i would appreciate a “hello”, a “please” and a “thank you”..
The SDCCH channel is a bit useless in GSM, when it comes to setting up a call. It is actually usefull to handle other tasks such as location update, SMS, etc… it allows to multiplex 8 channels on one timeslot, for simple operations, so there is a gain of capacity to put all small tasks on the same timeslots.
In GPRS, the only type of operation needed is the allocation of a TBF. And a GPRS timeslot (PDCH) can already be multiplexed among different TBF, if needed.