Hi Lydia,
Ok Let us get in to Serious Business.
MO Call :-
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Normally when there is a MO Call ( Doesnt Matter where the call goes ) the Call setup procedure will be carried out in BSS.The request for a Callsetp from MS to Switching System-MSC is carried through BSS.In this case BSS is responsible for Handling the Radio Interface.BSS=BTS+BSC.BSC Controls number of BTS and its resources.
When the Call reaches MSC Which is responsible for Hanldling the call switching as per the B+A Number+Location does some analysis and finds that the number belongs Same HPLMN then it routes the call to GMSC ( or Handles itself if it is a GMSC ).
MT Call :-
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When GSMC Functionality receives this Call it sends a Routing ENQUIRY to the responsible HLR.It finds that through Analysis.Then HLR Replies after checking its database.In reply HLR Gives a MSRN and which it got from the Presently serving MSC ( Lets say same MSC ).Then GMSC Routes thecall with that MSRN to the present MSC.Then the MSC After receiving that it knows which MS is receiving this call.So it pages..that MS.If MS Responds then it informs that there is a incoming call through Paging.So MS-B requests for MT-Call Setup.Which is processed by BSS Part and connects the call to MSC WHERE msc cONNECTS THIS lEG WITH THE OTHER MSRN CALL LEG.
If both Serving and Cal Originating MSC Are same Node then mostly Signalling happens internally.Depends upon Vendor and setup.Then while terminating the MS-B Can be anywhere…In the same SECTOR/CELL/BTS IN Same BSC/BTS in Other BSC.
Rgds
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