04.18 CHANNEL REQUEST for GPRS / EGPRS request
and 04.60 PACKET RESOURCE REQUEST message from an EGPRS capable MS (for establishment of a concurrent TBF)
The Channel Request is sent over the RSL, that’s correct.
Regarding the PDCH, it is highly depending on the vendor you’re using. The PCU can be located in the BSC, in the SGSN, or even be a standalone equipment (= MFS, in alcatel).
The PDCH allocation / deallocation can be performed by the PCU, but it is not “standard”. So there are many solutions to activate or deactivate a PDCH. It is safe to assume the PCU is always in charge of (de)allocating the PDCH’s, but it might need the confirmation of the BSC.
You are correct when you say the PDCH should be allocated before being used by the TBF.
The deallocation can be done with a timer, after the release of the TBF.