What is GSM Protocol Stack?
GSM protocol stack was designed for allowing communications between two different layers. Lower layers will assure services of the upper-layer protocols. Each layer contribute proper information to establish proper call flow in network.
GSM protocol stack mainly structured in 3 different layers.
- Physical layer
- Data link layer
- Network layer
It is denotes as Layer 1, which is a radio interface. It provides the functionality required to transfer the bit streams over the physical channels on the radio medium. The layer is responsible for below services.
■ Channel mapping (logical to physical)
■ Channel coding and ciphering
■ Digital modulation
■ Frequency hopping
■ Timing advance and power control
Data link layer:
Signalling Layer 2 is based on the LAPDm protocol, It is a modified version of the Link access protocol for the D channel (LAP-D) protocol used in ISDN, called Link access protocol on the Dm channel (LAP-Dm). The main task of LAPDm is to provide a reliable signalling link between the network and the mobile station. It is used across Um interface
LAP-Dm has the following functions:
- Connection less 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.
This layer takes care of signalling procedures between an MS and the network. It consists of three sublayers with distinct signalling procedures.
■ Radio resource management (RR)
■ Mobility management (MM)
■ Connection management (CM)
Radio resource management:
Radio resource management (RR) is responsible for procedures required to establish, maintain and release the dedicated radio connections between MS and MSC via BTS/BSC during a call. The RR sublayer functions include:
■ Channel assignment and release
■ Modification of channel modes, e.g., voice and data
■ Handover between cells
■ Frequency redefinition to enable frequency hopping
■ MS measurement reports
■ Power control and timing advance
■ Radio channel access
The mobility management (MM) sublayer handles functions and procedures related to mobility of the mobile user. This includes procedures for:
■ Authentication of user
■ Location registration and periodic updating of user
■ TMSI reallocation
■ IMSI detach/attach
This layer is responsible for call control function which ncludes procedures to establish, release, and access services and facilities. The CM consists of three sublayers, namely, call control (CC), supplementary services (SS), and short message services (SMS).
The supplementary service sublayer provides the procedures to support non-call-related supplementary services such as call forwarding and call waiting.