TCH Access is ability of MS to use TCH. It relates to TCH Blocking, HO Blocking, and Call Setup Success Rate.
For instance a SDCCH drop can be caused to interference, low coverage (uplink, downlink) settings of bss parameters, or an Abis failure.
As for SDCCH drop, user side will experience the call is try to connect yet not complete and MS return to normal. As SMS, the SMS won’t be sent out after certain while connected/sending.
For situation having high TCH Access FAilure Rate, when u make call, after “connecting” you will get call block/reject~ “network busy”.
For situation having high SD drop, you get immediate reject after press call and connecting.
Situation can be harder if the site facing both TCH Access Failure and SDCCH drop.