E1 is divided in 32 TS. Each TS = 64 kbps.
TS 0 is reserved for synchro or adressing (??? to be verified ???).
Then each TRX occupies 2 TS for its traffic, and need few kilobits for its signalling (the RSL)..
There are many possibilities regarding the RSL : mostly it can be carried on one quarter, one half, or on full TS.
Sometimes it can even be carried within the 2 TS reserved for the TRX traffic (that’s a special configuration, with restrictions on TRX configuration)
Most common configurations : 4 RSL within one TS, or 2 RSL within one TS (depending on how much signalling & SMS the cell is carrying)
FYI, the OML (link with OMCR, for supervision) is multiplexed with the RSLs, because it doesn’t require much bandwidth.
About Abis over IP, it is spreading. I believe some vendors already have this solution available in their portfolio. For alcatel, it will be available end of next year (release B11).