Unservice by PCU, it probably means you are having Packet service failures, due to PCU unavailability (or unservice)..
PRACH is used on the master channel for GPRS.. Are you using this feature ? As far as i know, there is no mobile which is supporting this feature.
Anyway, the RACH and PRACH messages are sent to the PCU transparently, so if the PCU is down, RACH assignments for packets will not work.