GSM Protocol Stack: Layer 3 Overview

This page delves into Layer 3 of the GSM protocol architecture.

It describes the responsibilities of Layer 3 in the GSM protocol stack, covering Radio Resource Management (RRM), Mobility Management (MM), Connection Management (CM), Signalling Connection Control Part (SCCP), Base Station System Management Application Part (BSSMAP), and BTS Management (BTSM).

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GSM Protocol Layer 3

• Layer 3 involves several protocols, each with specific functions:

• RRM (Radio Resource Management): Manages radio channel and handover functionalities. It’s responsible for assigning, maintaining, and releasing radio frequency carriers/channels.

• MM (Mobility Management): Handles location updates, registration, security, and authentication functionalities for the mobile station within the GSM network.

• CM (Connection Management): Manages call setup, call maintenance, and call termination functions for end devices.

• BTSM (BTS Management): Takes care of BTS administration and management functions under BSC (Base Station Controller) control.

• MAP (Mobile Application Part): Handles signaling between various network entities. MAP operates above the SCCP and MTP (Message Transfer Part) modules.

A Layer 3 message typically consists of three main fields: Type ID, Message Type, and Data field. The Type ID contains a 4-bit Protocol Discriminator (PD). This PD distinguishes between Call Control (CC), Mobility Management (MM), Radio Resource (RR), Short Message Service (SMS), and Supplementary Services (SS) management by using different values within the GSM system.

Protocol Discriminator values are unique and defined for the GSM Layer-3 modules:

• Call control (CC) messages: 0011 (0x3)
• Mobility Management (MM) messages: 0101 (0x5)
• Radio Resource management (RR) messages: 0110 (0x6)

The Message Type is an 8-bit field that identifies the specific type of message being sent, such as a channel request, paging request, or response.

GSM Protocol References

• GSM Layer 1 Protocols, 3GPP Technical Specification document TS 44.004 (latest version).
• GSM Layer 2 Protocols, 3GPP Technical Specification documents TS 44.005 & 44.006.
• GSM Layer 3 Protocols, 3GPP Technical Specification documents TS 24.007 & 44.018.