Global System for Mobile Communications(GSM)

7/27/2019 Global System for Mobile Communications(GSM)

1/52

Industrial Training Seminar Report

on

GLOBAL SYSTEM FOR MOBILE COMMUNICATION (GSM)

Submitted in partial fulfillment of the requirement

for the award of the

Degree of

Bachelor of Technology

in

Electronics and Communication Engineering

Submitted by:

Manisha Gupta

Enroll No.-070295

October 2010

Mody Institute of Technology and Science

(A deemed university u/s 3 of UGC Act 1956)


2/52

Lakshmangarh, Sikar 332311 (Rajasthan)

ACKNOWLEDGEMENT

I take this opportunity to express my sincere thanks and deep gratitude to all the

members of the technical department of AIRCEL LTD. All of them were extremely

co-operative and helping. They all have been very supportive of my work with their

encouragement and criticism. I am deeply indebted to all of them and welcome this

opportunity to benefit further from their contribution. I am also thankful to the

organization as a whole for providing me the opportunity to undergo training there.

I pay my sincere thanks to Mr. Shakti Shekhawat, Mr. Ankur Jain and Mr. Apoorva

Chauhan for their guidance.

2


3/52

ABSTRACT

Global System for Mobile Communication (GSM) is a globally accepted standard for

digital cellular communication. Before GSM networks there were public mobile radio

networks (cellular). They normally used analog technologies, which varied from country

to country and from manufacturer to another. These analog networks did not comply with

any uniform standard. There was no way to use a single mobile phone from one country

to another. The speech quality in most networks was not satisfactory. GSM became

popular very quickly because it provided improved speech quality and, through a uniform

international standard, made it possible to use a single telephone number and mobile unit

around the world.

GSM was designed to be platform-independent. The GSM specifications do not specify

the actual hardware requirements, but instead specify the network functions and

interfaces in detail. This allows hardware designers to be creative in how they provide the

actual functionality, but at the same time makes it possible for operators to buy

equipment from different suppliers.

In a GSM network, this decentralized intelligence is implemented by dividing the whole

network into three separate subsystems: Network Switching Subsystem (NSS), BaseStation Subsystem (BSS), and Network Management Subsystem (NMS). The actual

network needed for establishing calls is composed of the NSS and the BSS. The BSS is

responsible for radio path control and every call is connected through the BSS. The NSS

takes care of call control functions. Calls are always connected by and through the NSS.

The NMS is the operation and maintenance related part of the network and it is needed

for the control of the whole GSM network. The network operator observes and maintains

network quality and service offered through the NMS.

3


4/52

CONTENTS

Page no.

1. Introduction.....................................................................................

... 6

1.1 GSM overview...........................................................................

... 6

1.2 GSM features....................................................................

........... 7

1.3 GSM frequencies............................................................................

10

1.4 GSM network

structure................................................................... 11

2. GSM Network subsystem ........... 12

2.1. Mobile Station............................................................................

13

2.2. BSS......................................... 13

2.2.1 BSS interfaces.....................................................................

15

2.2.2 BSS

features........................................................................ 16

2.3.

NSS...............................................................................................

18

2.3.1 NSS functions......................................................................

19

2.3.2 NSS commands...................................................................20

2.4 NMS...............................................................................................

21

3. Traffic Management.........................................................................

23

4


5/52

3.1 Introduction............................................................................................... 23

3.2 Mobile Call setup....................................................................................... 23

3.3 Location Update......................................................................................... 28

3.4 Handover.................................................................................................... 30

3.5 Charging..................................................................................................... 33

3.6 Renting of services..................................................................................... 33

3.7 Services...................................................................................................... 33

3.8 OMC.......................................................................................................... 34

4. Transmission................................................ 36

4.1 E1..................................................................................................

36

4.2 Transmission Problems.................................................................

37

4.3 Solutions........................................................................................

38

5. Glossary........................................................................................................... 41

6. Software & tools.............................................................................................. 42

7. Snapshots........................................................................................................ 43

8. Appendix......................................................................................................... 479. References....................................................................................................... 49

5


6/52

1. INTRODUCTION

1.1GSM OVERVIEW

The Global System for Mobile Communications (GSM) is a set of recommendations

and specifications for a digital cellular telephone network (known as a Public Land

Mobile Network, or PLMN). These recommendations ensure the compatibility of

equipment from different GSM manufacturers, and interconnectivity between

different administrations, including operation across international boundaries.

GSM networks are digital and can cater for high system capacities. They are

consistent with the world-wide digitization of the telephone network, and are an

extension of the Integrated Services Digital Network (ISDN), using a digital radio

interface between the cellular network and the mobile subscriber equipment.

6


7/52

A cellular telephone system links mobile subscribers into the public telephone

system or to another cellular subscriber. Information between the mobile unit and the

cellular network uses radio communication. Hence the subscriber is able to move

around and become fully mobile.

The service area in which mobile communication is to be provided is divided into

regions called cells. Each cell has the equipment to transmit and receive calls from

any subscriber located within the borders of its radio coverage area.

1.2 GSM Features

1. Increased Capacity

The GSM system provides a greater subscriber capacity than analogue

systems.

GSM allows 25 kHz per user, that is, eight conversations per 200 kHz

channel pair (a pair comprising one transmit channel and one receive

channel).

Digital channel coding and the modulation used makes the signal resistant to

interference from cells where the same frequencies are re-used (co-channel

interference); a Carrier to Interference Ratio (C/I) level of 12 dB is achieved,

as opposed to the 18 dB typical with analogue cellular.

This allows increased geographic reuse by permitting a reduction in the

number of cells in the reuse pattern.

7


8/52

2. Audio Quality

Digital transmission of speech and high performance digital signal processors

provides good quality speech transmission.

Since GSM is a digital technology, the signals passed over a digital air

interface can be protected against errors by using better error detection and

correction techniques.

In regions of interference or noise-limited operation the speech quality is

noticeably better than analogue.

3. Use of Standardised Open Interfaces

Standard interfaces such as C7 and X25 are used throughout the system.

Hence different manufacturers can be selected for different parts of the

PLMN.

There is a high flexibility in where the Network components are situated.

4. Improved Security and Confidentiality

GSM offers high speech and data confidentiality.

Subscriber authentication can be performed by the system to check if a

subscriber is a valid subscriber or not.

The GSM system provides for high degree of confidentiality for the

subscriber. Calls are encoded and ciphered when sent over air.

The mobile equipment can be identified independently from the mobile

subscriber. The mobile has a identity number hard coded into it when it is

manufactured. This number is stored in a standard database and whenever a

call is made the equipment can be checked to see if it has been reported

stolen.

5. Cleaner Handovers

8


9/52

GSM uses Mobile assisted handover technique.

The mobile itself carries out the signal strength and quality measurement of

its server and signal strength measurement of its neighbors.

This data is passed on the Network which then uses sophisticated algorithmsto determine the need of handover.

6. Subscriber Identification

In a GSM system the mobile station and the subscriber are identified

separately.

The subscriber is identified by means of a smart card known as a SIM.

This enables the subscriber to use different mobile equipment while retaining

the same subscriber number.

7. Enhanced Range Of Services

Speech services for normal telephony.

Short Message Service for point to point transmission of text message.

Cell broadcast for transmission of text message from the cell to all MS in its

coverage area. Message like traffic information or advertising can be

transmitted.

Fax and data services are provided. Data rates available are 2.4 Kb/s, 4.8

Kb/s and 9.6 Kb/s.

Supplementary services like number identification, call barring, call

forwarding, charging display etc can be provided.

8. Frequency Reuse

There are total 124 carriers in GSM (additional 50 carriers are available if

EGSM band is used).

Each carrier has 8 timeslots and if 7 can be used for traffic then a maximum

of 868 (124 X 7) calls can be made. This is not enough and hence frequencies

have to be reused.

9


10/52

The same RF carrier can be used for many conversations in several different

cells at the same time.

The radio carriers available are allocated according to a regular pattern which

repeats over the whole coverage area.

The pattern to be used depends on traffic requirement and spectrum

availability.

1.3GSM FREQUENCIES

There are two popular GSM standards:

GSM-900 (Channels 125 operating band 900Mhz carrier spacing 200khz

spacing 45Mhz)

GSM -1800 (Channels 374 spacing 95Mhz)

NOTE: AIRCEL uses GSM-1800 standard

Figure 1.

GSM-900 FREQUENCIES

10


11/52

Uplink frequency range (transmitted by the MS and received by the BTS):

890MHz~915MHz.

Downlink frequency range (transmitted by the BTS and received by the MS):

935MHz~960MHz

The working bandwidth is 25MHz, the duplex interval (i.e., the interval

between the receiving and transmitting frequency) is 45MHz, and the carrier

interval is 200kHz, with 124 carrier channels in total.

Here, the carrier channel refers to a channel, and each channel has a

bandwidth of 200kHz.

GSM-1800 FREQUENCIES

GSM-1800 systems use radio frequencies between 1710-1785 MHz for

receive and between 1805-1880 MHz for transmit.

RF carriers are spaced every 200 kHz, allowing a total of 373 carriers.

There is a 100 kHz guard band between 1710.0 MHz and 1710.1 MHz and

between 1784.9 MHz and 1785.0 MHz for receive, and between 1805.0 MHz

and 1805.1 MHz and between 1879.9 MHz and 1880.0 MHz for transmit.

Transmit and receive frequencies are always separated by 95 MHz.

1.4 GSM - Network Structure

11


12/52

Figure 2. GSM Network Architecture

2. GSM Network Sub-System

12


13/52

Figure 3.GSM Network Sub-System

A GSM network is made up of three subsystems:

The Base Station Sub-system (BSS) comprising a BSC and several BTSs

The Network and Switching Sub-system (NSS) comprising an MSC and

associated registers

Network Management Sub-system

The interfaces defined between each of these sub systems include:

'A' interface between NSS and BSS

'Abis' interface between BSC and BTS (within the BSS)

'Um' air interface between the BSS and the MS

2.1 The Mobile Station (MS)

13


14/52

A mobile station may be referred to as a handset, a mobile, a portable terminal or

mobile equipment (ME). It also includes a subscriber identity module (SIM) that is

normally removable and comes in two sizes. Each SIM card has a unique

identification number called IMSI (international mobile subscriber identity). In

addition, each MS is assigned a unique hardware identification called IMEI

(international mobile equipment identity).

In some of the newer applications (data communications in particular), an MS can

also be a terminal that acts as a GSM interface, e.g. for a laptop computer. In this

new application the MS does not look like a normal GSM telephone.

The seemingly low price of a mobile phone can give the (false) impression that the

product is not of high quality. Besides providing a transceiver (TRX) for

transmission and reception of voice and data, the mobile also performs a number of

very demanding tasks such as authentication, handover, encoding and channel

encoding.

2.2 BSS (Base Station Sub-system)

To understand the paging process, we must analyze the functions of the BSS.

The Base Station Subsystem consists of the following elements:

BSC- Base Station Controller

BTS- Base Transceiver Station

TC- Transcoder

Fig. 4, The Base Station Controller (BSC) is the central network element of the BSS

and it controls the radio network. This means that the main responsibilities of the

BSC are : Connection establishment between MS and NSS, Mobility management,

Statistical raw data collection, Air and A interface signalling support.

14


15/52

Figure 4. BSC Figure 5. Flexi BTS

Fig. 5, The Base Transceiver Station (BTS) is a network element maintaining the Air

interface. It takes care of Air interface signalling, Air interface ciphering and speech

processing. In this context, speech processing refers to all the functions the BTS

performs in order to guarantee an error-free connection between the MS and the

BTS.

The TransCoder (TC) is a BSS element taking care of speech transcoding, i.e. it is

capable of converting speech from one digital coding format to another and vice

versa.

15


16/52

2.2.1 BSS interfaces

Figure 6. Image of the GSM network, showing the BSS interfaces to the MS, NSS and GPRS Core

Network.

Um The air interface between the MS (Mobile Station) and the BTS. This

interface uses LAPDm protocol for signaling, to conduct call control,

measurement reporting, Handover, Power control, Authentication,

Authorization, Location Update and so on. Traffic and Signaling are sent in

bursts of 0.577 ms at intervals of 4.615 ms, to form data blocks each 20 ms.

Abis The interface between the Base Transceiver Station and Base Station

Controller. Generally carried by a DS-1, ES-1, or E1 TDM circuit. Uses

TDM subchannels for traffic (TCH), LAPD protocol for BTS supervision and

telecom signaling, and carries synchronization from the BSC to the BTS and

MS.

A The interface between the BSC and Mobile Switching Center. It is used

for carrying Traffic channels and the BSSAP user part of the SS7 stack.

Although there are usually transcoding units between BSC and MSC, the

signaling communication takes place between these two ending points and

the transcoder unit doesn't touch the SS7 information, only the voice or CS

data are transcoded or rate adapted.

16
http://en.wikipedia.org/wiki/Um_Interfacehttp://en.wikipedia.org/wiki/Mobile_Stationhttp://en.wikipedia.org/wiki/Base_Station_Subsystem#Base_Station_Controllerhttp://en.wikipedia.org/wiki/Handoffhttp://en.wikipedia.org/wiki/Power_controlhttp://en.wikipedia.org/wiki/Authenticationhttp://en.wikipedia.org/wiki/Authorizationhttp://en.wikipedia.org/wiki/Link_Access_Procedures,_D_channelhttp://en.wikipedia.org/wiki/SS7http://en.wikipedia.org/wiki/SS7http://en.wikipedia.org/wiki/File:Gsm_network.pnghttp://en.wikipedia.org/wiki/Um_Interfacehttp://en.wikipedia.org/wiki/Mobile_Stationhttp://en.wikipedia.org/wiki/Base_Station_Subsystem#Base_Station_Controllerhttp://en.wikipedia.org/wiki/Handoffhttp://en.wikipedia.org/wiki/Power_controlhttp://en.wikipedia.org/wiki/Authenticationhttp://en.wikipedia.org/wiki/Authorizationhttp://en.wikipedia.org/wiki/Link_Access_Procedures,_D_channelhttp://en.wikipedia.org/wiki/SS7http://en.wikipedia.org/wiki/SS7


17/52

Ater The interface between the Base Station Controller and Transcoder. It

is a proprietary interface whose name depends on the vendor (for example

Ater by Nokia), it carries the A interface information from the BSC leaving it

untouched.

Gb Connects the BSS to the Serving GPRS Support Node (SGSN) in the

GPRS Core Network.

2.2.2 BSS Features:

The BTS, BSC and TC together form the Base Station Subsystem (BSS) which is a

part of the GSM network taking care of the following major functions:

1. Radio Path Control

In the GSM network, the Base Station Subsystem (BSS) is the part of the network

taking care of Radio Resources, i.e. radio channel allocation and quality of the radio

connection. For this purpose, the GSM Technical Specifications define about 120

different parameters for each BTS. These parameters define exactly what kind of

BTS is in question and how MSs may "see" the network when moving in this BTS

area. The BTS parameters handle the following major items:

Kind of handovers (when and why),

paging organization

radio power

level control

BTS identification.

2. BTS and TC Control

Inside the BSS, all the BTSs and TCs are connected to the BSC(s). The BSC

maintains the BTSs. In other words, the BSC is capable of separating (barring) a

BTS from the network and collecting alarm information. Transcoders are also

maintained by the BSC, i.e. the BSC collects alarms related to the Transcoders.

17
http://en.wikipedia.org/wiki/GPRS_Core_Networkhttp://en.wikipedia.org/wiki/GPRS_Core_Network


18/52

3. Synchronisation

The BSS uses hierarchical synchronisation which means that the MSC synchronises

the BSC and the BSC further synchronises the BTSs associated with that particular

BSC. Inside the BSS, synchronisation is controlled by the BSC. Synchronisation is a

critical issue in the GSM network due to the nature of the information transferred. If

the synchronisation chain is not working correctly, calls may be cut or the call

quality may not be the best possible. Ultimately, it may even be impossible to

establish a call.

4. Air & A Interface Signalling

In order to establish a call, the MS must have a connection through the BSS. This

connection requires several signaling protocols.

5. Connection Establishment between MS and NSS

The BSS is located between two interfaces; the Air and the A interface. From the call

establishment point of view, the MS must have a connection through these two

interfaces before a call can be established. Generally speaking, this connection may

be either a signaling type of connection or a traffic (speech, data) type of connection.

6. Collection of Statistical Data

The BSS collects a lot of short -term statistical data that is further sent to the NMS

for post processing purposes. By using the tools located in the NMS the operator is

able to create statistical "views" and thus observe the network quality.

A Base Station Subsystem is controlled by an MSC. Typically, one MSC contains

several BSSs. A BSS itself may cover a considerably large geographical area

consisting of many cells. (A cell refers to an area covered by one or more frequency

resources).

Equation (1), each cell is identified by an identification number called Cell Global

Identity (CGI) which comprises the following elements:

18


19/52

CGI = MCC + MNC + LAC + CI (1)

MCC -Mobile Country Code

MNC -Mobile Network Code

LAC -Location Area Code

CI -Cell Identity

2.3 The Network Switching Subsystem (NSS)

The NSS contains the following parts:

Mobile Switching Centre (MSC)

Home Location Register (HLR)

Visitor Location Register (VLR)

Authentication Centre (AuC)

Equipment Identity Register (EIR)

The Mobile Switching Centre (MSC): Acts like a standard exchange in a fixed

network and additionally provide all the functionality needed to handle a mobile

subscriber. The main functions are registration, authentication, location updating,

handovers and call routing to a roaming subscriber. The signalling between

functional entities (registers) in the network subsystem uses Signalling System 7

(SS7). If the MSC also has a gateway function for communicating with other

networks, it is called Gateway MSC (GMSC).

The Home Location Register (HLR): A database used for management of mobile

subscribers. It stores the international mobile subscriber identity (IMSI), mobile

station ISDN number (MSISDN) and current visitor location register (VLR) address.

The main information stored there concerns the location of each mobile station in

order to be able to route calls to the mobile subscribers managed by each HLR. The

19


20/52

HLR also maintains the services associated with each MS. One HLR can serve

several MSCs.

The Visitor Location Register (VLR): Contains the current location of the MS and

selected administrative information from the HLR, necessary for call control and

provision of the subscribed services, for each mobile currently located in the

geographical area controlled by the VLR. A VLR is connected to one MSC and is

normally integrated into the MSC's hardware.

The Authentication Centre (Auc): A protected database that holds a copy of the

secret key stored in each subscriber's SIM card, which is used for authentication and

encryption over the radio channel. The AuC provides additional security against

fraud. It is normally located close to each HLR within a GSM network.

The Equipment Identity Register (EIR): The EIR is a database that contains a list of

all valid mobile station equipment within the network, where each mobile station is

identified by its international mobile equipment identity (IMEI). The EIR has three

databases:

White list: for all known, good IMEIs

Black list: for bad or stolen handsets

Grey list: for handsets/IMEIs that are uncertain

2.3.1 The main functions of NSS are:

Call Control : This identifies the subscriber, establishes a call and clears the

connection after the conversation is over.

Charging : This collects the charging information about a call such as the

numbers of the caller and the called subscriber, the time and type of the

transaction, etc., and transfers it to the Billing Centre.

Mobility management : This maintains information about the location of the

subscriber.

20


21/52

Signalling with other networks and the BSS : This applies to interfaces with

the BSS and PSTN.

Subscriber data handling : This is the permanent data storage in the HLR and

temporary storage of relevant data in the VLR.

Locating the subscriber : This locates a subscriber before establishing a call.

2.3.2 Some basic NSS commands:

ZMIO: Gives particular details of subscriber including name, IMSI, IMEI, MSRN,

etc.

ZMGO: Teleservices

Some tele-services are as follows:

ZMSO: Call barring supplementary services provider.

ZMIS: Information regarding service centre.

21


22/52

2.4 Network Management Subsystem

The Network Management Subsystem (NMS) is the third subsystem of the GSM

network in addition to the Network Switching Subsystem (NSS) and Base Station

Subsystem (BSS) which we have already discussed. The purpose of the NMS is to

monitor various functions and elements of the network. These tasks are carried out

by the NMS/2000 which consists of a number of Work Stations, Servers and a

Router which connects to a Data Communications Network (DCN).

The functions of the NMS can be divided into three categories:

Fault Management

Configuration Management

Performance Management

These functions cover the whole of the GSM network elements from eth level

individual BTSs, up to MSCs and HLRs.

Fault Management

The purpose of Fault Management is to ensure the smooth operation of the network

and rapid correction of any kind of problems that are detected. Fault management

provides the network operator with information about the current status of alarm

events and maintains a history database of alarms. The alarms are stored in the NMS

database and this database can be searched according to criteria specified by the

network operator.

Configuration Management

The purpose of Configuration Management is to maintain up to date information

about the operation and configuration status of network elements. Specific

configuration functions include the management of the radio network, software and

22


23/52

hardware management of the network elements, time syncronisation and security

operations.

Performance Management

In performance management, the NMS collects measurement data from individual

network elements and stores it in a database. On the basis of these data, the network

operator is able to compare the actual performance of the network with the planned

performance and detect both good and bad performance areas within the network.

23


24/52

3. TRAFFIC MANAGEMENMT

3.1 Introduction

A connection between two people - a caller and the called person is the basic

service of all telephone networks. To provide this service, the network must be able

to set up and maintain a call, which involves a number of tasks: identifying the called

person, determining his location, routing the call to him and ensuring that the

connection is sustained as long as the conversation lasts. After the transaction, the

connection is terminated and (normally) the calling user is charged for the service he

has used.

3.2 Mobile Call-Setup

1. Fig. 7, a subscriber in a fixed network dials the number of a mobile station. This

can be either a national or an international number. Equation (2), the dialed number

is called an MSISDN (Mobile Subscriber International ISDN Number) which

contains the following elements:

MSISDN = CC + NDC + SN (2)

CC= Country code (33=France, 358=Finland, etc.)

NDC= National Destination Code

SN= Subscriber Number

Figure 7.PSTN originates the call

24


25/52

2. The PSTN exchange analyses the dialled number. The result of the analysis is the

routing information required for finding the mobile network (Public Land Mobile

Network, PLMN) in which the called subscriber has made his subscription. Fig. 8,

the PSTN identifies the mobile network on the basis of the NDC, after which it

accesses the mobile network via the nearest Gateway Mobile Services Switching

Centre (GMSC).

Figure 8. Incoming call from PSTN to GSM network

3. The GMSC analyses the MSISDN in the same way as the PSTN exchange did.

As a result of the analysis, it obtains the HLR address in which the subscriber is

permanently registered. Notice that the GMSC itself does not have any information

about the location of the called subscriber. The subscribers location can only be

determined by the two databases, the HLR and VLR. At this stage however, the

GMSC only knows the HLR address and so it sends a message (containing the

MSISDN) to the HLR. In practice this message is a request for locating the called

subscriber in order to set up a call. This is called an HLR Enquiry.

25


26/52

4. The HLR analyses the message. It identifies the called subscriber on the basis of

MSISDN and then checks its database to determine the subscribers location. The

HLR is informed every time the subscriber moves from one VLR area to another, i.e.

the HLR knows in which VLR area the subscriber is currently registered. It has to be

pointed out that the HLR does not handle network traffic at all. A traffic connection

requires two network elements that are able to provide speech connections. A speech

connection is a network service and it can be handled only by an MSC. Therefore, to

enable the traffic connection, maybe two MSCs will have to be connected. Fig. 9,

the first MSC is the Gateway MSC which is contacted by the PSTN exchange. The

HLR acts as a co-ordinator to set up the connection between the GMSC and the

destination MSC (which could of course be the GMSC itself).

There is also another identification number involved in the process known as the

International Mobile Subscriber Identity (IMSI). The purpose of IMSI is to identify

the subscriber in the mobile network. Equation (2), the total length of the IMSI is 15

digits and it consists of the following elements:

MSI = MCC + MNC + MSIN (3)

MCC = Mobile Country Code (three digits)

MNC = Mobile Network Code (two digits)

MSIN = Mobile Subscriber Identification Number (ten digits)

26


27/52

Figure 9. Routing the call inside the GSM network

5. Now the HLR interrogates the MSC/VLR that is currently serving the called

subscriber. But why do we need to interrogate instead of connecting right away?

First of all, the current status of the mobile station is stored in the VLR database and

we need to know the status to avoid setting up a call to a subscriber whose phone is

switched off. Secondly, we need to have some sort of information that enables the

GMSC to route the call to the target MSC, wherever in the world it may be.

6. Fig. 10, in terms of routing the call, the serving MSC/VLR is the destination of

the call. This means that we must direct the call to it by using the following

procedure:After receiving the message from the HLR, the serving MSC/VLR generates a

temporary Mobile Station Roaming Number (MSRN) and associates it with the

IMSI. Equation (4), the roaming number is used in initiating the connection and it

has the following structure:

MSRN = CC + NDC + SN (4)

CC = Country Code (of the visited country)

NDC = National Destination Code (of the serving network)

SN = Subscriber Number

27


28/52

Figure 10.MSRN request from HLR to second MSC

7. Fig. 11, the MSC/VLR sends the roaming number to the HLR. The HLR does not

analyse it because the MSRN is used for traffic transactions only and the HLR does

not handle traffic, it is only a database that helps in locating subscribers and co-

ordinates call set-up. Therefore, the HLR simply sends the MSRN forward to the

GMSC that originally initiated the process.

28


29/52

Figure 11.The HLR is giving the MSRN to the originating MSC.

8. When the GMSC receives the message containing the MSRN, it analyses the

message. The roaming number identifies the location of the called subscriber, so the

result of this analysis is a routing process which identifies the destination of the call -

the serving MSC/VLR

9. The final phase of the routing process is taken care of by the serving MSC/VLR.

In fact, the serving MSC/VLR also has to receive the roaming number so that it

knows that this is not a new call, but one that is going to terminate here - i.e. a call to

which it has already allocated an MSRN. By checking the VLR, it recognises the

number and so it is able to trace the called subscriber.

3.3 Location Update

In practice, there are three types of location updates:

Location Registration (power on)

Generic

Periodic

Location registration takes place when a mobile station is turned on. This is also

known as IMSI Attach because as soon as the mobile station is switched on it

informs the Visitor Location Register (VLR) that it is now back in service and is able

to receive calls. As a result of a successful registration, the network sends the mobile

station two numbers that are stored in the SIM (Subscriber Identity Module) card of

the mobile station. These two numbers are the Location Area Identity (LAI) and the

Temporary Mobile Subscriber Identity (TMSI). The network, via the control

channels of the air interface, sends the LAI. The TMSI is used for security purposes,

so that the IMSI of a subscriber does not have to be transmitted over the air interface.

The TMSI is a temporary identity, which regularly gets changed. A Location Area

29


30/52

Identity (LAI) is a globally unique number. A Location Area Code (LAC) is only

unique in a particular network.

Every time the mobile receives data through the control channels, it reads the LAI

and compares it with the LAI stored in its SIM card. A generic location update is

performed if they are different. Fig. 12, the mobile starts a Location Update process

by accessing the MSC/VLR that sent the location data.

Figure 12. Location update

A channel request message is sent that contains the subscriber identity (i.e.

IMSI/TMSI) and the LAI stored in the SIM card. When the target MSC/VLR

receives the request, it reads the old LAI which identifies the MSC/VLR that has

served the mobile phone up to this point. A signalling connection is established

between the two MSC/VLRs and the subscribers IMSI is transferred from the old

MSC to the new MSC. Using this IMSI, the new MSC requests the subscriber data

from the HLR and then updates the VLR and HLR after successful authentication.

Periodic location update is carried out when the network does not receive any

location update request from the mobile in a specified time. Such a situation is

created when a mobile is switched on but no traffic is carried, in which case the

mobile is only reading and measuring the information sent by the network. If the

subscriber is moving within a single location area, there is no need to send a location

update request. A timer controls the periodic updates and the operator of the VLR

sets the timer value. The network broadcasts this timer value so that a mobile station

knows the periodic location update timer values. Therefore, when the set time is up,

30


31/52

the mobile station initiates a registration process by sending a location update request

signal. The VLR receives the request and confirms the registration of the mobile in

the same location area. If the mobile station does not follow this procedure, it could

be that the batteries of the mobile are exhausted or the subscriber is in an area where

there is no network coverage. In such a case, the VLR changes the location data of

the mobile station to unknown.

31


32/52

Figure 13. Location Update procedures

3.4 Handover

In a mobile communications network, the subscriber can move around. Maintaining

the traffic connection with a moving subscriber is made possible with the help of the

handover function. The basic concept is simple: when the subscriber moves from the

coverage area of one cell to another, a new connection with the target cell has to be

set up and the connection with the old cell has to be released. There are two reasons

for performing a handover:

1. Handover due to measurements occurs when the quality or the strength of the

radio signal falls below certain parameters specified in the BSC. The deterioration of

the signal is detected by the constant signal measurements carried out by both the

mobile station and the BTS. As a consequence, the connection is handed over to a

cell with a stronger signal.

2. Handover due to traffic reasons occurs when the traffic capacity of a cell has

reached its maximum or is approaching it. In such a case, the mobile stations near the

edges of the cell may be handed over to neighbouring cells with less traffic load.

The decision to perform a handover is always made by the BSC that is currentlyserving the subscriber, except for the handover for traffic reasons. In the latter case

the MSC makes the decision. There are four different types of handover and the best

way to analyse them is to follow the subscriber as he moves:

1. Intra cell - Intra BSC handover

The smallest of the handovers is the intra cell handover where the subscriber is

handed over to another traffic channel (generally in another frequency) within the

same cell. In this case the BSC controlling the cell makes the decision to perform

handover.

2. Inter cell - Intra BSC handover

32


33/52

The subscriber moves from cell 1 to cell 2. In this case the handover process is

controlled by BSC. The traffic connection with cell 1 is released when the

connection with cell 2 is set up successfully.

3. Inter cell - Inter BSC handover

The subscriber moves from cell 2 to cell 3, which is served by another BSC. In this

case the handover process is carried out by the MSC, but, the decision to make the

handover is still done by the first BSC. The connection with the first BSC (and BTS)

is released when the connection with the new BSC (and BTS) is set up successfully.

4. Inter MSC handover

The subscriber moves from a cell controlled by one MSC/VLR to a cell in the

domain of another MSC/VLR. This case is a bit more complicated. Considering that

the first MSC/VLR is connected to the GMSC via a link that passes through PSTN

lines, it is evident that the second MSC/VLR can not take over the first one just like

that. The MSC/VLR currently serving the subscriber (also known as the anchor

MSC), contacts the target MSC/VLR and the traffic connection is transferred to the

target MSC/VLR. As both MSCs are part of the same network, the connection is

established smoothly. It is important to notice, however, that the target MSC and the

source MSC are two telephone exchanges. The call can be transferred between two

exchanges only if there is a telephone number identifying the target MSC.

Such a situation makes it necessary to generate a new number, the Handover Number

(HON). The generation and function of the HON are explained in the following text.

The anchor MSC/VLR receives the handover information from the BSS. It

recognises that the destination is within the domain of another MSC and sends a

Handover Request to the target MSC via the signalling network. The target MSC

answers by generating a HON and sends it to the anchor MSC/VLR, which performs

a digit analysis in order to obtain the necessary routing information. This information

allows the serving MSC/VLR to connect the target MSC/VLR. When the two MSCs

are connected, the call is transferred to a new route. In practice, the handover number

is similar to the roaming number. Moreover, the roaming number and the handover

33


34/52

number have a similar purpose, that is connecting two MSCs. Equation (5), the

structure of the handover number is shown below:

HON = CC + NDC + SN (5)

CC= Country Code

NDC= National Destination Code (of the serving network)

SN= Subscriber Number

Figure 14. Inter MSC handover procedure

3.5 Charging

Charging in GSM networks follows similar principles to that used in fixed telephone

networks. In addition to a standard fee, subscribers have to pay for the calls they

make and the services they use. However, there are a few differences in how the

costs are calculated and who is liable to pay them. The actual charging practices vary

considerably from one network operator to another.

3.6 Renting of Service

After the subscription has been made and the subscriber has become a customer ofthe particular network, he is usually charged for the availability of the network

services and the right to use them. This is a regular fee which is charged irrespective

of whether the subscriber makes any calls or not. This kind of charge is also known

as renting the service of the network.

34


35/52

3.7 Services

What Are Services?

In the broadest sense of the concept, any subscriber action that uses the facilities

provided and supported by the GSM system can be categorised as a service.

Therefore, a person who has access to a GSM mobile phone and wishes to make a

call, is trying to access the speech service provided by the system.

Classification of Services

GSM is a multiservice system that allows various types of communication that can

be distinguished by the nature of the transmitted information. Generally, based on

the nature of the transmitted information, services can be grouped as speech services,

where the transmitted data is speech and data services which cover the rest of the

information types such as text, facsimile, etc.

However, if a person registers as a GSM subscriber and buys a mobile station, he

takes it for granted that at least the speech service is guaranteed (after all that is the

reason why he bought the phone in the first place).

This raises another distinction in services:

Basic Services which are individual functions and may be automatically

available and included in the basic rights of the subscriber as soon as heregisters.

Supplementary Services which are extra services that are not included as

basic features, but are associated with the basic services by enhancing and/or

adding extra features to the basic services.

3.8 Operation and Maintenance Center (OMC)

The OMC is a management system that oversees the GSM functional blocks. The

OMC assists the network operator in maintaining satisfactory operation of the GSM

network. Hardware redundancy and intelligent error detection mechanisms help

prevent network down-time. The OMC is responsible for controlling and maintaining

the MSC, BSC and BTS. It can be in charge of an entire public land mobile network

(PLMN) or just some parts of the PLMN.

35


36/52

BTS/Infra Alarms Listing

SNO. Alarm type Alarm No Alarm Spec.

1 Critical 7403 Low Battery

2 Critical 7405 Low Fuel

3 Critical 7409 Fire/Smoke

4 Critical 7410 Shelter Temp.

5 Major 7408 DG LLOP

6 Major 7411 AC1 Fails

7 Major 7412 AC2 Fails

8 Major 7402 Rectifier Mod.

Fails9 Major 7406 DG Fails to Start

10 Major 7407 DG Fails to Stop

11 Minor 7401 Mains Fail

12 Minor 7404 Load on DG

Some Internal Alarms:

ALARM SPECIFICATION ALARM NO

BCF FAULTY 7600

BCF OPERATION DEGRADED 7601

BTS OPERATION DEGRADED 7604

TRX FAULTY 7606

TRX OPERATION DEGRADED 7607

OSCILLATOR ADJUSTING

TEMPORARY INTERRUPTED

7616

BCF INITIALIZATION 7701

PCM FAILURE 7704

LAPD FAILURE 7705

36


37/52

NOTE: To check all the alarm listing we use the command ZEOL.

4. Transmission System

Transmission systems form the backbone of any networks. Normally transmission

systems include SDH, PDH, ATM, Microwaves, leased lines.In GSM normally the core network is located in the same premises and is mostly

interconnected by fixed wireline. In huge network consisting of many MSC located

at different places the interconnection may be through any of the transmission

systems mentioned above.

The Access network consists of BSCs with many BTSs connected to them in

various transmission topologies. Normal practice is to connect various BSCc to the

MSC via fiber and different BTSs connected to BSC via microwave in Daisy chain,

star or any other topology. However there can be many different ways of

implementation.

4.1 E1

37


38/52

2.048 Mbps circuit provides high speed, digital transmission for voice, data,

and video signals at 2.048 Mbps.

2.048 Mbps transmission systems are based on the ITU-T specifications

G.703, G.732 and G.704, and are predominant in Europe, Australia, Africa,

South America, and regions of Asia.

The primary use of the 2.048 Mbps is in conjunction with multiplexers for

the transmission of multiple low speed voice and data signals over one

communication path rather then over multiple paths.

The most common line code used to transmit the 2.048 Mbps signal is known

as HDB3 (High Density Bipolar 3) which is a bipolar code with a specific

zero suppression scheme where no more then three consecutive zeros are

allowed to occur.

4.2 Transmission Problems

A number of problems can occur during the transmission of a radio signal. Some of

the most common problems are given below.

1. Path Loss: This occurs when the received signal becomes weaker and weaker due

to increasing distance between MS and BTS, even if there are no obstacles between

the transmitting and receiving antenna. But, the path loss problem seldom leads to a

dropped call because before the problem becomes extreme, a new transmission path

is established via another BTS.

2. Shadowing: Shadowing occurs when there are physical obstacles like hills and

buildings between the BTS and the MS. The obstacles create a shadowing effect,

which can decrease the received signal strength. A signal influenced by fading varies

in signal strength. Drops in strength are called fading dips.

38


39/52

3. Multipath Fading: Multipath fading occurs when there is more than one

transmission path to the MS or BTS and thus, more than one signal is arriving at the

receiver. This may be due to buildings or mountains, either close to or far from the

receiving device.

Rayleigh fading and time dispersion are two types of multipath fading. Rayleigh

fading occurs when a signal takes more than one path between the BTS and BTS

antennas. In this case, the signal is reflected off buildings, for example, and is

received from several indirect paths. It occurs when the obstacles are close to the

receiving antenna. Time dispersion is another multipath fading problem but here; the

reflected signal comes from an object far away from the antenna. It causes Inter-

Symbol Interference (ISI) where consecutive symbols interfere with each other

making the receiver hard to determine which is the is the correct signal.

4. Time Alignment: Each MS on a call is allocated a time slot on a TDMA frame. It

is the amount of time during which the MS transmits information to the BTS. The

information must also arrive at the BTS within that time slot. Time alignment

problem occurs when part of the information transmitted by an MS does not arrive

within the allocated time slot and usually occurs because of large distances between

the MS and BTS.

5. Combined Signal Loss: Signal strength as a global mean value decreases with the

distance (path loss) and finally results in a lost connection. Around the global mean,

slow variations are present due to shadowing effects and fast variations are present

due to Rayleigh fading.

4.3 Solutions to Transmission Problems

Channel Coding: In digital transmission, the quality of the transmitted signal is often

expressed in terms of how many of the received bits are incorrect. This is called Bit

Error Rate (BER) and is defined as the percentage of the total number of received

bits, which are incorrectly detected. Channel coding is used to detect and correct

39


40/52

errors in a received bit stream. It ads bits to a message. These bits enable a channel

decoder to determine whether the message has faulty bits, and to potentially correct

the faulty bits.

1. Adaptive Multi Rate (AMR): Channel coding provides a way of protecting digital

information over the air interface. With Adaptive Multi Rate (AMR), the rate of

channel coding bits and the underlying speech codec rate can be adapted to suit the

prevailing radio environment.

AMR consists of a number of different codecs, which together with the associated

channel coding has been optimized for different radio environments. Depending on

the measured Channel Interference Ratio (C/I) conditions, the best speech codec rate

for the present conditions is chosen, which results in a significant improvement in

speech quality.

2. Interleaving: Bit errors often occur in sequence, as caused by long fading dips

affecting several consecutive bits. Channel coding is most effective in detecting and

correcting single errors and short error sequences. It is not suitable for handling

longer sequences of bit errors. For this reason, a process called interleaving is used to

separate consecutive bits of a message so that these are transmitted in a non-

consecutive way.

3. Antenna Diversity: Antenna diversity increases the received signal strength by

taking advantage of the natural properties of radio waves. There are two primary

diversity methods: space diversity and polarization diversity.

Space Diversity Increased received signal strength at the BTS may be

achieved by mounting two receiver antennae instead of one. If the two Rx

antennae are physically separated, the probability that a deep fading dip at the

same time affects both of them is low. By choosing the best of each signal,

the impact of fading can be reduced. Space diversity offers slightly better

antenna gain than polarization diversity, but requires more space.

40


41/52

Polarization Diversity With the help of this, the two space diversity

antennae are replaced by one dual polarized antenna. This antenna has

normal size but contains two differently polarized antenna arrays. The most

common types are vertical/horizontal arrays and arrays in +-45 degree slant

orientation.

4. Adaptive Equalization: Adaptive equalization is a solution designed to counteract

the problem of time dispersion. Eight sets of predefined known bit patterns exist,

known as training sequences, which are known to the BTS and the MS. The BTS

instructs the MS to include one of these in its transmissions to the BTS. The other

party receives the transmission and examines the training sequence within it. The

received training sequence is compared with the known training sequence that is

used in this cell. The receiver begins a process in which it uses it knowledge of what

happened in the sequence to correct the speech data bits of the transmission.

5. Frequency Hopping: Fading dips from Raleigh fading occur at different places for

different frequencies. To benefit from this fact, it is possible for the BTS and MS tohop from frequency during a call. The frequency hopping of the BTS and MS is

synchronized.

6. Timing Advance: Timing advance is a solution specifically designed to counteract

the problem of time alignment. It works by instructing the mis-aligned MS to

transmit its burst earlier or later than it normally would.

41


42/52

5. Glossary

GSM: Global System for Mobile Communications

PLMN: Public Land Mobile Network

ISDN: Integrated Services Digital Network

BSC: Base Station Controller

BSS: Base Station Sub-system

MS: Mobile Station

TC: TransCoder

MCC: Mobile Country Code

MNC: Mobile Network Code

LAC: Location Area Code

CI: Cell Identity

NSS: Network Switching Subsystem

42


43/52

MSC: Mobile Switching Centre

HLR: Home Location Register

VLR: Visitor Location Register

AuC: Authentication Centre

EIR: Equipment Identity Register

CC: Country code (33=France, 358=Finland, etc.)

NDC: National Destination Code

SN: Subscriber Number

GMSC: Gateway Mobile Services Switching Centre

LAI: Location Area Identity

LAC: Location Area Code

SIM: Subscriber Identity Module)

TMSI: Temporary Mobile Subscriber Identity

6. Software & Tools

Net Tech: Alarm monitoring

Reflection: BSC login, Switching

Flexi BTS: BTS login

Flexi Hub: FIFA login

Cera View: Ceragon login

Nokia Hopper Manager: FIU manager

Metro Hub Manager: Hub sites management

Map Info: Information about various BTS/BSC/MSC sites

Teen Viewer: Remote login

Net Act Start: OSS management

43


44/52

7. Snapshots

1. To check the number of calls on a particular BTS sector.

44


45/52

Flexi EDGE BTS Manager

2. TRX test

45


46/52

1. FIU Login Performance

46


47/52

Nokia Hopper Manager

4. Ceragon

47


48/52

Cera View

8. Appendix

48


49/52

1. BTS Commands:

2. TRX Commands:

49


50/52

9. REFRENCES

GSM pocket guide by Wandel & goltermann

en.wikipedia.org/wiki/GSM

J. Schiller, Mobile Communications, Addison Wesley, 2000.

Mehrotra, GSM System Engineering, Artech House, 1997.

50


51/52

P. Struckmann, GSM Evolution, Wiley, 2003.

Y-B. Lin and I Chlamtac, Wireless and Mobile Network Architectures,

Wiley, 2001.

51


52/52

Documents

Global System for Mobile Communications(GSM)