you should give a try to wikipedia, there are realy interesting stuff in there.
– E1 is way to use a physical link:
2Mb/s, 32 timeslots, ts0 reserved for framing/sync.
– Ater is the interface between BSC and Transcoder, based on E1. You can see the Ater as the name of a link, and E1 is the protocol to “use” this link.
Basically, 1 Ater = 1 E1.
1 Ater = 32ts, but ts0 is already used –> 1 Ater = 31 useful ts.
1 call on the radio interface will occupy 1/4 of an Ater timeslot.
But you have also at least 1 signalling timeslot every 4 Ater interfaces.
Roughly, you can safely assume that 1 Ater = 120 calls.
1 BSC with 8 Ater means the BSC is connected to the transcoder with 8 E1 links. 8 * 120 = 960 simultaneous calls max.