5. GSM System

7/31/2019 5. GSM System

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GSM SystemGSM System

Prof. S.-I. Sou

,

Fall, 2008

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Outlines

• Introduction

• GSM Architecture

• Air Interface

•

• Mobility Management

• Summary

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GSM Architecture

MAP interface

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• Also called Mobile Terminal (MT)

• The MS consists of two parts: Subscriber Identity Module (SIM)

Mobile Equipment (ME)

Mobile Station (MS)

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• A SIM contains subscriber-related information A list of abbreviated and customized short dialing

numbers

Short messa e

SIM

Names of preferred Networks to provide service

• Personal Identity Number (PIN) .

• SIM information can be modified:

By the subscriber either by keypad or a PC using an RS232 connection

By sending codes through short messages

(network operators)6

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Mobile Equipment (ME)

• ME: non-customer-related hardware and

software specific to the radio interface

• ME can not be used if no SIM is on the MS.

Except for emergency calls

• e - es gn suppor s por a y: The MS is the property of the subscriber.

The SIM is the property of the service provider.

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Base Station System (BSS)

• The Base Station System (BSS) connects the

MS and NSS.

• BSS contains

Base transceiver station (BTS)

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• Network and Switching Subsystem (NSS)

Telephone switching functions

Subscriber profiles

Mobility management

•

NSS (1/2)

MSC : provide basic switching function

Gateway MSC (GMSC): route an incoming call to an MSC by interrogating the HLR directly.

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NSS (2/2)

• Components in NSS (continuous):

HLR and VLR maintain the current location of

the MS.

Authentication Center (AuC) is used in the security management.

qu pment ent ty eg ster s use or the registration of MS equipment.

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Mobility Databases

• The hierarchical databases used in GSM.

The home location register (HLR) is a database

used for MS information management.

The visitor location register (VLR) is the database of the service area visited by an MS.

MSC 1

HLR

VLR 1 VLR 2

MSC 2

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Home Location Register (HLR)• An HLR record consists of 3 types of

information:

Mobile station information • IMSI (used by the MS to access the network)

• MSISDN (the ISDN number－“Phone Number”of the MS)

Location information • ISDN number of the VLR (where the MS resides)

• ISDN number of the MSC (where the MS resides)

Service information

• service subscription• service restrictions

• supplementary services

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Visitor Location Register (VLR)

• The VLR information consists of three parts:

Mobile Station Information • IMSI

• MSISDN

• TMSI

Location Information • MSC Number

• Location Area ID (LAI)

Service Information

• A subset of the service Information stored in HLR

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Registration

• MS powers on.

• By using FCCH and SCH to

synchronize with the BTS.

BS 2

BS 1

BS 3

• From BCCH, the MS gets thecell global identity (CGI).

• MS registers with the MSC.

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GSM Call Origination

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GSM Call Termination

RACH(request signaling channel)

MS BSS

PCH(page MS)

SDCCH(respond to paging)

SDCCH message exchanges for call setup

AGCH(assign signaling channel)

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Location Tracking and CallLocation Tracking and Call

SetupSetup

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Location Area

• Location area (LA) is the basic unit for location

tracking.

• Location Area Identification (LAI)

MSC MSC

MSC

LA 1

LA 2LA 3

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Key Terms

• GSM uses some identifiers

Mobile system ISDN (MSISDN)

Mobile Station Roaming Number (MSRN)

International Mobile Subscriber Identity (IMSI)

Temporary Mobile Subscriber Identity (TMSI)

International Mobile station Equipment Identity (IMEI)

Location Area Identity (LAI)

Cell Global Identity (CAI)

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MSISDN

• Mobile System ISDN

MSISDN uses the same format as the ISDN

address (based on ITU-T Recommendation E.164).

HLR uses MSISDN to provide routing

reach the subscriber.

Country code

(CC)

National destination

code (NDC)

Subscriber

number (SN)

Total up to 15 digits

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MSRN

• Mobile Station Roaming Number

• The routing address to route the call to the MSthrough the visited MSC.

MSRN=CC+NDC+SN

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IMSI

• International Mobile Subscriber Identity

Each mobile unit is identified uniquely with an

IMSI. IMSI includes the country, mobile network,

mobile subscriber.

ota up to 15 g ts

Mobile country

code (MCC)

Mobile network

code (MNC)

Mobile subscriber

identification code (MSIC)

3 digits 1- 2 digits Up to 10 digits

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TMSI

• Temporary Mobile Subscriber Identify

TMSI is an alias used in place of the IMSI.

This value is sent over the air interface in place of the IMSI for purposes of security.

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IMEI

• International Mobile Station Equipment Identity

IMEI is assigned to the GSM at the factory.

When a GSM component passes conformance and interoperability tests, it is given a TAC.

Up to 15 digits

Type approval

code (FAC)

Final assembly

code (FAC) Serial number (MSIC)

3 digits 2 digits Up to 10 digits

Spare 1 digit 24

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LAI

• Location Area Identity

LAI identifies a location area (LA).

When an MS roams into another cell, if it is in the same LAI, no information is exchanged.

Total up to 15 digits

Mobile country

code (MCC)

Mobile network

code (MNC)Location area code (LAC)

3 digits 1-2 digits Up to 10 digits

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CGI

• Cell Global Identity

• CGI = LAI + CI

= MCC + MNC + LAC + CI

CI : Cell Identity

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Identifiers and Components

號碼 HLR VLR/MSC BSC BTS MS

MSISDN

MSRN

IMSI

TMSI

LAI

CGI

MSC號碼

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Cloud

Cloud

Other

HLR

1

GMSCVLR

1

1 12

2

The Mobile Call Termination

(Delivery) Procedure

MSRN MSRN

IMSI

MSISDN

IMSI

w c es

3

MSCCloud

CloudCloud

Other

Switches

3

3

依據PSTN正常程序建立電話

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MS Registration Process

HLR

35

認證成功後向HLR 進行location update

deregistration

消除VLR 內資料

Old

VLR

New

VLR

1 4

2

TMSI

TMSI

MS’s IMSI 及其他認證資料

new TMSI

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Periodically Registration

• The MS periodically send registration

messages to the network.

• The period is 6 minutes to 24 hours.

• Periodic registration is useful for fault-tolerance.

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Location Update Concept

• Registration: the location update procedure

initiated by the MS:

Step 1. BS periodically broadcasts the LAaddress.

Step 2. When an MS finds the LA of BS different from the one stored in it memory, it sends a registration message to the network.

Step 3. The location information is update.

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GSM Basic Location UpdateProcedure

• In GSM, registration or location update occurs

when an MS moves from one LA to another.

• Three cases of location update:

. -

Case 2. Inter-MSC Movement

Case 3. Inter-VLR Movement

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• Two LAs belong to the same MSC.

• Four major steps:

Step 1. MS sends a location update request message (MS →BTS →MSC) .

• Parameters included: Previous LA, revious MSC

Inter-LA Movement (1/2)

and previous VLR.• IMSI (International Mobile Subscriber Identity) is

used to identify MS.

• However, the MS identifies itself by the Temporary

Mobile Subscriber Identity (TMSI).• TMSI is used to avoid sending the IMSI on the

radio path.

• TMSI is temporary identity is allocated to an MS

by the VLR at inter-VLR registration. 34

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• The Process continues:

Step 2. The MSC forwards the location update

request to the VLR by a TCAP message,MAP_UPDATE_LOCATION_AREA.

• Parameter includes: Address of the MSC, TMSI,

Inter-LA Movement (2/2)

,

LAI, Other related information Steps 3 and 4.

• Part I. The VLR find that both LA1 and LA2 belongto the same MSC.

• Part II. The VLR updates the LAI field of the MS.

• Part III. The VLR replies an ACK to the MSthrough the MSC.

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Inter-MSC Registration

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• The two LAs belong to different MSCs of the same

VLR.

• The process is:

Inter-MSC Movement (1/2)

.

message (MS →BTS →MSC) .

Step 3.

• Part I. VLR1 finds that the LA1 and LA2 belong toMSC1 and MSC2, respectively. Two MSCs areconnected to VLR1.

• Part II. VLR1 updates the LAI and MSC fields of MS.

• Part III. The VLR1 derives the HLR address of theMS from the MS’s IMSI.

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• The process continues:

Step 3.• Part IV. The VLR1 sends the

MAP_UPDATE_LOCATION to the HLR.

•

Inter-MSC Movement (2/2)

, ,

VLR Address, other related information Step 4. HLR updates the MSC number field of

the MS. An acknowledgement is sent to VLR1.

Steps 5 and 6. The acknowledgement is

forwarded to the MS.

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I t VLR R i t ti M

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Inter-VLR Registration Message

Flow

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• Two LAs belong to MSCs connected to different

VLRs.

• The process is:

Step 1. MS sends a location update request.MSC2 sends MAP UPDATE LOCATION AREA

Inter-VLR Movement (1/2)

_ _ _

to VLR 2 with MS’s TMSI. Steps 2 and 3.

• VLR2 does not have the record of MS.

• VLR2 identifies the address the VLR1 and sends

MAP_SEND_IDENTIFICATION (with TMSI) toVLR1.

• VLR1 sends IMSI to VLR2.

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• The process continues:

Steps 4 and 5.

• VLR2 creates a VLR record for the MS.• VLR2 sends a registration message to HLR.

• HLR updates the record of the MS.

Inter-VLR Movement (2/2)

• HLR sends an acknowledge back to VLR2.

Step 6.

• VLR2 generates a new TMSI and sends it to theMS.

Steps 7 and 8.

• The obsolete record of the MS in VLR1 is deleted.

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• General Packet Radio Service (GPRS)

• A packet-switched protocol

• GPRS radio link protocol

To guarantee fast call setup procedure and low-

GPRS

w

and the BSs

• A new infrastructure is introduced to GPRS forthe packet services.

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GPRS Architecture

MS

TAF

TEMSC

PSTN

PSDN

ISDN

r adi o

i nt er f ace

SGSN GGSN

HLR

Signaling link

HLR : Home Location Register

VLR : Visitor Location Register

BSS : Base Station Subsystem

TAF : Terminal Adaption Function

TE : Terminal Equipment

BSS

PSTN : Public Switched Telephone Network

PSDN : Public Switched Data Network

MSC : Mobile Switching Center

SGSN : Serving GPRS Support Node

GGSN : Gateway GPRS Support Node

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5. GSM System