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Contents

GSM MOBILE ORIGINATING CALL

 

The below Description is a mobile-originated call that terminates outside the PLMN.

Request Access
The MS sends a Channel Request (CHAN_REQ) message on the RACH.
The BSS responds with a radio resource assignment (IMM_ASS_CMD) on the AGCH.
The MS sends a Service Request (CM_SERV_REQ) message to the BSS on the SDCCH.
Authentication
Before the network will provide any services to the MS, the network will require the MS to authenticate itself. The BSS sends an Authentication Request (AUTH_REQ) message to the MS. The RAND serves as the "challenge" for authentication.

The MS calculates the proper SRES based on the RAND that was given and sends the SRES to the BSS in an Authentication Response (AUTH_RESP) message.

The BSS verifies the SRES. If the SRES is correct then the MS is authenticated and allowed access to the network. The BSS will send a Service Accept (CM_SERV_ACC) message letting the MS know that the service request was received and processed.

Once authenticated, the BSS orders the MS to switch to cipher mode with the CIPH_MOD_CMD message.

Initial Call Setup
The MS will immediately switch to cipher mode and send a Cipher Mode Complete (CIPH_MOD_COM) message.
The MS then sends a Call Setup (SETUP) message to the BSS. The message includes the address information (MSISDN) of the called party.
The BSS assigns a TCH to the MS by sending an Assignment Command (ASS_CMD) message. This message includes which Transceiver (TRX) and which Time Slot (TS) to use.
The BSS does not actually assign a TCH to the MS until the MSC sends a Call Proceeding (CALL_PROC) message to the BSS indicating that the IAM has been sent.
The MS immediately switches to the assigned TCH. The MS sends an Assignment Complete (ASS_COM) message back to the BTS on the FACCH.
Remember that a FACCH is not a separate channel; it is simply a stolen time slot from the TCH that is used for signaling data instead of voice traffic.
 
Call Setup
The MSC sends an Initial Address Message (IAM) to the GMSC. The IAM contains the MSISDN of the called party as the MS dialed it.
The MSC will also send a Call Proceeding (CALL_PROC) message down to the BSS and this is when the BSS would assign a TCH to the MS, as described in step 10 above.
 Based on the dialed number, the GMSC decides where to route the IAM within the PSTN.
The PSTN will continue to route the IAM until it reaches the correct Switching Center and the call routing is complete. The PSTN will then establish the call circuit and send an Address Complete Message (ACM) back to the GMSC.
The GMSC then forwards the ACM back to the responsible MSC indicating that the call circuit has been established
 
Call Establishment
Once the MSC receives the ACM, it sends an ALERT message to the MS indicating that the call is going through. The BSS sends the ALERT message on the FACCH. Once the MS receives the ALERT, it will generate the ringing sound in the earpiece. The BSS sends an alerting message the subscriber will hear the line ringing.
Once the called party answers the phone, the PSTN will send an Answer message to the MSC. The MSC forwards this to the MS in a Connection (CON) message.
Once the MS receives the CON message, it switches over to voice and begins the call. All voice traffic occurs on the assigned TCH.
 
Call Termination
When either the caller or the called party hangs up, the call will be disconnected. Either party can initiate the disconnect. In this example, the MS initiates the disconnect. The MS sends a Disconnect (DISC) message to the BTS on the FACCH.
The BSS forwards the DISC to the MSC. Once the MSC receives the DISC message, it sends a Release (REL) message through the GMSC to the PSTN as well as down through the BSS to the MS.
The MS responds by sending a Release Complete (REL_COM) message to the BSS on the FACCH. The BSS forwards the REL_COM message up to the MSC. Once the MSC receives the REL_COM message the call is considered ended from the call control perspective.
Although the call has ended, the BSS still has a TCH allocated to the MS. The MSC sends a Channel Release (CHAN_REL) message to the BSS. The BSS forwards the CHAN_REL message to the MS.
The MS responds with a DISC (LAPDm) message and returns to an idle mode. The BSS reallocates the channel for other call or releases the TRX. 

External Resource

 

GSM Originating Call Flow

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. Dial the called person's number Send Button Begin RR Connection Establishment The user keys in the phone number for the landline subscriber and .. .. presses the Send button 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 TCH, SAPI = 0 Fixed Network PSTN PSTN EventStudio System Designer 4.0 13-Sep-08 21:38 (Page 2) SCCP CONNECTION REQUEST + MM CM SERVICE REQUEST SS7 Check subscriber authentication 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. 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 will 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 point on ciphering is enabled in both directions. BSS replies back to the MSC, indicating that ciphering has been successfully enabled. 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 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) CC CALL PROCEEDING Connecting... 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. 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 BSS responds back to the MSC. The MSC routes the call and sends the call towards the called subscriber Mode Modify allocate Voice circuit towards BSS BSSMAP ASSIGNMENT REQUEST Voice circuit RR CHANNEL MODE MODIFY RR CHANNEL MODE MODIFY ACKNOWLEDGE BSSMAP ASSIGNMENT COMPLETE ISUP INITIAL ADDRESS MESSAGE SS7, Dialed Digits CC ALERTING Alerting Tone 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 ISUP ANSWER SS7 The called subscriber answers the call. The MSC informs the mobile that the call has been answered. Acknowledge the recipt of CC CONNECT. Display that the call has been connected. CC CONNECT CC CONNECT ACKNOWLEDGE 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. 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 side. The MSC disconnects the voice path and also releases the voice circuit between the BSS and the MSC. Call Release End Button CC DISCONNECT ISUP RELEASE SS7 Disconnect Voice Path 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 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. 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. ISUP RELEASE COMPLETE SS7 CC RELEASE COMPLETE RR Connection Release BSSMAP CLEAR COMMAND RR CHANNEL RELEASE BSSMAP CLEAR COMPLETE RR DISC RR UA free TCH Call Released Indication