I've done some search and from what I understand, both in E1 and in T1 facilities,
some bits can be used to convey signaling information.
The bit positions differ, but the common base is
the capability to use 4 bits (namely the ABCD bits)
to represent 16 different situations.
Sometimes only 2 situations using the AB bits
or even 1 using the A bit might be actually needed.
As the above document says,
"International connections include both T1 and E1 trunk interfaces.
Channel associated signaling (CAS) bits, ABCD, differ on T1 and E1 interfaces.
These CAS bits carry signaling information that describe events, for example
off-hook, on-hook, idle, and so forth."
So, you might need to define a translation
of the signals received from and/or sent to the TDM.
When you apply the mapping to the signaling bits received from the TDM,
the bits are translated and then transmitted out the AAL2 trunk.
When you apply the mapping to the signaling bits received from the AAL2 trunk,
the bits are translated and then transmitted out the TDM interface.
I couldn't find what values to use for the parameters.
I inferred that this would depend on the actual network equipment being used,
but there are probably some standard procedures.
Perhaps if you tried to ask on a forum with voice experts,
you could get a more clear answer.
M.
