e MSC and transcoders; minimal blocking of about 0.1 % is permitted on the Aten interface. Based on factors such as capacity, coverage and quality, radio planners need to determine calculations such as the number of TRXs required per cell. Although the number of TRXs determines the 2 Mbps links that are required from the BTS to the BSC, topology and protection also play an important role in capacity planning. Interface Planning 3.6.1 Abis Planning One contains 32 timeslots of 64 kbps capacity. Each timeslot (TS) has four traffic channels of 16 kbps capacity. TSO is used for management. The remaining 31 timeslots can be freely to carry traffic and signalling channels (allocation can vary, as described later in the chapter). Generally TS31 is used for carrying management bits related to synchronisation, loop control, etc. This means that 30 TSs are used for user traffic and associated signalling. Each TRX needs two TSs for traffic and 0.25 TSs for signalling (if the signalling used is kbps). Using this principle, it can safely be assumed that for 12 TRXs, 24 TSs are required, and depending upon the topology and the number of BTSs connected on 1E1 (1 PCM), the remaining six TSs can be used for BCF and TRX signalling. Assume that a three sectored BTS-A site of 4 + 4 + capacity is connected to a BSC directly. This means that the 1E1 link would be enough to carry the traffic. The traffic can be carried by leased lines or a 2E1 PDH radio. However, if the same BTS-A site is followed by two more base stations (BTS-B and BTS- C) of similar capacity, then the traffic running on the BTS-BSC link would be three times the original, i.e. 3E1, thus increasing the radio requirements to that of a 4E1 PDH radio. However, if the BTS-B and BTS-C sites are of capacities 2 + 2 + 2 each, then the number of E1s required would reduce to 2E1. This is possible by 'grooming'. This means that the traffic channels are organised SO as to fill the 2 Mbit frame more efficiently. Once Abis is dimensioned, the link between the BSC and TCSM (transco