Hi Alexander,
This is how we determine the number of E1s on the Abis interface: Assuming an operator decides to add lets say 5000 subscribers to the network on one particular BSC, then we can say that on the Abis path,each subcriber(i.e. if FR is used)will utilise a bandwidth of 16kbps.This gives a total bandwidth of 80Mbps(16kbps x 5000) for the 5000 subscribers on the Abis interface. To find the total number of E1s that will be needed: 80Mbps/2Mbps=40E1. This 40E1 can be decided by the operator to evenly distribute it amongst the cell sites on that particular BSC. The same calculation is carried out for the other interfaces like Ater,and A.