gsm architecture overview - discount coupons gsm network is made up of three subsystems: ... section...

GSM Architecture Overview

Section 1 – GSM Architecture Overview

Introduction

It provides an overview of the GSM network architecture. This includes a brief explanation of the different network subsystems and a description of the functionality of the elements within each of the subsystems. Topics include:

• General architecture overview

• The Mobile Station (MS) Subsystem and Elements

• The Base Station Subsystem (BSS) and Elements

• The Network Subsystem (NSS) and Elements

• Introduction to network interfaces


A GSM network is made up of three subsystems:

• The Mobile Station (MS) • 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

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


Abbreviations:

MSC – Mobile Switching Center

BSS – Base Station Sub-system

BSC – Base Station Controller

HLR – Home Location Register

BTS – Base Transceiver Station

VLR – Visitor Location Register

TRX – Transceiver

AuC – Authentication Center

MS – Mobile Station

EIR – Equipment Identity Register

OMC – Operations and Maintenance Center

PSTN – Public Switched Telephone Network


Mobile Station The Mobile Station (MS) consists of the physical equipment used by a

PLMN subscriber to connect to the network. It comprises the Mobile Equipment (ME) and the Subscriber Identity Module (SIM). The ME forms part of the Mobile Termination (MT) which, depending on the application and services, may also include various types of Terminal Equipment (TE) and associated Terminal Adapter (TA).


• The IMSI identifies the subscriber within the GSM network while

the MS ISDN is the actual telephone number a caller (possibly in another network) uses to reach that person.

• Security is provided by the use of an authentication key and by the transmission of a temporary subscriber identity (TMSI) across the radio interface where possible to avoid using the permanent IMSI identity.

• The IMEI may be used to block certain types of equipment from accessing the network if they are unsuitable and also to check for stolen equipment.


MS and SIM


The mobile station consists of :

• mobile equipment (ME) • subscriber identity module (SIM)

The SIM stores permanent and temporary data about the mobile, the subscriber and the network, including :

• The International Mobile Subscribers Identity (IMSI) • MS ISDN number of subscriber • Authentication key (Ki) and algorithms for authentication check

The mobile equipment has a unique International Mobile Equipment Identity (IMEI), which is used by the EIR


Base Station Subsystem (BSS)


The BSS comprises:

• Base Station Controller (BSC)

• One or more Base Transceiver Stations (BTSs)

The purpose of the BTS is to:

• provide radio access to the mobile stations

• manage the radio access aspects of the system

BTS contains:

• Radio Transmitter/Receiver (TRX)

• Signal processing and control equipment • Antennas and feeder cables


The BSC:

• allocates a channel for the duration of a call

• maintains the call:

monitors quality

controls the power transmitted by the BTS or MS

generates a handover to another cell when required


Network Switching System (NSS)

The NSS combines the call routing switches (MSCs and GMSC) with database registers required to keep track of subscribers’ movements and use of the system. Call routing between MSCs is taken via existing PSTN or ISDN networks. Signaling between the registers uses Signaling System No. 7 protocol.


Functions of the MSC:

• Switching calls, controlling calls and logging calls

• Interface with PSTN, ISDN, PSPDN

• Mobility management over the radio network and other networks

• Radio Resource management - handovers between BSCs

• Billing Information


Interfaces

BSC

VLR

MSC

Um

Abis A

ISDN, TUP


Exercise

Q1. Name the interfaces used between

Mobile and BTS

BTS and BSC

BSC and MSC


Access Network

Section 2 – Access Network

Objective

The Trainee will be able to understand:

• Different BTS configuration commonly used in the network

• Advantages of the configuration and optimal use of the trunks

• Abis mapping


Introduction

Access network is a connection between MS and NSS, comprise of BTSs & BSCs. It is responsible for radio management.

BSC looks towards MSC through single A-interface as being the entity responsible for communicating with Mobile Stations in a certain area. The radio equipment of a BSS may support one or more cells.

A BSS may consist of one or more base stations, where an A-bis-interface is implemented.


BSS Configuration

• Collocated BTS

• Remote BTS

• Daisy Chain BTS

• Star Configuration

• Loop Configuration


Collocated BTS: BTS is situated along with BSC or the MSC and no

additional E1 link is required.

BSC

BTS


Remote BTS : BTS is situated in a stand alone position and additional E1

links are required to connect to BSC.

BSC

BTS


Daisy Chain

MSC

BSC

BTS 1

BTS 2

BTS 3

BTS 4


Star Configuration

MSC

BSC

BTS 1

BTS 2

BTS 3

BTS 4

BSC

BTS 3


Loop Configuration

MSC

BSC

BTS 1

BTS 2

BTS 3

BTS 4


Comparison of Different Configurations

• Daisy Chain: Easy to implement, effective utilization of transmission links but if one of the link fails, all the BTSs connected in the chain will went off.

• Star Configuration: Easy to implement but poor utilization of links. Each BTS require one E1 to connect to BSC. But if link goes down only individual BTS will be affected.

• Loop Configuration: Slightly difficult to implement but effective utilization of E1 links. Even if one link goes off BTS will continue to communicate with the network from the other side.


BSS Interfaces

• Air Interface: Radio Interface between the BTS and Mobile the supports frequency hopping and diversity.

• A Interface: Interface carried by a 2-Mb link between NSS and BSS. At this interface level, transcoding takes place.

• OMC Interface: X25 Link.


Abis Interface (BTS - BSC)

If the BTS and BSC are not combined, A-bis interface will be used. Otherwise, BS interface will be used. Several frame unit channels are multiplexed on the same PCM support and BSC and BTS can be remote from each other. Its main functions are:

• Conversion of 260 – bit encoded blocks (corresponding to 160x8 – bit samples for 20ms)

• Encoded block synchronization

• Vocal activity detection

• Alarm dispatch to BSC via PCM

• Test loop back operation


TRX 1

TRX 2


Exercise

Q1. In How many ways BTSs can be connected and which configuration gives the optimal solution?

Q2. What is a difference between BS interface and Abis interface?

Q3. How many time slots are occupied by 1TRX on a PCM frame?


NSS Topology

Section 3 – NSS Topology

Objective

The Trainee will be able to understand:

• Terminology used in Network Sub System

• Protocols and Interfaces inside NSS

• Call routing and circuit groups

• Switching modules

• Stand alone and integrated HLR

• Echo canceller and TRAU location

• Authentication, Ciphering, OMC, Billing center

• Transit Switch


Introduction

Network Sub System can be considered as a heart of the GSM Network. All the major activities like switching of calls, routing, security functions, call handling, charging, operation & maintenance, handover decisions, takes place within the entities of NSS.

Various kinds of interfaces are used to communicate between the different entities. Different methods are used to optimize and provide the quality network with the minimum operating cost.


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