THE STUDY OF THE INTERCONNECTION OF GSM MOBILE COMMUNCATION SYSTEM OVER IP BASED NETWORK+ Le-Pond Chin. Jyh-Hong Wen2, and Ting-Way Liu Department of Information Management Shih-Chien University, Taipei, Taiwan, 104. chink&scc 1.scc.cdu.tw 2Gepartment of Electrical Engineering National Chung Cheng University, Chia-yi, Taiwan. Abstract We in this paper propose a protocol to interconnect the GSM mobile communication networks and PSTN through IP based backbone. The basic functions of mobile communication such as call setup, call recovery, and handover together with its further applications such as conference call are taken into account. Not only establishment and closing of a conference call are discussed, but also the recovery procedures due to the failure of handover are also considered. Different feasible interconnection types are also studied. There are several possible connecting topologies, and can be divided into two categories: 1. MSC s of the same operator are connected in traditional architecture. Only the GMSC s of different operators are connected via IP network. 2. All MSC s are liked with IP network. The mechanisms suitable for the above connection will be studied in our work. I. Introduction The integration of tdecommunication network and computer network becomes a trend. The feasibility study and of replacement of a traditional telecommunication into a packet(1p based) network have been done. The standards such as H.323. H.245, H.225, H.450[1]-[4], and etc. are also proposed for this manner. H.323 is a series of Recommendations by the ITU-T to enable multimedia communications in packet-switched networks and is designed to extend the traditional circuit-based services including audiovisual and multimedia conferencing service into packet-based networks. Internet Telephony can be a subset of the H.323 for voice-only service. Therefore, one of the primary objectives of H.323 is the interoperability with the existing circuit-switching system (PSTN and ISDN). The basic elements defined in H.323 architecture are: the terminal, gateway, gatekeeper, multipoint control unit (MCU), in which the terminal, gateway and MCU are collectively referred as endpoint. This work was supported in the part by the Far EasTone Telecommunication Co., Ltd., under contract 90NWE037. The concepts of interconnecting GSM to IP based network were also proposed in [5]-[6]. The igsm means to interconnect GSM systems through IP networks following standards such as H.323, H.245, H.225, H.450, and etc and is illustrated in Fig. I. GSM I I H.323 network network Fig.l The system architecture In the current GSM system, the conference call is supported by call holding and multiparty supplement service[7]. In this mechanism, however, the conference member is limited to five in general. Another approach recommended in the GSM specification to support multi-user conference call is Group Call Service. This approach resolves the limitation of conference member number. It is still limited by some conditions such as service area and the scope of application. We propose a interconnection protocol between IP and GSM network to support multi-user conference and also study the problems of intemetworking. Our result can be generalized to accommodate other protocols such as Media Gateway Control Protocol [5] to integrate GSM network and hnctions of H.323 entities such as gateway and gatekeeper into GSM network without changing the GSM network. I1 System Architecture We divide the interconnection mechanism into two categories: 1. MSC s of the same operator are connected in traditional architecture. Only the GMSC s of different operators are connected via IP network and referred as case 1 in the remainder of this article. 2219 VI C OI
s;c; ITP I I P "UP MAC traditional architecture 2. All MSC's are linked to IP network ( plotted in Fig. 3): The gatekeeper is designed to equip with hnctions of VLR of GSM. The igsm gateway can connect to more than one MSC. This architecture is scalable[5]. We can put igsm gateways and gatekeepers into the system just like the multiple MSCs and VLRs in a GSM network and referred as Fig.3 All MSC's are liked with IP network. The igsm gateway, in Fig. 4, is used to perform the signaling and media translation between GSM and IP network. To simplify our discussion, we assume that both signaling and media gateways are integrated into the igsm gateway [5]. The basic functions of igsm gateway include: 0 Call Control Signaling conversion : to perform GSM / IP call setup aiid release 0 Control conversion: to create the corresponding media control channel between GSM and H.323 network. 0 Media code conversion: to perform encoding and decoding functions Layer 1 Fig. 4 igsm protocol stack The following functions have to be added so as to perform as a H.323 node: @ The igsm gateway is assigned an ISDN number as other MSCs that can be recognized by the GSM network switching subsystem. @ The igsm gateway assigns signaling transport address and keeps name service list between the MSISDN and 1P address. This list will be exchanged between GSM gateway and gatekeeper. @ GSM MAP unstructured supplementary services (UUSs)[S], the series of H.450 supplementary protocol, or other supplementary services can be translated. We translate unstructured supplementary services (UUSs) [9] message into specified control signal defined in H.450 or other standards. @ Functions of GSM MAP and corresponding hnctions of H.225 such as Registrations, Admission, and Status (RAS) protocol can be also translated. To implement the GSM MAP message in RAS protocol, the nonstandardata filed of an RAS message is used to indicate the "type" and the parameters of a GSM MAP operation[5]. Alternatively, the GSM MAP message can be implemented by the RAS non-standard messages. @ Supplementary functions perform system integration such as inquiring routing information The gatekeeper will also performs the following three major functions: @ To inform the signaling transport address of a subscriber who visits the 1P network through the igsm gateway. @ In the second connection situation, a gatekeeper in the H.323 network implements the functions similar to VLR of GSM. Thus, the gatekeeper is assigned an address that can be recognized by the HLR Every gatekeeper maintains GSM database that stores information for all GSM subscribers and add GSM VLR function. The update location and registration procedure is the same as GSM. We describe those procedures as follows: 2220 VTC'O I
Registration Procedure To implement the GSM MAP message in RAS protocol, the nonstandardata filed of an RAS message is used to indicate the "type" and the parameters of a GSM MAP operation. subscriber is registered in the H.323 network or in the GSM network. The igsm gateway implements the rocedures of roaming in order to avoid signaling overlay. JE" 6 W V.2 I.:E" :a:,x,eper -3,sa:way WX MSC igsm gatevay Gatekeeper IVLRJ HLR J ig. 7 message flow of inquiring routing information Fig. 8 shows message follow of Authentication. According to the Previous location area Id, the gatekeeper (new) exchange identification of subscriber with the old ;atekeeper. VS VLR ' icsv Gatekeepek Gatekeeper Vsc * gateway ivbri (old VLR: ig. 5 message flow of registration in VLR Fig. 6 shows the message flow of cancelink location. Based on the received GSM MAP CANCEL-LOCATION message with parameter IMSI, the igsm gateway uses the same mechanism as mentioned above, FL4S URQ/UCF message, to unregister from the gatekeeper. The gatekeeper update the database to indicate that the subscriber leaves this area. The igsm gateway and forwards this information to the HLR via CANEL LOCATION ack message. _..._..._ -, VLR I igsm Gatekeeper: HLR.Gateway (VLR) * CANCEL-LOCATION CANCEL-LOCATION-ack (CANCEL-LOCATION. (CANCEL-LOCATION-ack) I._.._...-..._. Fig. 6 message flow of canceling location. Fig. 7 shows message flow of inquiring routing information. The igsm gateway performance the interrogation procedure to inquire whether the GSM.. -...... g. 8 message flow of authentication. In order to establish a conference call, we discuss the procedures in MS, MSC, and H.323 network. Fig. 9 shows message flows of initialing conference call. According to the connection topologies, different mechanism is used. In the case I: The packet backbone provider is different from the operator. It is necessary for subscriber to get admission from H.323 network. The operator must negotiate signaling regulation with the service provider by using UNSTRUCTURED-SS-REQUEST or UNSTRUCTURED-SS-NOTIFY with associated type and data of the subscriber between the MSC and igsm gateway. Based on the received information, the igsm gateway initiates the endpoint registration to inform the gatekeeper of its transport address or alias address (i.e., MSISDN). The RAS Registration Request (RRQ) message is sent from the igsm gateway to the gatekeeper including the password in the nonstandarddata filed of the message. If subscriber is permitted to enter H.323 network, the gatekeeper responses RCF to the igsm gateway. This procedure is plotted with dotted line in Fig. 9. 222 1 VTC'O I
In the case 2: When the service provider is the same as the operator, based on the received the UNSTRUCTURED-SS-REQUEST with associated type of data, the igsm gateway takes responsibility of conference setup. In the H.323 network, we use a centralized multipoint conference in which all participating terminals communicate in point-to-point manner with an MCU. The terminals transmit their control signal, audio, video, and/or data streams to the MCU. The MC within the MCU manages the conference. The MP within the MCU processes the audio, video, andor data streams, and returns the results to each terminal. A conference call must have a MCU to control all participating terminals. The procedures required to create a point-to-point conference and to expand the conference is through the commands of invite and join. Therefore, a conference call must perform a series of point-to-point connections by the MCU, i.e. conference member will be invited through individual invitation. Detailed procedures will be plotted in Fig. 9. *S GMSC /MSC lgsm Gatekeeper Gateway IVLRI MCU 0 GSM Subscriber Fig. 10 shows message flows of inviting the member into onference. I I PI I I l l :ig. 10 message flow of conference invitation Ending the conference call The subscriber creating a conference call must dominate this conference until the call is terminated. Terminating a call may not terminate a conference. A I conference may be explicitly terminated by using an H.245 message, dropconference, sent by the dominator of conference. Fig. 11 shows the message flow of 'ig. 9 message flow of initialing conference call. Conference invitation procedure The member of a conference call may be in the domain of GSM, PSTN, or igsm and have to register to the gatekeeper. With different invitation domain, the procedures will be classified into the following categories: 0 PSTN subscriber 0 igsm Subscriber 1 I J-'e; *<,..".. c.">.-" I, r 'ig. 11 message flow of terminating conference call Interworking between H.323 and GSM circuit switch 2222 VTC"0 I
network H.323 allow the interoperability between IP network and switched circuit network through H.323 gateways, and can support a call across different type of cellular network. The current version of H.323, however. does not address the internetworking between cellular networks (i.e.$ wireless SCN part) and IP networks [6]. Under the current version of H.323, the mobile is allowed roaming only within the location area associated with the corresponding the igsm gateway while the conversation is proceeding. Because signaling in H.323 does not define mobility functions, it will not act properly for the GSM without proposing the capability to support mobility in a igsm gateway. For case 1 the mobility is done by the traditional GSM system. On the contrary, for case 2, the problems 01 mobility arise. In order to resolve this problem, we use the similar mechanism mentioned in [6] and the functions defined in H.450 to perform function of VolP mobility in hybird lp/cellular networks. The procedure of Internetworking between H.323 and GSM circuit switch network is described in Fig. 12. Fig. 12 message follow of interworking Call recovery One of the calls will be terminated improperly LE to the failure of handover, power going below a threshold, or operational mistake. There are two possible solutions: 1) MSC initiates the recovery, or 2) MCU initiates the recovery. 1. For the mechanism of MCU initiating the recovery If the igsm gateway receives the ISDN REL message from the MSC, it analyzes this message and finds abnormally terminated call. The igsm gateway performs the procedure of terminating ca!l and sends the Release Complete message including the reason to the MCU. Based on the received the Release Complete message, the MCU shall send TerminalLeftConference messages to the remaining terminals and try to re-establish the call without user intervention. The MCU re-invites subscriber to a conference call as a new call. 2. For the mechanism of a MSC initiating the recovery Based on received DISC message from MS, the MSC analyze this message and will find the abnormal termination. The MSC send the UNSTRUCTURED- SS-REQUEST to gateway with call-holding message. The igsm gateway translates this message to become call- holding supplement function of H.450.4 and forward it to the MCU. The MCU tries to re-establish the call without user intervention. After successhl recovery call the MSC sends the UNSTRUCTURED-SS-REQUEST with type retrieve call-holding message to the igsm gateway. The igsm gateway translates this message and forwards it to MCU. MCU therefore can recover this call successfully. References ITU-T H.323, Packet-Based Multimedia Communications System, February 2000Herman C.H. Rao, ITU-T H.225, Call Signalling Protocol and Media Stream Packetization for Packet Based Multimedia Communication Systems, February 1998 ITU-T H.450. I, Generic Functional Protocol for the support of supplementary Services in H.323, February 1998 ITU-T H.450.2, Call transfer Supplementary Service for H.323, February 1998 Yi-Bing Lin, and Sheng-Lin Cho, igsm: VoIP Service for Mobile Networks, IEEE Communications Magazine, pp. 62-69, April 2000Mouly, Wanjiun Liao and Jen-Chi, VoIP Mobility in IP/Cellular Network Internetworking, IEEE Communications Magazine, pp. 70-75, April 2000 M.and Pautet, M.-B, Current evolution of the GSM systems, IEEE Personal Communications, VOI 25, pp. 9-19, Oct.1995 ETSIITS Mobile Application Part (MAP) Specification, Version 3.0.0 GSM 09.02, 1998 ETSliEN Unstructured Supplementary Service Data GSM 04.90, Version 7.0.1 1998 0-780.3-6 728-610 I /$ I 0.00 C200 I I E 1 t: 2223 VTC O I