5.3 GSM Protocols Overview

Within a GSM network, different protocols are needed to enable the flow of data and
signalling between different GSM subsystems. The following diagram shows the interfaces
that link the different GSM subsystems
and the protocols used to communicate on each interface.

GSM Protocol Layers

• GSM protocols are basically divided into three layers:
• Layer 1: Physical layer
• Enables physical transmission (TDMA, FDMA, etc.)
• Assessment of channel quality
• Definition of physical links (e.g radio, PCM30 ISDN etc)
• Error detection (based on line coding)
• Layer 2: Data link layer
• Multiplexing of one or more layer 2 connections on control/signalling
channels
• Error detection (based on HDLC)
• Flow control
• Transmission quality assurance
• Routing
• Layer 3: Network Layer
• Connection management
• Management of location data
• Subscriber identification
• Management of Services

As GSM is a transport network, it is primarily only the lower 3 layers of the OSI Model that
are defined in the GSM Recommendations.
As GSM is predominantly a transport network it is less concerned with the end-to-end user
application layer (layers 4-7). Therefore this section of the course notes looks specifically at
the protocols used within GSM at layers 1-3.

5.3.1 LAYER 1 SERVICES

The Physical Layer (Layer 1) contains all the functions necessary for the transmission of bit
streams over the physical medium. It provides a transport service for the GSM logical
channels. Services offered at Layer 1 include:
· Access Capabilities. Layer 1 carries out the cell selection functions for MSs in idle
mode, in cooperation with the Layer 3 Radio Resource (RR) functions.
· Error Detection . Forward and backward error correction is implemented at layer 1
(see section on speech coding for details). Errored frames are not passed to Level 2
for processing.
· Encryption. Data encryption is also implemented at Layer 1 (see section on GSM
security for details).

5.3.2 LAYER 2 SERVICES

Here, the LAPDm protocol is used (similar to ISDN LAPD). LAPDm has the following
functions:
· Connectionless transfer on point-to-point and point-to-multipoint signalling channels,
· Setup and take-down of layer 2 connections on point-to-point signalling channels,
· Connection-oriented transfer with retention of the transmission sequence, error
detection and error correction.

5.3.3 LAYER 3 SERVICES

Layer 3 contains the following sublayers which control signalling channel functions (BCH,
CCCH and DCCH):
· Radio resource management (RR). The role of the RR management layer is to
establish and release stable connection between mobile stations (MS) and an MSC for
the duration of a call, and to maintain it despite user movements. The following
functions are performed by the MSC:
· Cell selection,
· Handover,
· Allocation and take-down of point-to-point channels,
· Monitoring and forwarding of radio connections,
· Introduction of encryption,
· Change in transmission mode.
· Mobility management (MM). Mobility Management handles the control functions
required for mobility including:
· Authentication
· Assignment of TMSI
· Management of subscriber location.
· Connection management (CM) is used to set up, maintain and clear call connections.
It comprises three subgroups:
· Call control (CC) - manages call connections,
· Supplementary service support (SS) - handles special services,
· Short message service support (SMS) - transfers brief texts.
Neither the BTS nor the BSC interpret CM and MM messages. They
are simply exchanged between the MSC and the MS using the Direct Transfer
Application Part (DTAP) protocol on the A interface (see below).
RR messages are mapped to or from the Base Station System Application Part (BSSAP) for
exchange with the MS.