VoIP Service for Mobile Networks Chapter 16 03, 05,06 winthory@yahoo.co.kr Network Computing LAB 김 응 도

VoIP is considered as a promising trend in telecommunication. Integrating mobile phone services with VoIP in particular has become an important issue. Telecommunications and Internet Protocol Harmonization over Network (TIPHON) specifies the mechanism (i.e. a mediation gatekeeper) to provide the service control functions for convergence of IP networks, mobile networks, fixed wireless networks, and PSTN. Several scenario are defined in TIPHON to illustrate different ways of integrating IP and mobile networks. We use GSM as an example of mobile networks to describe mobile/IP integration, where the mobile signaling protocol is GSM MAP

TIPHON IP and mobile integration scenario. Signaling Gateway MSC VLR HLR (VLR) Mediation Gatekeeper Mobile Network IP Network BSC/BTS

contents GSM on the Net The iGSM Wireless VoIP Solution The H.323 Network The iGSM Architecture iGSM Procedure and Message Flows Registration Deregistration Call Delivery to the IP network Implementation Issues Reducing GSM Tromboning Effect Misrouting Due to User Mobility

GSM on the Net Based on a concept similar to TIPHON, Ericsson’s GSM on the Net utilizes a corporate intranet to integrate an enterprise communication network with the public GSM network This system supports both terminal mobility and user mobility. Terminal mobility : whereby a terminal can be moved around the service area without losing contact with the system User mobility : whereby, using various types of terminals, a user can move around the service area without losing contact with the system Each of the elements in the corporate network is connected to a switched Ethernet with 10Mbps bandwidth. They communicate with each other using the H.323 family of protocols.

GSM on the Net VLR MSC HLR Corporate offices GSM Network IP Phone/PC Access node BSC/BTS MSC Intranet Gateway HLR Service node GSM/BTS

GSM on the Net Service node Access node Enables user mobility. Controls calls among different types of terminals. Translates addresses between PSTN and GSM on the Net. Provides authentication, resource management, least-cost routing, and administration for user identity and service profile. Access node Resembles the MSC, VLR, and BSC to manage connection, mobility, and radio resources. Does not provide trunks; instead, controls only the communication between the endpoints. Consist of network access controller (NAC), and the radio network server (RNS)

GSM/BTS Gateway Terminal equipment Provides wireless access for a GSM MS in the IP network Gateway Provides interfaces between GSM on the Net and other networks (particularly the GSM network) Consists of both voice and SS7 gateway functionality Terminal equipment Can be an IP phone, PC, DECT phone, or GSM mobile station.

The iGSM Wireless VoIP soultion We proposes the iGSM service that realizes another TIPHON scenario supporting user mobility for GSM subscribers to access VoIP service. GSM Network에서 GSM 서비스를 받던 사용자가 GSM MS없이 IP Network으로 이동하여 자신의 MSISDN 으로 수신되는 전화를 받기 위해서 H.323 terminal을 사용할 수 있다. iGSM solution is different from GSM on the Net Unlike GSM on the Net, iGSM is a value-added service to the public GSM Networks. iGSM network does not introduce wireless access equipment in the IP network; rather, iGSM service is implemented using standard platforms(general IP gateway / gatekeeper)

The iGSM Wireless VoIP soultion We describe the iGSM architecture and the protocols for location update and call delivery. We discuss how the tromboning effect in standard GSM systems can be avoided when accessing the IP Network. We investigate the problem of misrouting a call caused by user mobility.

H.323 Network The iGSM system consists of the GSM and H.323 (IP) networks. ITU-T H.323 covers the technical requirements for multimedia communications Over packet-based networks that may not provide a guaranteed quality of service H.323 Gatekeeper Gateway Multipoint Control Unit MC MP ISDN LAN PSTN H.323 Terminal (phone or PC) <H.323 Architecture>

H.323 architecture Terminal Gateway Gatekeeper Customer premises equipment (CPE) that provides audio, video, and data communications capability for point to point or multipoint conferences in the H.323 network Gateway Perform call control function and the communication protocol translation mechanism between an H.323 endpoint and endpoint of a circuit-switched network, such as ISDN,PSTN Gatekeeper Optional in an H.323 network May be colocated with a terminal, gateway, or multipoint control unit Provide these functions that address translation, admissions control, bandwidth control, and zone management.

H.323 architecture Multipoint control unit (MCU) Multipoint controller Utilizes multipoint controllers to support multipoint conferences Multipoint controller Provides control functions to support conferences between three or more endpoints in a multipoint conference. Every MCU contains an MC. Multipoint process (MP) Receives audio, video, and data streams from the endpoints involved in a multipoint conference. An MP is optionally included in a gateway, gatekeeper, or MCU

The iGSM Architecture iGSM gateway is implemented to perform two major functions besides the standard H.323 mechanisms GSM MAP and H.225 RAS (registration, admission, status) protocol translation. GSM/PSTN/IP call setup and release An iGSM gatekeeper is implemented to serve as the VLR for iGSM subscribers who visit the IP network. Every iGSM gatekeeper is assigned an ISDN number that can be recognized by the HLR. The iGSM gatekeeper maintains a list of all iGSM subscribers. Based on this list, the gatekeeper performs MSISDN-to- transport address translation and GSM roaming management procedures for the iGSM subscribers.

iGSM architecture MS BTS/BSC H.323 Terminal VLR MSC IP network iGSM HLR iGSM Gateway Gatekeeper iGSM VLR BTS/BSC H.323 Terminal MS

iGSM procedures and Message Flows Every iGSM subscriber has a record in the datebase. MSISDN of the MS Transport address of the H.323 terminal for the subscriber in the IP network. Password of the iGSM subscriber HLR address of the iGSM subscriber IMSI of the MS User profile which indicates the service features and restrictions the iGSM subscriber Presence indication of the iGSM subscriber in the IP network.

Registration HLR은 deregistration을 oldVLR에 알린다 MAP_CANCEL_LOCATION oldVLR은 iGSM MS의 기록을 지운다 H.323 terminal turn on  MSISDN and password 입력 Gatekeeper로 RAS RRQ(MSISDN , passwd) 전송 HLR은 registration 과정을 끝낸다 MAP_UPDATE_LOCATION_ack 을 iGSM gateway로 전달  iGSM Gateway는 RFC(Registration Comfirm) 를 gatekeeper로 전달 iGSM gatekeeper는 user profile을 기록  IRQ(InsSubDataAck) 을 iGSM gateway로 전달  iGSM Gateway는 ACK를 HLR로 전송 Pass를 통해 validate 검사  IRQ(UpdateLoc, IMSI, VLR, MSC num) 을 iGSM gateway로 전달  Gateway GSM MAP으로 변환 HLR로 MAP_UPDATE_LOCATION(IMSI, VLR, MSC num)전송 수신된 IMSI를 통해 HLR은 user profile 수정  MAP_INSERT_SUBSCRIBER_DATA (user profile)을 iGSM gateway로 전달  iGSM Gateway IRR(InsSubsData, user profile) 을 gatekeeper로 전송 iGSM user moves from the GSM network to the IP network MS BTS/BSC 1 VLR H.323 terminal MSC 7,8 iGSM Gateway IP network 3 4 2 HLR 3 GSM 5 4 Gatekeeper iGSM VLR 6 6 5

Message flow for iGSM registration H.323 terminal Gatekeeper (new VLR) Gateway HLR Old VLR PRQ(MSISDN) 2 IRQ 3 MAP_UPDATE_LOCATION 3 MAP_INSERT_SUBSCRIBERDATA 4 4 IRR MAP_CANCEL_LOCATION 7 IRQ 5 MAP_CANCEL_LOCATION_ack 8 MAP_INSERT_SUBSCRIBER_DATA_ack 5 MAP_UPDATE_LOCATION_ack 6 IRR 6 RCF 6

deregistration In this case of the iGSM gatekeeper is the “OLD VLR” iGSM subscriber moves from the IP Network to the GSM Network. H.323 terminal HLR Gateway Gatekeeper (old VLR) MAP_CANCEL_LOCATION(IMSI) MAP_CANCEL_LOCATION_ack IRR (CanLoc, IMSI) IACK (CanLocAck) URQ(unregister Request) UCF(Unregister Confirmation) 2 1

Call delivery to the IP network H.323 terminal HLR 12 11 3 8 2 7 2 3 iGSM Gateway IP network 10 GSM 6 9 Gateway MSC 5 3 Gatekeeper iGSM VLR 4 2 1 Switch Originating iGSM가입자의 MSISDN입력(0-936105401) 0 : special digit , 936 : GMSC  Originating switch send SS7 IAM message to GMSC H.323 terminal은 gatekeeper와 ARQ and ACF message를 교환(이때 ARJ(Admission Reject) 수신 가능 Getekeeper는 Gateway와terminal의 주소를 통해 MSRN 생성  RAS LCF(Location Confirm)를 gateway로 전달  gateway는 MAP_PROVIDE_ROAMING_NUMBER_ack with MSRN 을 HLR에 전달  HLR은 GMSC에 MAP_SEND_ROUTING_INFORMATION_ack GMSC query the location of iGSM subscriber to HLR  MSISDN을 통해 HLR은 가입자가 IP network에 있음을 확인  HLR send iGSM gateway(MAP_PROVIDE_ROAMING_NUMBER)  Gateway LRQ(Location Request)를 Gatekeeper에 전달. Gateway sends the H.225(Q.931) Set-up msg를 H.323 terminal에 전달  Call Proceeding (충분한 routing 정보를 받았다) message reply iGSM subscriber의 MSRN을 수신한 gateway는 ARQ (Admission Request) and ACF (Admissions Confirm) exchange  gatekeeper로부터 MSRN을 통해 terminal의 call-signaling channel transport address 획득 iGSM subscriber가 수신하면, H.323 terminal은 RAS Connect message 를 gateway에 전달  SS7 ANM로 변형되어 originating switch에 전달 대화 시작 얻은 MSRN을 통해 GMSC는 IAM message를 통하여 Gateway와 trunk 설정 Ringing tone is generated at the H.323 terminal  H.225 Alerting message is sent to gateway  gateway는 Alerting message를 SS7 ACM message로 변형하여 GMSC에 전달 1 PSTN

Message flow for iGSM call setup

Implementation Issues iGSM을 구현함에 있어서 고려해야 할 두가지 이슈 Reducing GSM Tromboning Effect Investigating misrouting of user mobility

Reducing GSM Tromboning Effect Tromboning occurs when the caller and the called MS are in the same city but the GMSC is in another city. Both GSM MSCs and iGSM gateways can serve as GMSCs for iGSM subscribers If an iGSM subscriber is assigned a GSM MSC as his GMSC, call delivery follows the procedure iGSM call delivery and tromboning occurs. If an iGSM subscriber is assigned a iGSM gateway as his GMSC, call delivery tromboning can be avoided

Reducing GSM Tromboning Effect PSTN iGSM IP network Switch iGSM Gateway Gatekeeper iGSM VLR 1 2 caller 3 4 5 iGSM user IP Network iGSM IP network iGSM Gateway Gatekeeper iGSM VLR 1 2 iGSM user caller Two kinds of subscribers are anticipated in iGSM The GMSCs of the iGSM subscribers are standard GSM MSCs. In this case, the subscibers typically subscribe to the standard GSM services at the beginning, and determine to include the iGSM service later The GMSC of the subscribers is the iGSM gateway. In this case, the subscribers typically subscribe to the iGSM service from the beginning <PSTN-to-IP Cal Setup> <IP-to-IP Cal Setup>

Reducing GSM Tromboning Effect The first kind of subscribers call delivery follows the standard GSM procedure A subscriber visits the IP network, tromboning may occur as in traditional GSM networks The GSM operator would prefer this scenario if the iGSM gateway and gatekeeper are owned by other ISPs. The second kind of subscribers Call-delivery tromboning avoided when the subscriber visits the IP network. The GSM operator is likely to own the iGSM gateway and gatekeeper

Misrouting Due to User Mobility To support user mobility, the subscriber needs to explicitly perform registration to inform the system in which location area he resides when the terminal has been changed. Missing  call deliveries may be misrouted This problem can be eliminated if the subscriber always turns off the MS when he moves to the H.323 terminal The turn-off action results in a GSM detach message, which deregisters the MS.

Misrouting Due to User Mobility 4 1 GSM MS 2 IP Phone VLR B p VLR B MSC GSM VoIP Gateway HLR p B HLR p A VLR A p VLR A 3 GSM VoIP network step3 step1 step2

Misrouting Due to User Mobility The misrouting problem is avoided if the subscriber explicitly or implicitly registers with the GSM MS step3. Implicit registration occurs in two cases The subscriber originates a call. In this case, VLR A finds that the VLR record for the subscriber does not exit. VLR A will ask the MS to perform a registration operation, as described in the VLR failure restoration procedure The subscriber moves to another LA in the GSM network. Registration is automatically initiated by the GSM MS. Both case, after the HLR has modified the subscriber’s record, it also cancels the subscriber’s VLR record in VLR B