signalling passes through unchanged. Sub-multiplexer then maps them into a single 2 Mbps channel and sends them to the base station. The traffic at the base station determines the number of the TRXs there. Traffic channels remain at 16 kbps on the Abis interface along with the addition of some signalling channels required for the transceivers and the base station signalling. Beyond the base station (i.e. on the air interface), the 16 kbps channels are converted into the 13 kbps traffic and 3 kbps inband signalling channels. As seen before, each TRX needs two time slots of 64 kbps and one signalling channel of 16 kbps for each TRX. One signalling channel is required for each base station, known as the base station control or BCF signalling. One TRX has a handling capacity of 12 transceivers (this figure may vary from equipment to equipment). Basically, TS allocation planning involves the allocation of the traffic channels (and time slots) for traffic and signalling apart from the pilot bits (for synchronisation) and loop control bits. There are a number of ways in which the transmission planner can allocate the TRANSMISSION NETWORK PLANNING AND OPTIMISATION time-slot and signalling channels. The allocation of the time slots should be done in such a way that there is a possibility for an upgrade from, for example, a 1 + 1 + 1 configuration to 3 + 3 + 3. While doing such an upgrade, there should be minimal changes made in the allocation table and the branching tables. Also, there should always be space for the management bits for the loop protection, synchronisation, etc. Time slots can be grouped either in linear fashion or in block fashion. Linear allocation means that the next free time slot is allocated for the new TRX. The main advantage of this technique is that it is simple to plan. But as the network starts varying, and a few changes are being made, it becomes difficult to keep a track, which is not very suitable for big and complicated networks. Block allocation of the time slots in the whole PCM is divided