels of cloud. The most famous astronomical satellite is the Hubble Telescope. Although now reaching the end of its life it has enabled scientists to see many things that would otherwise not have been possible. Nevertheless it did suffer some major design setbacks that were only discovered once it was in orbit.

Communications satellites - these satellites possible form the greatest number of satellites that are in orbit. They are used for communicating over large distances. The height of the satellite above the Earth enables the satellites to communicate over vast distances, and thereby overcoming the curvature of the Earth's surface.
Even within the communications field there are a number of sub-categories. Some satellites are used for point to point telecommunications links, others are used for mobile communications, and there are those used for direct broadcast. There are even some satellites used for mobile phone style communications. Even though these satellites did not take the market in the way that was originally expected because terrestrial mobile phone networks spread faster than was originally envisaged, some mobile phone satellite systems still exist.

Earth observation satellites - these satellites are used for observing the earth's surface and as a result they are often termed geographical satellites. Using these satellites it is possible to see many features that are not obvious from the earth's surface, or even at the altitudes at which aircraft fly. Using these earth observation satellites many geographical features have become obvious and they have even been used in mineral search and exploitation.

Navigation satellites - in recent years satellites have been used for accurate navigation. The first system known as GPS (Global Positioning System) was set up by the US DoD and was primarily intended for use as a highly accurate military system. Since then it has been adopted by a huge number of commercial and private users. Small GPS systems are available at costs that are affordable by the individual and are used for car navigation, and they are even being incorporated into phones in a system known as A-GPS (Assisted GPS) to enable accurate location of the phone in case of emergency.
Further systems are planned for the future. The Russian system known as Glonass and the European and Chinese system Galileo are planned for the future.

Reconnaissance satellites - these satellites, are able to see objects on the ground and are accordingly used for military purposes. As such their performance and operation is kept secret and not publicized.

Weather satellites - as the name implies these satellites are used to monitor the weather. They have helped considerably in the forecasting of the weather and have helped provide a much better understanding not only of the underlying phenomena, but also in enabling predictions to be made. A variety of these satellites are in use and include the NOAA series.

There are now many thousands of satellites in orbit around the Earth. Many are in operations, while some that have not yet fallen out of orbit are still circling the Earth. The operational satellites provide many of the services on which we rely today. Without them many of the services which we have come to accept as normal would not be so nearly to achieve by other means.

Satellite facts and information

2007-01-02T15:33:00.000+08:00

Facts about numbers of satellites in orbit
There are over 2500 satellites in orbit around the Earth
There are also over 10 000 man made objects orbiting around the Earth. These include a variety of pieces of satellite debris ranging from panels to disused equipment.

Facts about satellite firsts
The first satellite named Sputnik 1 was launched by the Soviet Union on 4th October 1957. It was a football sized globe that transmitted a "beep beep" sound as it orbited the Earth. The word Sputnik means satellite. It continued transmitting for about 21 days. It was followed four months later by the US satellite Explorer 1 which was launched on 31st January 1958.

Possibly one of the best known satellites was Telstar 1. Built by AT&T it was launched on July 10, 1962, and on the same day live television pictures originating in the United States were received in France.

Facts about satellite orbits
Most communications satellites use what is termed a geostationary orbit. These are at an altitude of, around 22,000 miles and as a result of their speed and the circumference of the orbit they travel round the Earth above the equator in 24 hours. As they travel at the same rate that the Earth rotates, they stay above the same point on the Earth's surface all the time.

In contrast, Low Earth Orbits are just above the Earth's atmosphere and are typically between 100 and 800 miles in altitude. Orbiting at this altitude, an object may only take about 90 minutes to completely circle the Earth, travelling at around 17,000 miles per hour. Low Earth Orbit is used by manned vehicles such as the space shuttle and the International Space Station. It is also used for weather and remote sensing satellites. On a clear night it is usually possible to see with the naked eye several satellites in low earth orbit passing overhear.

Facts about the Global Positioning System (GPS)
The GPS system is run by the US Department of Defense. It consists of 24 operational satellites although there are some extra in orbit as spares in case of catastrophic failure even though each satellite is built to last for ten years. The satellites are named Navstar satellites and each one weighs around 1860 pounds. They are about 17 feet across with the solar panels extended, and they transmit about 50 watts, although the solar panels generate around 700 watts.

The satellites are in one of six orbits. These are in planes that are inclined at approximately 55 degrees to the equatorial plane and there are four satellites in each orbit. The orbits that are roughly 20200 km above the surface of the earth and the satellites travel at a speed of around 14000 km / hour (i.e. about 8500 mph) which means they complete each orbit in roughly 12 hours.

The superheterodyne radio receiver

2006-12-30T12:53:00.000+08:00

The superhet radio or to give it its full name the superheterodyne receiver is one of the most popular forms of receiver in use today. Virtually all broadcast radios, televisions and many more types of receiver use the superhet or superheterodyne principle. First developed at the end of the First World War, with its invention credited to the American Edwin Armstrong, the use of the superhet has grown ever since the concept was first discovered.

Mixing
The idea of the superhet revolves around the process of mixing. Here RF mixers are used to multiply two signals together. (This is not the same as mixers used in audio desks where the signals are added together). When two signals are multiplied together the output is the product of the instantaneous level of the signal at one input and the instantaneous level of the signal at the other input. It is found that the output contains signals at frequencies other than the two input frequencies. New signals are seen at frequencies that are the sum and difference of the two input signals, i.e. if the two input frequencies are f1 and f2, then new signals are seen at frequencies of (f1+f2) and (f1-f2). To take an example, if two signals, one at a frequency of 5 MHz and another at a frequency of 6 MHz are mixed together then new signals at frequencies of 11 MHz and 1 MHz are generated.

The signals generated by mixing or multiplying two signals together

Concept of the superheterodyne receiver
In the superhet or superheterodyne radio, the received signal enters one input of the mixed. A locally generated signal (local oscillator signal) is fed into the other. The result is that new signals are generated. These are applied to a fixed frequency intermediate frequency (IF) amplifier and filter. Any signals that are converted down and then fall within the passband of the IF amplifier will be amplified and passed on to the next stages. Those that fall outside the passband of the IF are rejected. Tuning is accomplished very simply by varying the frequency of the local oscillator. The advantage of this process is that very selective fixed frequency filters can be used and these far out perform any variable frequency ones. They are also normally at a lower frequency than the incoming signal and again this enables their performance to be better and less costly.

To see how this operates in reality take the example of two signals, one at 6 MHz and another at 6.1 MHz. Also take the example of an IF situated at 1 MHz. If the local oscillator is set to 5 MHz, then the two signals generated by the mixer as a result of the 6 MHz signal fall at 1 MHz and 11 MHz. Naturally the 11 MHz signal is rejected, but the one at 1 MHz passes through the IF stages. The signal at 6.1 MHz produces a signal at 1.1 MHz (and 11.1 MHz) and this falls outside bandwidth of the IF so the only signal to pass through the IF is that from the signal on 6 MHz.

The basic concept of the superhet radio

If the local oscillator frequency is moved up by 0.1 MHz to 5.1 MHz then the signal at 6.1 MHz will give rise to a signal at 1 MHz and this will pass through the IF. The signal at 6 MHz will give rise to a signal of 0.9 MHz at the IF and will be rejected. In this way the receiver acts as a variable frequency filter, and tuning is accomplished.

Images
The basic concept of the superheterodyne receiver appears to be fine, but there is a problem. There are two signals that can enter the IF. With the local oscillator set to 5 MHz and with an IF it has already been seen that a signal at 6 MHz mixes with the local oscillator to produce a signal at 1 MHz that will pass through the IF filter. However if a signal at 4 MHz enters the mixer it produces two mix products, namely one at the sum frequency which is 10 MHz, whilst the difference frequency appears at 1 MHz. This would prove to be a problem because it is perfectly possible for two signals on completely different frequencies to enter the IF. The unwanted frequency is known as the image. Fortunately it is possible to place a tuned circuit before the mixer to prevent the signal entering the mixer, or more correctly reduce its level to acceptable value.

Fortunately this tuned circuit does not need to be very sharp. It does not need to reject signals on adjacent channels, but instead it needs to reject signals on the image frequency. These will be separated from the wanted channel by a frequency equal to twice the IF. In other words with an IG at 1 MHz, the image will be 2 MHz away from the wanted frequency.

Using a tuned circuit to remove the image signal

Complete receiver
Having looked at the concepts behind the superheterodyne receiver it is helpful to look at a block diagram of a basic superhet. Signals enter the front end circuitry from the antenna. This contains the front end tuning for the superhet to remove the image signal and often includes an RF amplifier to amplify the signals before they enter the mixer. The level of this amplification is carefully calculated so that it does not overload the mixer when strong signals are present, but enables the signals to be amplified sufficiently to ensure a good signal to noise ratio is achieved.

The tuned and amplified signal then enters one port of the mixer. The local oscillator signal enters the other port. The local oscillator may consist of a variable frequency oscillator that can be tuned by altering the setting on a variable capacitor. Alternatively it may be a frequency synthesizer that will enable greater levels of stability and setting accuracy.

Once the signals leave the mixer they enter the IF stages. These stages contain most of the amplification in the receiver as well as the filtering that enables signals on one frequency to be separated from those on the next. Filters may consist simply of LC tuned transformers providing inter-stage coupling, or they may be much higher performance ceramic or even crystal filters, dependent upon what is required.

Once the signals have passed through the IF stages of the superheterodyne receiver, they need to be demodulated. Different demodulators are required for different types of transmission, and as a result some receivers may have a variety of demodulators that can be switched in to accommodate the different types of transmission that are to be encountered. The output from the demodulator is the recovered audio. This is passed into the audio stages where they are amplified and presented to the headphones or loudspeaker.

Block diagram of a basic superheterodyne receiver

The diagram above shows a very basic version of the superhet or superheterodyne receiver. Many sets these days are far more complicated. Some superhet radios have more than one frequency conversion, and other areas of additional circuitry to provide the required levels of performance. However the basic superheterodyne concept remains the same, using the idea of mixing the incoming signal with a locally generated oscillation to convert the signals to a new frequency.

The Overview Digital Radio Mondiale

2006-12-15T13:28:00.000+08:00

Digital Radio Mondiale (DRM) is set to revolutionise broadcasting on the long, medium and short wave bands. Since the very earliest days of broadcasting these wavebands have been filled with signals that are amplitude modulated. These transmissions are of low audio quality and particularly in recent years there has been a move away from these bands to find higher quality transmissions. Broadcasts in the VHF FM band have received far more listeners with the result that audience figures are dropping for AM broadcasting. Now DAB Digital Radio is available in many countries and this has set new standards in broadcasting. The next stage is to improve the transmissions on the long medium and short wave bands. As the requirements are very different to those experienced on the higher frequencies the DAB standard is not applicable and as a result a totally new system has been developed. Known as DRM it provides many of the improvements that are badly needed along with the flexibility to allow for future developments.

What is DRM?
DRM itself is a consortium of broadcasters, network operators, equipment manufacturers, broadcasting unions, regulatory bodies and other organisations representing 29 countries. It was founded in Guangzhou, China in 1998 and now has its headquarters in Geneva. Now with 82 members, the wide base of its membership has been part of the reason for its success. It has been able to draw on the experience of the membership to ensure that the resulting standard met the requirements, and it has also drawn on the experience gained by the Eureka project that was set up to develop DAB Digital Radio. As a result the new system has come to fruition remarkably swiftly. A preliminary system was designed and tested within a laboratory and this was later extended to include field trials on air to ensure that the new system would successfully meet all the requirements.

The system
When the specification for DRM was being drawn up there were a number of key requirements that needed to meet. The main thrust of the development was to ensure that far greater audio quality could be achieved, but this needed to be achieved whilst keeping the transmissions in a form where they could operate alongside the existing AM transmissions. This meant having the ability for the transmissions to occupy a variety of different bandwidths dependent up the location and frequencies in use. In the Americas a 10 kHz channel spacing is used on the medium wave band whilst in Europe there is a 9 kHz spacing. On the short wave bands a 5 KHz channel spacing has been adopted. It is necessary for the new standard to be able to be compatible with these whilst offering the possibility of other bandwidth options for the future.

Data can also transmitted. Not only does this supply information required for decoding the signal but it also allows data to be transmitted in support of the programme. One particularly useful feature for the short wave bands is that a list of alternative frequencies is transmitted so that listeners can be transferred to better channels very easily as conditions change.

Another advantage of the new system is that it can support what is termed a single frequency network (SFN). This allows a single frequency to be re-used even within the coverage area of the first transmitter without mutual interference. Currently frequencies can only be re-used used outside the coverage area of the first transmitter to avoid interference problems. By using an SFN, far more efficient use can be made of the available channels. With spectrum bandwidth always in short supply, this is another important feature.

DRM transmissions
There are two main elements to the new transmission system. These are the audio coding and the RF modulation used.

The main audio encoding system employs two main techniques. The first is called Advanced Audio Coding (AAC). It is found that the ear does not perceive all the sounds that are heard. A strong sound on one frequency will mask out others close in frequency that may be weaker. AAC, therefore, analyses each section of the spectrum and only encodes those sounds that will be perceived.

However AAC on its own does not provide sufficient compression of the data to enable the transmissions to be contained within the narrow transmission bandwidths required. To provide the additional data compression required a scheme known as Spectral Band Replication (SBR) is employed. This analyses the sounds in the highest octave which are normally from sounds such as percussion instruments of those that are harmonically related to other sounds lower in frequency. It analyses them and sends data to the receiver that will enable them to be reconstituted later.

Data channels
Data to provide the different functions on the transmission is organised into a number of channels that are then applied to the overall modulating signal. The main payload for the signal is known as the Main Service Channel (MSC) and this includes the audio signal data. Two subsidiary channels are also available. These are known as the Fast Access Channel (FAC) that provides the essential data required to fully decode the signal and the Service Description Channel (SDC).

RF Signal
The transmitted signal uses a form of modulation known as Coded Orthogonal Frequency Division Multiplex (COFDM). This form of modulation is being used more frequently is very resilient to many common forms of interference and fading. Its main drawback has been that it requires a significant level of signal processing to extract the data from the carriers and reassemble it in the correct fashion. However signal processing ICs are now sufficiently powerful and at a reasonable cost to make the use of this form of modulation viable. Interestingly COFDM is also used by DAB Digital Radio.

The signal consists of several carriers, across which the data is spread equally. The carriers are spaced equally apart where the spacing is equal to the inverse of the symbol period of the data applied to the carrier. With this spacing it is found that the energy density in the sidebands has nulls or minimum points that correspond with the position of the next carrier. In this way the interference between the nearby carriers is eliminated and they are said to be orthogonal.

Technology for HD Radio

2006-12-12T14:14:00.000+08:00

Digital technology is being applied to many areas of radio communication including radio broadcasting as it offers some significant advantages. While DAB digital radio is becoming established in some areas of the globe, the system that has been chosen for use in the USA is known as HD, or High Definition, Radio. Using HD Radio, will enable high quality audio to be received along with the ability to incorporate many new features and facilities.

The HD Radio system has been developed by iBiquity, and has now been selected by the FCC in the USA. It will take the place of both the existing AM and FM transmissions, and offers many advantages for both listeners and broadcasters alike:

* Improved audio quality - it is claimed that HD Radio broadcasts on the AM bands will be as good as current FM services and those on the FM band will offer CD quality audio.

* Reduced levels of interference. AM transmissions in particular are prone to static pops and bangs as well as high levels of background noise. HD Radio will almost eliminate this.

* Opportunity to use additional data services. By using digital technology, HD Radio provides the opportunity to add data services such as scrolling programme information, song titles, artist names, and much more.

* There is also the possibility of adding more advanced services such as surround sound, multiple audio sources, on-demand audio services, etc.

* Easy transition for broadcasters and listeners. Although new HD Radio receivers are required to receive the new transmissions in their digital format there is considerable re-use of infrastructure and spectrum.

HD Radio basics
HD Radio uses a variety of technologies to enable it to carry digital audio in an acceptable bandwidth and with the new high quality that is required. The transmission uses COFDM combined with specialised codec to compress the audio.

Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each another there is no mutual interference. This is achieved by having the carrier spacing equal to the reciprocal of the symbol period.

This means that when the signals are demodulated they will have a whole number of cycles in the symbol period and their contribution will sum to zero - in other words there is no interference contribution. The data to be transmitted is split across all the carriers and this means that by using error correction techniques, if some of the carriers are lost due to multi-path effects, then the data can be reconstructed.

Additionally having data carried at a low rate across all the carriers means that the effects of reflections and inter-symbol interference can be overcome. It also means that single frequency networks, where all transmitters can transmit on the same channel can be implemented. Further information on OFDM can be found on this site under the Cellular telecoms section or by using the Search facility.

One of the requirements for HD Radio was that it would maintain compatibility with existing stations. To achieve this there are two versions; one HD Radio system for AM, and the other for FM.

In what is termed hybrid mode, the AM version has a data rate of 36 kbps for the main audio channel and the version of HD radio for the FM bands carries 96 kbps. In addition to this HD radio can also be used to carry multiple audio channels, and in addition to this secondary channels for services such as weather, traffic and the like may be added. However adding additional channels will reduce the available bandwidth for the primary channel and audio quality may be impaired.

In hybrid mode a radio receiver will first lock onto an analogue signal. If this is possible, then it will try to find a stereo component (FM only) and finally it will endeavour to decode a digital signal. If the digital signal is lost then it will fall back to the analogue signal. The success of this process depends upon the transmitting station being able to synchronise the digital and analogue signals. Often the digitisation process takes a noticeable amount of time and the digital and analogue signals may not be transmitted in time with each other.

Once HD Radio is fully established, the hybrid mode may be removed and at this point no analogue information will be transmitted. However it is envisaged that this will take some time as this can only be viable when very few analogue radios are in use.

Radio Frequencies Digital Audio Broadcasting

2006-12-09T09:34:00.000+08:00

DAB digital radio can be broadcast on a wide number of frequencies. There are both terrestrial and satellite allocations for Digital Audio Broadcasting (DAB). Currently the main frequencies where it is being deployed are within the Band III (Band 3) frequencies. Here a number of channels have been allocated. A complete table of the channels is given below, although in many countries the full number of channels is not available. Within the UK, the DAB multiplexes are being broadcast on channels 11B through to 12D inclusive.

Although it may appear that comparatively few channels are available, each multiplex is able to carry many stations. If high quality audio is required then fewer stations can be accommodated. However it is often possible to accommodate around four or five high quality broadcasts along with several lower quality ones. In addition to this data can also be carried.

Channel Frequency
MHz
5A 174.928
5B 176.640
5C 178.352
5D 180.064
6A 181.936
6B 183.648
6C 185.360
6D 187.072
7A 188.928
7B 190.640
7C 192.352
7D 194.064
8A 195.936
8B 197.648
8C 199.360
8D 201.072
9A 202.928
9B 204.640
9C 206.352
9D 208.064
10A 209.936
10B 211.648
10C 213.360
10D 215.072
11A 216.928
11B 218.640
11C 220.352
11D 222.064
12A 223.936
12B 225.648
12C 227.360
12D 229.072
13A 230.748
13B 232.496
13C 234.208
13D 235.776
13E 237.448
13F 239.200

How digital radio works

2006-12-07T09:32:00.000+08:00

DAB Digital Radio, which is also known as digital audio broadcasting, is an entirely new system for broadcasting and receiving radio stations. As the name indicates signals are broadcast in a digital format to enable CD quality to be achieved. People who have heard DAB digital radio have commented on the significantly better sound quality and "presence" of the new radio system. Also it does not suffer from the multipath effects often experienced on FM transmissions and as the system uses what it known as a single frequency network (SFN) there is no retuning required when moving from one coverage area to the next.

In addition to this many new services can be carried on these digital radio transmissions enabling the new system to be compatible with the 21st century. The digital radio signal carries data alongside the audio, and this enables text and images to be transmitted alongside the audio to enhance the listening experience. In this way it is possible to transmit the title of a track, and a picture of the artist whilst the some music is being transmitted. It is also possible to have news scrolling across the bottom of the screen used on the radio.

DAB digital radio is now well established in many countries around the world from the UK and Europe to Canada, Australia and many other countries. Wit the facilities that digital radio offers it is now being accepted and listeners are switching to these new digital radio transmissions in the areas where they are available.

To produce a digital system that operates satisfactorily under the conditions required for digital radio a large amount of work was undertaken in the development stages. Some existing digital techniques were investigated but it was realised these had significant limitations for this application. One of the major problems was that many receivers would use non-directional antennas and as a result they would pick up reflected signals. These would be delayed sufficiently for the data to become corrupted. Additionally the bandwidth required to accommodate a full stereo signal would need to be reduced to ensure efficient use of the spectrum. The technical standards for digital radio were developed under the auspices of the European Eureka Project 147. This consortium consisted of manufacturers, broadcasters research bodies and network operators.

There are two main areas of the system that are of interest in digital radio: namely the modulation system and the audio digital encoding and compression system.

The encoding and compression system is of paramount importance. For the system to be viable the data rate has to be considerably reduced from that of a standard CD. The digital radio system adopted reduces the data rate down to 128 kbits / sec, a sixth of the bit rate for a similar quality linearly encoded signal. To achieve these reductions the incoming audio signal is carefully analysed. It is found that the ear has a certain threshold of hearing. Below this the signals are not heard. Additionally if a strong sound is present on one frequency then weaker sounds close to it may not be heard because the threshold of hearing is modified. By analysing the incoming audio and only encoding those constituents that the ear will hear the significant reductions can be made. Further reductions in data rate can be achieved by reducing the audio bandwidth. This is implemented on some channels such as those used only for speech.

The other key to the operation of digital radio is the modulation system. Called Coded Orthogonal Frequency Division Multiplex (COFDM) it is a form of spread spectrum modulation that provides the robustness required to prevent reflections and other forms of interference from disrupting reception.

The system uses about 1500 individual carriers that fill around 1.5 MHz of spectrum. The carriers are spaced very close to one another. Interference between the carriers is prevented by making the individual signals orthogonal to each other. This is done by spacing each one by a frequency equal to the data rate being carried. In this way the nulls in the modulation sidebands fall at the position where the next carrier is located. The audio data is then spread across the carriers so that each carrier takes only a small proportion of the data rate. This has the advantage that if interference is encountered in one area then sufficient data is received to reconstitute the required signal. Guard bands are also introduced at the beginning of each symbol, and the combined effect is such that the system is immune to delays consistent with signals 60 km further away than the primary source.

With this level of immunity, the system can operate with other digital radio transmitters operating on the same frequency without any ill effects. This means that it is possible to set up a system where all the transmitters for a network operate on the same frequency. This means that it is possible to set up single frequency networks throughout an area in which a common "multiplex" is used. Even though it may appear that this is a recipe for poorer reception caused by several transmitters using the same frequency, the opposite is actually true. It is found that out of area signals tend to augment the required signal. It also means that small areas of poor coverage can have a small transmitter on exactly the same frequency filling in the hole and further improving reception in adjacent areas.

A further advantage of this digital radio system is that it requires less power than the more traditional transmitters. For example those that carry the main BBC FM networks from the main transmitting sites like Wrotham in the South East of England run at powers of around 100 kW for each of the four main services that are transmitted. The cost of the electricity alone is a significant factor in the BBC's running costs and the power reductions will bring huge savings, not to mention the environmental benefits.

Digital radio band allocations
In the UK a spectrum allocation between 217.5 and 230 MHz has been reserved for digital radio transmissions. This gives a total of seven blocks of 1.55 MHz, each able to carry a multiplex of services. In other countries as well spectrum is being made available. Within Europe spectrum is being made available either in Band III as in the UK or in L band between 1452 and 1467 MHz. The upper part of the band between 1467 and 1492 will be reserved for satellite delivery of digital radio.

Digital radio equipment
One of the main problems with the initial launch of digital radio was the availability of the equipment. A large investment had been required from the equipment manufacturers. The heavy reliance on digital signal processing techniques meant large development programmes were needed to develop the equipment. There were also problems with the fact that the early implementations required high current levels. These solutions would not have been suitable for portable receivers, and for in car and home applications heat dissipation was a problem. Furthermore the multi-chip solutions made the equipment large and bulky as well as making the manufacturing costs high.

Manufacturers soon solved the problem. Specific chip sets for DAB were developed and these enabled costs to be reduced dramatically from the initial ones that were seen so that DAB is no where near as high as it was when compared to FM receivers.

Many people now comment on the significant enhancements that DAB digital radio brings. One typical example was when a friend walked into a shop and noticed the music playing had an increased presence. He assumed it must be DAB, and this was confirmed when he asked. Others have noticed the seamless performance when in a car. None of the intermittent hissing when travelling through a marginal area between the two transmitters.

The Radio Data System ( RDS )

2006-12-04T13:18:00.000+08:00

RDS or Radio Data System is standard on most car radios and hi-fi tuners today. RDS is used on VHF FM radio broadcast transmissions and provides a number of facilities that are of great use to all radio listeners, but particularly to those radio listeners in cars. RDS enables traffic reports to be received more easily, and provides many facilities including enabling the radio station name to be displayed on the radio display.

The system has gained a considerable amount of popularity and is widely used in Europe where it has been established for a number of years.

How RDS Works
RDS operates by adding data to the baseband signal that is used to modulate the radio frequency carrier. The baseband signal consists of a number of components. Firstly there is the mono audio consisting of the left plus right (L+R) component that is transmitted at the normal audio frequencies up to 15 kHz. The stereo difference signal is then amplitude modulated as a double sideband suppressed carrier signal at 38 kHz. A pilot tone at 19 kHz (half the frequency of the stereo difference signal subcarrier) is also transmitted and this is used to enable the receiver demodulator to exactly recreate the 38 kHz subcarrier to decode the stereo difference signal.

The stereo difference signal is above the audio hearing range and as a result it does not detract from the normal mono signal. When adding anything new to a transmission, compatibility must be maintained with existing radios.

The RDS information is placed above the stereo difference signal on a 57 kHz subcarrier as shown. This happens to be three times the stereo pilot tone frequency. For stereo transmissions the RDS subcarrier is locked onto the pilot tone. It can either be in-phase with the third harmonic of the tone, or as in the case of the BBC it can be in quadrature.

The actual subcarrier that is used to carry the information is phase modulated to carry the data. It uses a form of modulation called Quadrature Phase Shift Keying (QPSK). This gives good immunity to data errors caused by noise whilst still allowing the data to be transmitted at a suitable rate. Combined with the fact that the subcarrier operates at a harmonic of the pilot tone, these facts minimise the possibility of interference to the audio signals.

Baseband Coding
The rate at which data is transmitted is 1187.5 bits per second. This is equal to the frequency of the RDS subcarrier divided by 48. By adopting this data rate the decoding circuits to operate synchronously. This reduces problems with spurious signals in the decoding circuits.

Data is transmitted in groups consisting of four blocks. Each block contains a 16 bit information word and a 10 bit check word as shown. This means that with the data rate of 1187.5 bit per second approximately 11.4 groups can be transmitted each second.

A 10 bit check word may seem to be long. However it is very important in view of the poor signal conditions which can exist. This can be particularly true for car or portable radios. The check word enables the radio decoder to detect and correct errors. It also provides a method for synchronisation.

The data groups are structured so that data can be transmitted as efficiently as possible. Different stations will want to transmit different types of data at different times. To cater for this there are a there are a total of 16 different group structures. Their applications are outlined in Figure 3.

Mixing of different types of data within groups is kept to a minimum. However the coding structure is such that messages which need repeating most frequently normally occupy the same position within groups. For example the first block in a group always contains the PI code and PTY and TP are to be found in block 2.

In order that a radio knows how to decode the data correctly, each type of group has to be identified. This function is performed by a four bit code occupying the first four bits in the second block.

Once generated the data is coded onto the subcarrier in a differential format. This allows the data to be decoded correctly whether the signal is inverted or not. When the input data level is "0" the output remains unchanged but when a "1" appears at the input the output changes its state.

With the basic signal generated the spectrum has to be carefully limited. This has to be done to avoid any cross talk in phase locked loop decoders. The power density close to 57 kHz is limited by the encoding each bit as a biphase signal. In addition to this the coded data is passed through a low pass filter.

Facilities
The RDS system offers a wide range of very useful facilities. The most widely publicised one is that of being able to provide travel news. This is available on most local radio stations. All of these stations transmit the TP code to identify that travel messages are flagged by RDS. When the radio is set for travel news it will only tune to stations which carry the TP indication. As the station is about to broadcast a travel announcement the TA code is transmitted. If a CD or cassette is being played then most sets will actually pause the CD or tape and then allow the travel announcement to be heard. In addition to this the volume may also be set slightly higher to allow the announcement to be heard more easily.

Autotuning
RDS brings intelligence into the tuning of a radio. The autotuning facility comes into its own on long journeys when the car moves from the service area of one transmitter to the next. Without RDS the radio has to be manually tuned to the next station. This is not always easy because it is difficult to reliably detect which is the strongest station.

An RDS set will look for the Programme Identification or PI code. A national network will be broadcast from a large number of different transmitters around the country. The station or network eg Radio 4 will have its own PI code. When the radio moves out of the range of one transmitter the radio will seek the strongest signal which has the same PI code, allowing the radio to remain tuned to the same programme.

When radios fitted with RDS store a station frequency, they also store the PI code along side it. This has the advantage that when the radio is turned on in a place outside the coverage area for the transmitter frequency which is stored then the radio will seek the strongest signal which has the correct PI code.

Local radio stations also have a PI code. In view of the local nature of these stations the PI code works slightly differently.

If the station has two or more transmitters then the PI code will operate in the normal way when it is range of these transmitters. However when the radio moves outside this coverage area it will retune to the strongest signal of the same type of station.

The PI code consists of four characters. The first indicates the country of origin and for the UK this is C. The next one indicates the type of coverage. The figure "2" indicates a national station, and the final two characters are the programme reference. For example Radio 3 has the PI code C203 and BBC GLR has C311.

Instant Tuning
It takes a number of seconds for the radio to search for the strongest signal with the correct PI code. During this time the radio would mute itself and the listener would have an annoying gap in listening. To enable the set to tune itself very quickly from one transmission to the next each transmitter broadcasts a short list of frequencies of adjacent transmitters. This vastly reduces the amount of seeking which the radio set has to perform. In addition to this a second front end is often employed to constantly detect the strength of the alternative frequency transmissions. This results in much faster changes in setting - to the extent that the listener should not be able to detect when the radio changes from one transmitter to another.

Another facility associated with tuning is called the Programme Service Name (PS). This enables the set to display the station name. This normally takes a second or two to come up on the display after the station has been tuned in. However it is a most useful facility with the ever-increasing number of stations on the air

New Facilities
A new feature which has been added to RDS is called Enhanced Other Networks (EON). This allows the set to listen to one station like a national network, but still be interrupted by travel news from a local radio station. This feature even allows announcements to be heard whilst travelling in silence or listening to a tape.

EON requires a large amount of co-ordination between the different stations. To achieve this, the BBC have a central computer specifically for this purpose. When a local radio station is about to transmit a traffic message the fact is flagged to the computer. In turn this directs the relevant national radio transmitters to indicate this fact, thereby enabling the radios to change frequency to the local radio station to receive the message. Once the message is complete the radio will return to its original station.

EON is relatively new and the first sets to have it included only appeared in 1991. Although it is being introduced on more sets, the majority still does not have it. However with manufacturers constantly bringing new sets onto the market EON should be included on far more sets in a year or two.

Broadcast VHF FM

2006-12-02T08:38:00.000+08:00

When broadcasting first started in the 1920s amplitude modulation was used because it was the obvious and the easiest way to transmit sound. However as radio technology developed its shortcomings became more obvious and the quest for higher quality transmissions lead to the introduction of wideband frequency modulation. Although the first commercial stations were set up in the USA around 1939, it was not until the 1950s that FM started to become really accepted. It was in 1954 that the BBC announced their intention to start FM broadcasting. Now VHF FM is the accepted medium for high quality transmissions, and stations that use AM on the medium and long wave bands have to work hard to retain listeners who prefer the higher quality of VHF FM.

What is FM?
Amplitude modulation, which is the simplest and most obvious form of modulation varies the amplitude of the carrier so that it carries the sound information. Frequency modulation is slightly more subtle and as the name indicates it varies the frequency of the carrier in line with the variations in the modulating audio signal. This as the modulating waveform increases in voltage, so the carrier will swing in one direction and as it decreases it will move in the other direction.

One of the important factors of FM is the degree by which the carrier changes. This deviation is usually expressed in kilohertz variation either side of the centre (no modulation) frequency. Typically a signal may have a deviation of +/- 3kHz if it varies up and down by 3 kHz. There are two main categories on FM. The first is called narrow band FM, and this is where the deviation is relatively small, possibly 5 kHz. This type of transmission is used mainly by VHF / UHF point to point mobile communications. To appreciate the full benefits of FM, wideband FM is used having a greater level of deviation. The standard for broadcasting is +/- 75 kHz. To fully accommodate these transmissions a bandwidth of 200 kHz is used.

The advantage of FM is that as the modulation is carried solely as frequency variations, much noise, which appears mainly as amplitude variations can be discarded in the receiver. Accordingly it is possible to achieve much better noise performance using FM. The upper audio frequency limit is generally taken as 15 kHz for these transmissions. This is quite adequate for most high quality transmissions.

Pre-emphasis and de-emphasis
One of the problems with the high quality VHF FM transmissions is that the increased audio bandwidth means that background noise can often be perceived. Even then it is considerably better than that obtained using and AM system. It is particularly noticeable towards the treble end of the audio spectrum, where it can be heard as a background hiss. To overcome this it is possible to increase the level of the treble frequencies at the transmitter. At the receiver they are correspondingly attenuated to restore the balance. This also has the effect of reducing the treble background hiss which is generated in the receiver. The process of increasing the treble signals is called pre-emphasis, and reducing the in the receiver is called de-emphasis. The rate of pre-emphasis and de-emphasis is expressed as a time constant. It is the time constant of the capacitor-resistor network used to give the required level of change. In the UK, Europe and Australia the time constant is 50 uS whereas in North America it is 75 uS.

Stereo
In recent years stereo transmission has become an accepted part of VHF FM transmissions. The system that is used maintains compatibility with mono only receivers without any noticeable degradation in performance. The system that is used is quite straightforward.

A stereo signal consists of two channels that can be labelled L and R, (Left and Right), providing one channel for each of the two speakers that are needed. An ordinary mono signal consists of the summation of the two channels, i.e. L + R, and this can be transmitted in the normal way. If a signal containing the difference between the left and right channels, i.e. L - R is transmitted then it is possible to reconstitute the left only and right only signals. Adding the sum and difference signals, i.e. (L + R) + (L - R) gives 2L, i.e. the left signal, and subtracting the two signal, i.e. (L + R) - (L - R) gives 2R, i.e. the right signal. This can be achieved relatively simply by adding and subtracting the two signals electronically. It only remains to find a method of transmitting the stereo difference signal in a way that does not affect any mono receivers.

This is achieved by transmitting the difference signal above the audio range. It is amplitude modulated onto a 38 kHz subcarrier. Both the upper and lower sidebands are retained, but the 38 kHz subcarrier itself is suppressed to give a double sideband signal above the normal audio bandwidth as shown below. This whole of the baseband is used to frequency modulate the final radio frequency carrier. It is the baseband signal that is regenerated after the signal is demodulated in the receiver.

To regenerate the 38 kHz subcarrier, a 19 kHz pilot tone is transmitted. The frequency of this is doubled in the receiver to give the required 38 kHz signal to demodulate the double sideband stereo difference signal.

The presence of the pilot tone is also used to detect whether a stereo signal is being transmitted. If it is not present the stereo reconstituting circuitry is turned off. However when it is present the stereo signal can be reconstituted.

To generate the stereo signal, a system similar to that shown in Fig. 8.5 is used. The left and right signals enter the encoder where they are passed through a circuit to add the required pre-emphasis. After this they are passed into a matrix circuit. This adds and subtracts the two signals to provide the L + R and L - R signals. The L + R signal is passed straight into the final summation circuit to be transmitted as the ordinary mono audio. The difference L - R signal is passed into a balanced modulator to give the double sideband suppressed carrier signal centred on 38 kHz. This is passed into the final summation circuit as the stereo difference signal. The other signal entering the balanced modulator is a 38 kHz signal which has been obtained by doubling the frequency of the 19 kHz pilot tone. The pilot tone itself is also passed into the final summation circuit. The final modulating signal consisting of the L + R mono signal, 19 kHz pilot tone, and the L - R difference signal based around 38 kHz is then used to frequency modulate the radio frequency carrier before being transmitted.

Reception of a stereo signal is very much the reverse of the transmission. A mono radio receiving a stereo transmission will only respond to the L + R signal. The other components being above 15 kHz are above the audio range, and in any case they will be suppressed by the de-emphasis circuitry.

For stereo receivers the baseband signal consisting of the stereo sum signal (L+R) and the difference signal (L-R) centred around 38 kHz and the pilot 19kHz tone are obtained directly from the FM demodulator. The decoder then extracts the Left only and Right only signals.

The block diagram of one type of decoder is shown below. Although this is not the only method which can be used it shows the basic processes that are required. The signal is first separated into its three constituents. The L + R mono signal between 0 and 15 kHz, the pilot tone at 19 kHz, and the stereo difference signal situated between 23 and 53 kHz. First the pilot tone at 19 kHz is doubled in frequency to 38 kHz. It is then fed into a mixer with the stereo difference signal to give the L - R signal at audio frequencies. Once the L + R and L - R signals are available they enter a matrix where they are added and subtracted to regenerate the L and R signals. At this point both signals are amplified separately in the normal way in a stereo amplifier before being converted into sound by loudspeakers or headphones.

Radio Broadcast Technology

2006-12-02T08:34:00.000+08:00

Radio broadcasting is an established use of radio technology. The first organised broadcasts taking place in the 1920s. Now there are many radio stations broadcasting all over the world using a variety of differernt types of transmission. Today, radio broadcast equipment from transmitters and receivers to antennas, studios and relay links are widely available, although with the new standards for transmission including DAB Digital Radio and DRM, new equipment is required. Nevertheless AM as well as FM with its RDS capability are still the most widely used.

VHF FM broadcasting
VHF FM is the most widely used form of broadcasting in areas of the world where the population is relatively high. Its bandwidth enables it to carry high quality transmissions, stereo, and other services such as RDS.
Broadcast VHF FM
RDS - Radio Data Service

Digital Audio Broadcasting (DAB)
DAB digital radio is now widely deployed in many countries around the globe, and now that the cost of radios has fallen, listener figures are rising. Although not available in many countries, it is certainly making a significant impact where it has been deployed, adding more flexibility and the possibility of near CD quality.
DAB digital radio
DAB digital radio Band III channel numbers and frequencies
HD Radio - the new digital radio system for the USA.

Digital Radio Mondiale
While DAB digital radio is focussed at bands at VHF and above there have been developments for the lower frequencies. Digital Radio Mondiale (DRM) is now being deployed and is a replacement for the amplitude modulation (AM) transmissions that have been on the airwaves for over 100 years. The new technology has now been extended for use up to 100 MHz and with many developments under way radios should soon be available.
Digital Radio Mondiale (DRM) - the new standard to replace AM broadcasting

Overview of TETRA Private Mobile Radio (PMR)

2006-11-29T09:11:00.000+08:00

TETRA is a modern standard for digital Private Mobile Radio (PMR) and Public Access Mobile Radio (PAMR). It offers many advantages including flexibility, security, ease of use and offers fast call set-up times. This makes it an ideal choice for many business communications requirements.

The name TETRA stands for TErrestrial Trunked RAdio. Aimed at a variety of users including the police, ambulance and fire services, it is equally applicable for utilities, public access, fleet management, transport services, and many other users. It offers the advantages of digital radio whilst still maintaining the advantages of a PMR system.

Tetra radio beginnings
Work started on the development of the TETRA standards in 1990 and has relied on the support of the European Commission and the ETSI members. Experience gained in the development of the highly successful GSM cellular radio standard, as well as experience from the development and use of trunked radio systems has also been used to fashion the TETRA standard. In addition to this the process has gained from the co-operation of manufacturers, users, operators and industry experts. With this combined expertise the first standards were ready in 1995 to enable manufacturers to design their equipment to interoperate successfully.

Tetra radio features
TETRA radio offers many new and valuable features. These include a fast call set-up time, which is a particularly important requirement for the emergency services. It also has excellent group communication support, direct mode operation between individual radios, packet data and circuit data transfer services, better economy of frequency spectrum use than the previous PMR radio systems and in addition to this it provides advanced security features. The system also supports a number of other features including call hold, call barring, call diversion, and ambience listening.

The TETRA radio system uses Time Division Multiple Access (TDMA) technology with 4 user channels on one radio carrier and 25 kHz spacing between carriers. This makes it inherently more efficient than its predecessors in the way that it uses the frequency spectrum. Data can be transmitted at 7.2 kbits per second for a single channel. This can be increased four fold to 28.8 kbits per second when multi-slot operation is employed.

For emergency services in Europe the frequency bands 380-383 MHz and 390-393 MHz have been allocated. These bands can be expanded to cover all or part of the spectrum from 383-395 MHz and 393-395 MHz should this be needed. For civil systems in Europe the frequency bands 410-430 MHz, 870-876 MHz / 915-921 MHz, 450-470 MHz, 385-390 MHz / 395-399,9 MHz, have been allocated.

TETRA radio trunking facility provides a pooling of all radio channels that are then allocated on demand to individual users, in both voice and data modes. By the provision of national and multi-national networks, national and international roaming can be supported, the user being in constant communication. TETRA supports point-to-point, and point-to-multipoint communications both by the use of the TETRA infrastructure and by the use of Direct Mode without infrastructure.

In addition to this it is possible for TETRA radio to operate in a secure format. The digital data can be encrypted before transmissions, making the system inherently secure. This may be required for some covert operations or for the police services.

TETRA radio operation
There are three different modes in which TETRA can be run:

* Voice plus Data (V+D)

* Direct Mode Operation (DMO)

* Packet Data Optimised (PDO)

The most commonly used mode is V+D. This mode allows switching between speech and data transmissions, and can even carry both by using different slots in the same channel. Full duplex is supported with base station and mobile radio units frequencies normally being offset by about 10 MHz to enable interference levels between the transmitter and receiver in the station to be reduced to an acceptable level.

DMO is used for direct communication between two mobile units and supports both voice and data, however full duplex is not supported in this mode. Only simplex is used. This is particularly useful as it allows the mobile stations to communicate with each other even when they are outside the range of the base station.

The third mode, PDO is optimised for data only transmissions. It has been devised with the idea that much higher volumes of data will be needed in the future and it is anticipated that further developments will be built upon this standard.

Data structures
TETRA radio uses TDMA techniques. This enables much greater spectrum efficiency than was possible with previous PMR systems because it allows several users to share a single frequency. As the speech is digitised, both voice and data are transmitted digitally and multiplexed into the four slots on each channel. Digitisation of the speech is accomplished using a system that enables the data to be transmitted at a rate of only 4.567 kbits/second. This low data rate can be achieved because the process that is used takes into account the fact that the waveform is human speech rather than any varying waveform. The digitisation process also has the advantage that it renders the transmission secure from casual listeners. For greater levels of security that might be required by the police or other similar organisations it is possible to encrypt the data. This would be achieved by using an additional security or encryption module.

The data transmitted by the base station has to allow room for the control data. This is achieved by splitting what is termed a multiframe lasting 1.02 seconds into 18 frames and allowing the control data to be transmitted every 18th frame. Each frame is then split into four time slots. A frame lasts 56.667 mS. Each time slot then takes up 14.167 mS. Of the 14.167mS only 14 milliseconds is used. The remaining time is required for the transmitter to ramp up and down. The data structure has a length of 255 symbols or 510 modulation bits. It consists of a start sequence that is followed by 216 bits of scrambled data, a sequence of 52 bits of what is termed a training sequence. A further 216 bits of scrambled data follows and then the stream is completed by a stop sequence. The training sequence in the middle of the data is required to allow the receiver to adjust its equaliser for optimum reception of the whole message.

The data is modulated onto the carrier using differential quaternary phase shift keying. This modulation method shifts the phase of the RF carrier in steps of ± pi /4 or ±3 pi /4 depending upon the data to be transmitted. Once generated the RF signal is filtered to remove any sidebands that extend out beyond the allotted bandwidth. These are generated by the sharp transitions in the digital data. A form of filter with a root raised cosine response and a roll off factor of 0.35 is used. Similarly the incoming signal is filtered in the same way to aid recovery of the data.

Additionally, TETRA radio uses error tolerant modulation and encoding formats. The data is prepared with redundant information that can be used to provide error detection and correction. The transmitter of each mobile station is only active during the time slot that the system assigns it to use. As a result the data is transmitted in bursts. The fact that the transmitter is only active for part of the time has the advantage that the drain on the battery of the mobile station is not as great as if the transmitter was radiating a signal continuously. The base station however normally radiates continuously as it has many mobile stations to service.

One important feature of TETRA is that the call set up time is short. It occurs in less than 300 mS and can be as little as 150 mS when operating in DMO. This is much shorter than the time it takes for a standard cellular telecommunications system to connect. This is very important for the emergency services where time delays can be very critical.

Further TETRA radio developments
While TETRA radio is a major improvement over the previous PMR systems in operation, additional data capacity is always needed. In view of the higher data capabilities now being offered by the cellular services, the TETRA radio standard is being updated to enable it to keep pace with other comparable technologies. In this way, TETRA will be able to offer commercial users the advantages of a PMR service alongside the data capabilities of a cellular network.

PMR Trunking using MPT1327

2006-11-28T09:02:00.000+08:00

A trunked version of the Private Mobile Radio (PMR) concept that is defined under the standard MPT 1327 (MPT1327) is widely used and provides significant advantages over the simpler single station systems that are in use. MPT1327 enables stations to communicate over wider areas as well as having additional facilities.

In view of the very high cost of setting up trunked networks, they are normally run by large leasing companies or consortia that provide a service to a large number of users. In view of the wider areas covered by these networks and the greater complexity, equipment has to be standardised so that suppliers can manufacture in higher volumes and thereby reduce costs to acceptable levels. Most trunked radio systems follow the MPT1327 format.

To implement trunked PMR a network of stations is set up. These stations are linked generally using land lines, although optical fibres and point to point radio are also used. In this way the different base stations are able to communicate with each other.

In order to be able to carry the audio information and also run the variety of organisational tasks that are needed the system requires different types of channel to be available. These are the control channels of which there is one in each direction for each base station or Trunking System Controller (TSC).

A number of different control channels are used so that adjacent base stations do not interfere with one another, and the mobile stations scan the different channels to locate the strongest control channel signal. In addition to this there are the traffic channels. The specification supports up to 1024 different traffic channels to be used. In this way a base station can support a large number of different mobile stations that are communicating at the same time. However for small systems with only a few channels, the control channel may also act as a non-dedicated traffic channel.

The control channels use signalling at 1200 bits per second with fast Frequency Shift Keying (FFSK) subcarrier modulation. It is designed for use by two-frequency half duplex mobile radio units and a full duplex TSC.

For successful operation it is essential that the system knows where the mobiles are located so that calls can be routed trough to them. This is achieved by base stations polling the mobile stations using the control channel.

To make an outgoing call the mobile transmits a request to the base station as requested in the control channel data stream from the base station. The mobile transmits its own code along with that of the destination of the call, either another mobile or a control office. The control software and circuitry within the base station and the central control processing area for the network sets up the network so that a channel is allocated for the audio (the traffic channel). It also sets up the switching in the network to route the call to the required destination.

To enable the mobile station to receive a call, it is paged via the incoming control channel data stream to indicate that there is an incoming call. Channels are allocated and switching set up to provide the correct routing for the call.

There is no method to "handover" the mobile from one base station to the next if it moves out of range of the base station through which a call is being made. In this way the system is not a form of cellular telephone. It is therefore necessary for the mobile station to remain within the service area of the base station through which any calls are being made.

The control channel signalling structure has to be defined so that all mobiles know what to expect and what data is being sent. Signalling on the forward control channel is nominally continuous with each slot comprising 64 bit code words. The first type is the Control Channel System Codeword (CSCC). This identifies the system to the mobile radio units and also provides synchronisation for the following address codeword. As mentioned the second type of word is the address codeword. It is the first codeword of any message and it defines the nature of the message. It is possible to send data over the control channel. When this occurs, botht he CSCC and the address codewords are displaced with the data appended to the address codeword. The mobile radio unit data structure is somewhat simpler. It consists fundamentally of synchronism bits followed by the address codeword.

There are a number of different types of control channel messages that can be sent by the base station to the mobiles:

Aloha messages -- Sent by the base station to invite and mobile stations to access the system.

Requests -- Sent by radio units to request a call to be set up.

"Ahoy" messages -- Sent by the base station to demand a response from a particular radio unit. This may be sent to request the radio unit to send his unique identifier to ensure it should be taking traffic through the base station.

Acknowledgements -- These are sent by both the base stations and the mobile radio units to acknowledge the data sent.

Go to channel messages -- These messages instruct a particular mobile radio unit to move to the allocated traffic channel.

Single address messages -- These are sent only by the mobile radio units.

Short data messages -- These may be sent by either the base station or the mobile radio unit.

Miscellaneous messages -- Sent by the base station for control applications.

One of the problems encountered by mobile signalling systems is that of clashes when two or more mobile radio units try to transmit at the same time on the control channel. This factor is recognised by the system and is overcome by a random access protocol that is employed. This operates by the base station transmitting a synchronisation message inviting the mobile radio units to send their random access message. The message from the base station contains a parameter that indicates the number of timeslots that are available for access. The mobile radio unit will randomly select a slot in which to transmit its request but if a message is already in progress then it will send its access message in the next available slot. If this is not successful then it will wait until the process is initiated again.

Although the data is transmitted as digital information, the audio or voice channels for the system are analogue, employing FM. However some work has been carried out to develop completely digital systems. The main systems are by Motorola, by Ericsson (EDACS) and Johnson (LTR). These systems have not gained such widespread acceptance.

Overview of Private Mobile Radio

2006-11-27T09:32:00.000+08:00

Private Mobile Radio (PMR) or as it is sometimes called Professional Mobile Radio is widely used for businesses as a very convenient way of communicating. The basic concept has been in use for many years and was firmly established prior to the introduction of the first cell phone systems, although systems including MPT1327 that provide trunking and TETRA enable far greater facilities.

The first PMR systems were initially set up to enable a set of mobile business users to maintain contact with a base. Organisations such as taxi firms, utility workers and the like all used these systems as they enabled them to maintain contact with their office. Additionally the emergency services used their own systems.

Initially the systems consisted of a base station with a number of mobile stations. Communication used a single frequency, with simplex push to talk transmissions. As pressure rose on the frequency allocations, often frequencies had to be shared. As the systems almost invariably used frequency modulation, squelch was employed so that the audio from the received was only switched on when a signal was present. Developments of this known as DTMF (dual tone multiple frequency) and CTCSS (continuous tone, coded squelch system) were used to enable only the required users to hear the call.

These systems were only able to communicate over relatively short distances. They used a single central base station to communicate with all the mobile stations. This considerably reduced their coverage area. To overcome this a system known as trunking was devised whereby several transmitters could be used and the signal was “trunked” to the correct station. Several systems are available for this but the one that has gained by far the widest use is specified as MPT 1327.

All the standards mentioned so far are analogue systems. The cellular telecommunications industry moved to digital technology to provide improved efficiency of spectrum usage along with a variety of new facilities. So too did the PMR industry with the induction of a system known as TETRA. . Originally the letters stood for Trans European Trunked RAdio, but as the system is now being used beyond Europe the abbreviation now stands for TErrestrial Trunked RAdio. This system provided a far more flexible service along with all the other advantages of using a digital system.

Overview of the DMB system, and in particular the T-DMB version to be used for mobile video broadcasts

2006-11-21T15:01:00.000+08:00

Digital Multimedia broadcasting, DMB is based on the Eureka 147 Digital Audio Broadcast or DAB system that is widely deployed in the UK and many other countries around the world for audio broadcasting. One of the advantages of using DMB is that it can be rolled out and used without much modification for mobile video applications, simply increasing the level of error correction to cope with the mobile environment.

In view of the different broadcasting platforms that could be used account needs to be taken of this. Eureka 147 allows for broadcasts both from terrestrial transmitters and from satellite based transmitters. For DMB both platforms are possible, but in view of the differing platforms and transmission requirements there would need to be some modifications between the two systems. For terrestrial based transmissions a flavour of the system designated as T-DMB (Terrestrial Digital Multimedia Broadcasting) is used, whereas for satellite broadcasting S-DMB (Satellite Digital Multimedia Broadcasting) is used.

RF signal characteristics
Like many other broadcasting systems, DMB and DAB use a form of transmission known as Orthogonal Frequency Division Multiplex (OFDM). This has been adopted because of its high data capacity and suitability for applications such as broadcasting. It also offers a high resilience to interference, can tolerate multi-path effects and is able to offer the possibility of a single frequency network, SFN.

DMB format
The transmissions for the form of DMB being deployed in many countries occupy approximately 1.5 MHz bandwidth and for the VHF broadcasts the transmission contains 1536 Carriers. However it is possible to use a variety of modes:

* 2K mode 1536 carriers

* 1K mode 768 carriers

* 0.5K mode 384 carriers

* 0.25K mode 192 carriers

Frequency allocations
It would be possible to utilise the DAB transmission system within the UK for DMB, however much of the capacity has been taken up, although some reserve capacity has been retained for future data transmissions of which DMB could be part.

A more likely solution for DMB is to use frequencies within the L-Band DAB allocation (1452 - 1467.5 MHz). This might be possible in some countries where the use of this broadcasting allocation could be used for this purpose with little legislation.

Using a new band it will not only be possible to use smaller antennas, an important element for mobile phones and PDAs, but it will also be possible to tailor the transmission to accommodate the Doppler shifts likely to be encountered by small mobile devices. This can be achieved by reducing the number of carriers. Despite the carrier number reduction, the maximum data rate of 1.152 Mbps is still retained. The drawback of using the L band frequencies is that they would require a much higher density of transmitters to provide the required coverage.

Battery consumption
One of the major requirements for any mobile video system such as DMB is that it shall not place a major load on the battery of the handheld device. With user expectations requiring that battery life shall be several days between recharges, this is a major consideration. While battery technology is improving, and IC technology has enabled current consumption of chips to be reduced, the basic technology can also play its part.

DMB is also ideally suited to the delivery of material to handheld devices. DAB inherently includes a technique known as time slicing by using an effectively using a Time Division Multiplexing delivery method. In this way the receiver is only switched on when it is required, thereby saving battery power.

It remains to be seen whether DMB or DVB-H will be the major standard that is adopted for mobile video. Some indicate that both schemes may be used in different countries around the world Accordingly many chip manufacturers whoa re addressing this market are catering for both schemes and developing systems that will be able to switch between the variety of bands that will be used around the globe.

In addition to this DMB trials are well advanced, particularly in Korea where it appears DMB will be adopted. For other countries, it remains to be seen what happens.

Overview of the DVB-H system to be used for mobile video broadcasts

2006-11-18T08:09:00.000+08:00

DVB-H or Digital Video Broadcast - Handheld, is one of the major systems to be used for mobile video and television for cellular phones and handsets. DVB-H has been developed from the DVB-T (Terrestrial) television standard that is used in many countries around the globe including much of Europe including the UK, and also other countries including the USA. The DVB-T standard has been shown to be very robust and in view of its widespread acceptance it forms a good platform for further development for handheld applications.

DVB-H development requirements
The environment for handheld devices is considerably different to that experienced by most televisions. Normally domestic televisions have good directional antenna systems and in addition to this the reception conditions are fairly constant. Additionally most televisions receiving DVB-T will be powered by mains supplies. As a result current consumption is not a major issue.

The conditions for handheld receivers are very different. In the first instance the antennas will be particularly poor because they will need to be small, and integrated into the handset in such a way that they either appear fashionable, or they are not visible. Additionally they will obviously be mobile, and this will entail receiving signals in a variety locations, many of which will not be particularly suitable for video reception. Not only will be signal be subject to considerable signal variations and multi-path effects, but it may also experience high levels of interference. Also some difficulties are presented by the fact that the handset could be in a vehicle and actually on the move. The operation of DVB-H has to be sufficiently robust to accommodate all these requirements.

"Note on multi-path effects:

Multi-path effects occur when signals reach the receiver via several different paths from the transmitter. This occurs because the signals leave the transmitter in a variety of directions - typically the transmitter may have an omni-directional radiation pattern so that it radiates signals equally in all directions. Accordingly some of the signal may travel directly to the receiver in what is termed the direct path, but some of the radiated may be reflected off a nearby hill, building or other object. In fact the received signal will consist of components reaching the receiver from the transmitter via a large number of paths. As the path length travelled by each of these components will be slightly different, each component will arrive at a slightly different time. If there are significant differences, this can cause the data being transmitted to be corrupted under some circumstances, although many modern receiver technologies can accommodate this and use the different signals travelling over different paths to reinforce one another."

While DVB-T proved to be remarkably robust under many circumstances, one of the major problems was that of current consumption. Battery life for handsets is a major concern where users anticipated the life between charges will be several days.

Operation of DVB-H
The DVB-H standard has been adopted by ETSI, European Telecom Standards Institute, and in this way the system can be truly international, and this will prevent compatibility problems caused by different countries and operators using different variants of the same system. The documents for the physical layer were ratified in 2004, with the upper layers defined in 2005.

DVB-H (Digital Video Broadcast Handheld) is based on the very successful DVB-T (Digital Video Broadcast Terrestrial) standard that is now used in many countries for domestic digital television broadcasts. DVB-H has taken the basic standard and adapted so that it is suitable for use in a mobile environment, particularly with the electronics incorporated into a mobile phone.

The DVB-H standard like DVB-T uses a form of transmission called Orthogonal Frequency Division Multiplex (OFDM). This has been adopted because of its high data capacity and suitability for applications such as broadcasting. It also offers a high resilience to interference, can tolerate multi-path effects and is able to offer the possibility of a single frequency network, SFN.

"Note on OFDM:

Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each another there is no mutual interference. This is achieved by having the carrier spacing equal to the reciprocal of the symbol period. This means that when the signals are demodulated they will have a whole number of cycles in the symbol period and their contribution will sum to zero - in other words there is no interference contribution. The data to be transmitted is split across all the carriers and this means that by using error correction techniques, if some of the carriers are lost due to multi-path effects, then the data can be reconstructed. Additionally having data carried at a low rate across all the carriers means that the effects of reflections and inter-symbol interference can be overcome. It also means that single frequency networks, where all transmitters can transmit on the same channel can be implemented. Further information on OFDM can be found on this site under the Cellular telecoms section or by using the Search facility."

There are a variety of modes in which the DVB-H signal can be configured. These are conform to the same concepts as those used by DVB-T. These are 2K, 4K, and 8K modes, each having a different number of carriers as defined in the table below. The 4K mode is a further introduction beyond that which is available for DVB-T.

Parameter 2K mode 4K mode 8K mode
Number of active carriers 1705 3409 6817
Number of data carriers 1512 3024 6048
Individual carrier spacing 4464 Hz 2232 Hz 1116 Hz
Channel width 7.61 MHz 7.61 MHz 7.61 MHz

Signal parameters for DVB-H OFDM Signal (8MHz Channel)

The different modes balance the different requirements for network design, trading mobility for single frequency network size, with the 4K mode being that which is expected to be most widely used.

The standard will support a variety of different types of modulation within the OFDM signal. QPSK (Quadrature Phase Shift Keying), 16QAM (16 point Quadrature Amplitude Modulation), and 64QAM (64 point Quadrature Amplitude Modulation) will all be supported, chipsets being able to detect the modulation and receive the incoming signal. The choice of modulation is again a balance, QPSK offering the best reception under low signal and high noise conditions, but offering the lowest data rate. 64QAM offers the highest data rate, but requires the highest signal level to provide sufficiently error free reception.

Time slicing
One of the key requirements for any mobile TV system is that it should not give rise to undue battery drain. Mobile handset users are used to battery life times extending over several days, and although battery technology is improving, the basic mobile TV technology should ensure that battery drain is minimised.

There is a module within the standard and hence the software that enables the receiver to decode only the required service and shut off during the other service bits. It operates in such a way that it enables the receiver power consumption to be reduced while also offering an uninterrupted service for the required functions.

The time slicing elements of DVB-H enable the power consumption of the mobile TV receiver to be reduced by 90% when compared to a system not using this technique. Although the receiver will add some additional power drain on the battery, this will not be nearly as much as it would have been had the TV reception scheme not employed the time slicing techniques.

Interleaving
Interleaving is a technique where sequential data words or packets are spread across several transmitted data bursts. In this way, if one transmitted burst or group is lost as a result of noise or some other drop-out, then only a small proportion of the data in each original word or packet is lost and it can be reconstructed using the error detection and correction techniques employed.

Further levels of interleaving have been introduced into DVB-H beyond those used for DVB-T. The basic mode of interleaving used on DVB-T and which is also available for DVB-H is a native interleaver that interleaves bits over one OFDM symbol. However DVB-H provides a more in-depth interleaver that interleaves bits over two OFDM symbols (for the 4K mode) and four bits (for the 2K mode).

Using the in-depth interleaver enables the noise resilience performance of the 2K and 4K modes to be brought up to the performance of the 8K mode and it also improves the robustness of the reception of the transmissions in a mobile environment.

MPE-FEC
In view of the particularly difficult reception conditions that may occur in the mobile environment, further error correction schemes are included. A scheme known as MPE-FEC provides additional error correction to that applied in the physical layer by the interleaving. Tjis is a forward error correction scheme that is applied to the transmitted data and after reception and demodulation, allows the errors to be detected and corrected.

Compatibility with DVB-T
DVB-H is a development of DVB-T and as a result it shares many common components. It has also been designed so that it can be used in 6, 7, and 8 MHz channel schemes although the 8MHz scheme will be the most widely used. There is also a 5MHz option that may be used for non-broadcast applications.

In view of the similarities between DVB-H and DVB-T it is possible for both forms of transmission to exist together on the same multiplex. In this way a broadcaster may choose to run two DVB-T services and one DVB-H service on the same multiplex. This feature may be particularly attractive in the early days of DVB-H when separate spectrum is not available.

DVB-H has been used in a number of trials and appear to perform well. It ahs support from a number of the major industry players and is likely to achieve a considerable degree of acceptance world-wide. Accordingly it is likely to be one of the major standards, if not the major standard used for mobile video.