1. The document describes different scenarios for GSM call routing including: when the called mobile subscriber is powered off, has call forwarding enabled to a PSTN number, has all incoming calls barred, or is unknown at the HLR. 2. It also provides an overview of the signaling process for a mobile originated call from requesting a channel to call setup, as well as the process for a mobile terminated call including paging the destination mobile and connecting the call upon answer. 3. Key network elements involved in call routing include the MS, BSS, MSC, VLR, HLR, GMSC and PSTN, and different messages are exchanged like address, routing info, and assignment messages to setup the

1. GSM Call Routing

2. 1. The called MS is powered off
GMSCMS BSS MSC VLR HLR GMSC
(
(
MSISDN
+91 9885 xxxxxx
Initial and Final
1 Address Message
2 Send Routing Info
3 Routing Info Ack
Subscriber Absent The Calling
Mobile hears the
announcement of
mobile is powered
off
AP Vodafone AP Airtel
MSISDN
+91 9885 xxxxxx

3. CFN
4 IFAM
MS BSS MSC VLR HLR GMSC PSTN
2. The called MS activates the CFU to a PSTN No.
Initial and Final
1 Address Message
2 Send Routing Info
3 Routing Info Ack
MSISDN
+91 9885 xxxxxx
MSISDN
+91 9885 xxxxxx
+91 40 66235307

4. 3. The called MS has been barred all incoming call.
MS BSS MSC VLR HLR GMSC
Initial and Final
1 Address Message
2 Send Routing Info
3 Routing Info Ack
GMSC
MSISDN
+91 9885 xxxxxx
MSISDN
+91 9885 xxxxxx
Call Barred

5. 4. The Called MS are not known at HLR
MS BSS MSC VLR HLR GMSC
Initial and Final
1 Address Message
2 Send Routing Info
3 Routing Info Ack
GMSC
MSISDN
+91 9885 xxxxxx
MSISDN
+91 9885 xxxxxx
Unknown Subscriber

6. Call Routing
Scenarios

7. 2 REQ. FOR SERVICE
1 CHANNEL REQUEST
DCCH ASSIGN
SIGNALING LINK
ESTABLISHED
6 COMPLETE CALL
CALL PROCEEDING
3 AUTHENTICATION
SET Cipher MODE
4 SET-UP
5 EQUIP. ID REQ.
MS BSS MSC VLR HLR PSTN
Mobile Originated Sequence
Signaling Link request
Signaling Link Granted
Call setup is in Process
Setup Voice Call
Dailed digits …+91 40 66235307
Channel request

8. <
MS BSS MSC VLR HLR PSTN
7 ASSIG. COMMAND
ASSIG. COMPLETE
Switch from Signaling
Mode to Voice
8.Initial and Final Address
Message (IFAM)
Alerting
MS hears ring
tone from land
phone
Address Complete(ACM)
9 Answer (ANS)Connect
Ring tone
stops
10 Connect Acknowldge
BILLING STARTSSpeech Started
Called Subscriber Answered
Switch Informs
Call Answered
Acknowledge Call
Answered received
Hi RK
Ring Alert
Passed to Mobile
Switch routes the call to PSTN
PSTN indicates to Switch
Called Subscriber Informed Via Ring
+91 40 66235307
Mobile Originated Sequence

9. <
MSISDN
+91 9885 xxxxxx
IMSI
404 13 xxxxxxxxxx
MSRN
LAI & TMSI)TMSI)TMSI)
PCH>
MSRN MSRN
MSRN
Initial and Final
1 Address Message
2 Send Routing Info
3 Routing Info Ack
Initial and Final
Address Message
4 Send Info For I/C
Call Setup
5 Page
Paging Request
MS BSS MSC VLR HLR GMSC GMSC
MSISDN
+91 9885 xxxxxx
091 9885 xxxxxx (roaming number)
091 9885 xxxxxx (roaming number)
LAI – 404 13 location Area Code
Mobile Terminated Sequence
AP Vodafone AP Airtel

10. MS BSS MSC VLR HLR GMSC GMSC
Billing
starts
7 Connect
Connect ACK ANS
Ringing stops
at Other
Mobile
Subscriber
picks up
Hello...
6 Assignment
Command
Assignment
Complete
Alert
Ringtone Alert Acknowledge
Ring Tone at
the Other
Mobile
Address Complete
Speech Started
Mobile Subscriber Answered
Mobile Terminated Sequence
AP Vodafone AP Airtel
Call Setup Request
Call Setup Confirmed