The H.323 Standard CM316 Multimedia Systems Coursework Department of Electronics and Computer Science University of Southampton, Southampton SO17 2GQ

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1 The H.323 Standard CM316 Multimedia Systems Coursework Department of Electronics and Computer Science University of Southampton, Southampton SO17 2GQ Abstract The H.323 recommendation by ITU-T is a standard for multimedia communications services over packet-based networks, including LAN s and IPbased networks. The H.323 standard is broad and comprehensive, specifying the components, protocols and procedures required for multimedia communications services (such as real-time audio, video and data communications), yet is still flexible and practical to implement. This paper looks into the elements, features and advantages of the H.323 standard. Keywords IP, VoIP, PSTN, CODEC, Terminal, Gateway, Gatekeeper, MCU. 1. Introduction The H.323 standard is a set of protocols which form a fundamental technology for real-time audio, video and data conferencing over packet-based networks. Packet based networks include IP-based networks; such as the Internet, Ethernet, ATM, TCP/UDP/IP or Internet packet exchange (IPX)-based LANs. The H.323 protocol stack is designed to operate above the transport layer of the underlying network [KARIM]. H.323 can be used to provide services such as Voice over IP (VoIP), also known as IP telephony; videoconferencing or multipoint-multimedia communications, i.e., fully fledged conferences from multiple locations using tools such as application sharing, electronic whiteboard or instant messaging. This paper covers the elements, protocols and procedures defined in the H.323 recommendation. 2. Background The H.323 protocol was first published in 1996, and was designed for local area networks. Companies were more interested in its use in wide area networks, and therefore this was the first use given to the protocol. The result of using the H.323 protocol over wide area networks was good, and in recognition of this, it was renamed H.323 Version 2 in 1998, with enhancements in security,

2 performance and scalability, as well as having supplementary services added to the protocol. Version 3 of the H.323 protocol added a few minor improvements mainly aimed at better Public Switched Telephone Network 1 (PSTN) integration and scalability. The current version of H.323, Version 4, was approved the 17 th of November 2000, and added numerous enhancements, mainly in the area of scalability, services provided, must have features and Generic Extensibility Framework. 3. Components of an H.323 system The H.323 standard defines the following types of components, which may be used in order to obtain point-to-point and point-to-multipoint multimediacommunications services: - Terminals - Multipoint Control Units (MCUs) - Gateways - Gatekeepers - Border Elements Not all of these components are required in order to use an H.323 system. 3.1 Terminals A terminal is used for real-time multimedia communication, and is the endpoint where H.323 data steams and signalling originate and terminate. They can be either a PC or a stand-alone H.323 enabled device, such as a USB IP phone. The basic service required by the H.323 protocol is audio transmission; therefore a terminal must be capable of audio communication, video and data transmission being optional. The primary goal of H.323 is to interwork with other multimedia terminals, therefore H.323 terminals are compatible with H.324 terminals on SCN and wireless networks, H.310 terminals on B ISDN, H.320 terminals on ISDN, H.321 terminals on B ISDN, and H.322 terminals on guaranteed QoS LANs. H.323 terminals may be used in multipoint conferences. 3.2 Multipoint Control Units (MCUs) The MCU is an optional component of an H.323-enabled network. It is responsible for managing multipoint conferences conferences between three or 1 Public Switched Telephone Network or PSTN refers to the international telephone system based on copper wires carrying analogue voice data, as opposed to the newer telephone networks based on digital technologies such as ISDN and FDDI

3 more H.323 terminals. All the terminals in an H.323 conference establish a connection with the MCU, which manages the conference resources, negotiates the CODEC 2 to use and may handle the media stream. MCUs consist of a mandatory multipoint controller (MC) and may contain multiple multipoint processors (MP). The multipoint controller manages the call signalling, while the multipoint processors handle media mixing, switching or other media processing. 3.3 Gateways A Gateway is an optional component on a single H.323 network, however it provides connectivity between dissimilar networks, and is therefore necessary for interconnectivity. The Gateway is in charge of translating protocols between the different networks, enabling communication between H.323 networks and other types of networks, such as SCN networks (all switched telephone networks, including PSTN) or H.32x networks. Gateways consist of a Media Gateway Controller (MGC) and a Media Gateway (MG), which may co-exist or exist separately. The MGC handles call signalling and other non-media-related functions, while the MG handles the media. 3.4 Gatekeepers Gatekeepers are another of the optional components of the H.323 network. They are in charge of providing various services to all the endpoint in its zone. These services include [IEC]: Address translation: A gatekeeper maintains a database for translation between aliases, such as international phone numbers, and network addresses. Admission and access control of endpoints: This control can be based on bandwidth availability, limitations on the number of simultaneous H.323 calls, or the registration privileges of endpoints. Bandwidth management: Network administrators can manage bandwidth by specifying limitations on the number of simultaneous calls and by limiting authorization of specific terminals to place calls at specified times. Routing capability: A gatekeeper can route all calls originating or terminating in its zone. This capability provides numerous advantages. First, accounting information of calls can be maintained for billing and security purposes. Second, a gatekeeper can re-route a call to an appropriate gateway based on bandwidth availability. Third, re-routing can 2. CODEC: [CNET] CODECs are used to encode and decode (or compress and decompress) various types of data--particularly those that would otherwise use up inordinate amounts of disk space, such as sound and video files.

4 be used to develop advanced services such as mobile addressing, call forwarding, and voice mail diversion. Billing and accounting. Both [IEC] and [KARIM] refer to the Gatekeeper as the brain of the H.323 network because of the central management and control services it provides. When a Gatekeeper exists, all endpoints (terminals, gateways and MCUs) must be registered with it. Registered endpoints control messages are routed through the Gatekeeper. The Gatekeeper and the endpoints it administers form a management zone. 3.5 Border Elements [JONES] also defines Border Elements as part of the H.323 protocol: Border Elements exchange addressing information and participate in call authorization between administrative domains. They are often co-located with a Gatekeeper and may aggregate address information to reduce the volume of routing information passed through the network. Border Elements may assist in call authorization/authentication directly between two administrative domains or via a clearinghouse. 3.6 Zones As described above, a zone consists of a Gatekeeper and all the endpoints it administers. There must be at least one terminal in a zone and it may include Gateways or MCUs. A zone is independent of network topology and can be composed of multiple network segments that are connected using routers or other devices, as is shown in the following figure [IEC]: 4. H.323 Protocols

5 The H.323 standard is independent of the packet network and transport protocols over which it runs and therefore does not specify them. The H.323 standard depends upon several other standards and recommendations to enable real-time multimedia communications. All the protocols quoted are specified in Annex A. All H.323 terminals must support the following standards [IEC]: H.245 for exchanging terminal capabilities and creation of media channels H.225 for call signalling and call setup RAS for registration and other admission control with a gatekeeper RTP/RTCP for sequencing audio and video packets H.323 terminals must also support the G.711 audio CODEC. Optional components in an H.323 terminal are video CODECs, T.120 data-conferencing protocols, and MCU capabilities. The protocols defined for audio, video and data conferencing by the H.323 standard are the following: 4.1 Audio CODECs Audio transmission is the basic service required by the H.323 standard and therefore there must be support for at least one audio CODEC present in all H.323 terminals, namely the one specified by the ITU-T G.711 recommendation. H.323 specifies a series of audio CODECS ranging in bit rates from kbs. The G.711, which must be supported by all terminals, produces bit rates of 56 and 64 kbs. This however is not ideal for communications over the Internet where bandwidths are much smaller. A popular choice is the G standard, which produces a good quality audio at 5.3 and 63 kbs. 4.2 Video CODECs H.323 specifies video support as optional and therefore the support of video CODECs is as well. However, an H.323 terminal providing video communications must support video encoding and decoding as specified by the ITU-T H.261 recommendation. Video CODECs are in charge of encoding the video from the camera for transmission on the H.323 terminal and decodes the received video code that is sent to the video display on the receiving H.323 terminal. The quality of video transmission strongly depends on compression techniques. Active work is on going in the development of more efficient CODECs like MPEG-4 and MPEG-7.

6 The architecture of H.323 is designed to allow the incorporation of new CODECs as they become available. 4.3 Data Conferencing This is required in order to provide for activities such as application sharing, electronic whiteboard or instant messaging, among others. The H.323 standard specifies recommendation T.120 to carry out this functionality. T.120 is a real-time data communication protocol designed specifically for conferencing needs. T.120 provides several advantages over regular data transmission such as [DATABEAM]: Multipoint conferencing support: T.120 supports multipoint data delivery, which enables group collaboration activities. The MCU handles the mixing and switching of data in a similar manner to that used for video and audio. Network and platform independence: T.120 operates on top of the transport layer of the underlying network. As such, it is transparent and independent of the network hardware and software. Interoperability: T.120 is referenced in all the H.32X conferencing standards. This cross-referencing, together with the network and platform independence, ensures a high degree of interoperability at the application level. Multicast support: T.120 supports multicast of data streams in multicastcapable networks. This support is flexible with mixed unicast and multicast also possible during a conference. Other benefits: T.120 provides error correction capability on top of the network transport ensuring reliable delivery. In general, T.120 has a scalable and extendible architecture with provisions for the addition of new applications that take advantage of real-time reliable and efficient data delivery among a group of collaborators. 4.4 Supplementary Services H.323 joins traditional telephone networks and media-rich packet-based networks, creating a huge potential for new services and applications that take advantages of the capabilities of both networks. H.323 provides a flexible architecture for supplementary services through the H.450.x series of recommendations. The H recommendation provides an essential mechanism for end-to-end control signalling between peer service entities. The H protocol is based on the QSIG protocol developed by International Organization for Standardization (ISO) for private ISDN networks. QSIG is the most common services control mechanism employed in call centres and PBXs (private branch exchange). Using

7 QSIG as a basis for H.323 supplementary services provides several advantages such as [KARIM]: Interoperability with QSIG based networks Existence of several basic services models (from ISDN) Extendibility and flexibility of QSIG Existence of implementation knowledge baseº Two supplementary services have been ratified by ITU-T. These are call transfer (H.450.2) and call diversion (H.450.3). Other services that are being developed include call hold (H.450.4), call park/pickup (H.450.5), call waiting (H.450.6), message waiting (H.450.7), name identification (H.450.8), and call completion on busy subscriber (H.450.9). 4.5 Security The security requirements for H.323 communications are specified by the H.235 recommendation. Four security services are provided: authentication, integrity, privacy and non-repudiation. 5. The Challenge Today the biggest market for H.323 applications is Voice over IP (VoIP), due to the fact that low bit-rate Internet connections make video and data intensive applications less appealing. Companies can provide VoIP services today at low cost and provide new competition to the incumbent carriers. The future will see video and data conferencing become more pervasive, as broadband connectivity increases and customers come to expect more than just audio communications. Some of the services made possible with H.323, and expected by clients are [JONES]: - Application Sharing - Electronic Whiteboard - File Transfer - Instant Messaging - Click to Dial - Internet Call Waiting - Web Call Parking - Call No-Waiting - Ad-Hoc Conferencing - Voic Anywhere - Unified Messaging - Service Portability

8 H.323 makes it possible to create and deploy new services quickly and to take advantage of multimedia capabilities. 6. Conclusion H.323 is a feature rich, powerful and useful system. It has received criticism for being too complex, however not every part has to be implemented in order to build a system. Multimedia by itself is already complex, therefore its transmission over IP is going to be necessarily so. Many of the features all ready implemented in H.323 are not currently in use, but will start to be so as video and data conferencing become more prevalent. H.323 uses ITU CODECs, although it allows the use of any CODEC. As put by [JONES], Almost all VoIP systems use ITU CODECs for a good reason they work very well. H.323 provides a flexible, comprehensive and scalable solution to videoconferencing for packet-based networks.

9 References [IEC] Trillium IEC Tutorial: H.323 Tutorial. [KARIM] Asim Karim, Ohio State University, H.323 and Associated Protocols. [CNET] CNET Glossary [JONES] Paul E. Jones, Introduction to H [DATABEAM] DataBeam Corp., "A Primer on the T.120 Series Standard,"

10 Recommendation # Audio Codecs Title (issue date) G.711 Pulse code modulation of voice frequencies (11/88) G khz audio coding within 64 kb/s (11/88) G G.728 G.729 Video Codecs H.261 Dual rate speech coders for multimedia communication transmitting at 5.3 and 6.3 kb/s (03/96) Coding of speech at 16 kb/s using low-delay code excited linear prediction (09/92) Coding of speech at 8 kb/s using conjugate-structure algebraic-code-excite linear-prediction (03/96) Video codecs for audiovisual services at p x 64 kb/s (03/93) H.263 Video coding for low bit rate communication (02/98) Data Conferencing T.120 Data protocols for multimedia conferencing (07/96) Control H.245 Control protocol for multimedia communication (09/98) H Real-time Transport RTP/RTCP Security H.235 Supplementary Services H H and Call signalling protocols and media stream packetization for packet-based multimedia communication systems (02/98) RFC1889 (01/96), RFC1890 (01/96), RFC2032 (10/96), RFC2429 (10/98) Security and encryption for H-Series (H.323 and other H.245-based) multimedia terminals (02/98) Generic functional protocol for the support of supplementary services in H.323 (02/98) Call transfer and call diversion supplementary services for H.323 (02/98)