How is call flow in gsm Network?
The user keys in the phone number forthe landline subscriber and ..Send Button.. presses the Send buttonBegin 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.RR CHANNEL REQUESTRR connection establishment istriggered by sending the ChannelRACHRequest 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 areply.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.allocateThe BSS allocates a Traffic ChannelTCH(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 network.RR IMMEDIATE ASSIGNMENTThe BSS transmits the radio resourceassignment to the Mobile via the AGCHAGCH, Radio_Resource = (TCH, Frequency,Timeslot), Time Correction, Frequency Correctionchannel. 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'smotion.Apply the time and frequency correctionsAdjust the frequency and timing basedon the advice from the BSS. This step isrequired so that transmissions from themobile reach the base station at theprecise time and with the correctfrequency.Tune to the frequency and timeslotThe mobile detunes from the AGCH andtunes to the specified radio channel.RR SABM + MM CM SERVICE REQUESTThis is the first message that is sentafter tuning to the channel. The MobileTCH, SAPI = 0initiates a LAPm connection with theBSC by sending a Set AsynchronousBalanced Mode (SABM) message. Theservice request message meant for theMSC is also sent in this message.GSM Call Flow (GSM Originating Call)CellMobile NetworkFixed NetworMobile StationBase StationsNSSPSTNUserMobileBSSMSC VLRPSTN13-Sep-08 21:38 (Page 2)RR UAThe BSS replies with UnnumberedAcknowledge (UA) to complete theTCH, SAPI = 0LAPm setup handshakeSCCP CONNECTION REQUEST + MM CM SERVICE REQUESTThe BSS receives the CM ServiceRequest message from the mobile andSS7forms a "BSSMAP COMPLETE LAYER 3INFORMATION". The BSS then piggybacks the message on the SCCPconnection request messageCheck subscriber authenticationMSC checks if the subscriber has beenauthenticated. In this case, thesubscriber has already beenauthenticated, so the authenticationprocedure is skipped.Enable CipheringBSSMAP CIPHER MODE COMMANDSince the subscriber has beensuccessfully authenticated, the MSCinitiates ciphering of the data being senton the channel. The channel is cipheredso as so protect the call fromeavesdropping.Expect 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 error.RR CIPHERING MODE COMMANDThe BSS sends the CIPHERING MODECOMMAND to the mobile. The mobilemode = CLEARwill be able to receive this message asthe transmission from the BSS is still inclear.Enable ciphering for received and transmittedAs a second step, the Mobile receivesdatathe 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 COMPLETECiphering has already been enabled, sothis message is transmitted withmode = CIPHEREDciphering. The BSS will receive thismessage as it is already expectingciphered data in the receive direction.Enable ciphering of data transmitted to theThe third and final step in the cipheringmobilehandshake. The BSS enables theciphering in transmit direction. From thispoint on ciphering is enabled in bothdirections.BSSMAP CIPHER MODE COMPLETEBSS replies back to the MSC, indicatingthat ciphering has been successfullyenabled.RR Connection Establishment CompletedAt 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 SetupCC SETUPThe Mobile sends the setup message toestablish a voice call. The messageDialed Digitscontains the dialed digits and otherinformation needed for callGSM Call Flow (GSM Originating Call)CellMobile NetworkFixed NetworkMobile StationBase StationsNSSPSTNUserMobileBSSMSC VLRPSTN13-Sep-08 21:38 (Page 3)establishment.CC CALL PROCEEDINGThe mobile is informed that the callsetup is in progress.Connecting...At this point, the mobile phone displaysa message on the screen to indicate thatcall setup is being attempted.Mode ModifyallocateThe MSC allocates a voice circuit on oneVoice circuit towards BSSthe digital trunks between the MSC andthe BSS.BSSMAP ASSIGNMENT REQUESTMSC informs the BSS about theallocated voice circuit. The call is alsoVoice circuitswitched from signaling to voice.RR CHANNEL MODE MODIFYThe BSS notifies the Mobile about thechangeover to voice mode.RR CHANNEL MODE MODIFY ACKNOWLEDGEMobile acknowledges.BSSMAP ASSIGNMENT COMPLETEThe BSS responds back to the MSC.ISUP INITIAL ADDRESS MESSAGEThe MSC routes the call and sends thecall towards the called subscriberSS7, Dialed DigitsISUP ADDRESS COMPLETE MESSAGE The PSTN indicates to the MSC that ithas received all the digits and the calledSS7subscriber is being rung.CC ALERTINGThe MSC informs the mobile that thecalled subscriber is being alerted via aringAlerting ToneISUP ANSWERThe called subscriber answers the call.SS7CC CONNECTThe MSC informs the mobile that the callhas been answered.CC CONNECT ACKNOWLEDGEAcknowledge the recipt of CC CONNECT.ConnectedDisplay that the call has been connected.ConversationSpeechThe call has entered the conversationphase. The speech path has been setupbetween the mobile subscriber and theland-line subscriber.Call ReleaseEnd ButtonThe mobile subscriber hits End to clearthe call.CC DISCONNECTThe mobile sends the disconnectmessage to the MSC.ISUP RELEASEThe MSC initiates release on the PSTNside.SS7Disconnect Voice PathThe MSC disconnects the voice path andalso releases the voice circuit betweenthe BSS and the MSC.freeVoice circuit towards BSSCC RELEASEThe MSC informs the Mobile that it hasinitiated call releaseGSM Call Flow (GSM Originating Call)CellMobile NetworkFixed NetworkMobile StationBase StationsNSSPSTNUserMobileBSSMSC VLRPSTN13-Sep-08 21:38 (Page 4)ISUP RELEASEDThe MSC informs the PSTN that the callrelease has been completed.SS7ISUP RELEASE COMPLETEThe PSTN informs that call release hasbeen completed at its end.SS7CC RELEASE COMPLETEMobile indicates that the call has beenreleased.RR Connection ReleaseBSSMAP CLEAR COMMANDCall release has been completed, nowthe RR connection is released by theMSC.RR CHANNEL RELEASEThe BSS initiates RR release with themobile.BSSMAP CLEAR COMPLETEThe BSS informs the the MSC that theRR connection has been released.RR DISCThe mobile sends a disconnect messageto release the LAPm connection.RR UAThe BSS replies with an UnnumberedAcknowledge message.freeThe BSS releases the TCH channel.TCHCall Released IndicationMobile goes back to the default displayto indicate that call has been completelyreleased.ECE Charanjit.
