The transmit ABCD signaling bits for the CAS channel.
aBit - the signaling bit in the 6th frame
bBit - the signaling bit in the 12th frame
cBit - the signaling bit in the 18th frame
dBit - the signaling bit in the 24th frame
the cBit and dBit are only valid for E1 or T1
interface is operating in extended superframe (ESF)
mode.