Call flow
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The document describes the call flow procedures for mobile originating and mobile terminating calls in a GSM network. For a mobile originating call, the MS requests a dedicated channel and indicates it wants to set up a call. The MSC receives the call setup message and checks for call barring before establishing a link with the BSC. The BSC assigns a traffic channel for the call. For a mobile terminating call, the call is routed to the GMSC serving the called subscriber's home network. The GMSC queries the HLR for routing information. The HLR provides a roaming number to route the call to the subscriber's current MSC. The MSC pages the subscriber through the BSCs in their
Call flow
- 1. CALL FLOW
- 2. CALL FROM MS Provided that the MS is listening to the system information in the cell and that it is registered in the MSC/VLR handling this cell, the MS can attempt to make a call. The procedures are shown in Figure1. • The MS requests a dedicated channel using the RACH. the MS gets information about the dedicated resource on the AGCH. • The MS indicates that it wants to set up a call. The identity of the MS, IMSI, is analyzed and the MS is marked as busy in the VLR. • Authentication is performed as described for location updating. ciphering may be initiated.
- 3. CALL FROM MS • The MSC receives a setup message from the MS. This information includes the kind of service the MS wants and the number (called the B number) dialed by the mobile subscriber. MSC checks that the MS does not have services like barring of outgoing calls activated. Barring can be activated either by the subscriber or by the operator. If the MS is not barred, the setup of the call proceeds. • Between the MSC and the BSC a link is established and a PCM TS is seized. The MSC sends a request to the BSC to assign a TCH. The BSC checks if there is an idle TCH , assigns it to the call and tells the BTS to activate the channel. The BTS sends an acknowledgment when the activation is complete and then the BSC orders the MS to transfer to the TCH.
- 4. • The BSC informs the MSC when the assignment is complete. The traffic control subsystem analyses the digits and sets up the connection to the called subscriber. The call is connected through in the group switch. • . An alert message is sent to the MS indicating that a ringing tone has been generated on the other side. The ringing tone generated in the exchange on the B subscriber side is sent to the MS via the group switch in MSC. The ringing tone is sent over the air, not generated in the MS. • When the B subscriber answers, the network sends a connect message to the MS indicating that the call is accepted. The MS returns a connect acknowledgment, which completes the call set-up. CALL FROM MS
- 5. Mobile originating call establishment
- 6. Call establishment (early assignment)
- 7. • A mobile terminated call is more complicated than a mobile originated call. This is due to the fact that the calling party does not know where the MS (called party) is located. The signaling is shown in Figure. • The number dialed by the calling part is called the Mobile Station ISDN number (MSISDN). In cases where a call is made from the fixed network, PSTN, the exchange analyses the number and determines that the call is for a GSM subscriber. The call is routed to the GMSC in the home PLMN of the called MS. • By analyzing the MSISDN, the GMSC finds out which HLR the subscriber is registered in. CALL TO MS
- 8. CALL TO MS • GMSC asks the HLR for information so the call can be routed to the MSC/VLR where the MS is temporarily registered. Using the MSISDN the HLR finds the IMSI and the data record for the subscriber. International Mobile Subscriber Identity (IMSI) is a subscriber number (see Appendix), only used in the signaling network. The address to the serving VLR is received at registration and is stored together with IMSI in the HLR. • The HLR contacts the VLR to get a roaming number. This is an ordinary telephone number to the MSC/VLR. The VLR sends the roaming number to the HLR. The HLR forwards the roaming number to the GMSC. • With help of the roaming number, the GMSC can route the call to the appropriate MSC.
- 9. • The call is routed to the MSC .If the system has the feature ‘Call Drop back’, and this feature is supported by the national signaling scheme, the GMSC may drop the call ‘back’ to the previous node. The decision is based on the received roaming number and the capabilities of the incoming route. Then, the routes marked (2) and (6a) are released. • In some situations, PSTN may be used to reroute the call. In countries where there is a poorly functioning or very expensive PSTN, it might be better to build a separate network with connections between the MSCs and GMSCs. CALL TO MS
- 10. Call to MS from PSTN
- 11. • The MSC knows which location area the MS is located in and sends a paging message to the BSCs handling this location area. In GSM, there are two places where information about which cells belong to the location area can be stored: the MSC, the BSC. • In GSM system it is the BSC that stores this information. The MSC sends the identity of the location area and MS to the BSCs in the LA. the BSC distributes the paging message to the BTSs in the LA. The BTSs page the MS using IMSI or TMSI. • The MS sends a request for an SDCCH when it detects its identity in the paging message. CALL TO MS
- 12. Mobile terminating call establishment
- 13. CALL TO MS • See Figure . The MSC performs authentication and starts ciphering as described earlier. The MSC may send information to the MS about the requested services. These can for example be speech, data, fax • The BSC orders the BTS to activate a TCH and to release the SDCCH. The MS is ordered to tune in to the frequency of the TCH. An Alerting message is sent from the MS indicating that a ringing tone has been generated in the MS. A ringing tone for the calling subscriber is generated in MSC. When the mobile subscriber answers, the MS sends a Connect message. The network completes the through connection path and sends a Connection acknowledgment to the MS. The duplex path for traffic is now open.
- 14. Immediate Assignment on TCH • In the two previous traffic cases, the BSC assigns an SDCCH in the Immediate Assignment message, to be used by the MS for call set-up signaling. After call initiation the network allocates a TCH to the MS. This way of performing call set-up is called “early assignment” in the GSM specifications. • The “Immediate Assignment on TCH” function makes it possible to have more than one resource allocation strategy at call set- up. Using this function, the BSC can allocate a TCH using the Immediate Assignment message. This can be useful for example, when there is temporary congestion on the SDCCH. It allows call set-up signaling to be performed on the TCH. When finished, the mode of the traffic channel is changed from signaling only to the mode necessary for traffic by employing a channel mode change procedure. See Figure .
- 15. Immediate Assignment on TCH • This means that the call set-up time can be slightly decreased, as the signaling messages can be sent on the timeslot when ever they are ready. In the case of using an SDCCH, a message can only be sent when the allocated sub channel occurs .For each traffic case indicated by the MS in the channel request message, the operator can control the allocation strategy via commands in the BSC. A choice of detail level can be made from one BSC down to individual cells. • In the specifications, this procedure is called “very early assignment”.
- 16. Immediate Assignment on TCH