Layers of GSM

In a GSM network, there are three functional areas:
Ø The Mobile Station such as a mobile phone which is carried by a subscriber,
Ø the Base Station Subsystem which controls the radio link with the Mobile Station,
Ø and the Network Subsystem which mainly performs the switching of calls between the mobile users, and between mobile and fixed network users.
The signalling protocol in GSM is structured into three general layers as shown in the following figure.
v Layer 1 (the Physical Layer) uses the data channel structures over the air interface.
v Layer 2 (the Data Link Layer) is a modified version of the LAPD protocol used in ISDN, called LAPDm. Across the A interface(bw BSS and MSC), the Message Transfer Part layer 2 (MTP2) of SS7 is used.
v Layer 3 is the GSM Signalling Protocol, which is divided into 3 sub-layers:
ü Radio Resources Management to control the setup, maintenance, and termination of radio and fixed channels, including handovers;
ü Mobility Management to manage the location updating and registration procedures, as well as security and authentication; and
ü Connection Management to handles general call control, similar to CCITT Recommendation Q.931, and manages Supplementary Services and the Short Message Service.

Signalling between the different entities in the fixed part of the network, such as between the HLR and VLR, uses the Mobile Application Part (MAP), one of protocols in the SS7/C7 stack.