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Protocol Architecture

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GSM Protocol Stack- In any telecommunication system, signaling is required to coordinate among the necessarily distributed

functional entities of the network.

- GSM uses the CCITT SS7 protocols for its signaling.

- The transfer of signaling information in GSM follows the layered OSI model.

Layer 1: Physical Layer :Radio Transmission

Layer2:Data Link Layer (DLL) : Provides error-free transmission between adjacent entities,based on the ISDNs LAPD protocol for the Um and Abisinterfaces, and on SS7s Message Transfer

Protocol (MTP) for the other Layer interfaces.

Layer 3: Networking or Messaging Layer: Responsible for the communication of networkresources, mobility and call-related management messages between various network entities.

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GSM Protocol Architecture

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Overview of Interfaces

Um

Radio interface between MS and BTS

each physical channel supports a number

of logical channels

Abis

between BTS and BSC

primary functions: traffic channel transmission,

terrestrial channel management, and radio channel

management

A

between BSC and MSC

primary functions: message transfer

between different BSCs to the MSC

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The data link layer over the radio link is based on a modified LAPD (Link Access Protocol

for the D channel) referred to as LAPDm.

On the Abis interface, the layer 2 protocol is based on the LAPD from ISDN.

The Message Transfer Protocol (MTP) level 2 of the SS7 protocol is used at the A interface.

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Physical Layer : Layer I

Handles all radio specific entities:

-creation of bursts

-multiplexing of bursts into TDMA frame

-synchronization with BTS

-detection of idle channels

Modulation TechniquesGaussian Minimum Shift Keying (GMSK) Channel Coding

Speech Coding

Interleaving

Power control methodologyto minimize the co-channel interference

Time synchronization approaches

At physical layer, the air interface uses RF radio transmission

A-bis interface between BTS and BSC uses 64 Kb/s on wire cable, optical fiber, ormicrowave links based on the best availability and ease of installation

All other interfaces use the SS7 Message Transfer Part Level 1(MTP 1)

with channel rates of 64 Kb/s and medium can be coaxial cable or fiber optic cable

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Data Link Layer : Layer II

Signaling and control data are conveyed through Layer II and Layer III messages in GSM

Purpose of Layer II is to check the flow of packets for Layer III

DLL checks the address and sequence for Layer III

Also manages Acks for transmission of the packets.

Data link layer uses ISDN Link Access Protocol D (LAP-D) and version called LAP-Dm onradio link to MS.

LAP-Dm uses GSM RF interface framing and synchronization scheme.

All other inter faces use the SS7 MTP level2 protocol for the data link layer.

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Layer III Message

Radio Resource Management (RR),

Mobility Management (MM) and

Connection Management (CM).

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Radio Resource Management (RR)

Handles all procedures necessary to establish, maintain and release dedicated radio

connections.

-Channel allocation

-Handover

-Power Control

-Frequency Hopping

Location management involves the procedures and signaling for location updating,

so that the mobiles current location is stored at the HLR, allowing incoming calls

to be properly routed.

Security involves the authentication of the mobile, to prevent unauthorized access

to the network, as well as the encryption of all radio link traffic.

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Mobility Management (MM)

- Assumes a reliable RR connection

- Responsible for- location management and

- Security

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Connection Management (CM)

The CM functional layer is divided into three sub layers.

- Call Control (CC)

- Supplementary Services

- Short Message Service

Call Control (CC) sub layer

- manages call routing, establishment, maintenance, andrelease and is closely related to ISDN call control.

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Connection Management (CM)

Supplementary Servicessub layer

- manages the implementation of the various supplementary

services (Call Forwarding/waiting/hold ) and also allows users

to access and modify their service subscription.

Short Message Servicesub layer

- handles the routing and delivery of short messages both from

and to the mobile subscriber.