Hi sham,
all messages on the A-bis interface use LAPD protocol enable safe transmission of information .
LAPD links serve for RSL &OML & L2ML.
Just go to internet and type structure of LAPD FRAME.
2/for one PCM time slot (TRH-TRXC signaling we can have the following structure :
RSL=SAPI=0
OML=SAPI=62
L2ML=SAPI=63
OML=SAPI=62
Just keep in mind that logical configuration is for RBS200 , just you have to find for RBS200.
then SAPI will adress diffrent entities layer mangement at bsc level which are controlled by L2ML who take the managemnt of data link sharing a physical connection whithin one phisycal entity ( TRX in TRU ) at BTS level .
GOODLUCK