e or data. In the GSM system the control channels can be of three basic types: broadcast control channels, common control channels and dedicated control channels. The usage is different as common channels are used in the idle mode and dedicated channels are allocated to the mobile when it is in the active mode. As indicated by their name, the broadcast channels broadcast the information to the mobile phones, whereas the dedicated and common control channels are of a point-to-multipoint type. In the following the logical channels of the GSM system are described in more detail. Traffic Channel (TCH) A traffic channel can be either full, half rate or enhanced full rate. In the case of a full rate channel the bit rate of the codec is 13 kbps with a half rate of 5.6 kbps. The enhanced full rate (EFR) works at 12.2 kbps. The TCH is a dedicated channel for one user. Another speech coding type is the adaptive multirate (AMR) specified by the 3GPP (3G partnership project). The idea of the AMR is to adapt dynamically the source and channel coding according to the network conditions. The better the channel, the less channel coding Capacity Planning is needed. Overall this improves the quality and saves capacity. AMR allows eight bit rates, which are 12.2, 10.2, 7.95, 7.40, 6.70, 5.90, 5.15 and 4.75 kbps. Broadcast Control Channels Broadcast Control Channel (BCCH) The BCCH broadcasts the system information continuously in the downlink direction, e.g. frequencies of the neighbouring cells and paging group information. Cell Broadcast Channel (CBCH) This broadcasting contains user information with no signalling. Frequency Correction Channel (FCCH) This is for the MS to find the FCCH burst, which contains the frequency information. Synchronisation Channel (SCH) This is used for synchronisation of the MS. The broadcast contains information of the BS identity code and TDMA frame number. Common Control Channels Access Grant Channel (AGCH) The AGCH channel is used to allocate the SDCCH for the MS and acknowledge the MS channel reque