Thomas Walter1, Burkhard Stiller1, Hans Hänni2, Bernhard Plattner1
Distributed Learning Scenarios supported by ATM – Experiences in Télépoly – 1
ETH Zürich, Institut für Technische Informatik und Kommunikationsnetze
[email protected],
[email protected],
[email protected] 2
ETH Zürich, Didaktikzentrum
[email protected]
Abstract Research in distributed learning, tele-teaching, and computer-based education evolved over the last five years. Initial approaches of using stand-alone workstations with tutorial programs have been refined to networked solutions, where a lecturer is integrated into a distributed scenario and workstations are applied to handle communication purposes only. Exploiting the rich area of available multimedia technology results in the usage of high-speed networks, e.g., Asynchronous Transfer Mode (ATM), for distributed learning scenarios to take real advantage of high-quality audio and video transmissions. This technology offers an acceptable and sufficient solution for virtual classrooms and remote teaching. 1
Introduction
In order to gain practical and didactic experiences in distributed learning scenarios, while exploiting multimedia technology and high-speed networks, a tele-teaching experiment has been established between the Swiss Federal Institutes of Technology in Zürich (ETHZ) and Lausanne (EPFL). Three main areas of application-oriented communication research and didactic research have been tackled within the Télépoly project. In particular, they cover: •
Network support for dialog-oriented, isochronous services;
•
Technical equipment to be utilized, such as advanced communication devices for ATM and workstations; and
•
Tele-teaching as a didactic concept in support of general education.
Télépoly involves the concept of distributed, synchronous, and interactive teaching. Télépoly is distributed teaching, since a traditional lecture is communicated to a single or multiple remote sites. In addition, the scenario involves a strong synchronous aspect, because people at all involved sites have to meet at the same time in dedicated and technologically equipped classrooms. Finally, Télépoly entirely is interactive due to the fact that students at either site may ask questions at any time during the lesson and can communicate interactively with the lecturer.
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This paper is organized in the following manner. Section 2 gives an overview of the Télépoly scenario. Section 3 presents a brief discussion of utilized teaching aids. In addition, Section 4 examines networking prerequisites, in particular the ATM network applied. Section 5 deals with details of the technical approach, focusing on multimedia technology in use. Section 6 outlines practical experiences gained, while Section 7 states didactic experiences. Allowing for a comparison with alternative tele-teaching approaches, Section 8 covers related work. Finally, Section 9 concludes and delivers a resume of the Télépoly project. 2
Télépoly Scenario
In support of networked tele-teaching for both sites in Lausanne and Zürich, a classroom is equipped with multimedia audio and video devices including cameras, monitors, microphones, and loudspeakers (cf. Figure 1) to capture and present audible and visible information, respectively. For digitizing analog audio and video a coder/decoder device (CODEC) is used, which in turn prepares these digital data for transmission (cf. Section 5). In addition, a workstation is employed for the exchange of electronic teaching aids, such as slides, graphics, or tables which are locally and remotely displayed by a beamer. The CODECs and the workstations at both sites are interconnected via a high-speed network (cf. Section 4). Given the situation that the lecturer is located at ETHZ, audio and video streams of the classroom in Zürich are transmitted to the remote site in Lausanne. Firstly, a mixed audio of the lecturer's voice and the local audience is sent to the remote site. Secondly, separate video streams of the lecturer and the local classroom are transmitted to Lausanne. From the remote classroom at EPFL, audio and video streams are received at ETHZ, where remote audio is directed to loudspeakers and the remote classroom view is displayed on two different TV screens. One TV screen in Zürich is directed to the lecturer, to ensure that she or he is aware of the situation at the remote site; in particular, if a student at the remote site is asking questions. The second monitor is used for the local audience to expose the remote audience. Transmission and presentation of audio and video is co-ordinated to convey that each site has a perceptible feeling of tele-presence, like being in a single classroom.
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Video
Audio Video
Video
Audio
Data
Beamer Outgoing Streams
Incoming Streams
Figure 1: Télépoly Scenario
The one-to-one tele-teaching scenario described above can be extended in principle to a manyto-many scenario. In practice, however, a scenario will cover a limited number of participating sites only – reasonably up to five – since the tele-teaching session has to remain manageable. After transmitting many lectures based on the one-to-one scenario, recently a Télépoly session between three sites was performed: the Swiss Center for Scientific Computing (SCSC) in Manno, ETHZ, and EPFL. As a consequence the scenario changed as follows: As before, from the local site a mixed audio and two separate video streams (lecturer and local classroom) are sent to all connected sites, technologically supported by networking features (cf. Section 4). In order to present audio and video from two remote sites, the local classroom additionally is equipped with a second set of loudspeakers and a TV screen so that audio and video are directed to these devices dedicated to the first and second remote site respectively. Each remote site receives two audio and three video streams: a mixed audio of the lecturer and the local classroom's audience as well as an audio from the second remote site, two separate video streams from the local site (lecturer and local classroom), and a video stream of the second remote classroom view. Audio and video streams are directed to two sets of loudspeakers and monitors. 3
Teaching Aids
Teaching aids, such as slides, graphics, or tables, in general are applied during lectures to view, display, or present issues and topics of a certain area. Usually, within traditional teaching, these teaching aids are applied in a hardcopy type of form, such as an overhead slide, paper sheets, or blackboard texts and drawings. Therefore, they are visible within a single classroom only utilizing a traditional overhead projector or a blackboard. Konferenz Hochschule und Weiterbildung - Mediengestützte wissenschaftliche Weiterbildung, Braunschweig (D), März 1997
In contrast, teaching aids for distributed learning or tele-teaching are accessible in an electronic form on workstations, such as an electronic slide in PowerPoint format, a spread-sheet in Excel format, or regular text in FrameMaker® format. These electronic documents have to be distributed between local and remote using an application sharing tool. The applied application sharing software runs on standard workstations on top of a TCP/IP protocol stack (Transmission Control Protocol/Internet Protocol). For interconnecting workstations two solutions have been handled and tested. In the first one, both workstations are connected to the Internet, which is in most cases straightforward. In the second solution, workstations are equipped with an additional interface and connected to the same network which also is used for the transmission of audio and video streams. 4
ATM Network
The networking infrastructure used for the Télépoly tele-teaching experiment is the Asynchronous Transfer Mode (ATM). Since ATM [1] supports data rates at high bandwidth and low delays with guaranteed quality of service, applications like tele-teaching are adequately supported, in particular to offer the required degree of interactivity as well as high quality audio and video.
Digital Dat a
AAL PDU
ATM Cell
ATM C ell
…
ATM Cell
Figure 2: Layered ATM Protocol Architecture
As depicted within Figure 2, the layered ATM protocol architecture for end-systems processes digital application data from upper protocol layers which are handed to the ATM adaptation layer (AAL) first and assembled into AAL Protocol Data Units (PDU) afterwards. In general, different ATM transport services are supported by distinct AAL protocols. In case of digitized audio or video, the AAL type 5 (AAL 5) protocol is applied. Within the next lower layer, the ATM Layer, AAL PDUs are segmented into ATM cells, which comprise the basic ATM transmission unit of 53 byte, containing 48 byte payload. Finally, these ATM cells are transferred to the directly attached ATM switch via an optical link, which is specified within the Physical Layer.
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Considering the current ATM network configuration in use (cf. Figure 3), the ATM switch in Zürich receives cells from the local end-system, which comprises in the Télépoly setting the CODEC. The switch is connected by a bi-directional 155 Mbit/s STM-1 link (Synchronous Transfer Module Level 1) to the ATM network of the Swiss Bundesverwaltung, also called KOMBV 3. At every participating remote site another ATM switch is connected by a 155 Mbit/s STM-1 link to KOMBV 3 as well. It receives transmitted ATM cells and forwards them to the remote end-system, again the CODEC, applying two different virtual circuits (VC) for audio and video streams, respectively. Each VC in use separately provides the bandwidth and cell delay variation required for these detached multimedia data. As ATM inherently supports the multicasting functionality during the ATM connection set up time, a many-to-many communication between n different sites is available. Without any major adaptations of the one-to-one Télépoly scenario and cabling of utilized devices (cf. Section 2), except for adding a supplementary set of loudspeakers and TV screens per additional remote site, two multicast permanent virtual circuits (PVC) are used instead of unicast VCs. In this case, the generated audio and video data, respectively, are duplicated n times transparently for the sender and all receivers inside the ATM switch. Every participating site receives multicast data streams likewise as unicast data streams. 5
Technical Approach
For simplicity reasons concerning the one-to-one tele-teaching scenario as depicted above, three TV-quality video streams, one single CD-quality audio stream, and one data stream are transmitted from Zürich to Lausanne (cf. Figure 1). This covers (1) one single video stream of the lecturer, (2) one single video stream of the local audience, and, in addition, (3) optionally one single video stream of a document camera. The use of a document camera is required, whenever the lecturer likes to present material which is not available electronically, for instance statistics from a present-day journal. In addition, a mixed audio of the lecturer's voice and of the local audience is transmitted. From Lausanne one video stream and one audio stream are received. Analog audio and video signals are fed into the CODEC (cf. Figure 3), transforming analog signals to digital ones and vice versa. The CODEC consists of three units, called CellstacksTM: one master and two slave units. Hosting an analog to digital converter, a single Cellstack is capable of processing incoming and outgoing streams, one audio and one video stream each. It performs full Motion JPEG (Joint Pictures Expert Group) compression on video. Each Cellstack assembles this compressed video into AAL 5 PDUs and segments them into ATM cells. Since only the master unit is connected to the ATM switch, all ATM cells from slaves and the master units are transferred via the interconnected STM-1 link to the ATM switch. Simultaneously to JPEG compressed video transmission, audio is transmitted in CD-quality: analog Konferenz Hochschule und Weiterbildung - Mediengestützte wissenschaftliche Weiterbildung, Braunschweig (D), März 1997
audio is sampled and a CD compliant 16 bit digital stereo data stream is produced. Digitized audio data is handled accordingly. Remote Cellstacks reversely transform ATM cells into AAL 5 PDUs, compressed data into the uncompressed form, and digital to analog audio and video. 2
2 2
Slave
Slave
2 2
2 2 2
Slave
Slave
2
Mast e r
CODEC
CODEC
Mast e r
155 Mb it/s KOMBV 3 155 Mb it/s STM-1 ATM Ne t work STM-1
A TM Switch ETHZ
2 2 2
A TM Switch EPFL
Figure 3: ATM Network Configuration
6
Practical Experiences
Télépoly committed two experimental phases. The first phase (finished February 1996) involved between both sites of ETHZ and EPFL – a point-to-point approach – the alternating distribution of weekly seminars, held in English and dedicated to topics of networking and multimedia systems. During the summer term of 1996 (phase 2) a 6th semester graduate course on "Computer Networking" has been transmitted from EPFL to ETHZ. Currently, another series of seminars held at both sites are transmitted. Further experiences have been gained in supporting special events in terms of transmitting a summer school between EPFL and ETHZ as well as dedicated talks as a triangular multipoint-to-multipoint approach between the two Federal Institutes in Zürich and Lausanne and the SCSC in Manno. For Télépoly it has been required to use technical equipment available on the market. Off-theshelf solutions are applied for digitizing, networking, and multimedia devices, offering straightforward approaches. However, the interconnection of this equipment requires timeintensive and exhaustive testing to achieve a practical and stable configuration, which can be used and maintained by non-networking specialists. In addition to this equipment, an automatic tracking system for the lecturer's camera has been employed. Sensors have been fixed to the lecturer's jacket and a horizontally and vertically movable Camera ManTM followed closely her or his movements in the classroom to obtain a clear and focused video picture detail. Concerning required data rates for audio and video streams, the measured audio data transmission rates have reached an average value of 1.75 Mbit/s (about 5% variance) due to packing Konferenz Hochschule und Weiterbildung - Mediengestützte wissenschaftliche Weiterbildung, Braunschweig (D), März 1997
audio samples into AAL 5 frames and ATM cells in turn. The JPEG compressed video transmission rate is dependent on the current content of the captured picture. An average rate of 8.5 Mbit/s per video stream has been measured, while variances (about 10%) are dependent on the motion of the transmitted video. Summarizing the one-to-one scenario, a 39 Mbit/s reservation for a KOMBV 3 VC including three video streams in total and a separate 2 Mbit/s reservation for an audio VC additionally has yielded in particularly sufficient results in terms of lossfree and high-quality motion video and CD-quality audio. Applying multimedia technology in terms of audio and video transmission always raises the question of synchronization issues. The achieved lip synchronization between every pair of received audio and video streams has been of high quality as well, due to an unaltered processing of multimedia data within a Cellstack and their timing constraints preserving transmission in the ATM network. Finally, referring to a classroom equipped for Télépoly, required multimedia devices, such as monitors, microphones, loudspeakers, and cameras need to be placed carefully. This encompasses, e.g., the location of the remote video next to the remote audio and the local camera within one single alignment. 7
Didactic Experiences
Apart from the above mentioned technical aspects, also important experiences have been collected during Télépoly phases 1 and 2 concerning the didactic and pedagogical aspects of interactive tele-teaching. Two main areas have to be mentioned: •
Firstly, the method of preparing and organizing teaching aids;
•
Secondly, the style of presenting teaching aids to students, given the restriction that, in order to enable electronic transmission, the used material either had to be stored electronically (documents, slides) or to be presented appropriately (e.g., on a view graph) in order to be visible and audible on the remote site.
Concerning the first issue, the use of a professional presentation package like Microsoft® PowerPoint has been beneficial, since it supports a clear structuring of teaching aids and provides a basic standardization of the layout. Furthermore, for lecturers not experienced with teleteaching it introduces the danger of choosing inappropriately small fonts, thin lines, and tiny graphic elements in order to concentrate as much information as possible onto a single slide. This, however, has to be avoided, since most of the projection devices, such as beamers and TV monitors, are still immature in resolution to display small text and picture elements in a clearly recognizable manner, especially when the projection occurs in a spacious auditorium.
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This problem can be reduced slightly by distributing paper copies of teaching aids to all students prior to every lecture. During phase 2 it has been observed that the following frequently observed weaknesses of conventional teaching become even more severe in case of tele-teaching: too little interaction, too brief animation, and inappropriate time management. Investigations mentioned below revealed that during tele-teaching sessions students in general have to concentrate extensively compared to traditional teachings to follow the lecturer and her or his explanations. Therefore, frequent interactions are a necessity. To trigger those, the lecturer has to motivate (especially remote) students more intensively compared to traditional teachings to ask or answer questions, since sitting in front of a TV screen instead of a lecturer personally often introduces an additional threshold to spontaneous interaction. This is supported by the tendency observed with a lot of students just to sit in the classroom, listening, consuming, and remaining silent. Furthermore, to prevent a quick fatigue and loss of concentration, appropriate and periodic animation has to be introduced during the lecture. Within Télépoly the didactic quality of tele-teaching has been evaluated together with the quality of technical aspects by means of several questionnaires distributed to participants. The obtained results showed that in general the quality of the audio and video transmission has been judged positively as well as the main quality of the computerized teaching aids. Surprisingly, students have been keen to get a direct eye contact with the lecturer. Thus, the video picture detail of the lecturer has to be visible in an appropriate size all the time. The use of the automatic tracking system for the camera (cf. Section 6) satisfied this requirement, while focusing on the speaker and following her or his movements. Concerning the concentration during lectures, the majority of students felt slightly more challenged compared to traditional teaching. This means that in tele-teaching a sufficient number of breaks has to be incorporated which in turn requires good time management. However, in case of Télépoly this was not always easy, since some lectures have been affected at the beginning by small delays introduced by handling all required technical devices. As a result, the amount of teaching material which could be covered in the tele-teaching approach has been considerably smaller compared to an identical course presented by traditional teaching previously. This problem has been amplified by the use of English as the lecturing language. Problems, such as waiting for the initial transmission to be ready between both sites or waiting for a microphone to be handed to a student, who wants to ask a question, remain to be important, as long as properly equipped tele-teaching classrooms are lacking. Permanent installations can be set up easily, while pressing a single button only, and they can avoid initialization problems.
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Related Work
An extract of related work covering issues within tele-teaching in general and a discussion of related projects can be found in [2] which was by itself a distributed event including four sites involved: Madeira (Portugal), Madrid (Spain), Sofia Antipolis (France), and Brussels (Belgium). A related project entitled "Tele-teaching within the Universities of Heidelberg and Mannheim" (Germany) is described in [3]. In addition, [4] deals with issues concerning the virtual university of the future. Furthermore, a description of an early tele-teaching experiment between Universities of Bern and Fribourg (Switzerland) is given in [5]. Finally, prior to its involvement in Télépoly, ETHZ participated in the BETEUS project (Broadband Exchange for Trans-European Usage) [6]. Due to preliminary tele-teaching experiences gained in BETEUS, a brief explanation of this approach is presented here. Similar to Télépoly, BETEUS has been a project exploring the usage of multimedia features in collaborative work scenarios. Functions developed in this project are best captured by the so-called virtual community paradigm, i.e., a group of people located in geographically distant locations with common interests participating in a set of operations towards reaching a common goal. In the case of BETEUS these operations are teaching, learning, project management, and technical design [7], particularly distributed classroom (as in Télépoly), tele-seminar, multimedia document archival and retrieval, tele-tutoring, and collaborative working environments.
Berlin (TUB) Sophia Antipolis
(EURECOM)
Stockholm (KTH)
ATM PILOT
Geneva (CERN)
Lausanne (EPFL)
Zürich (ETHZ)
Figure 4: BETEUS ATM Network
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The network for connecting BETEUS sites at CERN, EPFL, ETHZ, Technical University of Berlin, and Technical University of Stockholm was built on top of the European ATM pilot network. A fully meshed PVC network (Figure 4) was utilized to interconnect all participating sites. Since access to the ATM pilot was not possible for all partners at the same time, the fully meshed network was the optimum solution for launching tests as early as possible during the development of tele-teaching applications. Each site was ATM-connected with all other five sites by means of bi-directional PVC of 3 Mbit/s peak bandwidth. This peak bandwidth was sufficient for a video of 1/4 PAL resolution with 25 frames per second for the lecturer (about 2 Mbit/s), a video of 1/4 PAL resolution with 12 frames per second for the audiences (about 1 Mbit/s), a medium quality audio stream (about 64 kbit/s), and application sharing data (about 60 kbit/s). On top of these PVCs the Internet Protocol (IP) was running over AAL 5 [8]. The use of IP over ATM was straightforward, because ATM hardware interfaces applied supported the necessary mapping.
100 Mbit/s TAXI
155 Mbit/s STM-1
TAXI Parallax (video) SUN SPARC 10
1
VPI 2
VCI 200 and 205 VCI 201 and 206
FORE ASX 200 100 Mbit/s TAXI European ATM Pilot
2 SUN SPARC 10 Parallax (video) TAXI
Figure 5: BETEUS Site Hardware Configuration (e.g., the ETHZ Site)
All BETEUS partners were using identical equipment comprising ForeRunner™ ASX-200 ATM switches from FORE Systems™ [9] and SUN™ workstations (Figure 5). These ATM switches were connected to ATM by E3 or STM-1 interfaces and to workstations by 100 Mbit/s TAXI interfaces. These workstations were equipped with a PARALLAX™ video board which performed Motion JPEG compression on-board. In BETEUS audio processing was performed on workstations in software. In support of the tele-seminar scenario, which equals a face-toface meeting or a collaborative work environment, at least two workstations in every partner’s site were used (cf. Figure 5). To accept ATM connections between all workstations, at every Konferenz Hochschule und Weiterbildung - Mediengestützte wissenschaftliche Weiterbildung, Braunschweig (D), März 1997
site four VCs per connected remote site (2 VCs per workstations) were set up manually. This configuration obtained the possibility to access every workstation in the BETEUS network from every other workstation, to see and to interact with every other tele-seminar or collaborative working environment participant. Compared to the Télépoly technical approach (cf. Section 5), the use of workstations equipped with additional hardware for video processing defines the most remarkable difference between the two approaches. The BETEUS specific solution is motivated by the requirement to implement a platform, capable to support all the collaborative work mentioned above. However, field trials performed between BETEUS partners revealed that video and audio quality in BETEUS has been not sufficient. The lack of synchronization between video and audio streams has been particularly disappointing, especially with a delay up to a few seconds between video and audio while using one workstation only for video and audio processing and application sharing at the same time. Furthermore, due to restrictions of the employed hardware the use of beamers for displaying video was not possible. 9
Conclusions
The Télépoly project provides an excellent technical platform for the exchange of lectures between remote sites, in particular for application-oriented networking and didactic research. Although initially considered as a one-to-one scenario for the transmission of lectures between ETHZ and EPFL only, Télépoly has been and can be used successfully for the transmission of workshops, presentations, and conferences, even in a many-to-many environment. In particular, the technical Télépoly set up allows for lecturing topics on the broad variety of educational materials and is not limited to technical issues at all. Three of the main technologic prerequisites for Télépoly have to be considered, explicitely. This includes the availability of electronic teaching aids, e.g., PowerPoint slides, to apply the application sharing tool, a public ATM connectivity to interconnect with high quality networking to remote sites, and CODEC devices for handling multimedia data, such as audio and video. Concerning the lecturer and students involved, the above mentioned didactic requirements have to be taken into account, especially motivating concerns and time management aspects. Compared with traditional lectures, the usage of conventional blackboards is rather limited. However, the use of a document camera closes this gap providing a method to present paper documents and to develop hand-drawn sketches. Just recently having received the approvement of a concept to establish a Network for Educational Technology (NET) by the directorate of ETHZ, a few classrooms now can be equipped Konferenz Hochschule und Weiterbildung - Mediengestützte wissenschaftliche Weiterbildung, Braunschweig (D), März 1997
with the required technological infrastructure for new media based teaching and learning at ETHZ on a rather short term basis. References [1] M. de Prycker: Asynchronous Transfer Mode; 3rd Edition, Prentice Hall, London, England, 1995. [2] Proceedings of the First IEEE International Distributed Conference on High Performance Networking for Tele-teaching, November 16 – 17, 1995, Madeira, Portugal. [3] W. Effelsberg: Das Projekt TeleTeaching der Universitäten Heidelberg und Mannheim; Praxis der Informations- und Kommunikationstechnik (PIK), Vol. 18, No. 4, May/June 1995, pp 205 – 208. [4] F. Bodendorf, R. Grebner, C. Langenbach: Die Virtuelle Universität; Deutsches Forschungsnetz, DFN Mitteilungen 41, No. 6, June 1996, pp 7 – 10. [5] D. Hogrefe: Fernunterricht per Video-Konferenz, Tele-teaching-Experiment zwischen Bern und Fribourg; UNIPRESS 9/95, Pressestelle Universität Bern, pp 21 – 23. [6] S. Znaty, T. Walter, M. Brunner, J.-P. Hubaux, B. Plattner: Multimedia Multipoint Teleteaching over the European ATM Pilot; International Zürich Seminar, IZS ‘96, Edt. B. Plattner, Lecture Notes in Computer Science, No. 1044, Springer, Berlin, Germany, pp 225 – 238. [7] BETEUS Consortium: BETEUS Application Platform Detailed Specification; Deliverable D6, November 1994. [8] J. Heinanen: Multiprotocol Encapsulation over ATM Adaptation Layer 5; RFC 1483, July 1993. [9] FORE Systems Inc: FORE Runner ASX 200 ATM Switch User´s Manual; Software Version 3.2.x, May 1995.
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