5.1 RLC/MAC Block Structure

The RLC/MAC block is the basic transport unit on the air interface that is used between the mobile and the network. It is used to carry data and RLC/MAC signaling.

In the previous chapter, the structure of the 52-multiframe was presented, and the concept of radio blocks was introduced. A radio block is defined as an information block transmitted over four consecutive bursts on four TDMA frames on a given PDCH.

One RLC data block is mapped onto one radio block, which is always transmitted on a packet data subchannel (PDTCH). One RLC/MAC control block is transmitted into one radio block on a signaling subchannel (PACCH, PCCCH, PBCCH).

The RLC/MAC control block is used to transmit RLC/MAC control messages, whereas the RLC data block contains data. A MAC header is added at the beginning of each type of radio block. A block check sequence (BCS) for error control detection is added at the end of the radio block.

The block formats are presented in the following section, with a brief definition of the different fields. The use of these fields for the GPRS layers will be studied in detail later in the chapter.