GSM communication protocols

I'm in the midst of an assignment and we have to report on the protocols for GSM (phase 2+ or G2.5) used by mobile phones. Specifically, we have to draw the transfer of packets between the phone and base station when sending an SMS. I'm guessing that there is a protocol standard for this but as yet I haven't managed to find it.

Does anyone have any experience/knowledge in this area and might be able to guide me as to where to look?

Any help MUCH appreciated

I assume you haven't been given this assignment "cold turkey", without suitable in-class learning of related technical matters.. right?

The air interface for GSM is called "Um", and is shared with GPRS, which is what GSM 2.5 is known as. SMS is transmitted one of two ways to a mobile station (MS), depending upon whether the MS is on a voice call or not at the time.

For an idle MS, the SMS info is carried over the SDCCH; for one on a call it is sent over the SACCH logical channel.

I found this link which may help (or not). It takes a rapid tour through the layer-2 frame structuring, which sits on top of Um.

I wonder how detailed or not they want the answer to be..