GSM System Architecture

Block Diagram of GSM System

• BSS: Base Station Subsystem• BSC: Base Station Controller• BTS: Base Transceiver Station• MSC: Mobile Services Switching Center• OMC: Operation and Maintenance Center• AUC: Authentication Center• EIR: Equipment Identification Register• HLR: Home Location Register• VLR: Visitor Location Register• MS: Mobile Station• ISDN: Integrated Services Digital Network• PSTN: Public Switched Telephone Network• PSPDN: Packet Switched Public DataNetwork• PLMN: Public Land Mobile Network

Network and Switching Subsystem (NSS)

NSS Architecture

The network sub-system is divided into six functionunits:• Mobile Service Switching Center (MSC)• Home Location Register (HLR)• Visitor Location Register (VLR)• Authentification Center (AUC)• Equipment Identification Register (EIR)• Operation and Maintenance Center (OMC)

Mobile Services Switching Center (MSC)

As the core of a network, MSC provides switching functions, and connects mobile subscribers with fixed network subscribers, or with mobile subscribers.

Thus, it provides interfaces to fixed networks (such as PSTN, ISDN, etc.) and interfaces for interconnection with other MSCs.

Functions of MSC

MSC obtains all the data for processing subscriber

call requests from 3 types of databases: HLR, VLR

and AUC.

MSC can provides a series of services for subscribers:

-Telecom services, such as phone, fax, and emergency

calls

- Bearer services

- Supplementary services, such as call transfer, call

restriction.

Visitor Location Register( VLR)

VLR stores all related information of mobile subscribers

having entering into its coverage area,which enables

MSC to set up incoming/outgoing calls.It can be taken as

a dynamic subscriber database.

VLR obtains and stores necessary data from the HLR of a

mobile subscriber. Once a mobile subscriber leaves the

coverage area of this VLR, it will be reregistered in

another VLR, the temporarily recorded data of this mobile

subscriber stored in the original VLR will be deleted.

Home Location Register( HLR)

As the central database of the GSM system, HLR stores the related data of all existing mobile subscribers controlled by the same HLR. One HLR can control several mobile switching areas or the whole mobile communication network and the important static data of all subscribers are stored in the HLR, including MSISDN,IMSI, access capability, subscriber type and supplementary services. Furthermore, HLR also stores the location information of all subscribers, so that any incoming call is immediately sent to the called subscriber on a selected path.

Authentication Center( AUC)

As a function unit of HLR, AUC is specially used for

security management of the GSM system.

AUC stores authentication information and encryption

keys for subscriber authentication, encryption of voice,

data, signaling messages on radio interfaces, preventing

unauthorized subscribers access and guaranteeing the

safety of mobile subscriber communication.

Equipment Identification Register( EIR)

• EIR stores the international mobile equipment identifier

(IMEI) of mobile equipment.

• By checking 3 types of lists, i.e., white lists, black lists,

and gray lists, it respectively lists the mobile equipment

identifiers that are authorized, that should be monitored in

case of faults, and that are unauthorized in case of theft.

• Service operators can use such information to locate the

location of a stolen mobile station and block it.

BSS System Architecture

ZXG-10 BSS Structure

Base Station Controller (BSC)

• As the control part of BSS, BSC performs the switching function in BSS.

BSC may be connected with multiple BTSs at one end, and MSC

and OMC at the other end.

• BSC mainly manages radio network and radio resources,

supervises and manages radio base station, controls the

establishment, connection and disconnection of radio links in

MS and BTS and the location updating, hand-over and paging

of mobile station, provides functions such as voice encoding,

transcoding, rate adaptation, as well as the operation and

maintenance functions of BSS.

Base Transceiver Station (BTS)

• It is the radio tranmission part of the base station

system.

• Controlled by BSC, it serves cell radio transceiver

equipment, handles conversion between BSC and radio

channels, and performs radio transmission between BTS

and MS via air interfaces as well as related control

functions.

Mobile Station

Mobile stations are not fixed to one subscriber. On any

mobile station in the system, we can identify the

subscriber with the SIM card (Subscriber Identity

Module). The personal identification number (PIN) can be

used to prevent unauthorized use of the SIM card.

Each mobile station has its own identification number,

i.e., the International Mobile Equipment Identifier (IMEI).

IMEI mainly consists of the type permission code and the

related manufacturer product number.

Mobile Station

Each mobile subscriber has its own international mobile

subscriber identifier (IMSI), which is stored in the SIM

card and in HLR.

Network and Switching Subsystem (NSS)

The Network and Switching Sub-system mainly includes

switching functions of the GSM system, and database

functions used for subscriber data and mobility

management as well as safety management. It manages

the communications among GSM mobile subscribers and

those between GSM mobile subscribers and other

communication network subscribers.

TransCoder (TC)

TC mainly completes voice conversion between

the16kbit/s RPE-LTP (Regular Pulse Excited Long-Term

Prediction) codes and 64kbit/s A-law PCM codes.

In a typical application pattern, ZXG10-TC is located

between MSC and BSC.

Wireless Coverage Area Structure

Functional Layers of GSM

• Transmission: data transmission function, providing methods of carrying subscriber data and transmitting signalings between different entities in various segments along the communication path.• RR: radio resources management. setting up and releasing stable connections between mobile stations and MSC at the call setup stage, which is mainly performed by MS and BSC.Such as: assignment achannel ,handover,system informations ,measurment report.• MM: refers to mobility and safety management, mobile station processing -- environment changing, making choices of cells possibly belonging to different networks, so that the calling subscriber is able to set up a valid process; infrastructures are required to manage subscriber location data (location updating), authentification.• CM: refers to communication management, i.e., under subscriber requests, setting up connections between subscribers, maintaining and releasing calls (which can be divided into CC - call control, SSM – supplementary service management, and SMS - short messages service);OAM: Operation, Administration and Maintenance platform, providing operation methods for operators. The service is supplied by the transmission layer directly.

Um Interface Signaling Protocol Model Layer1: Transmission layer (physical layer), the lowest layer of Um

interface, provides transmission channel for radio link and provides

differently functional logic channels (control channel and traffic

channel ) for higher layer.

Layer2: Data link layer, provides reliable dedicated data link for and

between MS and BTS. It’s based on link access protocol of D channel

(LAPD), but add some protocols of mobile applications that are unique

to GSM.

• Layer3: Application layer, mainly performs controlling and

management functions.

It includes three sub-layers (CM, MM and RR), each realizes call control,

supplementary service management and short message management

respectively.

Abis Interface Signaling Protocol Model • The Abis interface complies with the requirements in

08.5X series of GSM standards.

• Layer1: Physical layer, usually adopts the 2Mbit/sPCM

link in accordance with the specifications stipulated in

ITU-T G.703 and G.704

Layer2: data link layer employs the LAPD protocol,which

is a point to multi-point communication protocol. This

protocol is to provides reliable transmission of the

information in physical layer.

Abis Interface Signaling Protocol Model

Layer3: The upper layer part, mainly transmitting the

application part of BTS, including the radio link

management (RLM) function and the operation and

maintenance function.

Through the Abis interface, BSC provides such signaling

control information as BTS configuration,BTS monitoring,

BTS testing, and service control.

A Interface Signaling Protocol Model

• The A interface is used in the message between BSC and MSC as well as the message coming into/out of MS.

• Layer1—Physical and electrical parameter and channel

architecture, defining the physical layer structure of MSC~BSC.

It is realized by employing the first level of the message transfer

part (MTP) in the common channel signaling system NO.7

(CCS7), adopting 2Mbit/s PCM digital link as the transmission

link.

A Interface Signaling Protocol Model

• Layer2: defining the data link layer and the network

layer, namely MTP2, MTP3 and SCCP.

- MTP2 is a variant of HDLC protocol.

- MTP3 and SCCP (Signal Connection Controlling Part)

chiefly

implement such functions as signaling route selection.

A Interface Signaling Protocol Model

Layer3: Application layer, including BSS application part

(BSSAP) and BSS operation and maintenance

application part (BSSOMAP), maintains and manages the

resources and the connections in BSS as well as controls

both the connection and the disconnection of service.The

BSSAP can be divided into two parts:DTAP （ Direct

Transmission Application Part ） and BSSMAP.

BSSAP

•DTAP:transfered transparently through BSS (MM +CM)

•BSSMAP:BSS management message (RR+BTSM)

Channel types

In a GSM system, channels are divided into logical and

physical channels. Time slots are basic physical

channels; thus each carrier frequency contains 8 physical

channels. Physical channels of radio subsystems support

logical channels which can be subdivided funcationally

into Traffic Channels (TCH) and Control Channels (CCH).

Traffic Channel (TCH)

• Traffic channel carries encoded speech or subscriber data, including full-rate traffic channel and half-rate traffic channel:

– Full-rate traffic channel (TCH/F): total rate is 22.8kbit/s

– Half-rate traffic channel (TCH/H): total rate is 11.4kbit/s

1) Speech channel

– TCH/FS: full-rate speech traffic channel

– TCH/HS: half-rate speech traffic channel

2) Data channel

– TCH/F9.6: 9.6kbit/s full-rate data traffic channel

Control Channel

Control channel carries signaling or synchronized

data.

There are three types of control channels:

1) Broadcasting Channels (BCH)

Broadcasting channels are used only as downward

channels, i.e., one-way transmission from the base

station to mobile stations.

Control Channel-BCH

BCH can be divided into three types of channels:

FCCH: Frequency Correction Channel,carrying

information used in MS frequency calibration.

SCH: Synchronization Channel,carrying information of

MS frame synchronization and Base Transceiver Station

(BTS) identification.

BCCH: Broadcasting Control Channel;this channel

broadcasts general BTS information.Among transceivers

at each base station, there is always one transceiver that

contains such a channel, so as to broadcast system

information to mobile stations.

Common Control Channel( CCCH)

2) Common Control Channel( CCCH) are shared by all mobile

stations in the network.

There are 3 types of such channels:

PCH: paging channels,used by a base station to page mobile

stations (downward).

RACH: random access channel, used by mobile stations for

random access network application,i.e.,requesting the allocation

of SDCCH channels (upward).

AGCH: access granted channel, used by a base station to

respond to random access requests of mobile stations, i.e., to

assign one SDCCH or directly assign one TCH (downward).

Dedicated Control Channel (DCCH)

3) Dedicated Control Channel (DCCH) In application,

the base station will assign it to a mobile station, so as to

make point-to-point transmission between the base

station and the mobile station.

There are 3 types of such channels:

SDCCH: a stand-alone dedicated control channel,used to

transmit such information as channel assignment.

Dedicated Control Channel (DCCH)

SACCH: slow-speed associated control channel,Used in

combination with one traffic channel or one SDCCH,to

send some specific subscriber information, e.g.,power

and frame adjustment control information, measurement

data, etc.

FACCH: fast associated control channel, combining with

one traffic channel to carry the same signals as SDCCH,

but it is assigned only when SDCCH has not been

assigned. Call connection is realized via frames borrowed

from traffic channels to transmit such commands as

“handover”.

Traffic

Traffic refers to the numbers of subscribers the network can

support and is usually described as follows:

A=n×T/3600

where,

n- calls made by a subscriber within an hour;

T- average duration of each call(in seconds)

A - Traffic, in Erlang

If one call is made by a subscriber within an hour and last 120 seconds, the traffic is calculated as: A=1×120/3600=33mErl?

For convenience of engineering calculation, the traffic is usually

defined as 25mErl per subscriber.

GOS

Grade of Service( GOS) : GOS refers to the degree of

network congestion or call loss rate.

GOS=2% means that 98% of subscribers can make calls

successfully and 2% of subscribers will end up with

unsuccessfulness.

For network operators, 2-5% of GoS is adopted.

Voice Transform Process

Speech Coding

• The coding mode is called Regular Pulse Excited-Long Term Prediction (RPE-LTP).It works as follow: 8KHZ of sampling is performed first, then divided into frames with 20ms; every frame has 4 sub-frames; the duration of every sub-frame is 5ms;and the pure bit rate is 13kbit/s.

Channel Encoding

In order to enhance channel antiinterference capacity and transmission quality, special redundancy technologies should be adopted to increase the bulk of transmitted information which can be inserted at a certain pattern (encoding) at thesending end and extracted at an agreed pattern (decoding) at the receiving end. This is called the encoding/decoding process of channels.

Channel Coding

Error Coding: Detect and correct error bits in the receivingend by adding redundant bits

Channel Interleaving

In radio telecommunication, error bits often occurs in the

burst pulse sequence because of deep signal fading.

Check and correction of errors cannot be efficiently done

merely by means of channel encoding mentioned above.

Interleaving technique is thus adopted, by which the

continuous bits in an information block are segmented

and transmitted individually according to certain rules.

That is, the originally continuous block in the transmission

process becomes discontinuous, forming a group of

interwoven message transmitting blocks, which are to be

recovered (de-interleaving) into the original information

blocks at the receiving end.

An example of interleaving

Channel Interleaving

First Level

Interleaving

Second Level

Interleaving