GSM Protocols

GSM Protocol is divided into mainly three Layers:

Layer-1 Protocol:-          MTP
Layer-2 Protocol:-          LAPDm
-          LAPD
Layer-3 Protocol:-          RR
-          MM
-          CM
-          BSSMAP
-          DTAP
-          MAP

 Layer-1 Protocol:

        MTP-Message Transfer Part:

–        The Message Transfer Part (MTP) is part of the Signaling System 7(SS7) used for communication in Public Switched Telephone Networks.

–        MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners.

–        The MTP provides a mechanism giving reliable transfer of signaling messages

–        The Media Transfer Protocol (MTP) is an extension to the Picture Transfer Protocol (PTP) communications protocol that allows media files to be transferred atomically to and from portable devices.

 Layer-2 protocols:

LAPDm-Link Access Protocol D Channel Mobile

LAPDm:

–        LAPDm stands for Link Access Procedure on D channel (modified). This is a modified version of LAPD and is optimized for the GSM Air interface.

–        It is said to be a lightweight LAPD protocol as it does not handle error correction/detection.

–        It handles:

–        Segmentation and reassembly of data and  

–        Acknowledges/unacknowledged data transfer

–         Re-sequencing of data frames and flow control!

The LAPD protocol is used for A and A-bis interface.

LAPDm is a data link layer protocol used in GSM cellular networks. LAPDm forms Layer 2 of the Um interface between the Base Transceiver Station and Mobile station which is to say that it is used in the radio link between the cellular network and the subscriber handset

     Functions:

– Organization of Layer 3 information into frames

–        Data flow Control

–        Acknowledged/Unacknowledged data transmission

–        Segmentation

–        Data re-assembly

– Peer-to-peer transmission of signaling data in defined frame formats

– Recognition of frame formats

– Establishment, maintenance, and termination of one or more (parallel) data links on signaling channels

Layer-3 Protocols:

• A number of mechanisms needed to establish, maintain and terminate a mobile communication session
• Layer III implements the protocols needed to support these mechanisms
• A signaling protocol, the registration process, is composed of a sequence of communication events or messages

RR: Radio Resource Management

MM-Mobility Management

CM-Connection Management

BSSMAP-Base Station Subsystem Mobile Application Part

DTAP-Direct Transfer Application Part

MAP-Mobile Application Part

MM:

-          Responsible for

-           location management and

-           Security

·         Location management involves the procedures and signaling for location updating, so that the mobile’s 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.

·         The protocols in the MM layer involve the SIM, MSC, VLR, and the HLR, as well as the AuC (which is closely tied with the HLR).

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, and release, and is closely related to ISDN call control.

Supplementary Services sub 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 Service sub layer

 - handles the routing and delivery of short messages, both from and to the mobile subscriber.

     BSSMAP:

–        BSSMAP Support all of the procedure between the MSC & BSS that require interpretation and processing of information related to single call and Resource management

–        BSSAP includes all messages exchanged between the BSC and the MSC that the BSC actually processes—examples include PAGING, HND_CMD, and the RESET message. More generally, BSSAP comprises all messages that are exchanged as RR messages between MSC and BSC, and messages that are used for call-control tasks between the BSC and the MSC.

–        The BSS Management Application sub-Part (BSSMAP) supports other procedures between the

MSC and the BSS related to the MS (resource management, handover control), or to a cell within the BSS, or to the whole BSS.

   DTAP:

–        The Direct Transfer part is used to transfer call control and Mobility management message between the MSC & MS.The DTAP information in these message is not interpreted by the BSS.

MAP:

•          It pprovides basic communication between HLR and other MSC

•          MAP functions:

–        Updating of location information in VLRs

–        Storing routing information in HLRs

–        Updating and supplementing user profiles in HLRs

–        Handoff of connections between MSCs

Source: From Google