This document provides an overview of call routing in GSM networks. It discusses key components like the Home Location Register (HLR) and Visitor Location Register (VLR) that store subscriber data. It then describes different call routing scenarios like mobile originated calls, mobile terminated calls, and roaming calls. It explains the signaling process and interactions between network elements like the mobile station, base station, MSC, HLR, and other switches. Finally, it briefly discusses the handover process to transfer calls between base stations when a mobile changes location.

1. GSM Call Routing
RK

2. Agenda
GSM-Introduction
Mobile Station (ME+SIM)
HLR, VLR
Location Update
Call Routing Scenarios
Mobile Originated Sequence
Mobile Terminated Sequence
Mobile-to-Mobile Long distance
Mobile in Roaming (Domestic, International)
Call Handover
Questions

3. Mobile Station(MS)

4. HLR / VLR
 Home Location Registers (HLR)
o permanent database about mobile subscribers in a large
service area(generally one per GSM network operator)
o database contains IMSI, MSISDN, prepaid/postpaid,
roaming restrictions, MSC/VLR, supplementary services.
 Visitor Location Registers (VLR)
o Temporary database which updates whenever new MS
enters its area, by HLR database
o Controls those mobiles roaming in its area
o Reduces number of queries to HLR
o Database contains IMSI, TMSI, MSISDN, MSRN, Location
Area, authentication key

5. GSM Architecture
EIR AuC

6. Brief Introdution to Location Update
Three types of Location Update :
IMSI Attach/
Detach
Normal Location Update
Periodic Location Update

7. Questions
Please give the sequence:
1.The called MS is powered off.
2.The called MS activates the CFU to a
PSTN No.
3.The called MS has been barred all
incoming call.
4.The Called MS are not known at HLR

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

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

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

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

12. GSM Architecture Cntd…

13. Call Routing
Scenarios

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

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

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

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

18. Mobile-to-Mobile Long Distance
2. Towers are serviced by
1. Cell phone transmits to a local base station
local towers
Radio tower
3. Basestations are
services by Mobil
Switching Station
Radio tower Base Station
Mobile Switching Station
5. Destination switch office
4. Regional Switch Office
receives call, and forwards
forwards calls
to Mobil Switching Station
Trunk
Regional Switch Office Regional Switch Office
Radio tower
Mobile Switching Station
Base Station
5. Mobile Switching
Station forwards to Base 6. Base Station forwards
Station to local towers.
Radio tower
7. Local towers complete
the loop

19. Mobile is in Roaming
2.Receiving Mobile on Roaming
3. Both Mobiles are on Roaming

20. International Roamer terminated call

21. Handover
MSC B MSC A
Location Area-089 Location Area-100
20 50 53
49 089 100 100
404 404 49 40 4 49
BSC1 089
21 BSC2 100
51 BSC3 54
49 100
404 49 404 49
404
089
22
100
52 0 55
9 10
404 49 404 49 404 4
Global Cell Identification
Country Network Location Cell INDIA-AP-Airtel
Code Code Area Identity MCC-MNC
404 49 Code 404 -49(INA49)

22. Questions ?