The approximate relation is 4:1. It means, if there are 64 Abis links in a BSC then it is recommended to have atleast 16 A links b/w BSC& MSC. This is due to the mux feature in Abis in which 4 subscribers share a single path. But in BSC/TRC with MSC, the A link allows each subscriber with a slot. Hope this clears. Thanks
What’s not clear is: if 4 subscribers share a E1 timeslot on Abis and a full E1 timeslot is required per subscriber on A, then to support all potential subscribers on Abis one would need 4 times as many trunks on A than A bis: 64 on Abis => 256 on A.
There is 4:1 mapping on Abis/Ater interface. In most cases, the BSC/TRC is co-located with the MSC in the same place for less expensive physical link between them. The below link has a document that would be very useful for you.