It was a pleasure.
I quickly went thru the GSM Rec 03.09 version 7.0.0 release 1998 on Handover procedures. It says, after Handover from MSC-A to MSC-B, the connection between MSC-A and MSC-B is released. I think anchor and relay MSCs are the names given for understanding the scenerios.
When you say LU will be executed after the end of call, I assume you are meaning after the speech phase is over.
Main components of LU are TMSI and LAI. VLR-B, HLR and SIM card of MS must be updated and VLR-A cleared. While the call is in speech phase and the handover has been completed, I think VLR-B should be updated with the LAI,HLR of MS must have the VLR-B number. I feel this is necessary coz if a call lands on MS while he has moved from MSC-A to MSC-B and is still in speech phase and the LU has not yet been done then the HLR of MS will show the presence of MS in VLR-A and VLR-A will have the old LAI and the response to the calling party would be that the MS is not answering the call(this would also be improper utilization of PCH in VLR-A area). Whereas, actually he is busy in VLR-B area. But if the VLR-B and HLR of MS are updated during speech phase immediately after handover, any caller to this MS will hear the announcement that the subscriber is busy. But don’t know what the GSM recs say.
Another thing is updating the SIM of MS. New TMSI and LAI should be given to MS. When will this be done? I feel this info can be exchanged over FACCH during speech phase but not sure. Any member familiar with this scenerio may please contribute.
And yes, roaming subscription checks of MS for Inter PLMN(even if it exists) shouldn’t be an issue. The handover request would first have been received by MSC-A of HPLMN. Right at this moment before communicating with MSC-B of VPLMN, the MSC-A can find out from HLR of MS(or may be keeping such records already within VLR-A) whether the MS has Inter PLMN roaming subscription(MSC-A by now knows the area/operator where the MS is heading to). If yes then it proceeds to call MSC-B. Now if roaming agreements are in place between MSC-A and MSC-B of different PLMNs, then MSC-B will accept the handover request of MSC-A. Well, I have just tried to apply logic to the above scenerio, not read these in GSM Recs. In case of error, please correct me.