budget or meeting attendees suffering from jet lag. If traveling is too cumbersome, dangerous (gulf war) or expensive and telephone talk is unsatisfactory, video.
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Desktop Video: Building Large-scale Services with Incompatible Products Manfred Bogen Christian Bonkowski Richard Rodriguez-Val1 Clemens Wermelskirchen
I. Why do We Need Desktop Video Conferencing Services?
Abstract As companies go international and decentralize, there is an urgent need to complement store-andforward, multimedia communication via electronic mail or World Wide Web with real-time collaborative tools which facilitate interaction and the sharing of concepts, resources, and information objects. Desktop video conferencing offers such tools. TCP/IP-, ISDN- or ATM-based networks can provide a powerful underlying network and the prices for multimedia-capable PCs have fallen dramatically.
Real-time collaboration and synchronous communication are the center of interest of researchers and technology developers worldwide having completely deployed electronic mail and the World Wide Web (WWW). The goal is to complement and to make more efficient the huge number of meetings with all the associated circumstances such as hours waiting at the airport, inconvenient seats in an airplane, hotel prices which are too high for a travel budget or meeting attendees suffering from jet lag. If traveling is too cumbersome, dangerous (gulf war) or expensive and telephone talk is unsatisfactory, video conferencing is an alternative.
There are many products for desktop video conferencing on the market and in principle it should be easy to work with them particularly as most of them pretend to be ’plug-and-play’ software: make a choice, install, and advance to a new world of desktop video conferencing. Unfortunately, things are not that simple.
When you can see the people you are talking to, communication is likely to be more productive. Though 90% of all communication is speech-based, images, pictures, and video play an important role in the interpersonal relationship and complement and optimize the speech by the real-time transmission of moving pictures. This is also applicable to the private sector, especially to old or disabled people living in rural or urban areas and tele-teaching and distance-learning can be seen as one special form of video conferencing.
The following paper is based on several years of experience in establishing and running value-added services at GMD and on national and international level. We had the idea to establish a regular desktop video conferencing service on easy terms but this did not work out. Most desktop video conferencing products are of less convincing quality and are not comparable and compatible with each other. Our paper is an approach to quality assessment and quality evaluation of desktop video conferencing systems and of building a value-added service with incompatible products. It describes our experiences and it should help decision-makers, planners, and service providers to set up their own desktop video conferencing service on easy terms.
Figure 1 shows that video conferencing in general will not replace face-to-face meetings. Basically, a big part of the communication among people can be done by email, fax or computer conferencing especially if routine work has to be done, multimedia contents are not needed and only some deterministic decisions have to be communicated to other people without any discussion. As soon as personal involvement is needed, as digital, text-based information becomes unsatisfactory and as a less formal, undeterministic behavior of the communication partners is to be expected, video conferencing is the last step before having a face-to-face meeting.
1 Now working for AOL
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content: structured, digital information
Communication Function
M. Bogen
relationship: unstructured, analog information
high Face-to-faceMeeting
undeterministic, individual, informal task
Videoconferencing Information Transfer (Richness)
Task Complexity Telephone
Email, Fax, Computer Conferencing
deterministic, routine, formal task
low uncertainty reduction
Communication Motivation
equivocality resolution
Figure 1: Communication Matrix [1]
available on the market as a good basis for a regular service and how we rate them in practice. The last section sums up the status of our work and presents parts of our present discussion.
The experiences made in the early eighties using special rooms (studios) for videoconferences showed that people do not like to make additional efforts to communicate or co-operate. A regular service around the clock and as simple as telephone is needed. What is needed for this? World-wide interoperability (‘I don‘t care about my communication partner's system and I don‘t know anything about it!‘), extensive, global and distributed directories with names, numbers and connect information (‘Where the hell is the IP address of the reflector?‘) in contrast to local address or phone books, and archives (‘Which price did he offer in our last videoconference?‘) to mention only a few.
Berlin 2 Institutes
Sankt Augustin
GMD is a distributed company with 1200 employees (figure 2). In this paper we describe our experiences in building a desktop video conferencing service. Our work is embedded in the international TERENA project ‘DeViCe‘ (Desktop Video Conferencing in Europe) [2].
4 Institutes
Darmstadt 2 Institutes
Section II of this paper gives a short introduction to the standards dealing with video conferencing. Section III describes our general quality assessment scheme, which can be used to evaluate any networked multimedia application. The next section shows our individual choice of desktop video conferencing products out of the eighty products [3]
Figure 2: GMD Locations
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Standard H.310
Network ATM
Release Date (11/96)
H.320
ISDN
(03/96)2
H.321
ATM
(03/96)
H.322
LAN
(03/96)
H.323
LAN
(11/96)
H.324
Telephone
(03/96)
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Title of the standard Broad band and audio-visual communication systems and terminals Narrow-band visual telephone systems and terminal equipment Adaptation of H.320 visual telephone terminals to BISDN environments Visual telephone systems and terminal equipment for local area networks which provide a guaranteed quality of service Visual telephone systems and equipment for local area networks which provide a non-guaranteed quality of service Terminal for low bit rate multimedia communication
Table 1: The H.32x Family
II. Standards and Related Interoperability Problems
The main difference between the recommendations is the underlying network, which is tightly coupled with the quality characteristics of the conference. First and foremost, the bandwidth of the network is the dominating characteristic. Bandwidth is first allocated for the critical audio channel; the rest is available for video and data channels.
The premier standardized recommendation for today’s desktop video conferencing is H.320 [4,5]. In fact, H.320 only covers the technical requirements for narrow-banded visual telephone services, references many other subordinate standards, and is itself part of the general framework for audiovisual services defined in H.200 [6]. The framework is structured in service definitions, infrastructure, and system and terminal equipment.
Figure 3 shows the classification of the different H.32x-recommendations. While H.320, H.321, H.322, and H.324 have a guaranteed bandwidth, the achievable quality is pretty foreseeable. This is not clear at all with H.323-based conferencing, which has no guaranteed bandwidth. This is subject to further investigation.
H.320 is part of the system and terminal equipment section. The service definition of desktop video conferencing is described in F.730 [7]. F.730 classifies the video conferencing into two main categories: basic video conferencing services and high-quality video conferencing services. Most issues for the high-quality video conferencing services are for further study–the goal being broadcast-like quality. The quality characteristics for the basic video conferencing service are picture quality, audio quality, differential delay between audio and video signal, overall delay and disturbances. We have taken this and other quality aspects for our classification of the products (see next section).
While H.321 is just an addition to H.320 for desktop video conferencing over ATM, the ITU-T also develops H.310 [8], which is currently in ballot. H.310 is a whole new approach for video conferencing using the quality-of-service features of native ATM. Future high-end video conferencing products will probably be based on H.310 and in
high
The H.320 recommendation is intended to cover the requirements for visual telephone services over similar isochronous digital networks, i.e. ISDN. Other H.32x standards exist for various types of networks as shown in table 1.
H.321 Video
The H.32x recommendations themselves do not define the details of protocols needed to ensure interoperability but instead refer to a collection of detailed recommendations which together provide the necessary framework. All of the H.32x recommendations work in the same way, but only use other recommendations.
H.322 H.320
low
H.324 low
H.323
Audio
high
Figure 3: The quality of the recommendations
2 There was a revision of H.320. The first recommendation
is from 03/93.
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Characteristic
Topic
Quantity
CPU
operating requirement
386/486/Pentium, RAM, monitor, disk space, swap space, ...
type and number of slots
EISA, ISA, PCI, PCMCIA, SCSI
window size
80 * 96 pixels to 1152 * 870 pixels
window colors
4-bit gray scale,..., 24 bit true color
bandwidth requirement
64 kbit/s (H.320) to 622 Mbit/s (ATM)
topology
TCP/IP, ISDN, ATM, POTS, LAN, FDDI, Frame Relay, SMDS, Satellite, Novell
Monitor
Network
Table 2: Infrastructure Characteristics theory be able to communicate with existing H.320based products.
We started by defining different classes of quality characteristics with related topics for a desktop video conferencing system. We concentrated on the quality of the communication and the interaction (figure 4) and we neglected on purpose other quality aspects such as the interface, the documentation, the installability and the handling of the product.
The latest important standard is T.120 [9]. T.120 also only refers to a collection of subsidiary recommendations. T.120 focuses on interoperability on the data channel, thus offering additional features such as whiteboard, file transfer, and others. The major infrastructure standards are T.124 [10], which covers Generic Conference Control (GCC), and T.122 [11] and T.125 [12], which together define the Multipoint Communication Service (MCS) with embedded security features [13].
Table 2 shows the quality characteristics important for any networked multimedia application on the Internet or in Intranets. One of the biggest issues with any video conferencing system is the bandwidth required to support simultaneous video, voice, and data transmission. In consequence, the underlying network infrastructure has to be as performant as possible.
III. Quality Characteristics for Desktop Video Conferencing Systems
Table 3 shows the most important characteristics for the desktop video conferencing systems/products themselves. If they are not convincing from the start nobody will use desktop video conferencing products or subscribe to a desktop video conferencing service.
To assess the quality of networked multimedia applications there are basically two approaches: identify and describe all quality characteristics of a system and assess the features of the systems/products under comparison or try to concentrate on the most important ones.
Characteristic
Topic
Quantity
system
H.200-conformity
yes/no
transmission delay
< 40 ms (H.323)
video/frame rate
1-30 frames per second (fps) (25 for PAL, 30 for NTSC)
video-codec
H.261, JPEG, MJPEG, MPEG, NV, CellB, SRC, INDEO, DVE2, AppleVideo
video resolution
CIF, FCIF, QCIF
video
video signal formats NTSC, PAL, SECAM, D2-MAC, HDTV audio
audio-codec
G.711 (3.1 kHz), G.722 (7 kHz), G.728 (3.1 kHz with 16 kbit/s)
packet loss end-to-end delay
< 200 ms (ITU)
audio card
half-duplex, full-duplex
echo suppress
-25 … -80 dB
Table 3: System, Video, and Audio Characteristics 922-4
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Feature
Table 4 shows the optional add-ons, which may or may not be important depending on the scenario. According to [13], application sharing is the people’s most popular additional feature. Where this is not sufficient, it is possible to integrate desktop video into your own application by using software development kits available from some manufacturers.
Quantity
T.120-conformance
yes/no
white board everywhere
yes/no
Remote camera control
H.281, H.224
multi-point: 1-1, 1-n, n-m
H.231-, H.243-, H.331
Call management tools
yes/no
Snapshots
yes/no
Background file transfer and email
yes/no, ft rate
API and plug-ins
software development kits (STK)
Picture in picture (pip)
yes/no
Complementary tools
MPEG player
Table 4: Add-ons
DTVC-System Video
Audio
Camera
Microphone
Monitor
Loudspeaker
VideoCodec
AudioCodec
CPU
H.323 MCU Non-Guaranteed QoS LAN
H.323 Gatekeeper
H.323 Gateway
Guaranteed QoS LAN
GSTN
V.70 Terminal
H.324 Terminal
Speech Terminal
H.322 Terminal
H.323 Terminal
N-ISDN
Speech Terminal
H.320 Terminal
Figure 4: Desktop Video Conferencing
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B-ISDN
H.321 Terminal
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small set of quality characteristics, which are dictated by our network infrastructure and by user demand (table 5).
IV. Products Evaluated by GMD At GMD, we tried to classify available desktop video conferencing products based on the criteria listed above. The goal was to obtain a representative set of products for each quality-level and platform— from low-end PCs and Macs to high-end workstations, from cheap, black-and-white QuickCam cameras to 155 Mbps ATM conferences. The result would form the basis for planning the further deployment of desktop video in our company.
In the following, we present three categories of products that we think share typical areas of use in the framework of our project. Clearly, our selection is not even close to being exhaustive. However, we do think that it does reflect the state of development of the desktop video market.
IV.A. Low-Bandwidth Desktop Video over TCP/IP
It turned out that despite of about 80 desktop video products being available at that time, it was an impractical approach. Sometimes the product specifications were inaccurate—often promising features to be included in future releases. Some products use special, usually higher-quality hardware to achieve superior results that cannot be compared to softwareonly products. You cannot compare such products without actually running them yourself. Finally, product availability in Germany was often limited. Talking to vendors, obtaining information, and actually buying a product was more difficult than expected.
Quite a number of popular products fall in this category, because most of the currently available Internet-based desktop video conferencing as well as video phone applications use efficient compression techniques to be used with a little as 28.8 kbit/s Internet connections. However, we find that results are not really satisfactory until you use at least ISDN throughput (64 kbit/s). Given the absence of standards for desktop video over TCP/IP until now, it is no surprise that, with a single exception noted below, there is no product interoperability. Because it is not possible to install each and every product that our potential conference participant may use, we concentrated on White Pine’s well-known and widely-used Enhanced CUSeeMe.
Besides all that, product interoperability is a key issue in our heterogeneous, multi-platform environment. Unfortunately, the only widely deployed standard is H.320 for video conferencing over digital isochronous networks (ISDN). Many products claim H.320 compliance (even some which obviously did not support ISDN at all...), but in general they do not guarantee interoperability with particular products. It is up to third-party institutions, such as the Telecom Test Center in Stuttgart, Germany, to perform independent H.320 interoperability tests to rely upon.
IV.A.1. Enhanced CU-SeeMe Enhanced CU-SeeMe is the commercial version of Cornell University’s popular CU-SeeMe product. Unlike the Cornell version, which is distributed free of charge and offers black-and-white video only, Enhanced CU-SeeMe includes color video, a whiteboard, and IP multicast support.
In the LAN area, releases of H.323-compliant products such as Microsoft NetMeeting, Intel ProShare, and PictureTel LiveLAN are expected. However, because of the lack of standardized protocols for desktop video conferencing on the LAN until now, all currently shipping LAN products employ private protocols.
Besides being available for free, CU-SeeMe features some key advantages, which made it so popular: it runs on almost any multimedia PC and Mac, there are low-cost video cameras such as the Connectix QuickCam available that do not require costly video capture hardware, and it does work with TCP/IP across the Internet.
Given the general lack of interoperability, we felt a product classification by the criteria in chapter III does not make sense because the products are not comparable. Instead, we concentrated on a very
A so-called reflector is used for multipoint group conferencing. Every client connects to the reflector,
Characteristic
Requirement
Audio
At least phone quality
Video
Either color or b/w
Network Bandwidth
Guaranteed vs. Non-guaranteed bandwidth, i.e. multilink ISDN or ATM vs. LAN
Usability
Product reliability and robustness; acceptance by the users
Cost
Cost efficiency
Platform
Windows PC, Apple Macintosh, various UNIX Table 5: Practical characteristics for categorizing desktop video products 922-6
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which redistributes traffic to all other recipients. While there are provisions in the product to limit the bandwidth used, sending replicated UDP traffic over a serial line may potentially affect other applications on this line that use congestion control, e.g. TCP. Enhanced CU-SeeMe’s support for IP multicast is a big step forward for efficient scalability.
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IV.B.2. ProShare™ Video System 200 ProShare™ Video System 200 by Intel was the first H.320-based desktop video conferencing product for the PC. Similar to PictureTel, the product is completely self-contained. However, we encountered difficulties while installing the hardware because the supplied boards for the ISA bus were not compliant with Intel’s plug-and-play standard. The user interface is dramatically different to Windows 95 standards.
Currently, we could not get CU-SeeMe to work with full-duplex audio boards. This is quite a problem, because half-duplex audio conferences are usually not acceptable to our users.
Otherwise, ProShare is a robust product, too. The results are analogous to those for PictureTel. When talking to another ProShare, it is possible to use Intel’s Indeo video codec, which showed the best video screen we saw for a H.320 product on a dual-channel ISDN connection (128 kbit/s).
IV.B. H.320-based Desktop Video All H.320-based desktop video product that we have in-house use vendor specific hardware for connecting to ISDN, audio, and video frame grabbing. Compared to low-end solutions that use standard, but poor-quality sound blaster connectors, the audio has greatly benefited from this. In particular, problems regulating the sound volume and managing the audio controls have been greatly reduced. Full-duplex audio is standard.
A ProShare software development kit is available from Intel.
IV.B.3. ERIS Manufactured by RSI Systems, the ERIS is a stand-alone box with all physical interfaces needed to run a videoconference—ISDN, audio, and video connectors—as well as a SCSI interface to a host. In other words, an ERIS box is a platform-independent desktop video solution. ERIS client software is currently available for MacOS and Windows 3.1 only, but Windows 95 and NT, and Sun Solaris clients will follow.
All of the products presented in this category have proven interoperability with the other H.320 products. However, it is quite common to use alternate audio and video codecs when the same product is used on both sides. For example, Intel ProShare to ProShare uses the Indeo video codec to deliver improved video quality.
IV.B.1. PictureTel Live200
We have successfully tested interoperability of ERIS on a Macintosh against ProShare and PictureTel on Windows.
Live200 is the Windows 95 version of PictureTel’s LiveWare family. The product is completely self-contained—it includes the software, a PCI ISDN-Audio-Video multi-function board, a camera, the speakers, microphone, and a headset (ear clipped microphone and speaker). However, it is not possible to use any of that equipment for anything else other than for PictureTel. In practice, we do need two sets of speakers and microphones—one for PictureTel and one for anything else. The headset is quite convenient to eliminate audio echo or background noise problems.
Initially, we considered QuickTime Conferencing for the Mac, but the results were disappointing and we did not further use this product. ERIS was the only H.320-compliant conferencing that we could find for the Macintosh. We intend to use the ERIS box for H.320-based conferencing for Sun Solaris systems in the future.
IV.C. H.323-based Desktop Video Early releases of H.323-based desktop video products are becoming available now. This includes Microsoft’s NetMeeting Beta 2 and Intel Videophone Beta. Both products do have limitations, but our preliminary results have been encouraging so far. In particular, Microsoft NetMeeting, being a software-only, standards-based, and full-featured product, will certainly become rather popular in the near future. We have successfully tested all of the currently supported features including H.323interoperability with Intel’s Videophone, and T.120based collaboration features.
In general, Live200 is a robust product. We only hit one problem in case the ISDN connector is not plugged in properly. Other than that, we were able to run our test series of conferences, including interoperability with ProShare and ERIS as well as application sharing with another PictureTel, with or without ISDN channel bundling, very smoothly. When talking to another PictureTel, the product uses a proprietary PictureTel codec to improve the video quality. A software development kit is available for this product that you can use to integrate PictureTel desktop video into your own application.
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our tests, hardware-based solutions generally satisfy higher expectations—at an increased cost.
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is because the video codec is implemented in software and the calculation work to do depends on the differences in the video frames.
IV.D. High-Quality Desktop Video
IV.D.2. Multi-Media Collaboration (MMC)
Currently, high-quality desktop video conferencing requires the use of dedicated workstationclass hardware and sometimes rather expensive frame grabbers such as a Parallax board for the Sun Sbus. Fortunately, the expensive hardware can be reused for all desktop video products discussed here. We used Sun SPARCstations with a Parallax PowerVideo board and Digital Alpha machines with Digital’s FullVideo Supreme video board. All machines were connected to Ethernet as well as our ATM network.
The MMC software was developed by a consortium of vendors under the auspices of the BERKOM project to provide a vendor independent crossplatform desktop video solution. The underlying protocol specification is available to members of the MMC consortium and every member develops his own product according to this specification. Therefore, every member offers his own MMC product. GMD routinely holds a weekly videoconference to New York using MMC over ATM with excellent results regarding the quality of audio and video, network performance, and convenience for the participants.
Similar to low-bandwidth desktop video above, there is no cross-product compatibility. However, products of this category utilize machine and network resources rather heavily to achieve the highest possible audio and video quality, so H.323 compatibility is not a suitable goal anyway. What we feel is needed is oriented towards H.321 or H.310—desktop video on native ATM using guaranteed quality-ofservice.
Currently, we are testing MMC for Sun Solaris, Digital UNIX, and Microsoft Windows. Unfortunately, MMC is not something to take out of the box, install, and it works—we had, and still have, quite a few difficulties to solve, before we can present our interoperability and usability results.
Two products to be included in this category, InSoft Communique! and Paradise Simplicity could not be evaluated, because of technical problems that are to be resolved by the manufacturers.
For the Mbone and MMC product, the excellent results were based on using a manually configured ATM permanent virtual circuit with guaranteed bandwidth between the participants. In fact, this is something that H.310 will do automatically behind the scenes.
IV.D.1. Mbone The well-known Mbone tools vic, vat, and the session directory tool sdr have been developed to distribute audio and video on the large scale over the Internet using IP multicast. When used in highspeed, low-delay, zero packet-loss networks, it is possible to conduct conferences with a good quality. GMD and the University of Geneva using a 4 Mbps ATM virtual circuit over the European ATM pilot network have demonstrated this.
V. Conclusion Today, a wide range of desktop video conferencing products is available both commercially and in the public domain. The questions to ask are: Do you need interoperability between platforms and operating systems? Do you want to deploy wide-area conferencing using ISDN and H.320? Of course, you may also have a LAN environment with sufficient bandwidth, so your goal is to achieve the highest quality possible. As it has been presented in the previous chapters, the answer is not easy and has many facets. Even more complexity is added to these questions if you have the intention of setting up a regular desktop videoconferencing service.
Beware that this is a completely different scenario compared to running a conference over the public multicast backbone, which will never provide the “perfect” network that we use locally and for demonstrations. In particular, the audio quality is quite vulnerable to packet-loss problems. Assuming adequate video frame grabbers, we found that the quality of the video is more limited by the processor, which handles all video encoding and decoding in software, than by the network. A saturated CPU can lead to jerks in the continuity of media streams. During our conferences with Geneva, we have rarely used more than 1 Mbps to avoid these problems.
A lot of information about desktop video conferencing products has been produced only for advertising, commercial and market-strategic purposes. But, no product supplier or dealer is really interested in selling just 2 software packages to a customer with the expectation to run into difficult discussions with scientists of a major research lab about the quality and about enhancements urgently needed for their software.
Also, the Mbone tools do have some disadvantages that are inherent in their design. First, there is no synchronization between the audio and the video, because the vat and the vic tools are completely independent of each other. Both do work stand-alone. Second, the achieved frame rate is not constant. This
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PictureTel, and Microsoft have stated their intent to provide standards-based desktop video conferencing products. Besides interoperability, the next generation products will support multi-point conferencing and will integrate with existing H.320-based infrastructure, thus protecting our investments in this area.
M. Bogen
[14] P. Watzlawick, J.H. Beavin, D.D. Jackson: Menschliche Kommunikation: Formen, Störungen, Paradoxien, Bern: Huber, 1971
Author Information Manfred Bogen has been active in the area of group communication, X.400 development, and X.400 standardization since 1983. In 1987 he became head of the research group ‘value-added services (VaS)’ being responsible for the establishment and provision of value-added services for the scientific community in Germany on behalf of the DFN association. He studied computer science at the University of Bonn and is co-author of two books about X.400 and distributed group communication. At present, he is the convener of the TERENA working group on quality management for networking (WGQMN), a member of the TERENA Technical Committee and the Internet Society and involved as program committee member in the organization of international networking conferences.
Finally, we have found a clear need for high-end desktop videoconferencing based on native ATM’s unique quality-of-service features. Therefore, we will consider H.321 and H.310-compliant products to satisfy high-end user requirements as soon as they become available. We are sure to have more results at the time of the JENC8 conference and we hope to get an opportunity to present them in addition to the issues described in this paper.
VI. References [1] Alexander Lautz: Video Conferencing: Theorie und Praxis für den erfolgreichen Einsatz im Unternehmen, PhD Thesis, St. Gallen, Hochschule für Wirtschafts-, Rechts- und Sozialwissenschaften, ISBN 3-927282-37-5, Mai 1995
Christian Bonkowski has finished his education as mathematic-technical assistant at GMD in 1992. He has been the postmaster of the DFN X400BITNET-Gateway. As a member of the research group ‘value-added services (VaS)’ he is at present involved with the introduction of desktop videoconferencing on a company-internal but also on an international level (TERENA).
[2] http://www.terena.nl/projects/device/ [3] http://www2.ncsu.edu/eos/service/ece/project/ succeed_info/dtvc_survey/ [4] Barry Aldred: Desktop Conferencing, McGraw-Hill Book Company, London 1996
Richard Rodriguez-Val has been working at GMD since 1987. He is responsible for GMD's VM systems and the technical support of all VM applications and networking software (TCP/IP, SNA and OSI). He has been actively involved in supporting EARN/BITNET in Germany since the early days. He is currently the postmaster of the official DFN Interbit gateway at GMD. Additionally he is involved in quality and performance measurement of PCs and high-speed networks such as ATM.
[5] ITU-T Recommendation H.320: Narrow-Band Visual Telephone Systems And Terminal Equipment (1993/1996) [6] ITU-T Recommendation H.200: Framework for Recommendations for Audiovisual Services (1993) [7] ITU-T Recommendation F.730: Videoconference Service - General (1993)
[11] ITU-T Recommendation T.122: Multipoint communication service for audiographics and audiovisual conferencing service definition (03/93)
Dr. Clemens Wermelskirchen is working at GMD since 1975. He received his master's degree in Physics in 1982 and his PhD in 1988 from the University of Bonn. He worked in the development and operational area of electronic mail services using OSI and Internet protocols (X.400, X.400, SMTP, and Gateways). As part of the research group ‘valueadded services (VaS)’ of GMD he is responsible for the establishment and introduction of a desktop videoconferencing service at GMD. Since 1995 he is also the chairman of the Network-SIG (special interest group) of the German chapter of DECUS (Digital Equipment Computer User Society).
[12] ITU-T Recommendation T.125: Multipoint communication service protocol specification (04/94)
The authors can be reached at GMD - German National Research Center for Information Technology, D-53754 Sankt Augustin.
[8] ITU-T Recommendation H.310: Broadband and audiovisual communication systems and terminals (in ballot) [9] ITU-T Recommendation T.120: Data Protocols for Multimedia Conferencing (1996) [10] ITU-T Recommendation T.124: Generic Conference Control (08/95)
[13] Louise Lindop (Tech. Editor): Here‘s looking at you, PC Magazine November 1995, pp. 191231 922-9
