1. International Roaming for GSM

2. Introduction
GSM supports roaming services that allow a subscriber in a GSM network to receive mobile telephony service when the user visits a different GSM network
GSM networks are within the same country
GSM networks are located in different countries

3. Introduction
International roaming issue from the customer’s perspective
Gopi is a subscriber of GSM service in Korea
Suppose that he travels from Korea to Japan ,which have a GSM roaming agreement
Gopi
Jenny

4. Three scenarios for call delivery to Gopi
If a person in Korea call Gopi
The caller is charged for a local GSM call
Gopi is charged for an international call from Korea to Japan
Scenarios #2
If the caller is from a third country (say, Hong Kong)
The caller is charged for an international call from Hong Kong to Korea
Scenarios #3
If the caller is in Japan
international call
This scenario is in fact a special case of scenario 2, and is referred to as tromboning (tromboning is where RTP media traffic originates at a certain point, and follows a path out into the network and back to a destination close to where the RTP traffic originated.)

5. International GSM Call Setup
The call delivery procedure to a GSM roamer is basically the same as the procedure described in the earlier Chapter (MOC/MTO), except that two international switch centers (ISCs) are involved in the voice path.
ISCs offer interworking functions between the national networks and the international network
The call path of every international call is composed of three segments:
one in origination country
another in the international network
the third in the destination country
These circuit segments are interconnected by two ISCs:
one in the origination country
the other in the destination country

6. International call setup procedure (Step 1)
JAPAN
Korea
ISC C
(Korea)
1.2
ISC B
(Japan)
1.3
GMSC D
1.1
Switch A
Jenny first dials the ISCA (international switch center access code), CC (country code), Gopi’s MSISDN
Based on country code, ISC B routes the call to Korea’s ISC C
ISC C interprets the prefix of the remaining digits, and sets up the voice trunk to GMSC D
Switch A interprets the ISCA, the first portion of the dialed digits, it identifies the call as an international call, then sets up the call using the IAM message to Japan ISC
VLR F
HLR E
MSC G 1
Gopi
Jenny

7. International call setup procedure (Step 2 ~ 6)
JAPAN
Korea
ISC C
(Korea)
1.2
ISC B
(Japan)
1.3
GMSC D
3.2
4.2
6.1
6.2
1.1
3.3 2
4.1
Switch A 5
3.1
6.3
4.3
VLR F
HLR E
MSC G 1
Step 2.
GMSC D queries HLR E to obtain the MSRN
Step 6
Based on the MSRN, GMSC D uses the IAM message to set up the trunk to MSC G
Step 3and 4
HLR E queries VLR F
Step 5
The MSRN is returned to GMSC D
6.3
Gopi
6.3
Jenny

8. Reducing the International Call Delivery Cost
To avoid unnecessary international trunk setups, an IAM message should not travel across country boundaries before the destination is known
Four solutions that follow this guideline
A basic restriction is that we should not introduce any new message types to the GSM MAP protocol defined in GSM specification
The solutions utilize only existing messages with the prefix MAP_
In the first three solutions, we utilize the concept of roamer location cache (RLC)
In solution 4, we introduce a special dialing code that leads the call to the GMSC of the visited GSM system
IAM-Initial Address Message
RLC-Roamer location cache
RTP-Real Time Protocol
MAP-Mobile Application Part/Protocol

9. RLC
The RLC in a visited system maintains a database containing the records of all international roamers who are presently in that visited system
From the perspective of a VLR in the visited system, RLC functions as the HLR of a roamer
From the perspective of the HLR in the home system of the roamer, RLC serves as the VLR in a visited system

10. Solution 1 (Registration)
JAPAN
Korea
ISC C
(Korea) 3
ISC B
(Japan) 4 5
RLC
Step 5.
RLC는 MAP_RESTORE_DATA : HLR에게 MSISDN
MAP_INSERT_SUBSCRIBER_DATA : MSISDN
Step 4.
At the same time, ISC B duplicates the message and forwards it to the RLC.
RLC creates a record to store the IMSI and VLR/MSC address
Step 1.
The MS registers to the VLR
Step 2.
The VLR sends MAP_UPDATE_LOCATION to the roamer’s HLR.
HLR은 외국에 있으므로 ISC B에게 라우트
Step 3.
ISC B는 메시지를 해석, roamer 등록절차인지를 식별,
The message is forwarded to Korean (ISC C) as usual 3 5 2
HLR
VLR
MSC 1 1 1

11. Solution 1 (Call delivery)
ISC B
(Japan)
PSTN
Switch A 1 2
Step 4.
By using the MSRN, ISC B routes the IAM message to Gopi
Step 1.
Jenny first dials the ISCA code, the CC code, then Gopi's MSISDN
스위치A는 다이얼의 앞 부분(ISCA+CC)을 해석
국제전화임을 확인하고 ISC B에게 라우트 (IAM message)
Step 2.
CC code와 남아있는 번호에 근거하여 ISC B는 국내에서 이뤄지는 통화임을 알게 된다
ISC B는 IAM 메시지에 의해 제공된 MSISDN을 이용하여 RLC를 찾는다
Step 3.
RLC Table에 Gopi entry가 있는 경우 VLR로 부터 MSRN을 가지고 온다.
RLC 1 4
Jenny 3
VLR 4
MSC
Gopi

12. Solution 1 (Advantage & Disadvantage)
Only ISC B needs to be modified
Other network elements, such as the VLR and HLR, remain the same
Disadvantage
Most ISCs are not equipped with the GSM MAP protocol (not be able to interpret the GSM MAP message)
ISCs typically belong to an international telephone carrier different from that of the GSM service provider

13. Solution 2 (Registration)
Korea
JAPAN
ISC C
(Korea) 3
ISC B
(Japan) 3 4
RLC 4 3 4
Switch D 2
HLR
VLR
Step 4.
등록과정이 끝난 후/
After registration is complete
, RLC는 /is MAP_RESTORE_DATA
With a message
Roamer의 / of MSISDN Get the
Step 2.
VLR은 international roamer의 등록임을 알고, MAP_UPDATE_LOCATION 메시지를 RLC에게 보낸다
RLC creates a record to store the IMSI and VLR/MSC address
Step 3.
RLC sends the MAP_UPDATE_LOCATION message to the roamer’s HLR
Step 1.
The MS registers to the VLR
MSC A 1 1 1

14. Solution 2 (Call delivery)
PSTN
Switch A
Switch D 1 2
Step 4.
By using the MSRN, ISC B routes the IAM message to Gopi
Step 2.
Switch D는 IAM 메시지에 의해 제공된 MSISDN을 이용하여 RLC를 찾는다
Step 3.
RLC Table에 Gopi entry가 있는 경우 VLR로 부터 MSRN을 가지고 온다.
Step 1.
The steps are the same as those for solution 1.
Except that Jenny dials the number of Switch D instead of the country code
RLC 1 4
Jenny 3
VLR 4
MSC
Gopi

15. Solution 2 (Advantage & Disadvantage)
The modifications are made only within the GSM network
Disadvantage
Extra modifications to the VLR
The caller must dial the number of Switch D, then the MSISDN
Dialing process id different from the ordinary international call dialing procedure
Sophisticated billing procedure

16. Solution 3 (Registration)
Korea
JAPAN
ISC C
(Korea) 2
ISC B
(Japan)
RLC 4 4
Switch D 2 3 2 4
extrator
HLR
VLR
Step 3.
foreign country의 HLR에게 location update 메시지가 보내질 때 동작
RLC creates a record to store the IMSI and VLR/MSC address
Step 4.
등록과정이 끝난 후, RLC는 MAP_RESTORE_DATA 메시지를 이용해 roamer의 MSISDN을 얻는다
Step 2.
The VLR sends MAP_UPDATE_LOCATION to the roamer’s HLR.
HLR은 외국에 있으므로 ISC B에게 라우트
Step 1.
The MS registers to the VLR
MSC A 1 1 1

18. Solution 3 (Advantage & Disadvantage)
It is transparent to the VLR
Disadvantage
A new network component (i.e., the extractor)

19. Solution 4 Dial NDC1+IRAC+CC+NDC2+SN
NDC1 : NDC or mobile network access code to the visited GSM system (National Destination Code )
IRAC : international roamer access code
CC : country code of the home country
NDC2 : NDC of the home GSM system
SN : subscriber number for the roamer
PSTN routes the call to a GMSC of the visited GSM system (NDC1)
GMSC는 international roaming call 임을 확인 / Check that the(IRAC)
HLR 에게 질의하는 대신 / Instead of querying the CC+NDC2+SN을 통하여/Through MSISDN 얻어냄/Obtain clearance
Signaling path between the GMSC and the HLR already exists
The GMSC query the HLR to obtain the MSRN
GMSC는 MSC에게 / to call을 라우트/The route , 목적한 / Purpose MS에게 도착한다/Arrive to

20. PSTN 5 4
HLR
VLR 1
GMSC 2 3 6
MSC

21. Solution 4 GMSC is a general-purpose switching system
roamer의 HLR을 억세스하기 위한 경로해석을 수행
목적지 MSC에게 직접적으로 콜을 라우트
Advantage
GSM call delivery procedure is not modified
No new elements are required
Cost-effective (no new network elements)

22. Reference:
Wireless Internet and Mobile Computing: Interoperability and Performance
By Yu-Kwong Ricky Kwok, Vincent K.N. Lau