Network Performance And RF Optimization Engineer

Mobile communications: Basic concepts

noreply@blogger.com (Unknown) — Fri, 14 Aug 2009 09:53:00 +0000

From ancient to modern times, mankind has been looking for means of long distance communications. For centuries, letters proofed to be the most reliable way to transmit information. Fire, flags, horns, etc. were used to transmit information faster.
Technical improvements in the 19th century simplified long distance communications: Telegraphy, and later on telephony. Both techniques were wireline.
In 1873, J. C. Maxwell laid the foundation of the electro-magnetic theory by summarising empirical results in four equations, which are still valid today. It would however be several decades before Marconi made economic use of this theory by developing devices for wireless transmission of Morse signals (about 1895). Already 6 years later, the first transatlantic wireless transmission of Morse signals took place. Voice was transmitted the first time in 1906 (R. Fesseden), and one of the first radio broadcast transmission 1909 in New York.

Figure 1. Transmission

The economically most successful wireless application in the first half of the 20th century was radio broadcast. There is one transmitter, the so-called radio station. Information, such as news, music, etc. is transmitted from the radio station to the receiver equipment, the radio device. This type of one-way transmission is called simplex transmission. The transmission takes place only in one direction, from the transmitter to the receiver. When we take a human conversation, a technical solution is required, where the information flow can take place in two directions. This type of transmission is called duplex transmission. Walky-talky was already available the early 30ies. This system already allowed a transmission of user data in two directions, but there was a limitation: The users were not allowed to transmit at the same time. In other words, you could only receive or transmit user information. This type of transmission is therefore often called semi-duplex transmission. For telephony services, a technical solutions is required, where subscribers have the impression, that they can speak (transmit) and hear (receive) simultaneously. This type of transmission solution is regarded as full duplex transmission.

The first commercial wireless car phone telephony service started in the late 1940 in St. Louise, Missouri (USA). It was a car phone service, because at that time, the mobile phone equipment was bulky and heavy. Actually, in the start-up, it filled the whole back of the car. But it was a real full duplex transmission solution. In the 50ies, several vehicle radio systems were also installed in Europe. These systems are nowadays called single cell systems. The user data transmission takes place between the mobile phone and the base station (BS). A base station transmits and receives user data. While a mobile phone is only responsible for its user’s data transmission and reception, a base station is capable to handle the calls of several subscribers simultaneously. The transmission of user data from the base station to the mobile phone is called downlink (DL), the transmission from the mobile phone to the base station uplink (UL) direction. The area, where the wireless transmission between mobile phones and the base station can take place, is the base stations supply area, called cell.

Figure 2. Single Cell System

Single cell systems are quite limited. The more and more distant the subscriber is from the base station, the lower the quality of the radio link. If the subscriber is leaving the supply area of the cell, no communication is possible any more. In other words, the mobile communication service was only available within the cell. In order to overcome this limitation, cellular systems were introduced. A cellular mobile communication system consists of several cells, which can overlap. By doing so, a whole geographical area can be supported with the mobile communication service.

Figure 3. Cellular System

But what happens, when a subscriber moves during a call from one cell to another cell? It would be very annoying, if the call is dropped. If the subscriber is leaving a cell, and in parallel is entering a new cell, then the system makes new radio resources available in the neighbouring cell, and then the call is handed over from on cell to the next one. By doing so, service continuation is guaranteed, even when the subscriber is moving. The process is called handover (HO).

Figure 4. Handover

A handover takes place during a call, i.e. when the mobile phone is in active (dedicated) mode. A mobile phone can also be in idle mode. In this case, the mobile phone is switched on, but no resources are allocated to it to allow user data transmission. In this mode, the mobile phone is still listening to information, broadcasted by the base station. Why?
Imagine, there is a mobile terminated call. The mobile phone is then paged in the cell. This means the phone receives information that there is a mobile terminated call. A cellular system may consist of hundreds of cells. If the mobile network does not know, in which cell the mobile phone is located, it must be paged in all of them. To reduce load on networks, paging in is done in small parts of a mobile an operators network. Mobile network operators group cells in administrative units called location areas (LA). A mobile phone is paged in only one location area.

But how does the cellular system know, in which location area the mobile phone is located? And how does the mobile phone know? In every cell, system information is continuously transmitted. The system information includes the location area information. In the idle mode, the mobile phone is listening to this system information. If the subscriber moves hereby from one cell to the next cell, and the new cell belongs to the same location area, the mobile stays idle. If the new cell belongs to a new location area, then the mobile phone has to become active. It starts a communication with the network, informing it about it new location. This is stored in databases within the mobile network, and if there is a mobile terminated call, the network knows where to page the subscriber. The process, where the mobile phone informs the network about its new location is called Location Update Procedure (LUP).

Figure 5.Location Update and Paging

With the introduction of cellular mobile communication systems, we refer to generations. First generation mobile communication systems are
• TACS (Total Access Communications System)
• NMT (Nordic Mobile Telephony)
• AMPS (Advanced Mobile Phone Service)
• C450
• etc.
All of them were commercially launched in the 80s of the last century.
The 1st generation mobile communication systems often offered national wide coverage. But there were limitations:
Most of them did not support roaming. Roaming is the ability to use an other operator’s network infrastructure. International roaming is the ability to go even to another country and use the local operator’s infrastructure.

Figure 6. Roaming

Most 1st generation mobile communication systems only support speech transmission, but not data transmission, such as fax. Supplementary services, well known from ISDN, were not available, such as number indication and call forwarding, when busy. The transmission takes place unprotected via the radio interface – as a consequence, eavesdropping is possible. Finally, mobile communication started to become a mass market. And the radio interface is the main bottleneck in terms of capacity. Improved solutions were urgently required. This led to the inauguration of the 2nd generation mobile communication systems, one of which is GSM.

Co-channel Interference in GSM Networks

noreply@blogger.com (Unknown) — Fri, 14 Aug 2009 04:39:00 +0000

In a GSM network, the number of frequencies available to the operator is limited. Therefore, the frequencies must be reused across the network area. Inevitably, carriers with the same frequency will interfere with each other. This is known as co-channel interference and is usually by far the most common type of interference encountered in a GSM network. (Disturbances also arise from many other sources, but they will not concern us here.)

The operator must strive to minimize the co-channel interference, first of all by judicious frequency planning ensuring that frequencies are reused as sparsely and intelligently as possible. As the traffic in the network increases, however, the operator is forced to make the reuse pattern tighter and tighter, raising interference levels and increasing the risk of performance degradation. Excessive interference gives rise to bad speech quality, dropped calls, low data throughput, and so on. When such problems arise they must be speedily rectified, for instance by changing the set of frequencies used in a certain cell.

Frequency reuse. All shaded cells use the same TCH ARFCN as the current serving cell A and are potential co-channel interferers. The cross marks the phone's current position.

Traditional methods of tracing interference sources are typically complicated and awkward. Often they involve practices such as temporarily shutting down base stations one by one in order to see when the interference ceases – something which disturbs network traffic and causes inconvenience to subscribers. Another very work-intensive method is to repeatedly modify the frequency plan and check for improvements.
With interferer identification, TEMS Investigation offers a way to spot interferers without resorting to such heavy-handed and costly methods, and indeed without any performance loss whatever in the network.

C/A Measurements In TEMS Investigation GSM

noreply@blogger.com (Unknown) — Mon, 10 Aug 2009 04:34:00 +0000

1. Introduction

1.1. Scope
This document describes the information element C/A and how it is measured in TEMS Investigation. It is applicable to all current versions of TEMS Investigation.

1.2. C/A definition
The carrier-to-adjacent ratio is defined as the signal-strength ratio between a serving carrier and an adjacent carrier. It is calculated according to the following formula:

C/A (dB) = Serving signal strength (dBm) – Adjacent signal strength (dBm)
The adjacent carrier could be located on both sides of the serving carrier and at a different index. The index represents the number of ARFCN offset from a carrier, which means that the index corresponds to a frequency offset of index multiplied by 200kHz (200kHz is the carrier spacing in GSM). For example, if the serving carrier ARFCN is 20 and the adjacent ARFCN is 19, the corresponding C/A value is called C/A-1, located 200kHz below the serving carrier.

Figure 1. The information element C/A-1 and C/A + 2 shown, serving carrier is ARFCN 20.

In
Figure 1 above:
C/A-1 = –70dBm – (-90dBm) = 20dB
C/A+2 = -70dBm – (-65dBm) = -5dB

GSM requirements
The GSM specification 05.05 sets the C/A performance requirement on a GSM mobile phone in terms of adjacent channel rejection. Actually, the specification states how much stronger an adjacent carrier could be without disturbing the serving carrier due to leakage in the receiver filter. If the power level on an adjacent carrier is to high, the filter cannot remove all the power and it will appear as interference in the receiver disturbing the serving carrier signal.

According to GSM specification 05.05, section 6.3:

Adjacent (200 kHz) interference: C/Ia1=- 9 dB
Adjacent (400 kHz) interference: C/Ia2=- 41 dB
Adjacent (600 kHz) interference: C/Ia3=- 49 dB

This means that an adjacent carrier with 200kHz offset could be 9dB stronger than the carrier, still fulfilling the FER/BER performance requirement.

1.3. When should C/A measurements be used?
The typical usage of C/A measurements is for troubleshooting bad speech quality or low data throughput. The very first step after having detected a problem with the speech quality (preferably the SQI value) is to check the C/I values for the used frequency (or frequencies in case of frequency hopping). A low C/I value in TEMS Investigation can be caused by several different reasons (see the document “Interpreting the Information Element C/I”, doc. EPL/T/TN-00:022 (file c_i_causes.pdf)).
Measuring the C/A values on a disturbed carrier can tell whether the problem is related to a too-strong adjacent carrier.

2. C/A measurements using a single TEMS mobile
In TEMS Investigation, it is possible to enable C/A measurements using a single mobile phone and measure during idle mode or, if frequency hopping is not in use, during dedicated mode as well. It is possible to select up to three adjacent carriers on both sides of the serving carrier that should be measured.
However, those measurements will, as explained below, have implications regarding the mobile phone behavior compared to a standard GSM phone.

2.1. Measuring the adjacent sample [A]
Dedidated mode
A typical single slot GSM mobile is designed to, on each TDMA frame (i.e. eight consecutive timeslots), perform three tasks.

Figure 2 Mobile station behavior on a TDMA frame.

In Figure 2 above, a mobile station is assigned timeslot 1. Note the 3 timeslot offset between RX and TX.
On one TDMA frame, the mobile first receives one burst and simultaneously measures the signal strength of the serving cell, then it transmits one burst and then it measures signal strength on one neighbor cell. Note that if the neighbor measurement is not synchronized to the TDMA frame of the measured neighboring cell, it will just tune into the frequency of the neighbor and measure the signal strength.
This means that an intermittent adjacent interferer could be missed if it occurs at the receiving timeslot, but never at the timeslot corresponding to the neighbor cell measurement slot.

So, for each TDMA frame, one neighbor cell can be measured. In dedicated mode during one SACCH multiframe, it is possible to take 100 neighboring samples (there are 104 TDMA frames per SACCH multiframe but the four IDLE frames will be used for SCH search instead of measurements). If the neighbor cell list (the BA list) contains 20 frequencies, the reported signal strength in the Measurement Report will be an average of 100/20 = 5 samples per neighbor.
If C/A measurements are enabled, the adjacent carriers will be added to the list of neighboring cells that should be measured (although they will of course not be reported in the Measurement Report sent up to the GSM network). This means that if two adjacent carriers on both sides of the carrier should be measured, four frequencies will be added to the list of frequencies that should be measured.
In this instance we will have 24 frequencies instead of 20. This means that each sample in the Measurement Report will be based on 100/24 = 4 samples instead of 5 as in the example above. Although it would hardly be noticeable, this in turn might lead to incorrect handover decisions and thus degrade handover performance. The worst case is when the BA list contains 32 neighbors and then three adjacent carriers on both sides should be measured. This will end up as 100/(32+6) = 2 samples per carrier. The GSM specification requires that one neighbor in the BA list is measured on each TDMA frame (except for the idle frame) resulting in, for a full-rate traffic channel, an average of at least three samples.
Each adjacent value will be based on the same number of samples as the neighbors reported in MEASUREMENT REPORT (as calculated above).
Number of samples per value = 100 / (BA list length + number of adjacent frequencies).
Where number of adjacent frequencies could be 0 (C/A disabled), 2, 4, or 6).
A multi-slot mobile will increase the number of timeslots used for receiving and transmitting, however, the neighboring measurements will still be limited to one neighbor per TDMA frame.

Idle mode
In idle mode, the adjacent carriers to be measured will, as in the dedicated mode case above, be added to the list of the neighboring cells. The difference is that the normal neighboring cell measurements will not be affected by any means of the extra measurements.
Each adjacent [A] value will be an average of 5 samples spread equally over the measurement period.

2.2. Measuring the carrier sample [C]
Dedicated mode
In order to calculate the C/A ratio, both the adjacent sample and the serving-carrier sample are needed. The signal strength sample of the serving carrier will be the signal-strength sub value (RXLEV SUB), which means that it will be measured during the reception of data (i.e. timeslot 1 in Figure 2). This means that the [C] sample will be an average of 12 samples (see the document “FER, RXQUAL, and DTX DL rate measurements in TEMS GSM.” doc. No. EPL/N/TB-01:019 for a detailed description of the subset of bursts used for the SUB values.

Idle mode
In idle mode, the signal strength used for the [C] value is the same value as the serving-cell signal strength (RXLEV) used for cell reselection. Each [C] value will be an average of 5 samples spread equally over the measurement period.

2.3. Calculating the C/A ratio
For each measured adjacent frequency, a corresponding C/A value is calculated by using the formula stated in chapter 1. The C/A will be calculated once for each SACCH multiframe (i.e. 480 ms) in dedicated mode and in idle mode, C/A will be calculated once every 10:th second.

3. C/A measurements using two synchronized TEMS mobiles
In order to be able to measure C/A during frequency hopping, two TEMS mobiles must be used. This is also the case when the serving carrier [C] and the adjacent [A] should be measured synchronously (at the same timeslot (but at different TDMA frames)). One of the mobiles (referred to as the master) will be used as a regular GSM phone in idle or dedicated mode but the other one (referred to as adjacent scanning) will enter frequency scanning mode and will be synchronized to the master mobile. This function is completely independent of the C/A measurement function described in chapter 2. The adjacent scanning mobile will be controlled from the frequency-scanning tool.
The adjacent scanning mobile requires an R520, R320, or T28 World mobile phone, and the master mobile must be a R520, R320, SH888, CF688, T28 World, or T28s.
See the TEMS Investigation manual for how to set up and synchronize the adjacent scanning mobile.

3.1. Measuring the adjacent sample [A]
The adjacent scanning function uses the adjacent scanning mobile to track the behavior of the master mobile in both idle and dedicated mode. In dedicated mode, both frequency-hopping and non-frequency-hopping configurations can be monitored and, in frequency-hopping mode, both cyclic and pseudo-random hopping is supported.
At any given moment, the adjacent scanning mobile measures the two closest adjacent carriers on both sides of the master mobile's serving cell carrier, as well as the serving carrier itself. The reason for the latter is that picking this data from the master mobile may be misleading if the receiving conditions differ between the two devices.
If frequency hopping is used in the network, a number of such five-carrier sets will be measured alternately, and all of these are presented in parallel.
The averaging procedure takes place in the mobile and will be performed in the dB domain (i.e. no watts translation). Each value presented in TEMS Investigation will be an average based on the number of samples according to the formula below:
Number of samples per averaged value = 30/(length of MA list)
In idle mode and non-frequency-hopping dedicated mode, the MA list length is 1. In frequency-hopping mode, the MA list length is the number of frequencies in the hopping set.

3.2. Measuring the carrier sample [C]

The carrier sample [C] used in the C/A calculation will be taken in exactly the same way as the adjacent [A] sample as described in chapter Error! Reference source not found.. Note that the signal strength of the master mobile will not be used as the [C] sample.

3.3. Synchronization
The adjacent scanning mobile and the master mobile are synchronized in such a way that each serving-cell carrier and its associated adjacent carrier are sampled throughout the precise time intervals during which the master mobile is on this serving carrier. When frequency hopping is used, this is crucial for the measurements to represent exactly the radio environment experienced by the master mobile. If you measure the same set of carrier manually (i.e. not using this synchronization), very often while the master mobile is on one carrier in the hopping list, the scanner will be busy measuring some other carrier which is not even adjacent to it. In other words, the timing of most measured samples will be slightly off, meaning that the fast fading patterns will differ considerably from those actually encountered by the master mobile.

4. Summary
In TEMS Investigation, there are two different ways to measure the C/A ratio, using a single mobile or using two mobiles where one mobile is synchronized to the other. C/A can be measured in both idle and dedicated mode.
Using two synchronized mobiles has not only the advantage of handling frequency hopping but also measures the C/A ratio exactly as experienced by the master mobile.

BSC nominal capacity and dimensioning

noreply@blogger.com (Unknown) — Fri, 17 Jul 2009 13:52:00 +0000

Described here are the traffic handling capacities of the Nokia GSM/EDGE Base Station Controllers, BSC2i and BSC3i, with certain average call mixes. Additionally, the whole BSS (BSC) overload protection is described. The overload protection has been implemented to protect the equipment and the system in exceptionally high traffic cases.

In the given network the BSC erlang (traffic handling) capacity must be checked so that the following nominal erlang capacity is not exceeded. This is the simplest case (that is, to check only erlang figures) in which the reference call mix can be used. In many cases the call mix in real networks differs from the nominal one. By a separate agreement Nokia can provide a separate statement of the BSC capacity for the call mix in question. The BSC traffic handling capacity in the case of GPRS should be noted separately.

BSC2i and BSC3i traffic handling capacity with the following circuit-switched reference call mix is stated in documents:

In the example below, we assume that the BSC is defined to one location area.

Example: BSC3i 2000 Traffic Handling Capactiy

BSC3i is configured with 2000 full rate TRXs, circuit-switched:

11880 erlangs total traffic handling capacity
25 mErl per subscriber, 475 200 subscribers
354 000 busy hour call attempts (BHCA)
120 seconds mean hold time
70% mobile-originated calls (MO)
30% mobile-terminated calls (MT)
1.5 handovers (HO) per call
2 location updates (LU) per call
0.1 IMSI detach per call
63% no paging response
1 SMS call rate subs/hour
(80% terminated SMS)

In the circuit-switched (CS) case, for example, the nominal BSC paging load for BSC3i 2000 would be (note that both MT calls and MT SMSs create pages):

354 000 x 0.3 (MT) + 475 200 x 0.8 (SMS MT) = 486 360 + re-paging 0.63 x 486 360 = 792 767 pagings per hour.

The nominal BSC3i 2000 RACH load (MO, MT, SMS, LUs) would be, for example:

354 000 (MO, MT) + 2 x 354 000 (LU) + 0.1 x 354 000 (IMSI detach) + 486 360 (SMS) = 1 583 760 RACHs per hour.

Example: BSC3i 660 Traffic Handling Capacity

BSC3i is configured with 660 full rate TRXs, circuit-switched:

3920 erlangs total traffic handling capacity
25 mErl per subscriber, 157 000 subscribers
117 000 busy hour call attempts (BHCA)
120 seconds mean hold time
70% mobile-originated calls (MO)
30% mobile-terminated calls (MT)
1.5 handovers (HO) per call
2 location updates (LU) per call
0.1 IMSI detach per call
63% no paging response
1 SMS call rate subs/hour
(80% terminated SMS)

In the circuit-switched (CS) case, for example, the nominal BSC paging load for BSC3i would be (note that both MT calls and MT SMSs create pages):

117 000 x 0.3 (MT) + 157 000 x 0.8 (SMS MT) = 160 700 + re-paging 0.63 x 160 700 = 261 941 pagings per hour.

The nominal BSC3i RACH load (MO, MT, SMS, LUs) would, be for example:

117 000 (MO, MT) + 2 x 117 000 (LU) + 0.1 x 117 000 (IMSI detach) + 160 700 (SMS) = 523 400 RACHs per hour.

Example: BSC2i Traffic Handling Capacity

BSC2i configured with 512 full rate TRXs, circuit-switched:

3040 erlangs total traffic handling capacity
25 mErl per subscriber, 120 000 subscribers
91 000 busy hour call attempts (BHCA)
120 seconds mean hold time
70% mobile-originated calls (MO)
30% mobile-terminated calls (MT)
1.5 handovers (HO) per call
2 location updates (LU) per call
0.1 IMSI detach per call
63% no paging response
1 SMS call rate subs/hour
(80% terminated SMS)

In the circuit-switched (CS) case, for example, the nominal BSC paging load for BSC2i would be (note that both MT calls and MT SMSs create pages):

91 000 x 0.3 (MT) + 120 000 x 0.8 (SMS MT) = 123 300 + re-paging 0.63 x 123 300 = 200 979 pagings per hour, provided that all cells are paged in that BSC and there is only one LA defined per BSC. Here the load is on each Abis link of a broadcast control channel (BCCH) TRX on the same LA.

The nominal BSC2i random access channel (RACH) load (MO, MT, SMS, LUs) would be, for example:

91 000 (MO, MT) + 2 x 91 000 (LU) + 0.1 x 91 000 (IMSI detach) + 123 300 (SMS) = 405 400 RACHs per hour. This is the total load per BSC which can be divided by the number of cells when calculating the number of RACHs per BCCH-TRX Abis.

With this reference call mix the BSC processor load still remains in the safe area. The maximum 60% CPU load is the target for dimensioning. This gives enough margin for peak load situations as well as for new software releases.

Some call mix is needed in order to get the main performance figures (erlangs, BHCAs) with maximum allowed processor load. With a different call mix, the BHCA value, for example, varies a lot because in a complex system such as GSM there are many other transactions, in addition to calls, which load the system.

Roughly it can be said that the erlangs per air channel are the largest contribution to the BSC processor load; the next largest ones are the number of call procedures (call set-up, clearing), SMSs and location updates (LUR, IMSI Attach/Detach are similar) and, lastly, all different types of handovers. By saying that erlangs as such are significant we mean that there is a load in the system even though there is one call with indefinite length on each channel without having any HOs, LURs, and so on.

Packet-switched capacity

BSC3i 2000: max. 100 PCU2 (50 physical PCU2 units per BSC, 2 logical PCUs in one PCU2 plug-in unit)
BSC3i 1000: max. 50 PCU2 (25 physical PCU units per BSC, 2 logical PCUs in one PCU2 plug-in unit)
BSC3i 660: max. 24 PCU1/PCU2 (12 physical PCU units per BSC, 2 logical PCUs in one PCU1/PCU2 plug-in unit)
BSC2i: max. 16 PCU1/PCU2 (16 physical PCU units per BSC, 1 logical PCU in one PCU1/PCU2 plug-in unit)

Table: Connectivity of logical PCUs

	16kbit/s Abis TSL	TRX	Cell/ Segments	BTS
Logical PCU2 (PCU2-D/PCU2-U)	256	256	64	128
Logical PCU1 (PCU-B/PCU-T)	256	128	64	64
Logical PCU1 (PCU/PCU-S)	256 (128 RTSL)

GSM SYSTEM

noreply@blogger.com (Unknown) — Tue, 07 Oct 2008 03:41:00 +0000

GSM: Network Architecture

The GSM technical specifications define the different entities that form the GSM network by defining their functions and interface requirements.
The GSM network can be divided into four main parts:
• The Mobile Station (MS).
• The Base Station Subsystem (BSS).
• The Network and Switching Subsystem (NSS).
• The Operation and Support Subsystem (OSS).
The architecture of the GSM network is presented in figure 1.

figure 1: Architecture of the GSM network

Mobile Station
A Mobile Station consists of two main elements:
• The Subscriber Identity Module (SIM): It is protected by a four-digit Personal Identification Number (PIN). In order to identify the subscriber to the system, the SIM card contains amongst others a unique International Mobile Subscriber Identity (IMSI). User mobility is provided through maping the subscriber to the SIM card rather than the terminal as we done in past cellular systems.
• Mobile equipment/terminal (ME): There are different types of terminals (MN) distinguished principally by their power and application:
o `fixed' terminals mainly installed in cars. Their maximum allowed output power is 20W
o portable terminals can also be installed in vehicles. Their maximum allowed output power is 8W.
o handheld terminals; their popularity is owed to their weight and volume, which is continuously decreasing. According to some specification these terminals may emit up to 0.8W. However, as technology has evolved their maximum allowed power ouput is limited to 0.1W.
o
Base Station Subsystem
The BSS provides the interface between the ME and the NSS. It is in charge of the transmission and reception. It may be divided into two parts:
• Base Station Controller (BSC): It controls a group of BTSs and manages their radio ressources. A BSC is principally in charge of handoffs, frequency hopping, exchange functions and power control over each managed BTSs.
• Base Transceiver Station (BTS) or Base Station: it maps to transceivers and antennas used in each cell of the network. It is usually placed in the center of a cell. Its transmitting power defines the size of a cell. Each BTS has between 1-16 transceivers depending on the density of users in the cell.
NSS
Its main role is to manage the communications between the mobile users and other users, such as mobile users, ISDN users, fixed telephony users, etc. It also includes data bases needed in order to store information about the subscribers and to manage their mobility. The different components of the NSS are described below.
• MSC: the central component of the NSS. The MSC performs the switching functions of the network. It also provides connection to other networks.
• GMSC: A gateway that interconnects two networks: the cellular network and the PSTN. It is in charge of routing calls from the fixed network towards a GSM user. The GMSC is often implemented in the same machines as the MSC.
• HLR: The HLR stores information of the suscribers belonging to the coverage area of a MSC; it also stores the current location of these subscribers and the services to which they have access. The location of the subscriber maps to the SS7 address of the Visitor Location Register (VLR) associated to the MN.
• VLR: contains information from a subscriber's HLR necessary to provide the subscribed services to visiting users. When a subscriber enters the covering area of a new MSC, the VLR associated to this MSC will request information about the new subscriber to its corresponding HLR. The VLR will then have enough data to assure the subscribed services without needing to ask the HLR each time a communication is established. The VLR is always implemented together with a MSC; thus, the area under control of the MSC is also the area under control of the VLR.
• Authentication Center (AuC): It serves security purposes; it provides the parameters needed for authentication and encryption functions. These parameters allow verification of the subscriber's identity.
• Equipment Identity Register (EIR): EIR stores security-sensitive information about the mobile equipments. It maintains a list of all valid terminals as identified by their International Mobile Equipment Identity (IMEI). The EIR allows then to forbid calls from stolen or unauthorized terminals (e.g, a terminal which does not respect the specifications concerning the output RF power).
• GSM Interworking Unit (GIWU): The GIWU provides an interface to various networks for data communications. During these communications, the transmission of speech and data can be alternated.
Operation and Support Subsystem (OSS)
It is connected to components of the NSS and the BSC, in order to control and monitor the GSM system. It is also in charge of controlling the traffic load of the BSS. It must be noted that as the number of BS increases with the scaling of the subscriber population some of the maintenance tasks are transferred to the BTS, allowing savings in the cost of ownership of the system.

Geographical areas
A cell, as identified by its Cell Global Identity (CGI) number, maps to the radio coverage of a BTS. Similarly an LA as identified by its Location Area Identity (LAI) number , is a cluster of cells served by a single MSC/VLR. A group of LA under the control of the same MSC/VLR defines the MSC/VLR area. A Public Land Mobile Network (PLMN) is the area served by one network operator.

Network operations
In this paragraph, the description of the GSM network is focused on the differents functions to fulfil by the network and not on its physical components. In GSM, five main functions can be defined:
• Transmission: of data and signaling. Not all the components of the GSM network are strongly related with both types of types of Tx. While the MSC, BTS and BSC, among others, are involved with data and signaling, components such as HLR, VLR or EIR registers, are only concerned with signaling.
• Radio Resources Management (RRM).
• Mobility Management (MM).
• Communication Management (CM).
• Operation, Administration and Maintenance (OAM).

Radio Resources Management (RRM)
The role of the RR function is to establish, maintain and release communication links between mobile stations and the MSC. The elements that are mainly concerned with the RR function are the MN and the BTS. However, since the RR component performs connection management also during cell handoffs, it also affects the MSC which is the handoff management component.
The RR is also responsible for the management of frequency resources as well as varying radio interface conditions. Main component operations are:
• Channel assignment, change and release.
• Handoff
• Frequency hopping.
• Power-level control.
• Discontinuous transmission and reception.
• Timing advance.

Handoff
The user movements may result a change in the channel/cell, when the quality of the communication is degrading; this is known as handoff. Handoffs occur between:
• between channels within a cell
• between cells controlled by the same BSC
• between cells under the same MSC but controlled by different BSCs
• between cells controlled by different MSCs.
Handoffs are mainly controlled by the MSC. However to avoid unnecessary signalling, the first two types of handoffs are managed by the respective BSC (thus, the MSC is only notified of the handoff).
To perform the handoff the mobile station controls continuously its own signal strengh and the signal strengh of the neighboring cells. The list of cells that must be monitored by the mobile station is given by the base station. Power measurements allow to decide which is the best cell in order to maintain the quality of the communication link. Two basic algorithms are used for handoffs:
• The `minimum acceptable performance' algorithm. When the quality of the transmission degrades, the power level of the mobile is increased, until the increase of the power level has no effect on the quality of the signal. Upon this link layer hint, a handoff is initiated.
The `power budget' algorithm. Here the handoff pre-empts the power increase, to obtain a good SIR.
Mobility Management (MM)
The MM component handles:
• Location Management: Location is managed through periodicaly or on-demand. At power-on time, the MH signals an IMSI attach. On-demand location updates are signalled when the MN moves to a different PLMN or new location area (LA). The signal is sent to the new MSC/VLR, which forwards it to the subscriber's HLR. Upon authorization in the new MSC/VLR, the subscriber's HLR removes the registration entry of the MN at the old MSC/VLR. If after the update time interval, the MN has not registered, it is then deregistered. On power-off, the MN performs an IMSI detach.
• security and authentication: Authentication involves the SIM card and the Authentication Center. A secret key, stored in the SIM card and the AuC together with a ciphering algorithm called A3, are used to authenticate the user. The MN and the AuCcompute a SRES through A3 using the secret key and a nonce generated by the AuC. If the two computed SRES are the same, the subscriber is authenticated. The different services to which the subscriber has access are also checked. Next the a security check is performed in the equipment identity (IMEI). If the IMEI number of the mobile is authorized in the EIR, the mobile station is allowed to connect the network. To assure user confidentiality, the user is registered with a Temporary Mobile Subscriber Identity (TMSI) after its first location update procedure. Enciphering is another option to guarantee a very strong security.

Communication Management (CM)
The CM component manages:
• Call control (CC): it controls call setup, management and tear-down in relation to management of type of service. Call routing is the primary task for this component. To reach a mobile subscriber, a user dials the Mobile Subscriber ISDN (MSISDN) number which includes:
o a country code
o a national destination code; this identifies the subscriber's operator
o a code mapping to the subscriber's HLR.
o The call is then passsed to the GMSC (if the call is originated from a fixed network) that 'knows' the HLR corresponding to the particular MSISDN number. The GMSC signals the HLR for call routing information. The HLR requests this information from the subscriber's current VLR. This VLR allocates temporarily a Mobile Station Roaming Number (MSRN) for the call. The MSRN number is the information returned by the HLR to the GMSC. It is latter that routes the call through the MSRN number, to the subscriber's current MSC/VLR. In the subscriber's current LA, the mobile is paged.
• Supplementary Services management: This involves the MN and the HLR.
SMS management: Here the GSM network contacts the Short Message Service Center through the two following interfaces:
o SMS-GMSC for Mobile Terminating Short Messages (SMS-MT/PP). It has the same role as the GMSC.
o SMS-IWMSC for Mobile Originating Short Messages (SMS-MO/PP).

Operation, Administration and Maintenance (OAM)
The OAM component allows the operator to monitor and control the system as well as modify the configuration of the elements of the system. Not only the OSS is part of the OAM, but also the BSS and NSS participate in functions such as:
• provide the operator with all the information it needs. This information is forwarded to the OSS to control the network.
• perform self-test tasks in addition to the OAM functions.
• control of multiple BTSs by the BSS.