Telecom Gang

SS7 Overview

2012-03-02T07:00:00.003-08:00

Common Channel Signaling System No. 7 (i.e., SS7 or C7) is a global standard for telecommunications defined by the International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T). The standard defines the procedures and protocol by which network elements in the public switched telephone network (PSTN) exchange information over a digital signaling network to effect wireless (cellular) and wireline call setup, routing and control. The ITU definition of SS7 allows for national variants such as the American National Standards Institute (ANSI) and Bell Communications Research (Telcordia Technologies) standards used in North America and the European Telecommunications Standards Institute (ETSI) standard used in Europe.
The SS7 network and protocol are used for:

basic call setup, management, and tear down
wireless services such as personal communications services (PCS), wireless roaming, and mobile subscriber authentication
local number portability (LNP)
toll-free (800/888) and toll (900) wireline services
enhanced call features such as call forwarding, calling party name/number display, and three-way calling
efficient and secure worldwide telecommunications

Signaling Links

SS7 messages are exchanged between network elements over 56 or 64 kilobit per second (kbps) bidirectional channels called signaling links. Signaling occurs out-of-band on dedicated channels rather than in-band on voice channels. Compared to in-band signaling, out-of-band signaling provides:

faster call setup times (compared to in-band signaling using multi-frequency (MF) signaling tones)
more efficient use of voice circuits
support for Intelligent Network (IN) services which require signaling to network elements without voice trunks (e.g., database systems)
improved control over fraudulent network usage

Signaling Points

Each signaling point in the SS7 network is uniquely identified by a numeric point code. Point codes are carried in signaling messages exchanged between signaling points to identify the source and destination of each message. Each signaling point uses a routing table to select the appropriate signaling path for each message.
There are three kinds of signaling points in the SS7 network (Fig. 1):

SSP (Service Switching Point)
STP (Signal Transfer Point)
SCP (Service Control Point)

SSPs are switches that originate, terminate, or tandem calls. An SSP sends signaling messages to other SSPs to setup, manage, and release voice circuits required to complete a call. An SSP may also send a query message to a centralized database (an SCP) to determine how to route a call (e.g., a toll-free 1-800/888 call in North America). An SCP sends a response to the originating SSP containing the routing number(s) associated with the dialed number. An alternate routing number may be used by the SSP if the primary number is busy or the call is unanswered within a specified time. Actual call features vary from network to network and from service to service.
Network traffic between signaling points may be routed via a packet switch called an STP. An STP routes each incoming message to an outgoing signaling link based on routing information contained in the SS7 message. Because it acts as a network hub, an STP provides improved utilization of the SS7 network by eliminating the need for direct links between signaling points. An STP may perform global title translation, a procedure by which the destination signaling point is determined from digits present in the signaling message (e.g., the dialed 800 number, calling card number, or mobile subscriber identification number). An STP can also act as a "firewall" to screen SS7 messages exchanged with other networks.
Because the SS7 network is critical to call processing, SCPs and STPs are usually deployed in mated pair configurations in separate physical locations to ensure network-wide service in the event of an isolated failure. Links between signaling points are also provisioned in pairs. Traffic is shared across all links in the linkset. If one of the links fails, the signaling traffic is rerouted over another link in the linkset. The SS7 protocol provides both error correction and retransmission capabilities to allow continued service in the event of signaling point or link failures.
Signaling gateways can be configured as an STP or SEP (Signaling End Point).

Tutorial : Basic Antenna Concepts - 2

2012-02-28T00:04:00.001-08:00

Antenna

An antenna is a device that transmits and/or receives electromagnetic waves. Electromagnetic waves are often referred to as radio waves. Most antennas are resonant devices, which operate efficiently over a relatively narrow frequency band.

An antenna must be tuned to the same frequency band that the radio system to which it is connected operates in, otherwise reception and/or transmission will be impaired.

Wavelength

We often refer to antenna size relative to wavelength. For example: a half-wave dipole, which is approximately a half-wavelength long. Wavelength is the distance a radio wave will travel during one cycle. The formula for wavelength is:

w = c/f ( W= wavelength, C= Speed of Light, F = Frequency)

Note: The length of a half-wave dipole is slightly less than a half-wavelength due to end effect. The speed of propagation in coaxial cable is slower than in air, so the wavelength in the cable is shorter.

The velocity of propagation of electromagnetic waves in coax is usually given as a percentage of free space velocity, and is different for different types of coax.

Impedance Matching

For efficient transfer of energy, the impedance of the radio, the antenna, and the transmission line connecting the radio to the antenna must be the same. Radios typically are designed for 50 ohms impedance and the coaxial cables (transmission lines) used with them also have a 50 ohm impedance. Efficient antenna configurations often have an impedance other than 50 ohms, some sort of impedance matching circuit is then required to transform the antenna impedance to 50 ohms. Radiall/Larsen antennas come with the necessary impedance matching circuitry as part of the antenna. We use low loss components in our matching circuits to provide the maximum transfer of energy between the transmission line and the antenna.

VSWR and Reflected Power

The Voltage Standing Wave Ratio (VSWR) is an indication of how good the impedance match is. VSWR is often abbreviated as SWR. A high VSWR is an indication that the signal is reflected prior to being radiated by the antenna. VSWR and reflected power are different ways of measuring and expressing the same thing. A VSWR of 2.0:1 or less is considered good. Most commercial antennas, however, are specified to be 1.5:1 or less over some bandwidth. Based on a 100 watt radio, a 1.5:1 VSWR equates to a forward power of 96 watts and a reflected power of 4 watts, or the reflected power is 4.2% of the forward power.

Bandwith

Bandwidth can be defined in terms of radiation patterns or VSWR/reflected power. The definition used in this book is based on VSWR. Bandwidth is often expressed in terms of percent bandwidth, because the percent bandwidth is constant relative to frequency. If bandwidth is expressed in absolute units of frequency, for example MHz, the bandwidth is then different depending upon whether the frequencies in question are near 150, 450, or 825 MHz.

Directivity and Gain

Directivity is the ability of an antenna to focus energy in a particular direction when transmitting or to receive energy better from a particular direction when receiving.
The relationship between gain and directivity:
Gain = efficiency/Directivity. We see the phenomena of increased directivity when comparing a light bulb to a spotlight. A 100 watt spotlight will provide more light in a particular direction than a 100 watt light bulb, and less light in other directions. We could say the spotlight has more "directivity" than the light bulb. The spotlight is comparable to an antenna with increased directivity. An antenna with increased directivity is hopefully implemented efficiently, is low loss, and therefore exhibits both increased directivity and gain. Gain is given in reference to a standard antenna. The two most common reference antennas are the isotropic antenna and the resonant half-wave dipole antenna. The isotropic antenna radiates equally well in "all" directions. Real isotropic antennas do not exist, but they provide useful and simple theoretical antenna patterns with which to compare real antennas. An antenna gain of 2 (3 dB) compared to an isotropic antenna would be written as 3 dBi. The resonant half-wave. dipole can be a useful standard for comparing to other antennas at one frequency or over a very narrow band of frequencies. To compare the dipole to an antenna over a range of frequencies requires an adjustable dipole or a number of dipoles of different lengths. An antenna gain of 1 (0 dB) compared to a dipole antenna would be written as 0 dBd.

Gain Measurement

One method of measuring gain is by comparing the antenna under test against a known standard antenna. This is technically known as a gain transfer technique. At lower frequencies, it is convenient to use a 1/2-wave dipole as the standard. At higher frequencies, it is common to use a calibrated gain horn as a gain standard, with gain typically expressed in dBi. Another method for measuring gain is the 3 antenna method. Transmitted and received power at the antenna terminals is measured between three arbitrary antennas at a known fixed distance. The Friis transmission formula is used to develop three equations and three unknowns. The equations are solved to find the gain expressed in dBi of all three antennas. Use the following conversion factor to convert between dBd and dBi: 0 dBd = 2.15 dBi.

Antenna Placement

Correct antenna placement is critical to the performance of an antenna. An antenna mounted on the roof will function better than the same antenna installed on the hood or trunk of a car. Knowledge of the vehicle may also be an important factor in determining what type of antenna to use. You do not want to install a glass mount antenna on the rear window of a vehicle in which metal has been used to tint the glass. The metal tinting will work as a shield and not allow signals to pass through the glass.

Radiation Patterns

The radiation or antenna pattern describes the relative strength of the radiated field in various directions from the antenna, at a fixed or constant distance. The radiation pattern is a "reception pattern" as well, since it also describes the receiving properties of the antenna. The radiation pattern is three-dimensional, but it is difficult to display the three-dimensional radiation pattern in a meaningful manner, it is also time consuming to measure a three-dimensional radiation pattern. Often radiation patterns are measured that are a slice of the three-dimensional pattern, which is of course a two-dimensional radiation pattern which can be displayed easily on a screen or piece of paper. These pattern measurements are presented in either a rectangular or a polar format.

Absolute and Relative Patterns

Absolute radiation patterns are presented in absolute units of field strength or power. Relative radiation patterns are referenced in relative units of field strength or power. Most radiation pattern measurements are relative pattern measurements, and then the gain transfer method is then used to establish the absolute gain of the antenna.

Near-Field and Far-Field Patterns

The radiation pattern in the region close to the antenna is not exactly the same as the pattern at large distances. The term near-field refers to the field pattern that exists close to the antenna; the term far-field refers to the field pattern at large distances. The far-field is also called the radiation field, and is what is most commonly of interest. The near-field is called the induction field (although it also has a radiation component). Ordinarily, it is the radiated power that is of interest, and so antenna patterns are usually measured in the far-field region. For pattern measurement it is important to choose a distance sufficiently large to be in the far-field, well out of the near-field. The minimum permissible distance depends on the dimensions of the antenna in relation to the wavelength. The accepted formula for this distance is: When extremely high power is being radiated (as from some modern radar antennas), the near-field pattern is needed to determine what regions near the antenna, if any, are hazardous to human beings.

Beamwidth

Depending on the radio system in which an antenna is being employed there can be many definitions of beamwidth. A common definition is the half power beamwidth. The peak radiation intensity is found and then the points on either side of the peak represent half the power of the peak intensity are located. The angular distance between the half power points traveling through the peak is the beamwidth. Half the power is —3dB, so the half power beamwidth is sometimes referred to as the 3dB beamwidth.

Antenna Pattern Types

Omnidirectional Antennas

For mobile, portable, and some base station applications the type of antenna needed has an omnidirectional radiation pattern. The omnidirectional antenna radiates and receives equally well in all horizontal directions. The gain of an omnidirectional antenna can be increased by narrowing the beamwidth in the vertical or elevation plane. The net effect is to focus the antenna’s energy toward the horizon. Selecting the right antenna gain for the application is the subject of much analysis and investigation. Gain is achieved at the expense of beamwidth: higher-gain antennas feature narrow beamwidths while the opposite is also true. Omnidirectional antennas with different gains are used to improve reception and transmission in certain types of terrain. A 0 dBd gain antenna radiates more energy higher in the vertical plane to reach radio communication sites that are located in higher places. Therefore they are more useful in mountainous and metropolitan areas with tall buildings. A 3 dBd gain antenna is the compromise in suburban and general settings. A 5 dBd gain antenna radiates more energy toward the horizon compared to the 0 and 3 dBd antennas to reach radio communication sites that are further apart and less obstructed. Therefore they are best used in deserts, plains, flatlands, and open farm areas.

Directional Antennas

Directional antennas focus energy in a particular direction. Directional antennas are used in some base station applications where coverage over a sector by separate antennas is desired. Point to point links also benefit from directional antennas. Yagi and panel antennas are directional antennas.

Antenna Polarization

Polarization is defined as the orientation of the electric field of an electromagnetic wave. Polarization is in general described by an ellipse. Two often used special cases of elliptical polarization are linear polarization and circular polarization. The initial polarization of a radio wave is determined by the antenna that launches the waves into space. The environment through which the radio wave passes on its way from the transmit antenna to the receive antenna may cause a change in polarization. With linear polarization the electric field vector stays in the same plane. In circular polarization the electric field vector appears to be rotating with circular motion about the direction of propagation, making one full turn for each RF cycle. The rotation may be right-hand or left-hand. Choice of polarization is one of the design choices available to the RF system designer. For example, low frequency (< 1 MHz) vertically polarized radio waves propagate much more successfully near the earth than horizontally polarized radio waves, because horizontally polarized waves will be cancelled out by reflections from the earth. Mobile radio systems waves generally are vertically polarized. TV broadcasting has adopted horizontal polarization as a standard. This choice was made to maximize signal-to-noise ratios. At frequencies above 1 GHz, there is little basis for a choice of horizontal or vertical polarization, although in specific applications, there may be some possible advantage in one or the other. Circular polarization has also been found to be of advantage in some microwave radar applications to minimize the "clutter" echoes received from raindrops, in relation to the echoes from larger targets such as aircraft. Circular polarization can also be used to reduce multipath.

Tips : IP subnetting

2012-02-27T21:43:00.000-08:00

The first IP address is usually used for gateway, and the last IP address belongs to host in the subnet.
- Network address is always an even number, the last bit is zero (0)
- The first usable address is always an odd number, the last bit is one (1)
- On the other side it is inverted, the broadcast is odd number, and the last host is always even number

Lets see this example

Subnet 192.168.1.32/27

Number of bits to mask = 27
Number of host bits = 32 - 27 = 5

First usable 192.168.1.33
last 192.168.1.62,
broadcast 192.168.1.63,
next network 192.168.1.64/27

The last octet:

00100000 - network
00100001 - first
00111110 - last
00111111 - broadcast

The next network is broadcast + 1 = 01000000 - 64/27

It will be very usefull to learn subnet numbers.
The networks are increasing on exponent of 2, in exactly specified blocks.

24 - 255.255.255.0 = 256 2^8
25 - 255.255.255.128 =128 2^7
26 - 255.255.255.192 = 64 2^6
27 - 255.255.255.224 = 32 2^5
28 - 255.255.255.240 = 16 2^4
29 - 255.255.255.248 = 8 2^3
30 - 255.255.255.252 = 4 2^2
31 - 255.255.255.254 = 2 2^1
32 - 255.255.255.255 = 1 2^0
For a address range in 3. octet is the same situation, where you can use prefix minus 8...
Example:
255.255.0.0 /16

Tutorial : RADIO LINK CONTROL

2012-02-27T21:22:00.001-08:00

Radio Link Control is the interface between Radio Resource Control and Logical channels. As you can see from the diagram.

The primary function of RLC is to transfer user data and signaling between the upper layers and the MAC layer. Data is transferred into the RLC in data blocks called Service Data Units (SDU). Data is transferred out of the RLC in data blocks called Protocol Data Units (PDU).

Three data transfer modes are defined:

Transparent Mode (TM) – PDUs are transferred with no additional header information.
Unacknowledged Mode (UM) – A header containing length and sequence number fields are added to the PDU before it is transmitted.
Acknowledged Mode (AM) – PDUs are delivered according to a bidirectional protocol that includes in-sequence delivery, flow control, error detection, and duplicate detection.

Data flow to and from the upper layers are called Radio Bearers and may carry either signaling data (Signaling Radio Bearers) or user data (Radio Access Bearers). Data flow to and from the MAC are called Logical Channels.

RLC Transparent Mode Functions

In Transparent Mode (TM), PDUs are transferred with little interaction by RLC. No header is attached to the PDU. The functions performed by TM RLC are:

Segmentation and Reassembly – If the SDU size is too large to fit in a single PDU, it may be segmented on the transmitting side and reassembled on the receiving side.
SDU Discard – The transmitting TM entity may be configured to perform timer-based SDU discard. An SDU that is not transmitted after a configurable period of time is discarded. Note that ciphering for Transparent Mode Logical Channels is performed by the MAC layer. Voice and some signaling channels use transport mode.

RLC Transparent Mode Functions

RLC Unacknowledged Mode Functions

In Unacknowledged Mode (UM), RLC provides slightly more reliable service than in Transparent Mode. A small header, which contains information about segmentation, concatenation, and the sequence number, is attached to each PDU. The functions performed by UM RLC are:

Segmentation and Reassembly – If the SDU size is too large to fit in a single PDU, it may be segmented on the transmitting side and reassembled on the receiving side.
Concatenation/Padding – Multiple SDUs may be combined into a single PDU to fill the available PDU size, and padding bits may be added to fill the available PDU.
Sequence Number Check – The RLC header contains a sequence number, which is used during reassembly to detect corrupted SDUs. • Ciphering – If enabled, ciphering of UM Logical Channel data streams is performed by RLC.
SDU Discard – The transmitting UM entity may be configured to perform timer-based SDU discard. An SDU that is not been transmitted after a configurable time is discarded.

RLC Unacknowledged Mode Functions

RLC Acknowledged Mode Functions

The Acknowledged Mode (AM) protocol is a complex protocol that provides reliable service. Both positive and negative acknowledgment are supported. The functions performed by AM RLC are:

Segmentation and Reassembly – If the SDU size is too large to fit in a single PDU, it may be segmented on the transmitting side and reassembled on the receiving side.
Concatenation/Padding – Multiple SDUs may be combined into a single PDU to fill the available PDU size, and padding bits may be added to fill the available PDU.
Error Correction – PDUs received in error are retransmitted. The receiver may poll the transmitter to prompt it to send an acknowledgment status report. In-sequence Delivery – PDUs are delivered to the upper layers in the same order as they were submitted to the transmitting side RLC.
Duplicate Detection – A PDU is delivered only once to the upper layers.
Flow Control – Configurable transmit and receive window sizes perform flow control.
Ciphering – Ciphering of AM Logical Channel data streams is performed by RLC.
Protocol Error Detection and Recovery – AM RLC may be reset to recover from protocol errors.
SDU Discard – Several mechanisms are defined for SDU discard in AM RLC, including timerbased and maximum data retransmissions.

Acknowledged Mode

Quick Review : IP addressing

2012-02-27T20:29:00.003-08:00

IP addressing

– Class A addresses 8 bits network number

– Class B addresses 16 bits network number

– Class C addresses 24 bits network number

Distinguished by leading bits of address

– leading 0 => class A (first byte < 128)

– leading 10 => class B (first byte in the range 128191)

– leading 110 => class C (first byte in the range 192223)

Quick Review : Types of Networking Devices

2012-02-27T20:27:00.001-08:00

Layer 3 Network Routers

Layer 2 Data link Switches, bridges, NICs

Layer 1 Physical Hubs and repeaters

Repeater – Amplifies the signal received on input and transmits it on output

Modem – Accepts a serial stream of bits as input and produces a modulated carrier as output (or vice versa)

Hub

– Connect nodes/segments of a LAN

– When a packet arrives at one port, it is copied to all the other ports

Switch

– Reads destination address of each packet and forwards appropriately to specific port

– Layer 3 switches (IP switches) also perform routing functions

Bridge:

– “ignores” packets for same LAN destinations

– forwards ones for interconnected LANs

Router:

– decides routes for packets, based on destination address and network topology

– Exchanges information with other routers to learn network topology

The Basics of Antenna

2012-02-27T20:03:00.000-08:00

Antenna is the basis of Wireless communications. Its the key point which involves reception and transmission of data to and fro from any device used in Mobile or Wireless Communications.

In this tutorial, we will learn more about the basics of antenna system.

First of all we must understand , what antenna actually is.

In generic term we can say that an antenna is the converter between two kinds of electromagnetic waves.

cable bounded waves ⇔ free space waves

So we can say that an antenna is an electrical device that converts electric current into radio waves.

Technically speaking, antenna is a quad-pole device with the second termination connected to free space. Hope this image will make things clear for you.

To understand how antenna works , we first must know how wave propogates through the air. A wave is a permanent conversion from electrical into magnetical energy and vice versa. In this way a magnetic waves lead to an electrical wave and in same way an electrical wave leads to a magnetic wave.

There are lot of parameters related to antenna system, which we cover in later part of series.