it only depends on the equipment use for transmission. It it supports E1 and IP, then you just have to switch the mode of transmission :
BTS -> Abis Equipment : Ethernet link
Abis Equipment 1 –> Abis Equipment 2 : Radio, Optical, Satellite, etc.
In terms of link capacity : the voice is now “packetized”. when an user is not talking, zero packet are sent from BTS to TC.
On top of that, the whole capacity of the IP link is shared among all the TRXs. With E1, each TRX has its own TS, all the time, even when there are no calls on the TRX.
So altogether, the gain is about 50% : 2Mb/s in E1 can handle only S444. 2Mb/s in IP could handle S888 (to be calculated).