“BTS to release the channel by sending the RF_channel rel message”
===> This is a SD ASSIGNMENT FAILURE.
Not a SD DROP.
The SD DROP is not impacted by T3101.
If an indicator is called “SD DROP DUE TO T3101 EXPIRY”, then it is a mistake. It should be called “SD ASSIGN FAIL”.
What is the difference :
After a SD DROP, the call setup is failed. Subscriber must re-attempt by himself.
After a SD ASG FAIL, the MS will automatically retry, by sending additional CHANNEL REQUEST.