what hour is the peak of “dummy” SDCCH assignments ?
what is the cause of failure ?
you might experience massive Location Updates due to a site from another operator that becomes unavailable. All subscribers reselect your site for a while, creating a massive SDCCH congestion
1/ some external equipments (army radar, jammers, etc.) generate fake “RACH” Channel Request, that are “failed”, because the BTS sends an immediate assignment command, but there is no answer from this external equipment (that’s normal)
2/ BTS failure : the BTS generates electromagnetic noise that’s decoded as a channel requests…
That might give you some idea of investigation : check the type of sdcch assignment (for LU, for call, for reestab, for SMS ???)
check the cause of failure (radio, hardware, congestion ???)
check other counters (RACH counts, channel activation, channel activation ack, immediate assignment, etc.)