CH.2 TCH.3 TCH.4 TCH.5 TCH.6 TCH.7 TCH.8 TCH.1 TCH.2 TCH.3 TCH.4 TCH.5 TCH.6 TCH.7 TCH.8 TRXSIGI OMUSIGI TRXSIG2 OMUSIG2 TRXSIG3 OMUSIG3 TRXSIG4 OMUSIG4 TRXSIG5 OMUSIG5 TRXSIG6 OMUSIG6 TRXSIG7 OMUSIG7 TRXSIG8 OMUSIG8 TRXSIG9 OMUSIG9 TRXSIG10 OMUSIGIO TRXSIG11 OMUSIGII TRXSIG12 OMUSIG12 Figure 3.5 Example of a PCM This PCM also contains the O&M signalling for the base station. Depending upon the control. topology, this PCM signal might also contain the bits for synchronisation and loop 3.3.2 PCM Requirements on the Abis and Ater Interface Chapter 2 defined the blocking for the air interface. Blocking parameters need to be defined also for the Abis and Ater interfaces. Typically, the value of the air-interface blocking is 1-2% and that of the Ater interface is 0.1-0.5%. The Abis interface should have no blocking apart from no blocking between the TCSM and MSC. Radio planning engineers use the air interface for dimensioning the radio network, while transmission planning engineers use the Abis and Ater interfaces. The transmission capacity is determined by factors such as traffic generated at the air interface, the grooming ability of the equipments, topologies, the and the locations of the BSCs and TCSMs. Spare capacity plays an important part in taking future requirements of the network into account. Radio planning engineers determine the number of BTS and transceivers in the network. Transmission planners start with information about the radio planning dimensioning, i.e. the TRANSMISSION NETWORK PLANNING AND OPTIMISATION number of transceivers per base station. As shown in Figure 3.5, one TRX needs 2 X 64 kbps time slots, while for the signalling 16, 32 or 64 kbps time slots may be required. A lower signalling rate means that more subscribers can use the network at one time. The object dimensioning for the Ater interface is to find the number of traffic channels required. Once that is done, then based on the number of channels that can be multiplexed on the Ater, the capacity of the Ater interface can be calculate