hi rf opt,
here is your mistake : the TRX doesn’t need 128kbps.
It needs as much kbps as the TCH + PDCH require !
So yes, if you have a BTS with 12 TRXs and if the EGPRS traffic is high, then 1 Abis is not enough. You’ll need 2.
This is a dimension study you need to do. But don’t think “per TRX”, indeed it should be done “per BTS”, because the EGPRS traffic of one BTS will share the usage of the whole Abis. The Abis is not “split” per TRX.
Cheers
pix