the cause of the request (cause establishment) is received by the BTS in the Channel Request from the MS, but is not forwarded to the BSC in the Channel REquired.
I just found out about this thanks to your question.
So on the abis, you cannot know what is the cause of the establishment. However you can see it in the “Establish indication” which is sent a bit later. It is in the encapsulated protocol “DTAP”, “Message Type”.
The GSM phase 2/2+ MS may indicate in the LOCATION UPDATING REQUEST message that it has a
follow-on request pending. If the network reports to the MS in the LOCATION UPDATING ACCEPT (which arrives at the end of the SDCCH “DTAP” phase, after authentication and ciphering)that the MS can proceed its follow-on request then the MS sends a CM SERVICE
REQUEST to the network (message transparent to the BSS).
In the Estab Indication, do you see “follow-on” = 1?
In the Loc Upd Accept, do you see the field related to “follow-on request”?
It is the MSC that decides to accept the follow-on or not.
If MSC accepts it, then the MS will send a CM Service Request after it received the Loc Upd Accept from the network.