Protocols for multimedia multiparty conferencing

1 Introduction

Among telecommunications people the term conferencing hasbeen associated with audio conferencing, that is audio only, orvideo conferencing, that is audio and moving image.

The term conferencing is also used by data communicationpeople. Data conferences may be synchronous, that is real-timeconferences, or asynchronous, that is messages are sent to theother participants of the conference.

The introduction of ISDN and multimedia communication haveled to a need to add data communication functionality (e.g. textor graphics) to audio and video conferencing. In the same waypeople would like to add audio and moving images to data con-ferences.

Thus, the term multimedia conferencing is introduced. Fur-thermore, there is a need for multiparty multimedia confer-ences. The ITU-T T.120 series recommendations are definingthe data communication service for use in a multipoint multi-media conferencing environment.

This article presents an overview of the existing and plannedITU-T recommendations, and highlights some technologicaland market trends.

2 The basic principles

2.1 Area of applications

The ITU-T T.120 series of recommendations primarily address-es multiparty conferencing.

The initial application was audiographic conferencing, i.e.speech and data/graphics. The introduction of video codingalgorithms enabling the transmission of moving images both onthe ISDN and recently on the PSTN, led to an expansion of theapplication area. The recommendations now address multimediaconferencing, i.e. simultaneous transmission of speech, videoand data/graphics.

Single medium subsets (data conference or audio only confer-ence) are of course possible.

To set up a multiparty conference an MCU (Multipoint ControlUnit) is used as described in Figure 1. Point-to-point connec-tions are established between each conference participant andthe MCU. The protocols can also be used in point-to-point com-munications between two users, however, some of the protocolsare addressing multiparty functionalities.

The protocols also cover conference scenarios where two ormore MCUs are used to establish the conference as described inFigure 1.

The protocols are designed to set up and control multimediaconferences. The functions and protocols defined are free fromplatform or network dependence. The present versions areaddressing the data part of a multimedia conference and theconference control, while the audio and moving image parts ofthe multimedia conference need to be predefined similar to thepresent video conference service offered by a number of serviceproviders.

The following types of conferences can be described:

• Data conference*

• Audio conference

• Audiographic conference (i.e. audio and data/graphics)*

• Video conference (i.e. audio and moving image)

• Multimedia conference (i.e. audio, moving image anddata/graphics)*.

The T.120 protocols can offer control functionalities, e.g. chair-man control, to all these types of conferences, and in additionoffer data communication functions to those types indicated byan asterix.

2.2 Structure

The T.120 protocols can be separated into three groups as de-scribed in Figure 2. These groups are:

• Network related protocols

• Protocols addressing multipoint communication service andconference control

• Application protocols.

2.2.1 Network related protocols

ITU-T Recommendation T.123 specifies the network relatedprotocols. Protocol stacks are defined for:

• ISDN.This protocol stack is based on an MLP data channel estab-lished by the audio-visual framing and in-band signalling sys-tem defined in ITU-T Recommendation H.221. All terminalsthat are conforming to ITU-T Recommendation H.320 supp-ort this in-band signalling system.

• Line-switched data networks (X.21).The MLP channel shall be established by the audio-visualframing and in-band signalling system defined in ITU-T Rec-ommendation H.221.

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Protocols for multimedia multiparty conferencing – a presentation of the ITU-T T.120 series recommendations

B Y T R O N D U L S E T H

Telektronikk 3/4.1996

Terminal Terminal

MCU MCU

Terminal Terminal

Terminal

TerminalMCU

Figure 1 A conference set-up using three MCUs

• Packet-switched data networks (X.25).

• The analogue telephone network (PSTN) viamodems.

• Other networks or leased lines where audio-visual framing and in-band signalling systemdefined in ITU-T Recommendation H.221 isused.

• ATM-based networks (B-ISDN).

• Local Area Networks.

The protocol stacks covered by this recommenda-tion are described in Figure 3.

The protocol stacks for ATM-based networks andLAN are in the 1996 version of the recommenda-tion.

By using these protocols conferences between usersattached to different networks can be set up. Theseaspects are further elaborated in Section 2.4.

It is worth noting that the standardised protocolstack for ISDN communication requires use of theframing and in-band signalling system defined inITU-T Recommendation H.221. The 1996 versionof ITU-T Recommendation T.123 describes somealternative profiles,

• Based on ITU-T Recommendation Q.922 (i.e.without the MLP data channel)

• Based on the telematic protocols defined inITU-T Recommendation T.90

• Based on start/stop characters as defined inITU-T Recommendation V.120.

None of these alternatives are fully standardised inthe Recommendation. Furthermore, there is no pro-cedure to identify the profile used apart from theprocedures described in the referenced recommend-ations.

Multimedia applications will normally use theH.221 framing and in-band signalling and will thushave no problems. But there is a problem for bothdata conferences and applications where there is nosimultaneous audio and moving image communica-tion.

2.2.2 Protocols for multipoint communi-cation and conference control

The Multipoint Communication Service is de-scribed in ITU-T Recommendation T.122, and theassociated protocols are defined in ITU-T Recom-mendation T.125.

ITU-T Recommendation T.124 (Generic Confer-ence Control – GCC) provides a set of services forsetting up and managing the multipoint conference.It provides access control, chair control functionali-ties and control of priorities for the data communi-

96 Telektronikk 3/4.1996

T.127 (MBFT)

User application(s)(Using both standard and non-standard application protocols

User application(s)(Using std. appl. protocols)

Nodecontroller

User application(s)(Using non-std. protocols)

T.126 (SI)Application protocol entity

ITU-T T.120Application protocolrecommendations

Generic Conference Control (GCC)T.124

Multipoint Communications Service (MCS)T.122/125

Network specific transport protocolsT.123

......

Non-standard applicationprotocol entity

Figure 2 Structure of the T.120-series protocols

X.224/0

null + SCF

Q.922

H.221 MLPISDN

T.125 Multipoint communication service

X.224/0

null + SCF

Q.922

H.221 MLPX.21/X.21bis

X.224/0

X.25

X.21X.21 bis

X.224/0

null + SCF

Q.922

Modemstart/stop

X.224/0

null + SCF

Q.922/AAL5

I.361I.432

X.224/0

TPKT

LAN

LANMedium

Figure 3 Protocol stacks defined in ITU-T Recommendation T.123

cation between the participants in the conference. Four levels ofpriority are defined; the highest level is reserved for the confer-ence control based on GCC.

2.2.3 Applications

So far, two applications are standardised:

• Still Image (ITU-T Recommendation T.126).The Recommendation includes JPEG encoded colour stillimages, JBIG encoded black/white still images and facsimilegroup 3/4. Five profiles are defined. Procedures for creatingdrawings and the control of pointers in a multipoint environ-ment are defined,

• Multipoint Binary File Transfer (ITU-T RecommendationT.127).The Recommendation defines procedures for file distributionand file retrieval. Multiple files can be transferred simultane-ously to a number of addresses that can be defined for eachfile.

There will be few application recommendations among theITU-T T.120 series of recommendations. The objective is tostandardise the principles for the communication between non-standard applications and the standardised protocols for the ser-vice. ITU-T Recommendation T.121 describes a model forapplication protocols. The recommendation is presented in Sec-tion 2.3.

2.3 Model for application protocols

To assist application developers ITU-T Recommendation T.121Generic Application Template (GAT) presents a generic modelfor T.120-conforming applications.

Figure 4 describes the principle of the generic model.

The model consists of two functionally distinct parts:

• The Application Resource Manager (ARM) which is respons-ible for managing GCC and MCS resources

• The Application Service Element (ASE) which providesapplication-protocol-specific functionality.

All standardised application protocols are based on this model.It is recommended to use this model when developing non-standard applications.

The use of this model ensures that all non-standard applicationshave equal structure. It will simplify the communication be-tween non-standard applications developed by several compa-nies, but it cannot guarantee the communication between theseapplications; the guarantee can only be achieved by standardis-ing the applications.

2.4 Network independence

The T.120 series recommendations support a broad range oftransport options, including the Public Switched Telephone Net-work (PSTN), Integrated Switched Digital Networks (ISDN), orLocal Area Networks. Conferences might be set up between

97Telektronikk 3/4.1996

Node Controller

GenericConference

Control

User application(s)

ApplicationResourceManager(ARM)

Multipoint Communication Service

ApplicationService

Elements(s)(ASE)

Figure 4 The Generic Application Template Model

terminals of diverse capability, on multiple different networksand across administrations. An example is described in Figure 5.

Apart from the ITU-T standardised protocol stacks, DataBeamoffers solutions for communication on Novell Netware IPX net-works and on TCP/IP networks (e.g. Internet or Intranet) as de-scribed in Figure 6. The scenario described in Figure 5 cantherefore be extended to include Novell networks and TCP/IPbased networks. Since the version of ITU-T RecommendationT.123 approved at WTSC was completed by ITU-T SG 8,ITU-T has decided to accept normative references to IETF doc-uments (i.e. RFCs). The next version of T.123 will thereforeinclude a protocol stack for TCP/IP.

The network bandwidth restriction may create problems. Thebandwidth of an ISDN B-channel is 64 kbit/s, while the band-width when using standardised modems for the PSTN is re-stricted to maximum 33.6 kbit/s when using the recently app-roved extension to ITU-T Recommendation V.34. However, themodem bitrate depends on the quality of the connection; itmight be lower. On Local Area Networks the delay may createproblems for real-time communication.

As stated previously the present versions of the recommenda-tions focus on data communication and conference control.Bandwidth variations or delay introduces no serious problemsfor that part of a conference, but the impact on the audio andmoving image parts of a multimedia conference may be dra-matic. Furthermore, different video coding algorithms may cre-ate problems, transcoding between different video coding algo-rithms might be expensive and introduces a significant increaseto the delay of the connection.

2.5 T.120 implementor’s guide

Even though all possible effort has been made to avoid errorsand/or phrases that can be interpreted incorrectly, such prob-lems can hardly be avoided. ITU-T has no procedures for cor-recting errors in an approved document; a new edition has to beprepared.

The approval process of a new edition takes sometime. For the benefit of the users ITU-T has issuedan ‘Implementor’s Guide for the ITU-T T.120 Rec-ommendation Series Data Protocols for MultimediaConferencing’. This document is a compilation ofreported defects. It is a ‘living’ document freelyavailable. The document resolves defects in the fol-lowing categories:

• Editorial errors• Technical errors such as omissions or inconsis-

tencies• Ambiguities.

In addition, the document may include explanatorytext found necessary as a result of interpretationdifficulties apparent from the defect reports.

The document does not address proposed additions,deletions or modifications to the recommendationsthat are not strictly related to implementation diffi-culties in the above categories.

As already mentioned this is a living document.The latest version can be obtained at IMTC’s ftp-site at the address

ftp://ftp.imtc-files.org/imtc-site/IMPGUIDE

or ITU-T’s Web-site

http://www.itu.ch/itudoc/itu-t/com8/implgd.html.

Usually, new versions are available on the IMTC ftp-site earlierthan on the ITU-T Web-site.

3 Market actors and products available

3.1 Participants in the standardisation activities

North-American companies have been the major contributors tothe standardisation of the T.120 protocols. Mr. C.J. Starkey,Vice President of DataBeam and co-founder of this company,has been a key person and was rapporteur for the questionaddressing the T.120 protocols in CCITT (now ITU-T) for 6years until the middle of 1993.

Other major contributors have been AT&T, Bellcore, BJ Com-munications, BT (British Telecom), France Telecom, Picture-Tel, PolyCom and VideoServer. Other contributors are Intel,BNR (Nortel), ConferTech, Multilink and VTEL. MicroSoft hasbeen participating in 1996.

Apart from BT and France Telecom, European companies havebeen less active. GPT Video Systems has attended most of themeetings addressing the T.120 protocols. Other European com-panies attending some of the meetings are Teles, KPN (DutchPTT) and Austrian PTT. Other European network operators aswell as Japanese and South Korean companies have been pre-sent at a few meetings.

IMTC (The International Multimedia Teleconferencing Con-sortium, Inc.) is an important actor that is founded to facilitateand promote the creation and adoption of international standardsfor Multipoint Document and Video Teleconferencing, specific-


LAN A

LAN GatewayRouter

ISDN

MultiportTerminal

MultiportTerminal

Terminal

MCU

Terminal

MCU

Terminal

PSTN

PSTN

PSTN

ISDN

ISDN

Public Networks

TerminalTerminal

LAN B

LAN GatewayRouter

TerminalTerminal

ISDN

Figure 5 Example of a mixed-network conference topology

X.224/0

null + SCF

Q.922

COM.DRV

T.125 Multipoint communication service

X.224/0

null + SCF

Q.922*

NWIPXSPX.DLL

RFC 1006X.224/0

WINSOCK.DLL

X.224/0

null + SCF

Q.922

H.221 MLP

PSTN IPX TCP/IPISDN

Figure 6 Protocol stacks supported by DataBeam

ally the ITU T.120 and H.320 standards suites. The work onsupporting the T.120 series recommendations was initiated byCATS (The Consortium for Audiographics TeleconferencingStandards, Inc.). In 1994 CATS and MCCOI (The MultimediaCommunications Community of Interest) were merged to formIMTC.

Most of the CATS members were North-American companies,while the membership of MCCOI was based on both North-American and European companies. Telenor was a member ofMCCOI, and is now a member of IMTC.

In December 1995, IMTC merged with PCWG (the PersonalConferencing Work Group), and the major network operators,suppliers of video conferencing equipment as well as compa-nies like MicroSoft and Intel are now participating in the workof IMTC.

It is worth noting that, unlike other fora, IMTC is no alternativeto the standards organisations, the fundamental goal is to bringall organisations involved in the development of multimediateleconferencing products and services together to help createand promote the adoption of the required standards.

3.2 DataBeam

3.2.1 Introduction

DataBeam is one of the active companies in the work on theITU-T T.120-series recommendations. Co-founder and VicePresident of DataBeam, Mr. C.J. Starkey, was rapporteur for thework in CCITT, and is currently president of the IMTC.

DataBeam has established agreements with several partners onjoint activities. Examples are:

• Agreement with MCI to deliver new real-time services formulti-point collaboration via the Internet

• Relationship with Cisco on new Internet server with confer-encing capabilities

• Agreement with MicroSoft to facilitate the development anddeployment of real-time multi-point data applications in cor-porate Intranets and the global Internet

• Strategic co-operation between Cisco, MCI, MicroSoft andSun on development and marketing of neT.120

• Agreement with AT&T Paradyne and Multilink on develop-ment and marketing of technology for multiparty audiograph-ic conferencing.

The products offered by DataBeam are:

• CCTS (Collaborative Computing Toolkit Series), a develop-ment tool

• FarSite, a software product

• neT.120 Conference server, a data conferencing server.

3.2.2 CCTS

CCTS is an integrated set of software development tools basedon the T.120 standards. It is organised in 6 modules whichcorresponds to one or several T.120-series recommendations.

A toolkit on application sharing has been launched, but is nowwithdrawn, probably because ITU-T now is working on a rec-ommendation on application sharing (T.SHARE).

The modules are:

• Multi-Point Communications Application Toolkit (MCAT).MCAT is the basis module which provides a general-purposefacility for establishing, servicing and managing the simul-taneous delivery of data between multiple users connected viaLAN, WAN, and dial-up networks. MCAT is based on theITU-T Recommendation T.122/125 and T.123.

• Generic Conference Control Application Toolkit (GCCAT).This module offers facilities for establishing and managingmeetings, as well as controlling the communications re-sources required by the conference applications.

• Node Controller Toolkit (NCT). This module streamlines theestablishment and management of conference connections.NCT furnishes a generic standard for an undefined portion ofthe T.120 infrastructure.

• Session Manager Toolkit (SMT). This module offers a flex-ible abstraction of the session management functions com-mon to all T.120-based applications. SMT is based on ITU-TRecommendation T.121, and secures a standardised commu-nication between the applications.

• Shared Whiteboard Application Toolkit (SWAT). This mod-ule allows developers to rapidly add powerful T.126 compli-ant document conferencing or whiteboarding functions totheir applications. SWAT is based on ITU-T Recommend-ation T.126,

• File Transfer Application Toolkit (FTAT). This module offersa simple way to embed multi-point background file transferfunctions into an application. FAFT is based on ITU-T Re-commendation T.127.

CCTS can be considered as an implementation of the principlesdescribed in ITU-T Recommendation T.121, and is probably theonly commercial available tool for development of T.120-appli-cations.


Non-standardapplications

SMT (T.121) Node controller

SWAT (T.126)

FTAT (T.127)

GCCAT (T.124)

MCAT (T.122, T.123, T.125)

Figure 7 CCTS modules

3.2.3 FarSite

FarSite is a software which offers the tools needed for remotecollaboration based on the T.120 protocols. FarSite providesboth application sharing and shared whiteboarding, in point-to-point connections and in a multipoint conference.

Multipoint Binary File Transfer conforming to ITU-T Recom-mendation T.127 is offered. The whiteboarding function in thepresent version (FarSite 3.0) is probably not fully conforming toITU-T Recommendation T.126, although this is the objective ofDataBeam.

FarSite is available as a separate product and is included in sev-eral multimedia conferencing terminals and systems.

3.2.4 neT.120

The neT.120 Conference Server is a data conferencing serverbased on T.120 technology. It can be connected to the networksidentified in Figure 6, that is ISDN, PSTN, IPX-based networks(Novell) and TCP/IP-based networks (Internet and Intranet).However, the present marketing of neT.120 indicates thatDataBeam strategic decision is to offer products for dataconferencing on Internet and Intranet. The agreement withCisco, MicroSoft and Sun clearly documents this decision.

A software-only server, the neT.120 Conference Server lever-ages existing web browsers and desktop applications. The mini-mum requirement for participating in a neT.120 based confer-ence is an Internet browser such as MicroSoft Internet Explorer,Netscape’s Navigator or NSCA Mosaic.

By using terminals conforming to the T.120-series recommend-ations more functionalities can be offered.

3.3 Suppliers of T.120-based products

In the past standardisation has not been given high priority bycomputer and software manufacturers. Usually, the marketforces, strategic alliances, etc. have been used to establish defacto standards. In the development of video conferencing term-inals several suppliers have developed alternatives to the stand-ardised solutions. However, the last year a trend to choosestandardised solutions can be observed. Examples are:

• Intel ProShare 2.0 which conforms to ITU-T Recommenda-tion H.320, and offers T.120 functionality. Although not offi-cially confirmed, Intel will probably use MicroSoft developeddata conferencing functionalities. Intel’s development will berestricted to the audio and video part. Co-operation betweenIntel and VideoServer is established, and the VideoServerMCU is compatible with Intel ProShare 2.0.

• Apple’s video conferencing system, QuickTime is also con-forming to ITU-T Recommendation H.320. Apple has alicence agreement with DataBeam, and plans to introduceQuickTime with T.120 functionality in 1996. The licenceagreement with DataBeam allows the QuickTime users todevelop T.120-based applications without an extra licensingagreement with DataBeam.

• MicroSoft has indicated that support of T.120 protocols willbe a part of the successor of Windows 95, and that the T.120

protocols will be used in other MicroSoft products (e.g.MicroSoft NetMeeting). MicroSoft and DataBeam have sign-ed an agreement on development of T.120 application for useon Internet.

• PictureTel, one of the major suppliers of video conferencingequipment, is active in the ITU-T work on the T.120 recom-mendations. PictureTel is also active in the interoperabilitytesting events organised by IMTC (Event 120).

• The major supplier of video conferencing MCUs,VideoServer, is actively participating in the ITU-T work onthe T.120 recommendation, and offers some T.120 function-ality in software version 5 for their MCUs.

4 Trends

4.1 Market trends

No doubt the T.120 protocols will be a success. Several observ-ations supporting this statement can be made:

a) Important market actors such as MicroSoft and Intel are nowactively participating in the standardisation work, and areactively promoting these protocols.

b)Computer Supported Co-operative Work, teleworking andmultiparty multimedia conferencing have become importantstrategic elements for a number of companies.

Another aspect is the introduction of the T.120 protocols in theInternet environment. So far, real-time multiparty video confer-ences can be set up by using MBONE, a virtual network usingIP multicast on Internet. The information is distributed to allhosts who have expressed interest in becoming a member of acertain multicast group. Like the present version of Internet,there is no Quality of Service guarantee. By the use of the TimeTo Live (TTL) value the distribution of the information can berestricted.

The introduction of neT.120 is the first step towards multimediaconferences on the Internet that include a well-defined group ofparticipants. New functions such as chairman control can beoffered. Extensions to the present IP protocols which offerguaranteed bandwidth are available, and the Internet Engineer-ing Task Force is working on a new version of the Internet Pro-tocol (IPv6) which can guarantee Quality of Service. When thenew version of IP is implemented, world-wide multimedia con-ferences offering functionalities and Quality of Service similarto those offered on public networks, can be set up. This servicewill at the beginning be introduced on corporate networks(Intranets). Multimedia conferences set up on Intranets wheresome participants may be attached via public networks will bean important market segment.

From a technology point of view multimedia conferencingincluding moving images is the most exciting application. Fromthe users’ point of view audiographic conferencing and dataconferencing might be more interesting on the short term.Today, video conferences where the quality of the movingimage is acceptable for professional use requires connections setup via ISDN or networks offering higher bitrates. Audiographicconferences or data conferences can be set up on PSTN, Internetor a combination of PSTN and Internet (e.g. the audio connec-tion on the PSTN, the data connection on the Internet). The


required investment in terminal equipment will usually be low,a telephone, a modem and a PC are required. New modemstandards offering simultaneous transmission of voice and dataon a single telephone line may also be an alternative.

Another important trend is the evolution from the businessmarket to the domestic market. In the future multiparty gamesmight be an interesting market segment.

4.2 Network platforms

As already described standardised protocol stacks for publictelecommunications networks (PSTN and ISDN) and for publicdata networks are available. Protocol stacks for LANs andTCP/IP networks are also available.

ITU-T is currently working on a recommendation on audio-visual terminals on LANs without a guaranteed Quality of Ser-vice (Recommendation H.323).

A recommendation on videophones attached to the PSTN isapproved by ITU-T (Recommendation H.324). However, thesize of the screen as well as the audio and video quality areprobably not acceptable for business users. Real-time videotele-phony on PSTN or Internet has for the time being several short-comings, due to time delay as well as available bandwidth.Today, the only realistic network platform for multimedia com-munication where moving images are included, is the ISDN. Itis possible to set up conferences where both audio, movingimages and data are transported on two ISDN B-channels, i.e.128 kbit/s. For more demanding applications the bandwidth canbe increased to e.g. 384 kbit/s (6 B-channels).

Both data conferences and audiographic conferences can be setup on the ISDN. However, the standardised protocol stack forISDN communication is based on the in-band signalling systemspecified for audio-visual communication. Data conferencing onthe ISDN not using this in-band signalling system is therefore aproblem for the time being.

These considerations lead to the following scenarios:

1. Multimedia on the ISDNThis scenario is requiring video conferencing terminals,usually a PC, with T.120 software. Another option is a desk-top videophone or a video conferencing terminal with a PCinterface. Some videophones are equipped with a PC inter-face, but few support the necessary function on the data linklayer to enable T.120 communication.

2. Audiographic conferencesThis scenario may be based on ISDN communication. Thereis a standard specifying a wideband (7 kHz bandwidth) ISDNtelephone where up to 14.4 kbit/s can be reserved for datacommunication. The in-band signalling system specified inITU-T Recommendation H.221 is used. The terminal must beequipped with an interface to a PC, similar to that of desktopvideophones.

A more realistic short term scenario is to set up two PSTNconnections, one for audio, the other for data. A telephonemay be used for the audio communication; usually a hands-free solution is preferred. An alternative might be a headset.

3. Data conferencesThis scenario will be based on PSTN or Internet communica-tion.

Combinations are of course possible. One alternative is to useInternet for the data part of a multimedia or audiographic con-ference.

Where guaranteed Quality of Service can be offered Internet/Intranet is a realistic alternative for multimedia conferences too.

4.3 Interoperability testing

Experiences gained by the introduction of new protocols such asfacsimile and videophony indicate that the risks for misinterpre-tations of the requirement of the recommendations, errors in therecommendations or shortcomings of the recommendations aresignificant. There is always a need for a verification of a stand-ard or recommendation, as well as a need for verifying thatequipment produced by different manufacturers is able to inter-work.

IMTC is addressing these issues. An interoperability testingevent called Event 120 is organised by IMTC. The first eventwas held in Santa Clara, California, 25–27 March 1996. 25manufacturers participated. Approximately 6,000 test sequenceswere conducted, covering equipment connected to ISDN,PSTN, LAN and Internet.

A second session was held in Gaithersburg, Maryland 18–20 Sep-tember 1996. At this session 3,000 test sequences were performed.

It is claimed that 95 % of these tests were successful. Althoughthese results are promising, it is worth pointing out a fewfavourable conditions:

• One is the leading role of DataBeam. Most of the implement-ation was based on the DataBeam products or DataBeam’sdevelopment toolkit. When software or development toolkitfrom other suppliers is available, the situation could be worse.

• Another is that the tests were predefined. It is therefore likelythat attempts have been made to keep clear of problem areas.

The initiative to start Event-120, however, is positive, and animportant condition for interoperability between terminal equip-ment produced by different manufacturers.

In Europe, EV (European Videoconferencing); an organisationwhere European network operators are participating (amongthese Telenor), has carried out both interoperability testing andconformance testing of H.320 terminals.

4.4 Standardisation

4.4.1 Applications

As already stated, there will be few recommendations on appli-cations among the ITU-T T.120 series of recommendations.Some proposals for application recommendations have beenforwarded. One of these is to expand to T.190 series recom-mendations on Co-operative Document Handling (CDH) toinclude multiparty scenarios. A proposal for organisation of thework is described in Figure 8.


Work in ITU-T SG 8 on XAPI (Extended Applications Pro-grammable Interface) has so far been addressing point-to-pointCDH. The extension to a general use in a T.120-environment isnow considered.

Other applications related issues are security and encryption inan T.120 environment.

The most important proposal, to develop a recommendation onapplication sharing in a T.120 conference (T.SHARE), is pre-sented by MicroSoft, PictureTel and PolyCom. The proposal issupported by a number of other North-American companies.The proposal is based on technology used in MicroSoft Net-Meeting and PictureTel LiveShare Plus.

The principles of this recommendation are described in Figure 9.

The application runs on a host. The screen information is trans-mitted to the other participants of the conference. The otherparticipants can control the application by using mouse and key-board. Windows is not required. However, if Windows is used,all participants may switch between applications as if the appli-cations were running on their PC. The only restriction is thatinformation from the clipboard of other PCs than the host can-not be copied into the application(s).

The transmission of the screen information and control informa-tion will use traditional T.120 channels, and full T.120 function-ality is available.

T.SHARE will in principle specify two protocols:

• The main protocol is the protocol which will be maintainedand further processed


• In an annex a protocol which secures compatibility withMicroSoft NetMeeting and PictureTel LiveShare will bespecified. This protocol will not be maintained, and will bewithdrawn after a period to be defined (e.g. three years).

Several observations can be made on this proposal:

a) It is proposed to standardise a new application, a logicalevolution to secure interoperability between applications, butin conflict with the argument that applications are a subjectfor competition, not standardisation.

b)One of the sources of the proposal is MicroSoft, a companywhich is large enough to establish de facto standards.

c) It is proposed to standardise principles which are already usedin Internet products. This is an indication that in certain areasstandards designed for traditional telecommunications appli-cations will also be used in products designed for Internet. Inother words, the communication between traditional telecom-munications networks and Internet will be simplified.

d)The proposal, supported by a number of North-Americancompanies, is to some extent in conflict with the decisionmade by ITU-T SG 8 to develop CDH protocols for multi-party conferences.

e) Intel has accepted that a protocol which is not compatiblewith their application sharing protocol, is standardised. Fur-thermore, Intel has declared that there will not be further re-vision of the present application sharing software, the newsoftware will be based on T.SHARE.

f) An underlying question is – should protocols covering all rel-evant scenarios be standardised, or should attempts be madeto standardise protocols which are simpler and which arefunctioning although they may not cover all possiblescenarios.

By tradition the Internet environment has prepared their ownstandards, RFCs. However, some standards developed by ISOor ITU-T on algorithms for coding of audio and moving imagehave been used. There is also a trend to remove the border be-tween RFCs and formal standards developed by ITU-T andother standardisation organisations.

T.127T.126

Extend Applications Programmable Interface(XAPI)

Applications

SDPADPJSET.121

T.120 Multipointprotocol stack

Point-to-pointprotocol stack

Responsibilities of multipoint group

Responibilities of document transfer and CDH group

JSE : Joint Synchronous EditingADP : Asynchronous Document ProductionSDP : Sequential Document Production

Figure 8 Proposal for organisation of the work on CDH

Host

A

C

Host

B

Screen information Screen information

Moucekeyboard

Moucekeyboard

Figure 9 Principles of T.SHARE

4.4.2 Network interworking and applications inter-working

Network interworking is usually no problem for data conferenc-es and for audiographic conferences where two separate connec-tions are set up. The present standards for multiparty video con-ferences are based on the condition that all participants whoreceive the moving image have to have the same video codingalgorithm, and to transmit/receive the coding image, audio anddata at the same speed. In most cases these participants are con-nected to the same network. However, gateways do exist be-tween LANs and ISDN which support the principles of the ITU-T H.320 standard, and thus offer the user a possibility to partici-pate in a multiparty conference from a terminal on the local net-work. Participants may also be connected to the MCU via leas-ed lines. These conferences where the video coding algorithmand the transmission speed are identical are identified ashomogenous conferences.

Conferences where the participants are connected to differentnetworks (ISDN, Internet, ATM, etc.), different video codingalgorithms are used, and where also graphics and audio mightuse different coding algorithms, are described as heterogeneousconferences.


User applications

Realtime

audiodevice

Realtimevideodevice

Network specificmappings (T.131) Node controller

Remote Device Control(T.RDC))

Non-standardapplication

protocol entities

Audio-visual-control(T.132/T.133) GCC (T.124)

MCS (T.122/125)

Network specificcontrol entity

Network transport protocols(T.123)

Network multiplex

controlvideostream(s)

audiostream(s)

control data

ITU-T Standardapplication

protocol entities

Figure 10 Relations between the T.120 series recommendations and the planned T.130 series recommendations

Standards addressing these issues are on the way. Initially, asingle recommendation identified as T.AVC was planned. Nowthe objective is to prepare a set of recommendations identifiedas the ITU-T T.130 series. The structure of Figure 2 can beextended as indicated in Figure 10.

The number of recommendations in the T.130 series is not yetfixed. There will be an introductory recommendation, T.130.There is work on

• T.132 – Audio Video Control: Infrastructure Management• T.133 – Audio Video Control: Service Management.

Drafts are available. It is planned to approve these recommenda-tions at study group level in 1997. However, there are still anumber of complex, unsolved issues; the plan may fail.

A network related recommendation, T.131 – Protocol for Net-work Specific Mapping of Real Time Audio Video Elements, isplanned to be approved in 1998.

One of the issues discussed is whether control of remote devices(cameras, microphones, VCRs, etc.) should be an integral part

of the already planned recommendations, or be a separate rec-ommendation (T.RDC).

Most of the functionalities specified in the T.130 series arebased on the use of an MCU or a server. Scenarios where the in-band signalling is based on ITU-T Recommendation H.221(equipment connected to ISDN or leased lines), or ITU-T Re-commendation H.245 (PSTN, ATM or LAN) will be covered.

A control protocol for network interworking will be defined.Control mechanisms for transcoding (e.g. between H.261 andMPEG) as well as mechanisms enabling the user to specify andcontrol Quality of Service, will be prepared.

Whether T.131 and T.132 should be merged or not is anotherissue. A decision to merge these recommendations will probab-ly result in a delay.

ITU-T recommendation T.133 will cover

• Video switching• Video processing (e.g. Continuous Presence)• Mixing of audio information• Identification of audio and video sources• Chairman control.

An example is described in Figure 11. A Continuous Presencefunction presents images from several sites (e.g. five sites). Oneof the images is larger than the others. Additional information(video, documents, etc.) can also be presented.

The present Telenor video conference service (TelenorMøtepunktet®) offers a Continuous Presence function whereimages from four sites are presented on one screen. However,when the new recommendations are in force more flexibilitycan be offered. Some of the functions overlap with functionsspecified in the T.120 series recommendations. One solution isto extend ITU-T Recommendation T.124 to cover T.130-basedfunctionalities.

5 Closing remarks

The importance of standards can be illustrated by the success ofthe facsimile. Prior to the creation of the facsimile standards, afacsimile machine could only communicate with other machinesmade by the same manufacturer. The consequence was a few,expensive machines on the market. Today, people take it forgranted that when they send a fax, it will work no matter whatbrand of machines are sending and receiving. The price of a faxmachine has reached a level where it is interesting to homeusers as well as business users.

In much the same way, the T.120 series of standards defines ameans to communicate multimedia across various networks. Ascompanies develop software products based on these standards,they will be able to communicate with each other. The T.120series of standards are also creating a bridge between severalnetworks, PSTN, ISDN, Internet/Intranet, ATM and LAN. TheT.120 series standards will be an important element in the mul-timedia applications that will be developed in the coming years.


Site 1 Site 2 Site 3 Site 5

Site 3Document

camera

Site 3 stream 2Site 3 stream 1

Figure 11 A Continuous Presence presentation

Documents

Protocols for multimedia multiparty conferencing