As per my understanding:
MaxRetrans relates to RACH retry for different Access Cause.
As our OMCR server only pegs/trigger (increment)the counters on the basis of Abis messages or Ater messages. So, in real we cant calculate our RACH Success Ratio. That is one of the reason “RACH Success Rate” is calculated using the response from the BTS with message “Channel Act Ack” for “Channel Act” send by the BSC. “Immediate Assignment Reject” directly relates to the Hadware fault as this transparent message is send after recieving the “Channel Required” from BTS and when in the BSC allocation table required resource is not present.