d. Example 1: Capacity requirements on the Ater interface Assume, that radio planners have decided that there are five sites of 2 + 2 configuration under a single BSC. Air interface blocking is 2% and Ater 5 sites of 2 + 2 configuration = 10 cells, each having 15 TCH Air interface blocking = 2% Using Erlang B tables, 15 TCH support = 9.01 Erl of traffic. Traffic offered to the BSC = 10 X 9.01 = 90.1 Erl. If Ater blocking probability is 0.1%, then the number of traffic channels supported = 117 (approx.) If the number of traffic channels that can be multiplexed on the Ater=120 Then Ater interface capacity would be = 117/120=0.975~ 3.3.3 Equipment Location The base station and TCSM locations need to be decided during the nominal planning phase itself. As shown in Figure 3.3, the transmission capacity required between the TCSM and MSC is more compared with the connection towards the BSC. Thus, if the TCSMs were placed physically near the MSC, it would save transmission costs. When it comes to BSC locations, they can be located far from the MSC or co-located with the MSC. Both the remote and co-located BSC locations have their own advantages and disadvantages. A remote BSC location is chosen to cater for the needs of the huge number of BTSs that are located very far from the MSC. In this way, transmission costs are saved, though there may be heavy usage of the Abis links. This is usually preferred for rural regions. A co-located BSC is generally chosen for metropolitan areas where there is a heavy concentration of traffic. 3.3.4 Network Topology Selection of the most appropriate network topology can lead to huge savings in the trans- mission network. There are four main topologies, as shown in Figure 3.6. Point-to-point topology: As the name suggests, this is a point-to-point connection be- tween the base stations and the BSC. This is quite prevalent during the early phase of network evolution as the implementation is easy and quick. However, the capacity may or may not be fully utilised. Also, equipment protection me