Hi Ater being the dynamic path-i.e circuts get allocated only on need basis.So better ,if its to be calculated on the basis of carried traffic(Erlang) rather than number of circuits because if 90% TCH are configured as TCHD(dual rate-HR/FR),then total Ater requirement comes very high.In india still Abis is on TDM based PCMs,ofcourse Gb path genertally configured as Gb over IP.
U can send ur email id so that i can send an excel sheet with calculation(formulaes also there)