INTERNET DRAFT Gene Ma Category: Informational tti (telecom technologies inc.) Date: October 1998 Expires: April 1999 H.323 Signaling and SS7 ISUP Gateway: Procedure Interworking Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). Abstract This document describes protocol procedure interworking in designing signaling gateway between H.323 and SS7 ISUP, especially the procedure and message mapping between H.225/H.245 and SS7 ISUP. Table of Contents 1.0 Introduction 2.0 Acronyms 3.0 Basic Call Control and Signaling Overview 3.1 SS7 IUSP Protocol 3.2 H.225 and H.245 Protocols 4.0 H.323 to ISUP Interworking 4.1 Successful Basic Call Setup 4.2 Unsuccessful Basic Call Setup 4.3 Basic Call Release 5.0 Acknowledgements 6.0 References 7.0 Author 1.0 Introduction One of the most important components in VoIP to ISDN/PSTN gateway is the signaling gateway, which is to perform protocol procedure interworking, message mapping, and gateway screening or firewall. H.225 and H.245 have been used by H.323 in VoIP for call signaling and media control protocols. In PSTN Common Channel Signaling, all the circuits used for data or voice calls are controlled (i.e., setup and teardown) by the SS7 ISUP protocol. Because of the fundamental difference between packet-based IP network and circuit-based ISDN/PSTN, the signaling protocols for these two networks are conceptually different. The H.323 to SS7 ISUP signaling gateway needs to resolve the difference and interwork two protocols in a smooth way, which is addressed in this document. 2.0 Acronyms ACM - Address Complete Message ANM - Answer Message B-ISUP - Broadband ISUP CIC - Circuit Identification Code GK - Gate Keeper IP - Internet Protocol ISDN - Integrated Services Digital Network ISUP - ISDN User Part PSTN - Public Switched Telecommunication Network RAS - Registration, Admission, and Status REL - Release message RLC - Release Complete Message TSAP - Transport Service Access Point VoIP - Voice over IP 3.0 Basic Call Control and Signaling Overview 3.1 SS7 IUSP Protocol The SS7 ISUP protocol was created as the interexchange signaling between ISDN access and PSTN , and also used as the interworking between ISDN access and egress in PSTN. For an unrestricted 64-kpbs circuit switched example, the ISDN Q.931 and SS7 ISUP interworking can be showed in Figure 1. +--------+ +------+ +------+ +-------+ | Called | Q.931 | Orig.| SS7 ISUP | Dest.| Q.931 |Calling| | Party | | Exch.| | Exch.| | Party | +--------+ +------+ +------+ +-------+ | | | | |--------SETUP------>| | | | |--------IAM-------->| | |<-----CALL PROC-----| |------SETUP------->| | | | | | | |<--CALL PROC (opt)-| | | | | | | |<-----ALERT--------| | |<-------ACM---------| | |<-------ALERT-------| |<-----CONN---------| | |<-------ANM---------| | |<-------CONN--------| |------CONN ACK---->| | | | | |---CONN ACK (opt)-->| | | | | | | +-------------------------------------------------------------+ | Call Information Flow | +-------------------------------------------------------------+ | | | | | | |<-----DISC---------| | |<-------REL---------| | |<------DISC---------| |------REL--------->| | |--------RLC-------->| | |-------REL--------->| |<-----REL COMP-----| | | | | |<------REL COMP-----| | | | | | | Figure 1. Q.931-ISUP interworking for an ISDN call control One of the important information elements in the Q.931 SETUP message is the B-channel Bearer Capability, which is put in the ISUP User Service Information parameter to indicate the requested service from the network. The ISUP uses the CIC parameter in all of its messages to identify the physical circuit(s) being set up based on the bearer request from the called party. In telecommunication industry, any emerging networks, including ISDNs, have to interwork with other existing networks, notably the PSTNs that are the largest and most numerous among the existing networks. For example, a digital telephone set connected to an ISDN basic rate (2B+D) interface certainly needs to be able to talk with a conventional set to the conventional PSTN, which the bearer capability of speech or 3.1-kHz audio is requested from the network to set up the call. By using ISUP, there is no change required for either ISDN or PSTN, and protocol conversion is provided at the ISDN interworking exchanges. 3.2 H.225 and H.245 Protocols As we know, the packet-based IP network has the fundamental differences from the circuit-based network PSTN. One of the important differences is that there is no actual channel, such as ISDN B-channel or PSTN trunk circuit, in the IP network. In other words, the H.323 terminals can have more "channels", logical channels to carry more than one application, such as video, audio or data, in ONE call. Because of this difference, the call control in VoIP specified in ITU H.323 is more complicated. In circuit-based network, the call signaling (i.e., setup and teardown) and bearer capability control are processed in one protocol, Q.931 for ISDN and ISUP for SS7 network. However, since different H.323 terminals in VoIP may have different logical channel capabilities, ITU H.323 has specified the call signaling and bearer capability control in two separate protocols, H.225 and H.245. The call signaling specified in H.225 begins by exchanging the RAS information on the unreliable channel. This processing is followed by the call setup sequence through a reliable channel, which is based on Q.931 protocol with modifications. One modification is the bearer capability information element in SETUP message can be ignored if the call is bounded in the packet-based network, in which the bearer is set up by H.245. Another difference is that closing H.225 call signaling channel is not ending the call unless the H.245 channel is also closed. The Release and DISConnect messages in Q.931 are forbidden in H.225 for a H.323 entity. H.245 uses the reliable H.225 call signaling channel to perform H.245 control functions, such as master/slave determination, terminal capability exchange, open/closing logical channel, and others. During the terminal capability exchange procedure, both ends are based on their own terminal capability to set up the mutual acceptable bearer service to the network. Then logical channel(s) are opened to handle the bearer capability, for example, two channels for video and one for audio. This procedure is very similar to the bearer capability request in the ISDN Q.931 and SS7 ISUP. 4.0 H.323 to ISUP Interworking After having a brief overview on call signaling and control for both circuit-based and packet-based networks, interworking between SS7 ISUP and H.323 (actually H.225 and H.245) becomes quite straightforward. As mentioned in Section 3, the bearer capability set up within the packet- based network is done by H.245. However, for a signaling gateway between packet-based network and circuit network, the information from the bearer capability information element in the H.225 SETUP message is enough to set up a call in the ISDN or PSTN through SS7 ISUP. H.245 is just used for logical channel set up based on bearer capability carried in the H.225 SETUP message. The following sections describes the procedure interworking between ISUP and H.225/H.245. 4.1 Successful Basic Call Setup In Figure 2, H.245 control channel procedure may only contain logical channel setup messages since the H.323 terminal that initiate the call knows only one logical channel with the requested bearer capability needed for the call. Remember that circuit-based ISDN or PSTN has only one bearer capability for a call. +------+ H.323/ +-----------+ SS7 ISUP +------+ | H323 | H.225/ | Signaling | | Dest.| | GK | H.245 | Gateway | | Exch.| +------+ +-----------+ +------+ | | | |----H.225 SETUP (with bearer)-->| | | |------------IAM----------->| |<---H.225 CALL PROC-------------| | | |<-----------ACM------------| |<---H.225 ALERT-----------------| | | |<-----------ANM------------| |<---H.225 CONN------------------| | | | | |<==H.245 Capability Exchanges==>| | |<==H.245 Logical Channel Setup=>| | | | | +------------------------------------------------------------+ | Media Information Flow | +------------------------------------------------------------+ Figure 2. Successful Call Setup by Signaling Interworking The signaling gateway can, as optional, immediately start the H.245 logical channel setup procedure based on the TSAP address given in the H.225 SETUP message without waiting for the CONNect message, which can reduce the entire call setup time. However, media information will not start to flow before the called party, a H.323 terminal in this example, received the H.225 CONNect message, even H.245 has finished the logical channel setup. We highly recommend the implementation of signaling gateway to use this option for H.245 control channel setup, which is showed in the following figure. +------+ H.323/ +-----------+ SS7 ISUP +------+ | H323 | H.225/ | Signaling | | Dest.| | GK | H.245 | Gateway | | Exch.| +------+ +-----------+ +------+ | | | |----H.225 SETUP (with bearer)-->| | | |------------IAM----------->| |<---H.225 CALL PROC-------------| | | | | |<==H.245 Logical Channel Setup=>| | | |<-----------ACM------------| |<---H.225 ALERT-----------------| | | | | |<---H.225 CONN------------------|<-----------ANM------------| | | | +------------------------------------------------------------+ | Media Information Flow | +------------------------------------------------------------+ Figure 3. Successful Call Setup with a H.245 Option For the call initiated from ISDN or PSTN side to a H.323 terminal, the call setup procedure flow will be similar to Figure 2 or 3. Also, the H.245 logical channel setup procedure can be performed based on the bearer capability information in the ISUP IAM without waiting for the H.225 CONNect message. Figure 4 shows a successful call setup from an ISDN party to a H.323 terminal. +------+ H.323/ +-----------+ SS7 ISUP +------+ | H323 | H.225/ | Signaling | | Dest.| | GK | H.245 | Gateway | | Exch.| +------+ +-----------+ +------+ | | | | |<-----------IAM------------| |<---H.225 SETUP (with bearer)---| | | | | |<==H.245 Capability Exchanges==>| | |<==H.245 Logical Channel Setup=>| | | | | |----H.225 ALERT---------------->| | | |------------ACM----------->| |----H.225 CONN----------------->| | | |------------ANM----------->| | | | +------------------------------------------------------------+ | Media Information Flow | +------------------------------------------------------------+ Figure 4. Successful Call Setup Initiated from ISDN It is noted that no H.245 capability exchanges procedure is required in the Figure 4. When a H.323 terminal initiates the call to an ISDN called party, it knows its own terminal capability that is carried by the bearer capability information element in the H.225 SETUP message sent the signaling gateway. If the gateway has the compatible terminal capability, it forms an ISUP IAM and sends to the ISDN called party through SS7 ISUP protocol. 4.2 Unsuccessful Basic Call Setup In ISDN or PSTN, many causes are attributed to unsuccessful call setup, such as called party busy, resource unavailable, bearer service not implemented, and others. After the destination exchange receives the IAM and determines that the call is not able to complete, it sends back an ISUP REL message with the reason. At the originating exchange, the ISUP REL message is mapped into the Q.931 DISConnect message and sent to the calling party to end the call. However, a H.225 reliable call signaling channel is not set up until the H.225 CONNect message is received. Therefore, in an unsuccessful call setup across between ISDN (or PSTN) and packet-based network, no H.225 call signaling channel is established since no H.225 CONNect message mapped from the ISUP ANM is received. Instead, H.245 session end procedure is used to end the call. The Figures 5 and 6 show the unsuccessful call setup caused by either the H.323 terminal or an ISDN party. +------+ H.323/ +-----------+ SS7 ISUP +------+ | H323 | H.225/ | Signaling | | Dest.| | GK | H.245 | Gateway | | Exch.| +------+ +-----------+ +------+ | | | |----H.225 SETUP (with bearer)-->| | | |------------IAM----------->| |<---H.225 CALL PROC-------------| | | | | |<==H.245 Logical Channel Setup=>| | | |<-----REL (with cause)-----| |<==H.245 End Session Command===>| | | | | Figure 5. Unsuccessful Call Setup caused by the ISDN party +------+ H.323/ +-----------+ SS7 ISUP +------+ | H323 | H.225/ | Signaling | | Dest.| | GK | H.245 | Gateway | | Exch.| +------+ +-----------+ +------+ | | | | |<-----------IAM------------| |<---H.225 SETUP (with bearer)---| | | | | |<===H.245 Capability Setup======| | | | | |====H.245 Capability Reject====>| | | |------REL (with cause)---->| |<===H.245 End Session Command==>| | | | | Figure 6. Unsuccessful Call Setup caused by H.323 Terminal 4.3 Basic Call Release In packet-based network, the H.225 Release Complete message and the H.245 session end procedure are used to release an established call. The call release initiated by the H.323 terminal through the signaling gateway is showed in Figure 4. +------+ H.323/ +-----------+ SS7 ISUP +------+ | H323 | H.225/ | Signaling | | Dest.| | GK | H.245 | Gateway | | Exch.| +------+ +-----------+ +------+ | | | |===H.245 End Session Command=====>| | | |------------REL--------->| |<==H.245 End Session Command======| | | |<-----------RLC----------| |----H.225 Release Complete------->| | | | | Figure 7. Basic Call Release by Signaling Interworking The call release initiated from the ISDN or PSTN side is showed in Figure 6, which is the same as that is showed in Figure 5. +------+ H.323/ +-----------+ SS7 ISUP +------+ | H323 | H.225/ | Signaling | | Dest.| | GK | H.245 | Gateway | | Exch.| +------+ +-----------+ +------+ | | | | |<-----------REL----------| |<==H.245 End Session Command======| | |===H.245 End Session Command=====>| | | |------------RLC--------->| |<---H.225 Release Complete--------| | | | | Figure 8. Basic Call Release by Signaling Interworking 5.0 Acknowledgement The author would like to thank Wesley Hicks for his comments on this document. 6.0 Further Studies The H.323 to ISUP interworking presented in this document is based on version 1 of the ITU-T H.323 standards. The next step is to consider the fast connect procedures specified in the version 2 of the H.323 for the signaling gateway design. It is also important to extend the current work for signaling interworking between H.323 and B-ISUP. 7.0 References [1] ANSI T1.113-1993, "Signaling System No. 7 (SS7) - Integrated Services Digital Network (ISDN) User Part" [2] ITU-T Recommendation Q.931-1993, "Digital Subscriber Signaling System No. 1 (DSS 1) - ISDN User-Network Interface Layer 3 Specification for Basic Call Control" [3] ITU-T Recommendation H.323-1996, "Visual telephone systems and equipment for area networks which provide a non-guaranteed quality of service" [4] ITU-T Recommendation Q.225-1996, "Media stream packetization and synchronization on non-guaranteed quality of service LANs" [5] ITU-T Recommendation Q.245-1996, "Control protocol for multimedia communication" 8.0 Author Gene Ma tti (telecom technologies inc.) 1700 North Collins Blvd. Richardson, Texas 75080 Tel: 972-301-4987 Email: gene.ma@ttimail.com