Typical Call Flow in GSM

The typical signaling sequences shows the following features-:

• Location updating

• Mobile call origination

• Mobile call termination

• Authentication and ciphering

• Inter-MSC call handoff

Location Updating

1. The MS sends a location update request to VLR (new) via the BSS and MSC.

2. The VLR sends the location update message to HLR serving the MS which include the address of the VLR(new) and IMSI of the MS. This updating of HLR is not required if the new LA is served by the same VLR.

3. The service and security related data for the MS is downloaded to the new VLR.

4. The MS is sent an ackowledgement of successful location update.

5. The HLR requests the old VLR to delete data related to relocated MS.

Mobile Call Origination

1. The MS sends the dialed number indicating service requested to the MSC (via BSS)
2. The MSC checks from the VLR if the MS is allowed to requested service. If so, MSC asks the BSS to allocate neccessary resources for the call.
3. If the call is allowed , the MSC routs the call to GMSC (gateway mobile switching centre).
4. The GMSC routs the call to local exchange of called user.
5. The LE (local exchange) alerts (applies ringing ) the called terminal.
6. Answer back (ring back tone) from the called terminal to LE.
7. Answer back signal is routed back to the MS through the serving MSC which also completes the speech path to MS.

Mobile call termination

1. The PSTN (public switched telephone network) user dials the MSISDN of the called user in GSM.

2. The LE routes the call to GMSC of the called user .

3. The GMSC uses the dialed MSISDN to determine the serving HLR for the GSM user and interrogates it to obtain the required routing number.

4. The HLR requests the current serving VLR for the called MSfor a MSRN (MS roaming number) so that the call can be routed to correct MSC.

5. The VLR passes the MSRN to the HLR.

6. The HLR passes the MSRN to the GMSC.

7. Using the MSRN the GMSC routes the call to the serving MSC.

8. The MSC interrogates the VLR for the current location area identity (LAI) for the MS.

9. The VLR provides LAI for the MS.

10. The MSC pages the MS via appropriate BSS. The MS responds to the page and sets up the neccessary signaling links.

11. When the BSS has established the neccessary radio links , the MSC is informed and the call is delivered to the MS.

12. When the MS answers the call , the connection is completed to the calling PSTN user.

Authentication and Encryption

1. At terminal location update , VLR sends IMSI to HLR

2. HLR returns security triplets (RAND,SRES,Kc) to the VLR.

3. For authentication and ciphering the VLR sends RAND to the MS.

4. Using stored A3 algorithm and secret key Ki stored in the SIM, and RAND provided by the VLR, the MS calculates the SRES and return it to VLR. Using A8 algorithm and Ki , the MS also calculates the cipher key Kc.

5. If the SRES returned by the MS matches with the stored SRES in the VLR , the VLR sends the cipher key Kc to the BTS which uses Kc for ciphering the radio path(downlink).

The MS uses it's Kc to cipher the radio path (uplink) using encryption algorithm A5.

Inter-MSC Handoff

1. BSC A informs MSC A that MS needs handover from BTS A to BTS B.

2. MSC A informs MSC B that handover from BTS A to BTS B is underway.

3. MSC A commands BSC A/BTS A to proceed with handover to BTS B.

4. BTS A commands MS to change to a specified channel on BTS B.

5. MS informs BTS B that it is on specified channel.

6. BTS B informs BSC A/MSC A that handover is complete.

7. MSC B informs MSC A that handover to BTS B is complete.