The first layer is the physical layer, which is marked as L1 and is the lowest layer. This layer provides the radio link needed in transmission of bit stream. It defines the radio access capability of the GSM system and provides the most fundamental radio channel (logical channel) for  the information transmission of higher-layer , including the traffic channel and control channel.

Um Interface

The second layer, marked as L2,  is the data link layers and it is the middle layer. It applies the LAPDm protocol. This layer includes various types of data transmission structures. It controls the data transmission so as to ensure the reliable dedicated data links which are set up between the mobile station and base station. The LAPDm protocol is based on the D channel link access protocol (LAPD) in ISDN. For LAPDm, the radio transmission and control characteristics are suitable to the signal transmission at the Um interface.

The third layer is the network application layer, which is marked as L3 and is the top layer. It includes various types of messages and programs for control and management of the services. That is to say, in this layer, specific messages of the mobile station and the system control processes are packed into different protocols and mapped to logical channels.

L3 includes three sub-layers: the Radio Resources management (RR), Mobility Management (MM) and Communication Management (CM). These are the major contents of the messages transmitted via the Um interface. The CM sub-layer includes three major parts: CC (call control service), SS (supplementary service) and SMS (short message service).