GSM Call Flow (GSM Originating Call) Cell Mobile Station User Mobile Mobile Network Base Stations NSS BSS MSC VLR Fixed Network PSTN PSTN EventStudio System Designer 4. 0 13-Sep-08 21:38 (Page 1) LEG: GSM Mobile Originated Call This sequence diagram was generated with EventStudio System Designer 4. 0 (http://www. EventHelix. com/EventStudio). Copyright © 2008 EventHelix. com Inc. All Rights Reserved. The EventStudio source files for this document can be downloaded from http://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. The user keys in the phone number for the landline subscriber and .. .. presses the Send button Dial the called person’s number Send Button Begin RR Connection Establishment Call related information needs to be transported from the mobile phone to the Mobile Switching Center (MSC). This requires the establishment of a Radio Resource (RR) connection to MSC. The first phase of the call setup just sets up this RR connection. RR CHANNEL REQUEST RACH RR connection establishment is triggered by sending the Channel
Request message. This message requests the Base Station System (BSS) for allocation for radio resources for the RR connection setup. The mobile now waits for an assignment on the Access Grant Channel (AGCH). At this point the mobile is listening to the AGCH for a reply. 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 the channel at the same time, their messages will be lost in a collision.
The mobiles will detect the collision via a timeout and retransmit the message after a random back off. allocate TCH RR IMMEDIATE ASSIGNMENT AGCH, Radio_Resource = (TCH, Frequency, Timeslot), Time Correction, Frequency Correction Apply the time and frequency corrections Tune to the frequency and timeslot RR SABM + MM CM SERVICE REQUEST TCH, SAPI = 0 The BSS allocates a Traffic Channel (TCH) to the mobile. The TCH allocation assigns a specifies a frequency and a timeslot on that frequency. After the mobile receives this message, the mobile shall only use the specified resources for communication with the mobile network.
The BSS transmits the radio resource assignment to the Mobile via the AGCH channel. The message also contains the time and frequency corrections. The time corrections allow the mobile to time it’s transmissions so that they reach the BSS only in the specified slot. The frequency corrections correct for the Doppler shift caused by the mobile’s motion. Adjust the frequency and timing based on the advice from the BSS. This step is required so that transmissions from the mobile reach the base station at the precise time and with the correct frequency. The mobile detunes from the AGCH and tunes to the specified radio channel.
This is the first message that is sent after tuning to the channel. The Mobile initiates a LAPm connection with the BSC by sending a Set Asynchronous Balanced Mode (SABM) message. The service request message meant for the MSC is also sent in this message. GSM Call Flow (GSM Originating Call) Cell Mobile Network Mobile Station Base Stations NSS User Mobile BSS MSC VLR RR UA Fixed Network PSTN PSTN TCH, SAPI = 0 SCCP CONNECTION REQUEST + MM CM SERVICE REQUEST SS7 Check subscriber authentication EventStudio System Designer 4. 0 13-Sep-08 21:38 (Page 2) The BSS replies with Unnumbered Acknowledge (UA) to complete the
LAPm setup handshake The BSS receives the CM Service Request message from the mobile and forms a “BSSMAP COMPLETE LAYER 3 INFORMATION”. The BSS then piggy backs the message on the SCCP connection request message. LEG: Skip Authentication Procedure MSC checks if the subscriber has been authenticated. In this case, the subscriber has already been authenticated, so the authentication procedure is skipped. Enable Ciphering BSSMAP CIPHER MODE COMMAND Expect ciphered data from the mobile RR CIPHERING MODE COMMAND mode = CLEAR Enable ciphering for received and transmitted data RR CIPHERING MODE COMPLETE mode = CIPHERED
Enable ciphering of data transmitted to the mobile BSSMAP CIPHER MODE COMPLETE Since the subscriber has been successfully authenticated, the MSC initiates ciphering of the data being sent on the channel. The channel is ciphered so as so protect the call from eavesdropping. Ciphering on the radio link is enabled in three steps. As a first step, the BSS starts expecting ciphered data from the mobile but continues to send data in clear. Since the mobile has not been informed about the ciphering, all data received from the mobile will be in error. The BSS sends the CIPHERING MODE COMMAND to the mobile. The mobile ill be able to receive this message as the transmission from the BSS is still in clear. As a second step, the Mobile receives the message and enables ciphering in transmit and receive directions. This action will result in all BSS data being received in error. (The BSS is still transmitting data in clear. ) Ciphering has already been enabled, so this message is transmitted with ciphering. The BSS will receive this message as it is already expecting ciphered data in the receive direction. The third and final step in the ciphering handshake. The BSS enables the ciphering in transmit direction. From this oint on ciphering is enabled in both directions. BSS replies back to the MSC, indicating that ciphering has been successfully enabled. RR Connection Establishment Completed 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 for transporting the signaling messages between the Mobile and the MSC. Call Setup CC SETUP Dialed Digits The Mobile sends the setup message to establish a voice call. The message contains the dialed digits and other information needed for call GSM Call Flow (GSM Originating Call) Cell Mobile Station
User Mobile Mobile Network Base Stations NSS BSS MSC VLR Fixed Network PSTN PSTN EventStudio System Designer 4. 0 13-Sep-08 21:38 (Page 3) establishment. The mobile is informed that the call setup is in progress. At this point, the mobile phone displays a message on the screen to indicate that call setup is being attempted. CC CALL PROCEEDING Connecting… Mode Modify allocate Voice circuit towards BSS BSSMAP ASSIGNMENT REQUEST Voice circuit RR CHANNEL MODE MODIFY RR CHANNEL MODE MODIFY ACKNOWLEDGE BSSMAP ASSIGNMENT COMPLETE The BSS responds back to the MSC. ISUP INITIAL ADDRESS MESSAGE SS7, Dialed Digits
CC ALERTING The MSC allocates a voice circuit on one the digital trunks between the MSC and the BSS. MSC informs the BSS about the allocated voice circuit. The call is also switched from signaling to voice. The BSS notifies the Mobile about the changeover to voice mode. Mobile acknowledges. The MSC routes the call and sends the call towards the called subscriber ISUP ADDRESS COMPLETE MESSAGE The PSTN indicates to the MSC that it has received all the digits and the called SS7 subscriber is being rung. The MSC informs the mobile that the called subscriber is being alerted via a ring Alerting Tone ISUP ANSWER
The called subscriber answers the call. SS7 CC CONNECT The MSC informs the mobile that the call has been answered. Acknowledge the recipt of CC CONNECT. CC CONNECT ACKNOWLEDGE Connected Display that the call has been connected. Conversation Speech The call has entered the conversation phase. The speech path has been setup between the mobile subscriber and the land-line subscriber. Call Release End Button CC DISCONNECT ISUP RELEASE SS7 Disconnect Voice Path LEG: Mobile initiates call release The mobile subscriber hits End to clear the call. The mobile sends the disconnect message to the MSC. The MSC initiates release on the PSTN ide. The MSC disconnects the voice path and also releases the voice circuit between the BSS and the MSC. free Voice circuit towards BSS CC RELEASE The MSC informs the Mobile that it has initiated call release GSM Call Flow (GSM Originating Call) Cell Mobile Station User Mobile Mobile Network Fixed Network Base Stations NSS PSTN BSS MSC VLR PSTN ISUP RELEASED SS7 ISUP RELEASE COMPLETE SS7 CC RELEASE COMPLETE EventStudio System Designer 4. 0 13-Sep-08 21:38 (Page 4) The MSC informs the PSTN that the call release has been completed. The PSTN informs that call release has been completed at its end.
Mobile indicates that the call has been released. RR Connection Release BSSMAP CLEAR COMMAND RR CHANNEL RELEASE BSSMAP CLEAR COMPLETE RR DISC RR UA free TCH Call Released Indication Call release has been completed, now the RR connection is released by the MSC. The BSS initiates RR release with the mobile. The BSS informs the the MSC that the RR connection has been released. The mobile sends a disconnect message to release the LAPm connection. The BSS replies with an Unnumbered Acknowledge message. The BSS releases the TCH channel. Mobile goes back to the default display to indicate that call has been completely released.