International Roaming for GSM Kwangwoon Univ. Network Computing lab 2003. 04. 15 김형민...

International Roaming for International Roaming for GSMGSM

Kwangwoon Univ. Network Computing labKwangwoon Univ. Network Computing lab2003. 04. 152003. 04. 15

김형민 김형민 ([email protected])([email protected])

Kwangwoon Univ.http://netcom.kw.ac.kr

Wireless and Mobile Network Architectures

IntroductionIntroduction

GSM GSM supports roaming services that allow a supports roaming services that allow a subscriber in a GSM network to receive subscriber in a GSM network to receive mobile telephony service when the user mobile telephony service when the user visits a different GSM networkvisits a different GSM network

GSM networks are within the same country GSM networks are located in different countries



IntroductionIntroduction

International roaming issue from the International roaming issue from the customer’s perspectivecustomer’s perspective John is a subscriber of GSM service in Korea Suppose that he travels from Korea to Japan ,which

have a GSM roaming agreement

John

Jenny



Three scenarios for call delivery to JohnThree scenarios for call delivery to John

Scenarios #1Scenarios #1 If a person in Korea call John

The caller is charged for a local GSM call John is charged for an international call from Korea to Japan

Scenarios #2Scenarios #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

John is charged for an international call from Korea to Japan Scenarios #3Scenarios #3

If the caller is in Japan international call This scenario is in fact a special case of scenario 2, and is referre

d to as tromboning



International GSM Call SetupInternational GSM Call Setup

The call delivery procedure to a GSM roamer is basically the same The call delivery procedure to a GSM roamer is basically the same as the procedure described in Chapter 9, except that two as the procedure described in Chapter 9, except that two internatiointernational switch centers (ISCs)nal switch centers (ISCs) are involved in the voice path. are involved in the voice path.

ISCs offer interworking functions between the national networks anISCs offer interworking functions between the national networks and the international networkd the international network

The call path of every international call is composed of three segmeThe call path of every international call is composed of three segments:nts: one in origination country another in the international network the third in the destination country

These circuit segments are interconnected by two ISCs:These circuit segments are interconnected by two ISCs: one in the origination country the other in the destination country



Korea

International call setup procedure (Step 1)International call setup procedure (Step 1)

Radio tower

1 2 3

4 5 6

7 8 9

* 8 #

HLRE

VLRF

GMSC D

MSC G

SwitchA

ISC B(Japan)

ISC C(Korea)

John Jenny

1

JAPAN

Jenny first dials the ISCA (international switch center access code), CC (country

code), John’s MSISDN

1.1

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

1.2

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

1.3



Korea

International call setup procedure (Step 2 International call setup procedure (Step 2 ~ 6)~ 6)

Radio tower

1 2 3

4 5 6

7 8 9

* 8 #

HLRE

VLRF

GMSC D

MSC G

SwitchA

ISC B(Japan)

ISC C(Korea)

John Jenny

1

JAPAN

1.1

1.21.3

2

Step 2.

GMSC D queries HLR E to obtain

the MSRN

Step 3and 4

HLR E queries VLR F

3.1

3.2

3.34.1

4.2

4.3

Step 5

The MSRN is returned to GMSC

D

5

Step 6Based on the

MSRN, GMSC D uses the IAM

message to set up the trunk to MSC G

6.16.2

6.3

6.3

6.3



Reducing the International Call Delivery Reducing the International Call Delivery CostCost

To avoid unnecessary international trunk To avoid unnecessary international trunk setups, an IAM message should not travel setups, an IAM message should not travel across country boundaries before the across country boundaries before the destination is knowndestination is known

Four solutions that follow this guidelineFour 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 09.02.

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



RLCRLC

The RLC in a visited system maintains a The RLC in a visited system maintains a database containing the records of all database containing the records of all international roamers who are presently in international roamers who are presently in that visited systemthat 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



Solution 1 (Registration)Solution 1 (Registration)

Radio tower

Korea

HLRVLR

MSC

ISC B(Japan)

ISC C(Korea)

JAPAN

RLC

1

1

1

Step 1.

The MS registers to the VLR

2

Step 2.

The VLR sends MAP_UPDATE_LOCATION to

the roamer’s HLR.

HLR 은 외국에 있으므로 ISC B 에게

라우트

3

3 5

54

Step 3.

ISC B 는 메시지를 해석 , roamer

등록절차인지를 식별 ,The message is

forwarded to Korean (ISC C) as usual

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 5.

RLC 는 MAP_RESTORE_DATA : HLR 에게 MSISDN 요구 MAP_INSERT_SUBSCRIBER_DATA : MSISDN 을 돌려받는다



Solution 1 (Call delivery)Solution 1 (Call delivery)

Radio tower

VLR

MSC

ISC B(Japan)

RLC

4

1

2

4

1 2 3

4 5 6

7 8 9

* 8 #

3

PSTNSwitch

A

1

Jenny

John

Step 1.

Jenny first dials the ISCA code, the CC code, then John's MSISDN

스위치 A 는 다이얼의 앞 부분 (ISCA+CC) 을 해석

국제전화임을 확인하고 ISC B 에게 라우트 (IAM message)

Step 2.

CC code 와 남아있는 번호에 근거하여 ISC B는 국내에서 이뤄지는 통화임을 알게 된다

ISC B 는 IAM 메시지에 의해 제공된 MSISDN 을 이용하여 RLC 를 찾는다

Step 3.

RLC Table 에 John entry가 있는 경우 VLR 로 부터 MSRN 을 가지고 온다 .

Step 4.

By using the MSRN, ISC B routes the IAM message to John



Solution 1 (Advantage & Disadvantage)Solution 1 (Advantage & Disadvantage)

AdvantageAdvantage Only ISC B needs to be modified Other network elements, such as the VLR and HLR, remain the

same

DisadvantageDisadvantage 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 differe

nt from that of the GSM service provider




Radio tower

Korea

HLRVLR

MSCA

ISC B(Japan)

ISC C(Korea)

RLC

1

1

1Step 1.


3

3 4

4

JAPAN

SwitchD

4

3

2

Step 2.VLR 은 international roamer 의 등록임을 알고 , MAP_UPDATE_LOCATION 메시지를 RLC 에게 보낸다RLC 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 4.

등록과정이 끝난 후 , RLC는 MAP_RESTORE_DATA 메시지를 이용해 roamer의 MSISDN 을 얻는다



Solution 2 (Call delivery)Solution 2 (Call delivery)

Radio tower

VLR

MSC

RLC

4

1

2

4

1 2 3

4 5 6

7 8 9

* 8 #

3

PSTNSwitch

A

1

Jenny

John

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

Step 2.

Switch D 는 IAM 메시지에 의해 제공된 MSISDN 을 이용하여 RLC 를 찾는다

Step 3.

RLC Table 에 John entry가 있는 경우 VLR 로 부터 MSRN 을 가지고 온다 .

Step 4.

By using the MSRN, ISC B routes the IAM message to John

SwitchD




AdvantageAdvantage The modifications are made only within the GSM

network

DisadvantageDisadvantage 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




Radio tower

Korea

HLR

VLRMSC

A

ISC B(Japan)

ISC C(Korea)

RLC

1

11

Step 1.


2

2 4

4

JAPAN

SwitchD

4

Step 2.

The VLR sends MAP_UPDATE_LOCATION to the roamer’s HLR.

HLR 은 외국에 있으므로 ISC B 에게 라우트

Step 3.

foreign country 의 HLR 에게 location update

메시지 가 보내질 때 동작RLC creates a record to store the IMSI and VLR/MSC address

Step 4.

등록과정이 끝난 후 , RLC는 MAP_RESTORE_DATA 메시지를 이용해 roamer의 MSISDN 을 얻는다

2

extrator

3




AdvantageAdvantage It is transparent to the VLR

DisadvantageDisadvantage A new network component (i.e., the extractor)



Solution 4Solution 4

Dial Dial NDC1+IRAC+CC+NDC2+SNNDC1+IRAC+CC+NDC2+SN NDC1 : NDC or mobile network access code to the visited GSM system 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 PSTN routes the call to a GMSCGMSC of the visited GSM system (NDC1) of the visited GSM system (NDC1) GMSCGMSC 는 는 international roaming callinternational roaming call 임을 확인 임을 확인 (IRAC)(IRAC) HLRHLR 에게 질의하는 대신 에게 질의하는 대신 CC+NDC2+SNCC+NDC2+SN 을 통하여 을 통하여 MSISDNMSISDN 얻어냄얻어냄 Signaling path between the GMSC and the HLR already existsSignaling path between the GMSC and the HLR already exists The GMSC query the HLR to obtain the The GMSC query the HLR to obtain the MSRNMSRN GMSCGMSC 는 는 MSCMSC 에게 에게 callcall 을 라우트을 라우트 , , 목적한 목적한 MSMS 에게 도착한다에게 도착한다



HLR VLRGMSC

MSC

PSTNPSTN

1 2 3

4 5 6

7 8 9

* 8 #

1

2 3

45

6



Solution 4Solution 4

GMSC is a general-purpose switching systemGMSC is a general-purpose switching system roamerroamer 의 의 HLRHLR 을 억세스하기 위한 경로해석을 수행을 억세스하기 위한 경로해석을 수행 목적지 목적지 MSCMSC 에게 직접적으로 콜을 라우트에게 직접적으로 콜을 라우트

AdvantageAdvantage GSM call delivery procedure is not modified No new elements are required Cost-effective (no new network elements)

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