with half rate:
on air interface: 2 calls share 16kb/s
on abis interface, 2 calls share 16kb/s
on ater interface, 1 call on 16kb/s
on a interface, 1 call on 64kb/s
therefore, the design of a and ater interfaces is based on the number of calls, whichever the radio codec used.
Regards,
Pix