associated control channels and traffic channels. While a call is in progress, there is a need to carry signalling at the same time as user data. For that, an SACCH is allocated to each TCH. SACCH (Slow Associated Control Channel) is a bidirectional channel that transports two messages per second, mainly used for nonurgent procedures. Other more urgent procedures make use of a TCH, called an FACCH. FACCH (Fast Associated Control Channel) Radio Network Planning and Optimisation Radio Resource (RR) Management Data Link Layer Physical Layer Figure 2.59 Layer 1 interfaces in the GSM steals 20 ms of user data blocks to transmit signalling. The stealing flag, a kind of binary information transmitted on the TCH, allows the receiver to distinguish user data from signalling. Outside a call, there is still a need for signalling (for location update or short messages, for example). Since there are only a few exchanged messages for this context, there is no need to waste a whole TCH resource; the SDCCH is used instead. The SDCCH uses a low rate equivalent to one-eight of a TCH. All these channels introduced here constitute the transport media on the air interface (physical layer in the GSM). In the GSM, information flow mainly consists of traffic and signalling. Layer 1 is responsible for: constituting and multiplexing bursts into TDMA frames and transmitting frames over the available physical layer; searching for a BCCH using the MS; evaluating channel quality and signal level; error detection and correction; synchronisation with frame transmission; Encoding of the data stream. For more details on the physical layer, the reader can refer to the GSM recommendations. Linking Layer Data link protocols depend on the interface. For an MS-BTS connection, a new protocol inspired from LAPD (Link Access Procedure on the D channel) but with some individual adaptation is used (LAPDm, where m stands for mobile). For the BTS-BSC interface, the LAPD protocol, which is oriented to the ISDN standards, is used. The BSC-MSC connection uses t