All radio timeslots are mapped to equivalent throughput on the Abis.
For example :
1 TCH = 1 Abis nibble
1 PDCH = n Abis nibble (PDCH throughput can range from 8kb/s to 59kb/s so need more than 1 nibble to carry all that !)
As a rule of thumb:
4 TRX require 1 ABIS TS to signalling
1 TRX require 2 ABIS TS for data (TCH & PDCH)
And for PDCH, you can define additional ABIS TS to allow bigger throughputs to users & allow more users at the same time.
1 BTS 3 cells at 4 TRX each = 3 + 24 = 27 ABIS TS for sig + TCH.
You can add 4 ABIS TS for EGPRS, to fill up the whole 31 ABIS TS available.