The signaling protocol in GSM is structured into three general layers depending on the interface.

Must Read>> GSM Architecture

Layer 1 is the physical layer that uses the channel structures over the interface. Layer 2 is the data link layer. Across the Um interface the data link layer is a modified version of the Link Access Protocol D (LAPD) used ISDN. It is called as the LAPDm.

Across the A interface, the message transfer part layer 2 of signal system number 7 (SS7) is used. The GSM air interface consists of TDMA/FDMA time slots and frequency bands.

LAPDm is used over the air interface between the base station trans-receiver and the mobile device.

To transmit the information to the desired destination, some additional control information is required apart from the actual data. It is called a signaling message.

The signaling channels are multiplexed on an aggregate of the TDM slots. Layer 3 of the GSM signaling protocol is divided into 3 sub-layers.

  1. Mobility Management (MM).
  2. Radio Resource Management (RRM) and
  3. Connection Management (CM) for calls routing.

For the purpose of mobility, communication of network resources, code format, and call-related management messages between the different network entities, the layer 3 protocol is used.

Between the Mobile Station and the Base Station Subsystem (BSS), Radio Resource Management (RRM) can be implemented. Mobility Management and Connection Management is the communication between the Mobile Station and MSC (Mobile Switching Centre).

Must Read>>Mobile Station

Image shows the Signaling Protocol Structure in GSM.


The A-interface uses an SS7 protocol called Signal Correction Contro Protocol (SCCP) of SS7 that supports communication between the MSC and BSS and the network messages between the individual subscribers and the MSC.

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