GSM Originating Call - EventHelix.com

1 GSM call Flow (GSM Originating call )CellMobile NetworkFixed NetworkMobile StationBase StationsNSSPSTNUserMobileBSSMSC VLRPSTNE ventStudio System Designer 21:38 (Page 1)LEG: GSM Mobile Originated call This sequence diagram was generated with EventStudio System Designer ( ). Copyright Inc. All Rights Reserved. The EventStudio source files for this document can be downloaded 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 Inc. All Rights Reserved. Dial the called person's numberThe user keys in the phone number forthe landline subscriber and ..Send presses the Send buttonBegin RR Connection Establishment call related information needs to be transported from the mobile phone to the Mobile Switching Center (MSC).

2 This requires the establishmentof a Radio Resource (RR) connection to MSC. The first phase of the call setup just sets up this RR connection. RR CHANNEL REQUESTRACHRR connection establishment istriggered by sending the ChannelRequest message. This messagerequests the Base Station System (BSS)for allocation for radio resources for theRR connection setup. The mobile nowwaits for an assignment on the AccessGrant Channel (AGCH). At this point themobile is listening to the AGCH for : 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.

3 The mobiles will detect the collision via a timeout and retransmit themessage after a random back off. allocateTCHThe BSS allocates a Traffic Channel(TCH) to the mobile. The TCH allocationassigns a specifies a frequency and atimeslot on that frequency. After themobile receives this message, themobile shall only use the specifiedresources for communication with themobile IMMEDIATE ASSIGNMENTAGCH, Radio_Resource = (TCH, Frequency,Timeslot), Time Correction, Frequency CorrectionThe BSS transmits the radio resourceassignment to the Mobile via the AGCH channel. The message also contains thetime and frequency corrections. The timecorrections allow the mobile to time it'stransmissions so that they reach theBSS only in the specified slot. Thefrequency corrections correct for theDoppler shift caused by the mobile' the time and frequency correctionsAdjust the frequency and timing basedon the advice from the BSS.

4 This step isrequired so that transmissions from themobile reach the base station at theprecise time and with the to the frequency and timeslotThe mobile detunes from the AGCH andtunes to the specified radio SABM + MM CM SERVICE REQUESTTCH, SAPI = 0 This is the first message that is sentafter tuning to the channel. The Mobileinitiates a LAPm connection with theBSC by sending a Set AsynchronousBalanced Mode (SABM) message. Theservice request message meant for theMSC is also sent in this call Flow (GSM Originating call )CellMobile NetworkFixed NetworkMobile StationBase StationsNSSPSTNUserMobileBSSMSC VLRPSTNE ventStudio System Designer 21:38 (Page 2)RR UATCH, SAPI = 0 The BSS replies with UnnumberedAcknowledge (UA) to complete theLAPm setup handshakeSCCP CONNECTION REQUEST + MM CM SERVICE REQUESTSS7 The BSS receives the CM ServiceRequest message from the mobile andforms a "BSSMAP COMPLETE LAYER 3 INFORMATION".

5 The BSS then piggybacks the message on the SCCP connection request : Skip Authentication Procedure Check subscriber authenticationMSC checks if the subscriber has beenauthenticated. In this case, thesubscriber has already beenauthenticated, so the authenticationprocedure is Ciphering BSSMAP CIPHER MODE COMMANDS ince the subscriber has beensuccessfully authenticated, the MSCinitiates ciphering of the data being senton the channel. The channel is cipheredso as so protect the call ciphered data from the mobileCiphering on the radio link is enabled inthree steps. As a first step, the BSSstarts expecting ciphered data from themobile but continues to send data inclear. Since the mobile has not beeninformed about the ciphering, all datareceived from the mobile will be in CIPHERING MODE COMMAND mode = CLEARThe BSS sends the CIPHERING MODECOMMAND to the mobile.

6 The mobilewill be able to receive this message asthe transmission from the BSS is still ciphering for received and transmitteddataAs a second step, the Mobile receivesthe message and enables ciphering intransmit and receive directions. Thisaction will result in all BSS data beingreceived in error. (The BSS is stilltransmitting data in clear.)RR CIPHERING MODE COMPLETE mode = CIPHEREDC iphering has already been enabled, sothis message is transmitted withciphering. The BSS will receive thismessage as it is already expectingciphered data in the receive ciphering of data transmitted to themobileThe third and final step in the cipheringhandshake. The BSS enables theciphering in transmit direction. From thispoint on ciphering is enabled in CIPHER MODE COMPLETEBSS replies back to the MSC, indicatingthat ciphering has been Connection Establishment Completed At this point a connection has been setup between the Mobile and the MSC.

7 From this point onward, the BSS is just acting as a conduit fortransporting the signaling messages between the Mobile and the MSC. call Setup CC SETUPD ialed DigitsThe Mobile sends the setup message toestablish a voice call . The messagecontains the dialed digits and otherinformation needed for callGSM call Flow (GSM Originating call )CellMobile NetworkFixed NetworkMobile StationBase StationsNSSPSTNUserMobileBSSMSC VLRPSTNE ventStudio System Designer 21:38 (Page 3) call PROCEEDINGThe mobile is informed that the callsetup is in this point, the mobile phone displaysa message on the screen to indicate thatcall setup is being Modify allocateVoice circuit towards BSSThe MSC allocates a voice circuit on onethe digital trunks between the MSC andthe ASSIGNMENT REQUESTV oice circuitMSC informs the BSS about theallocated voice circuit.

8 The call is alsoswitched from signaling to CHANNEL MODE MODIFYThe BSS notifies the Mobile about thechangeover to voice CHANNEL MODE MODIFY ACKNOWLEDGEM obile ASSIGNMENT COMPLETEThe BSS responds back to the INITIAL ADDRESS MESSAGESS7, Dialed DigitsThe MSC routes the call and sends thecall towards the called subscriberISUP ADDRESS COMPLETE MESSAGESS7 The PSTN indicates to the MSC that ithas received all the digits and the calledsubscriber is being ALERTINGThe MSC informs the mobile that thecalled subscriber is being alerted via aringAlerting ToneISUP ANSWERSS7 The called subscriber answers the CONNECTThe MSC informs the mobile that the callhas been CONNECT ACKNOWLEDGEA cknowledge the recipt of CC that the call has been SpeechThe call has entered the conversationphase.

9 The speech path has been setupbetween the mobile subscriber and theland-line Release LEG: Mobile initiates call release End ButtonThe mobile subscriber hits End to clearthe DISCONNECTThe mobile sends the disconnectmessage to the RELEASESS7 The MSC initiates release on the Voice PathThe MSC disconnects the voice path andalso releases the voice circuit betweenthe BSS and the circuit towards BSSCC RELEASEThe MSC informs the Mobile that it hasinitiated call releaseGSM call Flow (GSM Originating call )CellMobile NetworkFixed NetworkMobile StationBase StationsNSSPSTNUserMobileBSSMSC VLRPSTNE ventStudio System Designer 21:38 (Page 4)ISUP RELEASEDSS7 The MSC informs the PSTN that the callrelease has been RELEASE COMPLETESS7 The PSTN informs that call release hasbeen completed at its RELEASE COMPLETEM obile indicates that the call has Connection Release BSSMAP CLEAR COMMANDCall release has been completed, nowthe RR connection is released by CHANNEL RELEASEThe BSS initiates RR release with CLEAR COMPLETEThe BSS informs the the MSC that theRR connection has been DISCThe mobile sends a disconnect messageto release the LAPm UAThe BSS replies with an UnnumberedAcknowledge BSS releases the TCH Released IndicationMobile goes back to the default displayto indicate that call has been completelyreleased.