Synopsison

GSM Network Structure and Performance

UNDERTAKEN AT

“Idea Cellular Ltd.”

Submitted as a part of course curriculum for

Bachelor of TechnologyIn

Electronics and Communication EngineeringIn

E.C.E.

Under the Guidance of: Submitted By: Name: Rahul Chauhan Name: Sandeep GoyalDesignation: BTS Engineer College Roll No.: EC/09/9403Department: O&M . University Roll No.: 90420413400

INSTITUTE OF ENGINEERING AND TECHNOLOGY-BHADDALPO: MIANPUR, District: Ropar, Punjab

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1. OBJECTIVE –

In this paper I have outlined the reasons GSM started and how, the architecture

that the GSM is built on, the signaling and Transmission used, the Air and Abis Interface

and last but not the least future prospects and enhancements possible.

2. HISTORY –

In 1980’s the analog cellular telephone systems were growing rapidly all

throughout Europe, France and Germany. Each country defined its own protocols and

frequencies to work on. For example UK used the Total Access Communication System

(TACS), USA used the AMPS technology and Germany used the C-netz technology.

None of these systems were interoperable and also they were analog in nature.

In 1982 the Conference of European Posts and Telegraphs (CEPT) formed a study

group called the GROUPE SPECIAL MOBILE (GSM) The main area this focused on

was to get the cellular system working throughout the world, and ISDN compatibility

with the ability to incorporate any future enhancements. In 1989 the GSM transferred the

work to the European Telecommunications Standards Institute (ETSI.) the ETS defined

all the standards used in GSM.

3. BASICS OF WORKING AND SPECIFICATIONS OF GSM –

The GSM architecture is nothing but a network of computers. The system has to

partition available frequency and assign only that part of the frequency spectrum to any

base transreceiver station and also has to reuse the scarce frequency as often as possible.

GSM uses TDMA and FDMA together. Graphically this can be shown below

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Fig 1.Representation of a GSM signal using TDMA & FDMA with

respect to the transmitted power.

Some of the technical specifications of GSM are listed below –

Multiple Access Method TDMA / FDMAUplink frequencies (MHz) 933-960 (basic GSM)Downlink frequencies (MHz) 890-915 (basic GSM)Duplexing FDDChannel spacing, kHz 200Modulation GMSKPortable TX power, maximum / average (mW) 1000 / 125Power control, handset and BSS YesSpeech coding and rate (kbps) RPE-LTP / 13Speech Channels per RF channel: 8Channel rate (kbps) 270.833Channel coding Rate 1/2 convolutionalFrame duration (ms) 4.615

GSM was originally defined for the 900 Mhz range but after some time even the 1800

Mhz range was used for cellular technology. The 1800 MHz range has its architecture

and specifications almost same to that of the 900 Mhz GSM technology but building the

Mobile exchanges is easier and the high frequency Synergy effects add to the advantages

of the 1800 Mhz range.

4. ARCITECTURE AND BUILDIGN BLOCKS –

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GSM is mainly built on 3 building blocks. (Ref Fig. 2)

GSM Radio Network – This is concerned with the signaling of the system. Hand-

overs occur in the radio network. Each BTS is allocated a set of frequency

channels.

GSM Mobile switching Network – This network is concerned with the storage of

data required for routing and service provision.

GSM Operation and Maintenance – The task carried out by it include

Administration and commercial operation , Security management, Network

configuration, operation, performance management and maintenance tasks.

Fig.2 The basic blocks of the whole GSM system

Explanations of some of the abbreviations used –

Public Land Mobile Network(PLMN) The whole GSM system

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Mobile System (MS) The actual cell phone that we use

Base Transceiver Station (BTS) Provides connectivity between network and

mobile station via the Air- interface

BaseStationController(BSC) Controls the whole subsystem.

Transcoding Rate & Adaption Unit

(TRAU)

This is instrumental in compressing the Data that

is passed on to the network, is a part of the BSS.

Mobile Services Switching Center

(MSC)

The BSC is connected to the MSC. The MSC

routes the incoming and outgoing calls and

assigns user cannels on the A- interface.

Home Location Register (HLR) This register stores data of large no of users. It is

like a database that manages data of all the users.

Every PLMN will have atleast one HLR.

Visitor Location Resigter (VLR) This contains part of data so that the HLR is not

overloaded with inquiries. If a subscriber moves

out of VLR area the HLR requests removal of

data related to that user from the VLR.

Equipment Identity Register (EIR) The IMEI no. is allocated by the manufacturer

and is stored on the network in the EIR. A stolen

phone can be made completely useless by the

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network/s if the IMEI no is known.

6. INTERFACES –

The two main interfaces are the AIR and the ABIS interface. The figure shows the

signaling between them.

AIR INTERFACE – signaling between MS and BTS

ABIS INTERFACE – signaling between BTS and BSC

Fig.5 Signaling between Air and Abis Interface

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2.1GSM ARCHITECTURE:-

2.2 Basic Architecture:

A GSM system is basically designed as a combination of four major subsystems:1. Radio subsystem (RSS)2. Network (switching) subsystem (SSS)

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3. Operation and maintenance subsystem (OMS)

2.3 Base Station System (BSS)

In GSM, the Base Station System is a term given to a BSC (Base StationController) and the BTS (Base Transceiver Station) associated with it. Thenumber of BTS associated with a BSC is dependent on the manufacturer.Although not mandatory, through interpretation of the Abis interface standardBTS and BSC employed within a BSS will always be supplied by the same Manufacturer.

The BSC, the TRAU and BTS form a unit, which is called Base Station System (BSS) in the GSM terminology. A BSC can control several BTS. Every BSC Contained in the network controls one BSS. The interface between BSC and BTS is called Abis - interface. A interface as being the entity responsible for communicating with MSs in a certain area. The radio equipment of a BSS may be composed of one or more cells. A BSS may consist of one or more BS.

The base station subsystem (BSS) is the section of a traditional cellulartelephone network which is responsible for handling traffic and signalling between a mobile phone and the network switching subsystem. The BSS carries out transcoding of speech channels, allocation of radio channels to mobile phones, paging, quality management of transmission and reception over the air interface and many other tasks related to the radio network.

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2.4 Base Station Controller (BSC)

A BSC is a network component in the PLMN that function for control of one or more BTS. It is a functional entity that handles common control functions within a BTS. BSC within a mobile network is a key component for handling and routing information. The BSC provides all the control functions and physical links between the MSC and BTS. It is a high-capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in base transceiver stations. A number of BSCs are served by an MSC.

2.5 Base Terminal Station (BTS)

The BTS handles the radio interface to the mobile station. The BTS is the radioequipment (transceivers and antennas) needed to service each cell in the network. A group of BTSs are controlled by a BSC.

A BTS is a network component that serves one cell and is controlled by a BSC.BTS is typically able to handle three to five radio carries, carrying between 24 and 40 simultaneous communication. Reducing the BTS volume is important to keeping down the cost of the cell sites.

A BTS compares radio transmission and reception devices, up to and includingthe antennas, and also all the signal processing specific to the radio interface. A single transceiver within BTS supports eight basic radio channels of the same TDM frame.

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Figure- Star BTS

2.6 Functions of BTS

The primary responsibility of the BTS is to transmit and receive radio signals from a mobile unit over an air interface. To perform this function completely, the signals are encoded, encrypted, multiplexed, modulated, and then fed to the antenna system at the cell site. Transcoding to bring 13-kbps speech to a standard data rate of 16 kbps and then combining four of these signals to 64 kbps is essentially a part of BTS, though; it can be done at BSC or at MSC. The voice communication can be either at a full or half rate over logical speech channel. In order to keep the mobile synchronized, BTS transmits frequency and time synchronization signals over frequency correction channel (FCCH and BCCH logical channels. The received signal from the mobile is decoded, decrypted, and equalized for channel impairments.

Random access detection is made by BTS, which then sends the message to BSC. The channel subsequent assignment is made by BSC. Timing advance is determined by BTS. BTS signals the mobile for proper timing adjustment. Uplink radio channel measurement corresponding to the downlink measurements made by MS has to be made by BTS.

2.6 Network operations GSM functions:

The description of the GSM network is focused on the different functions to fulfill by the network and not on its physical components. In GSM, five main functions can be defined: • Transmission• Radio Resources Management (RRM).• Mobility Management (MM).• Communication Management (CM).• Operation, Administration and Maintenance (OAM).

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2.6.1 Transmission:-

Transmission means sending and receiving of data and signalling bits. 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 signalling, components such as HLR, VLR or EIR registers, are only concerned with signalling. The GSM standard also provides separate facilities for transmitting digital data. This allows a mobile phone to act like any other computer on the Internet, sending and receiving data via the Internet Protocol.

2.6.1.1 SDH - Synchronous Digital Hierarchy

o A Modular, Layered & Organized Architecture using Synchronous

Multiplexing Technique.

o A Standardized hierarchical Set of Digital Transport Structures for

transport of suitably Adapted Payloads

oA Set of Improved & Standardized Management Interfaces and Functions, allowing

Digital Transmission Systems to inter-work in a multi-vendor environment.

2.6.1.2 SDH Network Configurations:-

1. Point-to-Point Configuration

2. Linear Add/Drop (or Chain) Configuration

3. Ring Configuration

4. Star Configuration

5. Mesh Configuration

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