Luis Bellido - Universidad Politécnica de Madrid
Dpto. de Ingeniería de Sistemas Telemáticos. Spain.
lbt@dit.upm.es
Abstract
This paper presents a review of the current standards to support videoconferencing over broadband networks. Videoconferencing has emerged as a new means of communication with many applications such as telemeeting, remote education or telemedicine. In the last years, the best way to access videoconferencing has been the use of equipment based on the H.320 standard. However, the deployment of broadband network technologies and advances in the techniques for audio and video compression is making better quality videoconferencing possible. This paper will analyse and compare different ways of supporting videoconferencing over broadband networks. Specifically, the paper will review ITU recommendations H.321, H.310 and H.323.
1. Introduction
Videoconferencing has emerged as a new means of communication with many applications such as telemeeting, remote education or telemedicine. In a videoconference, two or more people communicate synchronously by means of audio, video and very often, using data applications such as electronic whiteboards or chat. Videoconferencing is important in the various application fields, because it is a direct way of supporting co-operation. It allows simultaneous communication for geographically dispersed participants, and it accomplishes this task without imposing a rigid structure on the communication, thus leading to interactions that resemble as close as possible natural communication in a face-to-face situation. Videoconferencing communication is thus a key issue in multimedia cooperative systems (Bellido et al, 1996).
The increasing facilities available in videoconferencing systems are also important. From systems that allowed a uni-directional communication between two sites, we are now in a situation where it is possible to have several users engaged in a N to M communication. The addition of facilities such as electronic whiteboards, transfer of documents and window sharing has increased the potential utility of videoconferencing systems. The term multimedia conferencing has been proposed to describe the integration of the different data applications to basic audio and video communication. In the LEVERAGE project [Lev01], for example, a multimedia conferencing system has been developed, to support collaborative, task-based foreign language learning between students in Cambridge, Paris and Madrid. Videoconferencing is also used in other learning scenarios such as in the EDUBA project (Pastor et al, 1995).
Until recently, the best way to access to videoconferencing (or multimedia conferencing) has been the use of equipment based on the H.320 standard [H.320]. H.320 is an ITU (International Telecommunications Union) standard, defining the equipment and protocols needed to support videoconferencing over narrowband ISDN (Integrated Services Digital Networks). However, the deployment of broadband network technologies and advances in the techniques for audio and video compression is making better quality videoconferencing possible. This paper will analyse and compare different alternatives to support videoconferencing over broadband networks. Specifically, the paper will review ITU recommendations H.321, H.310 and H.323 ([H.321], [H.310] and [H.323]).
2. The first standard
H.320 has been the starting point for the current development of videoconferencing systems. It is the first standard ("Recommendation" in ITU terminology) to provide a total system for videoconferencing, allowing the interoperation of equipment built by different manufacturers.
H.320 is supported by narrow-band ISDN (N-ISDN), which provides digital channels with different bandwidths: 64 kbit/s (B channel), 384 kbit/s (H0 channel) and 1536/1920 kbit/s (H11/H12 channel). The Recommendation stipulates several other Recommendations in which constituent elements, such as audio coding, video coding, multimedia multiplexing and system control, are defined. For audio coding, G.711 [G.711] is the basic standard, but other Recommendations define different qualities with different bandwidth requirements. For video coding and compression, ITU-T defines the H.261 standard, which can work from 64 kbit/s to 2 Mbit/s in multiples of 64 kbit/s.
3. ATM for videoconferencing systems
The arrival of broadband network technology has created the need to use these technologies to support videoconferencing which requires high bandwidth. ATM is the chosen technology for future broadband ISDN, therefore the ITU-T concentrated on the standardization of videoconferencing systems in ATM environments. Some characteristics of ATM make it an excellent supporting infrastructure for videoconferencing systems, among them [Okubo97]:
In the definition of the different standards for videoconferencing over any kind of network, ITU-T has always considered interoperability with systems connected to other networks a key issue. This has influenced the different standards developed by ITU-T for videoconferencing over ATM, as will be described in the following sections.
3.1. Adapting to broadband networks
H.321 defines the technical specifications for adapting H.320 terminals to broadband ISDN environments. This recommendation allows the interworking of terminals over broadband ISDN and terminals over narrowband ISDN. To facilitate interworking, the basic technologies used for audio and video codification and compression are also G.711 and H.261, the same as in H.320.
Figure 1 illustrates the interconnection between an H.321 terminal in B-ISDN and an H.320 terminal in N-ISDN, which is supported by the interworking function at the boundary of the two networks according to Recommendation I.580 [I.580].
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Figure
H.321 focus primarily on the interworking aspect with H.320, using the same audio and video coding formats and the same multiplexing (H.221) as the recommendation for N-ISDN. This imposes some limitations that make H.321 unable to take full advantage of the characteristics of the supporting network. Firstly, it limits the amount of bandwidth used for video communications to 2 Mbps, which is the limit for H.261. Higher quality video can be obtained with other compression techniques that do not have the limitation ion bandwidth, though H.261 CIF gives a resolution of 352x288 pixels, which is adequate for many videoconferencing applications. Additionally, the use of AAL1 (ATM Adaptation Layer 1), with a constant bit rate service, is a limitation, because a videoconferencing system could take advantage of the variable bitrate (VBR) service defined for ATM, due to the characteristics of its traffic.
3.2. Taking full advantage of broadband
Recommendation H.310 has been recently approved. H.310 covers the requirements for systems and terminals to carry out audiovisual communications over ATM networks. The recommendation defines uni-directional and bi-directional terminals and classifies different types of terminals, depending on the ATM Adaptation Layer used (AAL1 or AAL5). H.310 not only covers conversational services such as videoconferencing, but also retrieval, messaging, video-on-demand, broadcast TV, video transmission and surveillance services.
H.321 is included in H.310 as one of the operating modes, for interworking with other networks. However, H.310 solves some of the limitations of H.321 by defining a Native ATM mode. Though it keeps the basic H.261 and G.711 as the codings for interoperability in video and audio respectively, it also defines the use of MPEG audio and video, which will provide a better quality in Native Mode operation. Figure 2 shows the H.310 protocol reference model, which consists of the following protocol stacks:
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Figure
As previously stated, H.310 defines different types of terminals depending, among other things, on the ATM Adaptation Layer supporting the videoconference. Terminals over AAL1 can benefit from the functionality of the AAL1 Convergence Sublayer (CS), such as clock recovery, jitter removal, bit error correction, cell loss recovery and data structure preservation. In addition, AAL1 is needed for H.321 compatibility. On the other hand, AAL5 CS provides less functionality -bit error detection, cell loss detection and data structure preservation-, but the wide availability of chips implementing it and the fact that it is already present for User to Network signaling means it too is highly suitable for videoconferencing.
3.3. H.323 over ATM
H.323 is the ITU recommendation for videoconferencing over LANs and Internet. H.323 is applicable to any packet-switched network regardless of the underlying physical layer. It uses H.261 and G.711 as basic video and audio coding formats to provide interoperability with other networks.
H.323 will typically use a reliable delivery mechanism such as TCP to carry control information and an unreliable delivery mechanism such as UDP to carry the audio and video packets without the overhead and latency of retransmission, as defined in [H.225.0]. Thus, H.323 can be used on ATM, by making use of an IP over ATM method. In this way, the use of ATM is transparent for H.323 protocols. However, this would be less efficient than using AAL5 directly for the transport of the audio and video streams. Also, the use of native ATM AAL5 can take advantage of a virtual circuit with QOS. H.323 version 2 specifies this approach as an annex.
The proposal consists of carrying audio and video packetized using RTP/RTCP over AAL5, while control and reliable data continues to use an IP over ATM method, as it is illustrated in Figure 3.
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Figure
Having RTP/RTCP directly over AAL5 would allow the possibility of using the quality of service (QoS) capabilities of ATM. Thus, the problem of using the QoS in IP over ATM methods is solved for the data flows that demand a better service level than plain best-effort, i.e., audio and video flows.
Still, this proposal has some problems that have to be solved:
4. Conclusions
This paper gives an overview of the different standards available for videoconferencing over broadband networks. It is clear that broadband networks offer great potential for applications that make use of videoconferencing, because of the requirements of real-time audio and video communication. ATM is the chosen technology for broadband networks, and the different standards for videoconferencing take advantage, to a greater or lesser degree, of ATM capabilities. One of the potential advantages of ATM for videoconferencing, the use of VBR services, is still not supported by current ATM networks.
5. References
Bellido L, D Fernández & E Pastor, Architectural Issues for Multimedia Cooperative Systems. Proceedings of the 3rd International Workshop on Protocols for Multimedia Systems (PROMS): 33-47 (1996)
[G.711] ITU-T Recommendation G.711: Pulse code modulation (PCM) of voice frequencies (1988)
[H.225.0] ITU-T Recommendation H.225.0: Media Stream Packetization and Synchronization for Visual Telephone Systems on Non-Guaranteed Quality of Service LANs (1996)
[H.245] ITU-T Recommendation H.245: Control Protocol for Multimedia Communication" (1996)
[H.261] ITU-T Recommendation H.261: Video codec for audiovisual services at p x 64 kbit/s (March 1993)
[H.310] ITU-T Recommendation H.310: Broadband audio-visual communications systems and terminal equipment (1996)
[H.320] ITU-T Recommendation H.320: Narrow-Band Visual Telephone Systems And Terminal Equipment (1993/1996)
[H.321] ITU-T Recommendation H.321: Adaptation of H.320 Visual Telephone Terminals to B-ISDN Environments (1996)
[H.323] ITU-T Recommendation H.323: Visual Telephone Systems and Equipment for Local Area Networks which provide a Non-Guaranteed Quality of Service (1996)
[I.580] ITU-T Recommendation I.580: General Arrangements for Interworking Between B-ISDN and 64 kbit/s Based ISDN (1993)
[Lev01] LEVERAGE: ACTS Project AC109. http://www.dit.upm.es/~leverage
S Okubo et al (eds), ITU-T Standardization of Audiovisual Communication Systems in ATM and LAN Environments. IEEE Journal on Selected Areas in Communications, 15/6 (August 1997)
Pastor E, D Fernández & L Bellido Cooperative Learning Over Broadband Networks 6th Joint European Networking Conference (JENC6), Tel-Aviv (May 1995)
[T.120] ITU-T Recommendation T.120: Data protocols for multimedia
conferencing (July 1996)
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