International Roaming for GSM Kwangwoon Univ. Network Computing lab 김형민

Kwangwoon Univ. Wireless and Mobile Network Architectures Introduction  GSM supports roaming services that allow a subscr iber in a GSM network to receive mobile telephony s ervice when the user visits a different GSM network  GSM networks are within the same country  GSM networks are located in different countries

Kwangwoon Univ. Wireless and Mobile Network Architectures Introduction  International roaming issue from the customer’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

Kwangwoon Univ. Wireless and Mobile Network Architectures Three scenarios for call delivery to John  Scenarios #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 #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 #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

Kwangwoon Univ. Wireless and Mobile Network Architectures International GSM Call Setup  The call delivery procedure to a GSM roamer is basically the same as the procedure described in Chapter 9, 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

Kwangwoon Univ. Wireless and Mobile Network Architectures Korea International call setup procedure (Step 1) HLR E VLR F GMSC D MSC G Switch A 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

Kwangwoon Univ. Wireless and Mobile Network Architectures Korea International call setup procedure (Step 2 ~ 6) HLR E VLR F GMSC D MSC G Switch A ISC B (Japan) ISC C (Korea) John Jenny 1 JAPAN Step 2. GMSC D queries HLR E to obtain the MSRN Step 3and 4 HLR E queries VLR F Step 5 The MSRN is returned to GMSC D 5 Step 6 Based on the MSRN, GMSC D uses the IAM message to set up the trunk to MSC G

Kwangwoon Univ. Wireless and Mobile Network Architectures 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

Kwangwoon Univ. Wireless and Mobile Network Architectures 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

Kwangwoon Univ. Wireless and Mobile Network Architectures Solution 1 (Registration) Korea HLR VLR MSC ISC B (Japan) ISC C (Korea) JAPAN RLC Step 1. The MS registers to the VLR 2 Step 2. The VLR sends MAP_UPDATE_LOCA TION to the roamer’s HLR. HLR 은 외국에 있으므 로 ISC B 에게 라우트 Step 3. ISC B 는 메시지를 해 석, roamer 등록절차인 지를 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_DAT A : HLR 에게 MSISDN 요구 MAP_INSERT_SUBSC RIBER_DATA : MSISDN 을 돌려받는 다

Kwangwoon Univ. Wireless and Mobile Network Architectures Solution 1 (Call delivery) VLR MSC ISC B (Japan) RLC PSTN Switch 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 번 호에 근거하여 ISC B 는 국내에서 이뤄지는 통화 임을 알게 된다 ISC B 는 IAM 메시지에 의해 제공된 MSISDN 을 이용하여 RLC 를 찾는다 Step 3. RLC Table 에 John entry 가 있는 경우 VLR 로 부 터 MSRN 을 가지고 온 다. Step 4. By using the MSRN, ISC B routes the IAM message to John

Kwangwoon Univ. Wireless and Mobile Network Architectures Solution 1 (Advantage & Disadvantage)  Advantage  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

Kwangwoon Univ. Wireless and Mobile Network Architectures Solution 2 (Registration) Korea HLR VLR MSC A ISC B (Japan) ISC C (Korea) RLC Step 1. The MS registers to the VLR JAPAN Switch D Step 2. VLR 은 international roamer 의 등록임을 알고, MAP_UPDATE_LOCATI ON 메시지를 RLC 에게 보낸다 RLC creates a record to store the IMSI and VLR/MSC address Step 3. RLC sends the MAP_UPDATE_LOCATI ON message to the roamer’s HLR Step 4. 등록과정이 끝난 후, RLC 는 MAP_RESTORE_DATA 메시지를 이용해 roamer 의 MSISDN 을 얻는다

Kwangwoon Univ. Wireless and Mobile Network Architectures Solution 2 (Call delivery) VLR MSC RLC PSTN Switch 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 메시지 에 의해 제공된 MSISDN 을 이용하여 RLC 를 찾 는다 Step 3. RLC Table 에 John entry 가 있는 경우 VLR 로 부 터 MSRN 을 가지고 온 다. Step 4. By using the MSRN, ISC B routes the IAM message to John Switch D

Kwangwoon Univ. Wireless and Mobile Network Architectures Solution 2 (Advantage & Disadvantage)  Advantage  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

Kwangwoon Univ. Wireless and Mobile Network Architectures Solution 3 (Registration) Korea HLR VLR MSC A ISC B (Japan) ISC C (Korea) RLC Step 1. The MS registers to the VLR JAPAN Switch D 4 Step 2. The VLR sends MAP_UPDATE_LOCATI ON to the roamer’s HLR. HLR 은 외국에 있으므로 ISC B 에게 라우트 Step 3. foreign country 의 HLR 에 게 location update 메시지 가 보내질 때 동작 RLC creates a record to store the IMSI and VLR/MSC address Step 4. 등록과정이 끝난 후, RLC 는 MAP_RESTORE_DATA 메시지를 이용해 roamer 의 MSISDN 을 얻는다 2 extrator 3

Kwangwoon Univ. Wireless and Mobile Network Architectures Solution 3 (Advantage & Disadvantage)  Advantage  It is transparent to the VLR  Disadvantage  A new network component (i.e., the extractor)

Kwangwoon Univ. Wireless and Mobile Network Architectures Solution 4  Dial NDC1+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 GMSC of the visited GSM system (NDC1)  GMSC 는 international roaming call 임을 확인 (IRAC)  HLR 에게 질의하는 대신 CC+NDC2+SN 을 통하여 MSISDN 얻어냄  Signaling path between the GMSC and the HLR already exists  The GMSC query the HLR to obtain the MSRN  GMSC 는 MSC 에게 call 을 라우트, 목적한 MS 에게 도착한다

Kwangwoon Univ. Wireless and Mobile Network Architectures HLRVLR GMSC MSC PSTN

Kwangwoon Univ. Wireless and Mobile Network Architectures Solution 4  GMSC is a general-purpose switching system  roamer 의 HLR 을 억세스하기 위한 경로해석을 수행  목적지 MSC 에게 직접적으로 콜을 라우트  Advantage  GSM call delivery procedure is not modified  No new elements are required  Cost-effective (no new network elements)