“Will the BTS do something different if the CHANNEL REQUEST indicates a packet data related cause?”
nothing different : the (e)gprs channel request is sent as a channel required in the RSL. The content of the channel required will indicate to the BSC that the channel required is for (e)GPRS.
I can’t find anything about “data indication (pdu)” in this case.
“Who is then responsible for generating this IMMEDIATE ASSIGNMENT, the BSC or the PCU?…”
The PCU generates a Channel Assignment, which is sent to the BSC. The BSC conveys it as a “Immediate Assignment Command” to the BTS. The BTS sends a “Immediate Assignment” on the PCH or on the AGCH, depending on the DRX mode of the mobile.
“how can the PCU have access to the RSL link?”
If the PCU is external to the BSC, then it is connected to the BSC via a dedicated interface (called ATER PS). The BSC will route all GPRS-related message from BTS (RSL or not) to the PCU, quite transparently.
As i said before, keep in mind the PCU implementation is totally vendor dependent. The solution I’m describing here might not apply for Ericsson, Nokia, Huawei, etc.
Thanks for your questions, they are interesting ! Not very useful on the field, unless you decide to build your own PCU 😀 but interesting to understand how the system works.