1-MS_1 is engaged in on call with MS_2
2-MS_3 is calling MS_1
3-MS_1 hold the call with MS_2 and try to answer the call from MS_3(sending CONNECT message to MSC)
4-MSC send the second Assignment Request to BSC serving MS_1
5-BSC serving MS_1 reply Assignment Failure with cause value #96(Protocol Error between BSS and MSC) and call from MS_3 to MS_1 is dropped
We are concerned about step-4, when MS-1 is already on-Call with MS-2 and Holding a TCH, why MSC is sending 2nd assignment request. We did Trace under Huawei MSC and found there is NO 2nd assignment request. Ericson claims that 2nd assignment request is according to 3GPP and MSC can decide for a new TCH, I went through 3GPP Doc and honestly i couldnt understand it fully.