Timeslot assignment in group calls

Hi all, I'm modifying the code to provide more information on group calls, and a more "friendly" display of a call from setup to completion. One doubt I have and can't seem to find in the Air Interface specs is how/where a particular timeslot is assigned to a particular call. For example, a call may be assigned a Usage Marker of 16, but so could a second all to a different group that takes place simultaneously. With which PDU is the TN assigned to a particular call? Best, Mike