GSM - Architecture, Protocols and Services, Third Edition

Chapter 4. Air interface – physical layer

The GSM physical layer, which resides on the first of the seven layers of the Open Systems Interconnection (OSI) Reference Model (Tanenbaum, 1996), contains very complex functions. The physical channels are defined here by a TDMA scheme. On top of the physical channels, a series of logical channels are defined, which are transmitted in the time slots of the physical channels. Logical channels perform a multiplicity of functions, such as payload transport, signaling, broadcast of general system information, synchronization and channel assignment.

The structure of this chapter is as follows: In section 4.1, we describe the logical channels. This serves as a foundation for understanding the signaling procedures at the air interface. The realization of the physical channels, including GSM modulation, multiple access, duplexing and frequency hopping follows in section 4.2. Next, section 4.3 covers synchronization. The mapping of logical onto physical channels follows in section 4.4, where the higher-level multiplexing of logical channels into multiframes is also covered. section 4.5 contains a discussion of the most important control mechanisms for the air interface (channel measurement, power control, disconnection and cell selection). The conclusion of the chapter is a power-up scenario with the sequence of events occurring, from when a MS is turned on to when it is in a synchronized state ready to transmit (section 4.9).

Logical channels

On Layer ...