CALL ROUTING IN GSM

UNIT - 2

Prepared By:- NITIN PANDYA

Assistant Professor,

SVBIT.

Call Routing

Call Originating from MS

Call termination to MS

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Outgoing Call

1. MS sends dialled number to BSS

2. BSS sends dialled number to MSC

3,4 MSC checks VLR if MS is allowed the requested service. If so,MSC asks BSS to allocate resources for call.

5 MSC routes the call to GMSC

6 GMSC routes the call to local exchange of called user

7, 8,

9,10 Answer back(ring back) tone is routed from called user to MS via GMSC,MSC,BSS

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Incoming Call 1. Calling a GSM subscribers

2. Forwarding call to GSMC

3. Signal Setup to HLR

4. 5. Request MSRN from VLR

6. Forward responsible MSC to GMSC

7. Forward Call to current MSC

8. 9. Get current status of MS

10.11. Paging of MS

12.13. MS answers

14.15. Security checks

16.17. Set up connection

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PLMN INTERFACE (private land mobile network)

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NETWORK ASPECTS IN GSM

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For roaming purpose a mobile need registration, authentication, call routing and location update.

The layer dependencies in GSM is structured into three general layers :

Layer 1 is physical layer

Layer 2 is DLL used for interface, LAPD(link access procedure-D) protocol is used.

Layer 3 is used for signaling purpose, it includes

Radio resources Management : setup control, maintenance

Mobility Management : registration procedures

Connection Management : includes services like SMS

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GSM FREQUENCY ALLOCATION

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Frequency Resource

GSM900 :

up: 890~915MHz

down: 935~960MHz

duplex interval: 45MHz

bandwidth: 25MHz，

frequency interval: 200KHz

GSM1800 :

up: 1710-1785MHz down:

1805-1880MHz

duplex interval: 95MHz， working

bandwidth: 75MHz， frequency interval: 200KHz

EGSM900 :

up: 880~890MHz

down: 925~935MHz

duplex interval: 45MHz

bandwidth: 10MHz，

frequency interval: 200KHz

GSM1900MHz:

up:1850~1910MHz

down:1930~1990MHz

duplex interval: 80MHz， working

bandwidth: 60MHz， frequency interval: 200KHz

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Mobile to base : Ft(n)=890.2 + 0.2(n-1)MHz

Base to mobile : Fr(n)=Fr(n) + 45 MHz

To share bandwidth for multiple users, GSM uses a combination of Time

Division Multiplexing and Frequency Division Multiplexing

encoding.

One or more carrier frequency is assigned to each base station.

Each of these carrier frequencies divided in time using TDMA scheme.

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CARRIER FREQUENCIES AND TDMA FRAMES 13 NITIN PANDYA

ORGANIZIATION OF BURSTS,TDMA FRAMES

SACCH-slow associated control channel

A TCH (traffic channel) carry 26 multiple frames at one instance.

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Mobility Management

Paging

Location Update

Handover

Authentication & Security Issues during Handover

Roaming

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Paging

For a mobile terminated call the MS needs to be traced,

located and then the call connected.

The MS is traced through the Paging process within a

location.

Mobile call reception is the process of the GSM

system sending paging messages on a paging control

channel to alert mobile devices that they are receiving a

call. Mobile devices listen for paging messages with their

identification code on a paging channel.

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Location Update

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As a MS moves around it is constantly monitoring the signal

strength of its current BTS, as well as neighboring BTS's to

determine if the neighbors have a stronger signal.

When the MS is in idle mode (not in a call), it will determine

for itself when to move from its current BTS to a more

attractive one.

When the MS switches from a BTS in one VLR to a BTS in a

different VLR, it must do an location update, so the network

knows which MSC/VLR the MS is currently using.

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Temporarily Mobile Subscriber Identification Number

(TMSI)

To insure the IMSI security, the VLR will assign an

unique TMSI number for the accessed subscriber. It is

used locally only and is a 4-byte TMSI number BCD

code.

Numbering Arrangement

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Handovers

Between 1 and 2 – Inter BTS /

Intra BSC

Between 1 and 3 –

Inter BSC/ Intra MSC

Between 1 and 4 –

Inter MSC

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Security in GSM

On air interface, GSM uses encryption and TMSI instead of IMSI.

SIM is provided 4-8 digit PIN to validate the ownership of SIM

3 algorithms are specified :

- A3 algorithm for authentication

- A5 algorithm for encryption

- A8 algorithm for key generation

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Authentication Concept

Random Number

Generator

Authentication

Algorithm Authentication

Algorithm

Secret Data Secret Data

Random Number

Authentication

Response

Yes

No

Authentication

Response

=

Mobile Station Serving Network

Grant

Access

Deny

Access

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Authentication in GSM (A3 Algo.)

During the authentication process the MSC challenges the

MS with a random number (RAND).

In SIM the RAND number is stored with a secret key Ki.

Both RAND & Ki are 128 bits long.

Using A3 algorithm with RAND & Ki a 32 bit output is

generated by SIM called SRES(signature response).

Using same algorithm AuC also generates a SRES as same as

old one, to compare with SIM (SRES & Ki).

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Authentication in GSM (A3 Algo.)

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Algorithm for Encryption (A5 Algo.)

Uses same cryptography methods as used in Internet

Network.

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Key generation and Encryption(A8)

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Characteristics of GSM Standard

Fully digital system using 900,1800 MHz frequency band.

TDMA over radio carriers(200 KHz carrier spacing.

8 full rate or 16 half rate TDMA channels per carrier.

User/terminal authentication for fraud control.

Encryption of speech and data transmission over the radio path.

Full international roaming capability.

Low speed data services (upto 9.6 Kb/s).

Compatibility with ISDN.

Support of Short Message Service (SMS).

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Advantages of GSM over Analog system

Capacity increases

Reduced RF transmission power and longer battery life.

International roaming capability.

Better security against fraud (through terminal validation and user

authentication).

Encryption capability for information security and privacy.

Compatibility with ISDN,leading to wider range of services

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GSM Applications

Mobile telephony

GSM-R

Telemetry System

- Fleet management

- Automatic meter reading

- Toll Collection

- Remote control and fault reporting of DG sets

Value Added Services

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Future Of GSM

2nd Generation

GSM -9.6 Kbps (data rate)

2.5 Generation ( Future of GSM)

HSCSD (High Speed ckt Switched data)

Data rate : 76.8 Kbps (9.6 x 8 kbps)

GPRS (General Packet Radio service)

Data rate: 14.4 - 115.2 Kbps

EDGE (Enhanced data rate for GSM Evolution)

Data rate: 547.2 Kbps (max)

3 Generation

WCDMA(Wide band CDMA)

Data rate : 0.348 – 2.0 Mbps

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