MSC-VLR Call Flow - EventHelix.com

MSC_VLR Interfaces (GSM Originating Call)Cell Mobile Network Fixed NetworkMobile Station Base Stations NSS PSTNUser Mobile BSS MSC VLR PSTNEventStudio System Designer 4.013-Sep-08 21:38 (Page 1)LEG: GSM Mobile Originated CallThis sequence diagram was generated with EventStudio System Designer 4.0 (http://www.EventHelix.com/EventStudio). Copyright © 2008EventHelix.com Inc. All Rights Reserved. The EventStudio source files for this document can be downloaded fromhttp://www.eventhelix.com/call-flow/gsm-call-setup.zip.This scenario describes the call setup for a GSM originating call. A mobile user calling a land line subscriber is covered here.Copyright © 2000-2008 EventHelix.com Inc. All Rights Reserved.Begin RR Connection EstablishmentCall related information needs to be transported from the mobile phone to the Mobile Switching Center (MSC). This requires the establishmentof a Radio Resource (RR) connection to MSC. The first phase of the call setup just sets up this RR connection.Note: The RR CHANNEL REQUEST is sent on a Random Access Channel (RACH). This is a slotted aloha channel that can be used at random,without any coordination between the mobiles. Any mobile can transmit on this channel whenever it wishes. If two mobiles transmit on thechannel at the same time, their messages will be lost in a collision. The mobiles will detect the collision via a timeout and retransmit themessage after a random back off.Enable CipheringRR Connection Establishment CompletedSCCP CONNECTION REQUEST + MM CM SERVICE REQUESTSS7Check subscriber authenticationBSSMAP CIPHER MODE COMMANDBSSMAP CIPHER MODE COMPLETEThe BSS receives the CM Service Requestmessage from the mobile and forms a "BSSMAPCOMPLETE LAYER 3 INFORMATION". The BSSthen piggy backs the message on the SCCPconnection request message.LEG: Skip Authentication ProcedureMSC checks if the subscriber has beenauthenticated. In this case, the subscriber hasalready been authenticated, so the authenticationprocedure is skipped.Since the subscriber has been successfullyauthenticated, the MSC initiates ciphering of thedata being sent on the channel. The channel isciphered so as so protect the call fromeavesdropping.BSS replies back to the MSC, indicating thatciphering has been successfully enabled.At this point a connection has been setup between the Mobile and the MSC. From this point onward, the BSS is just acting as a conduit fortransporting the signaling messages between the Mobile and the MSC.Call SetupMode ModifyCC SETUPDialed DigitsCC CALL PROCEEDINGallocateVoice circuit towards BSSBSSMAP ASSIGNMENT REQUESTVoice circuitBSSMAP ASSIGNMENT COMPLETEThe Mobile sends the setup message to establisha voice call. The message contains the dialeddigits and other information needed for callestablishment.The mobile is informed that the call setup is inprogress.The MSC allocates a voice circuit on one thedigital trunks between the MSC and the BSS.MSC informs the BSS about the allocated voicecircuit. The call is also switched from signaling tovoice.The BSS responds back to the MSC.ISUP INITIAL ADDRESS MESSAGE The MSC routes the call and sends the callSS7, Dialed Digits towards the called subscriberISUP ADDRESS COMPLETE MESSAGEThe PSTN indicates to the MSC that it hasSS7received all the digits and the called subscriber isbeing rung.

MSC_VLR Interfaces (GSM Originating Call)Cell Mobile Network Fixed NetworkMobile Station Base Stations NSS PSTNUser Mobile BSS MSC VLR PSTNCC ALERTINGCC CONNECTCC CONNECT ACKNOWLEDGEISUP ANSWERSS7EventStudio System Designer 4.013-Sep-08 21:38 (Page 2)The MSC informs the mobile that the calledsubscriber is being alerted via a ringThe called subscriber answers the call.The MSC informs the mobile that the call hasbeen answered.Acknowledge the recipt of CC CONNECT.ConversationCall ReleaseCC DISCONNECTDisconnect Voice PathISUP RELEASESS7LEG: Mobile initiates call releaseThe mobile sends the disconnect message to theMSC.The MSC initiates release on the PSTN side.The MSC disconnects the voice path and alsoreleases the voice circuit between the BSS andthe MSC.freeVoice circuit towards BSSCC RELEASECC RELEASE COMPLETEISUP RELEASEDSS7ISUP RELEASE COMPLETESS7The MSC informs the Mobile that it has initiatedcall releaseThe MSC informs the PSTN that the call releasehas been completed.The PSTN informs that call release has beencompleted at its end.Mobile indicates that the call has been released.RR Connection ReleaseBSSMAP CLEAR COMMANDBSSMAP CLEAR COMPLETECall release has been completed, now the RRconnection is released by the MSC.The BSS informs the the MSC that the RRconnection has been released.