GSM protocol stack
GSM Layer 3
At layer-3 there are many protocols involved as mentioned below with their respective functions.
RRM- Stands for Radio Resource Management, takes care of radio channel and handover functionalities. Assign, maintain and release radio frequency carriers/channels.
MM- Stands for Mobility management, takes care of location update,registration,security and authentication functionalities of mobile station with the GSM network.
CM- Stands for Connection Management, takes care of call setup, call maintenance and call termination funtions of end devices.
BTSM- stands for BTS Management, takes care of BTS admin and management functions under BSC control.
MAP(Mobile Application Part),takes care of signaling between various network entities. MAP works above SCCP and MTP modules.
A layer 3 message consists of three fields,Type ID,Message Type and Data field.The Type ID consists of a 4 bit Protocol Discriminator or PD.
This PD differentiates between CC,MM,RR,SMS(Short Message Service) and SS (Supplementary Services) management by having different values in the GSM system.
Protocol Discriminator values are unique and specified below for GSM Layer-3 modules.
Call control(CC) messages - 0011 (0x3)
Mobility Management(MM) messages- 0101 (0x5)
Radio Resource management messages- 0110 (0x6)
Message Type is 8 bit that identifies the type of message sent for example channel request,paging request or response etc.
For GSM Layer 1 Protocols, Refer 3GPP Technical Specifications document TS 44.004 version.
For GSM Layer 2 Protocols, Refer 3GPP Technical Specifications document TS 44.005 & 44.006
For GSM Layer 3 Protocols, Refer 3GPP Technical Specifications document TS 24.007 & 44.018