Workload of a Media-Enhanced Classroom Server - CiteSeerX

Workload of a Media-Enhanced Classroom Server Nissim Harel, Vivekanand Vellanki, Ann Chervenak, Gregory Abowd, Umakishore Ramachandran

College of Computing, Georgia Tech Email: fnissim,[email protected], [email protected], fabowd,[email protected] Address: Georgia Tech College of Computing, 801 Atlantic Drive, Atlanta GA 30332-0280 Fax: (404) 894-9442 Phone: (404) 894-3982, (404) 894-9760 * Address: Information Sciences Institute University of Southern California 4676 Admiralty Way, Suite 1001 Marina del Rey, CA 90292-6601 (310) 822-1511 ext. 225

Phone:

Contact Person: Nissim Harel Email: [email protected] Fax: (404) 894-9442 Phone: (404) 894-3982

Abstract

We charaterize a workload of media-enhanced classrooms. Such classrooms include equipment for presenting multimedia streams and for capturing streams of information (audio, video and notes) during a lecture. We present detailed quantitative performance measurements of one media-enhanced classroom system, Classroom 2000. We characterize the workload from the point of view of a server that supports multiple classrooms. The workload includes server bandwidth, network bandwidth and server storage requirements. We identify patterns in user behavior, and demonstrate how the number of simultaneous study sessions varies with time of day and with the proximity of a speci c date to exams.

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Workload of a Media-Enhanced Classroom Server 1 Introduction The ways we teach and learn will be dramatically aected by current, unprecedented rates of improvement in computational power and network bandwidth, as well as the development of innovative user interfaces and virtual environments. Already, some are predicting the demise of traditional universities and the rise of virtual classrooms and institutions. Media-enhanced classrooms are a rst step toward using technology in innovative ways in education. Such classrooms are designed both to improve the classroom experience with multimedia display and to capture an enriched record of lectures for later study. Multimedia display equipment in the classroom allows a teacher to project prepared materials, notes written during class, and material fetched over the World Wide Web. Capture devices such as cameras, microphones and electronic whiteboards record streams of information produced during a lecture. These streams might include audio and video recordings, notes written by the teacher or students, and the output of simulations run during class. The system may further process captured lecture material, for example, to extract keywords from recorded audio streams and construct search indexes. Students may then access the recorded lecture material in a variety of ways. Students in a traditional campus environment may use the system to view a missed lecture, study a dicult concept or prepare for an examination. Students in a distance learning program may use the system to study entire courses over a computer network at a remote geographical location.1 In this paper, we characterize the workload of a media-enhanced classroom server called Classroom 2000 (or C2000). Classroom 2000 has been used for three years in a university environment and has supported over 60 dierent graduate and undergraduate classes and seminars. The C2000 prototype is being used for a variety of research, including human-computer interaction studies and education research. Our work characterizes the Classroom 2000 server workload from a computer systems perspective. We present the C2000 server workload for the Spring 1998 quarter, when the system supported ten courses that included over 300 students. We describe the type and size distributions of les stored on the C2000 server, as well as patterns of le access during four modes of operation: advance preparation of lecture materials by professors, capture of data streams during a lecture, automatic postprocessing of lecture data, and student accesses. We show that media les dominate both the bytes stored on the classroom server and the bytes read from the server. Continuous media les captured by cameras and microphones account for less than 2% of les stored on the server, but for over 86% of the bytes stored. Accesses to continuous media les account for over 79% of all bytes transferred from the server. We also characterize length of media stream accesses and how students \jump" or change their viewing position within a media stream. This workload study also includes access patterns over the length of an academic quarter and over the course of one day. Quarter-long access studies show distinct peaks associated with midterm and nal examinations. Finally, and perhaps predictably, students accesses reach peak levels between the hours of noon and midnight, with the number of simultaneous study sessions rising in the late morning hours and dropping o between midnight and 3:00 a.m.

2 The Classroom 2000 System In this section, we discuss the hardware and software environment of Classroom 2000. Figure 1 shows a picture of the Classroom 2000 environment. At right, an electronic white-board either displays prepared material or acts as a blank white-board where the teacher writes notes. The gure shows two additional non-interactive 1

We do not model the distance learning application in this paper.

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Figure 1: The Classroom 2000 prototype environment in

use. The instructor uses an upright electronic white-board system to present a lecture. The middle screen provides an extended white-board facility. The leftmost display shows output from an instrumented Web browser.

Figure 2: The user interface of The Classroom 2000 system for an undergraduate lecture in Spring 1998.

projected displays. The room also contains an array of 6 ceiling microphones and a wireless lapel microphone to capture audio. A camera is mounted in the rear of the room to capture video. Audio and video are routed to a central audio/video cabinet and distributed to various analog and digital encoding machines. Classroom 2000 digitally encodes audio and audio-plus-video (AV) streams using commercial streaming products from RealNetworks. The main server for the Classroom 2000 system is a Sun Enterprise 450 dual processor, 250-MHz machine. The server's storage system is a RAID 3 disk array with 100 gigabytes of magnetic disk storage. The server runs customized Classroom 2000 system software, called Zen*, as well as commercial software including the RealServer streaming media software from RealNetworks and an Apache Web server. The Apache Web server was con gured to record all accesses into a log le. The Zen* software system includes a ZenMaster server and several Zen* clients. ZenMaster is a multithreaded server that supports multiple physical classrooms simultaneously. In each classroom, a number of Zen* clients provide presentation and recording capabilities. One client is ZenPad, an interactive white-board that displays static background images such as prepared slides and simple multicolored pen annotations. Pen strokes are time-stamped by ZenPad and stored by ZenMaster. Another Zen* client, ZenViewer, displays portions of the lecture on non-interactive, projected displays. A third Zen* presentation client is an instrumented Web browser that keeps a time-stamped history of URLs visited. After a lecture is complete, ZenMaster collects and stores all captured data and launches postprocessing software called StreamWeaver. StreamWeaver adds a teacher's annotations to prepared slides images and creates HTML les with links to the audio and AV streams. Students may then use a timeline interface to browse the slides, notes and Web links presented in class or to access continuous media recordings. Figure 2 shows the Classroom 2000 user interface for an undergraduate lecture.

3 Characterization of Files on C2000 Server In this section, we characterize le type and size distributions for the les created in dierent phases of Classroom 2000 operation. Files are written to the server during three phases of operation: advanced preparation, capture and postprocessing of lecture materials.

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Class cs1501 cs3302a cs3302b cs3361 cs4324 cs4391 cs6393 cs6397 cs6410 cs7322

Prepared Material Pen Strokes Modi ed Slides Created Slides Number Size Total Size Number Pen stroke Number Pen stroke Modi ed Avg. Data Created Avg. Data 0.75 7.30 7.95 0 0 0 6.05 0 28.8 1.67

(KB) 12.29 36.37 36.31 0 0 0 16.68 0 13.29 44.89

(KB) 9.21 265.5 288.66 0 0 0 100.91 0 372.12 74.96

(KB) 0 13.09 13.32 0 0 0 3.11 0 3.94 3.04

0 7.30 7.95 0 0 0 5.24 0 28.8 1.67

6.15 2.39 3.14 9.85 8.10 14.05 2.05 7.22 3.05 9.28

(KB) 64.49 41.53 35.81 41.22 41.11 47.16 41.60 85.63 41.72 31.93

(KB) 396.61 194.81 218.34 406.01 332.99 662.59 101.58 618.24 251.35 301.39

Table 1: Slide and pen stroke information for a lecture. Dierentiates between slides created in advance and those created during class. Also dierentiates between pen strokes on prepared slides and on newly-created slides.

Class cs1501 cs3302a cs3361 cs4324 cs4391 cs6393 cs6397 cs6410 cs7322

Lecture Length Media Files Pen Stroke Files URL Files Total Per Lecture (Minutes) 80 50 80 80 50 50 80 80 80

(KBytes) 13486.1 10547.2 16588.8 15165.4 11950.1 8990.7 17141.8 16660.5 15503.4

(KBytes) 396.61 194.81 406.01 332.99 662.59 101.58 618.24 251.35 301.39

(KBytes) 6.99 2.27 2.82 3.99 2.69 3.04 3.69 2.04 4.57

(KBytes) 13889.7 10744.3 16997.6 15502.4 12615.4 9095.34 17763.7 16913.9 15809.3

Table 2: Measured capacity requirements for C2000 capture phase.

3.1 C2000 Advance Preparation

A professor may prepare class materials such as PowerPoint slides in advance and preload them onto the classroom server. The server software then converts these slides to gif images, which the professor can display on the electronic whiteboard during a lecture. Six courses in Spring 1998 utilized prepared slide materials. Some classes used prepared slides in each lecture, while others used them rarely. The left side of Table 1 includes statistics on prepared slide materials. The table shows that the average size per slide is relatively small, from 12 to 45 kilobytes. The number of prepared slides per lecture varies widely, from less than one slide per lecture to more than 28 slides. For classes that make extensive use of prepared slides, the total amount of data generated in the advance preparation phase is on the order of several hundred kilobytes.

3.2 Capture

The majority of write trac to the C2000 server occurs during the capture of classroom lectures. Microphones and cameras record audio and video streams of the lecturer and students. Capture also includes storing discrete data such as pen strokes generated when a teacher writes notes on an electronic white-board. C2000 also records the URLs of World Wide Web sites a lecturer visits during class. A professor using C2000 during a lecture can write notes on the electronic whiteboard with an electronic pen. These annotations are written either on top of displayed images of prepared slides or on blank slides that are created during class. The annotations are also stored in pen stroke les. Table 1 shows the average size of these pen stroke les for a lecture. In the table, pen stroke data written on prepared slides are labeled \Modi ed Slides", while data for notes on newly-created blank slides are labeled \Created Slides". As might be expected, the four courses that used a substantial number of prepared slides (cs3302a, cs3302b, cs6393 and 4

cs6410) generated less pen stroke data, an average of 100 to 250 kilobytes per lecture. Courses that rarely used prepared slides generated from 300 to 660 kilobytes of pen stroke data per lecture. Other le types created during the capture phase include audio and video recordings of the lecture. Classroom 2000 records audio and AV streams using the Real Audio and Real Media formats. In Spring 1998, Real Audio was recorded at 6.5 kilobits per second and Real Media streams were recorded at 20 kilobits per second for. Given these data rates, the expected le sizes for captured media streams are 9.7 megabytes for a 50-minute lecture and 15.6 megabytes for an 80-minute lecture. In practice, media stream le sizes vary, since recording begins and ends according to the professor's command. The left side of Table 2 shows the average combined size for audio and video les for captured lectures. Total media le sizes range from about nine to seventeen megabytes per lecture. A nal le type created during the capture phase is a record of URLs that the professor visits during the lecture. Classroom 2000 records the hyperlinks followed in class but does not record the data retrieved. A typical lecture generates only a few kilobytes of this data, as shown in Table 2. The nal column in Table 2 gives the average number of total bytes captured per lecture. This value includes the size of media les, pen stroke data, and records of visted URLs. Not surprisingly, the total data stored per lecture is dominated by the recorded media les.

3.2.1 Postprocessing

Some additional le data is written to the classroom server during the postprocessing stage. After capture is complete, Classroom 2000 performs postprocessing, or computation on the captured data. C2000 postprocessing involves integrating text, graphics and media streams for easy access by students. The software to perform this postprocessing, called StreamWeaver, builds a time-line that connects images of prepared slides, annotations on prepared and created slides, links to visited web sites, and audio and AV streams. Streamweaver leaves media stream les unchanged. However, it creates new gif images for slides that incorporate pen strokes written during class. These gif images are created at lower resolution that the original slide images, so the les created during the postprocessing stage are fairly small, ranging from 30 to 155 kilobytes. (Future versions of Classroom 2000 may include more sophisticated post-processing, including converting audio and video streams to dierent formats, performing speaker recognition on video streams or automatic transcription of audio streams. Postprocessing might also include building database indexes on features extracted from the lecture data to facilitate searching.)

3.3 File Size and Type Distributions

A typical class using Classroom 2000 in Spring 1998 required approximately 300 megabytes of storage on the server's disk array to store all les related to a quarter-long course. Table 3 shows the distribution of le sizes and types for les stored on the Classroom 2000 server. The table presents aggregate statistics for the ten courses that used C2000 in Spring 1998. Total storage for these courses was approximately 3 gigabytes. Although audio and AV le types account for less that 2% of the total number of les stored on the C2000 server, they are responsible for the over 86% of all bytes stored, with approximately 26% in audio les and 60% in AV les. Table 3 shows that the average le size of media (and zip) les is two to three orders of magnitude larger than for other le types. As Classroom 2000 moves toward higher-resolution audio and AV formats, the dominance of these continuous media les over other le types will only increase. The largest numbers of les stored on the C2000 server are of three types: html les used to present lecture materials, prepared slide les, and post-processed gif images of slides and penstrokes. Collectively, these le types represent approximately 84% of all les. However, the average size of these les is small, ranging from 9 to 31 kilobytes. In all, these les consume only about 300 megabytes of storage space, or about 10% of total storage.

4 Accessing Classroom 2000 In this section, we characterize how the les created during the advanced preparation, capture and postprocessing stages are accessed by students. Students can access captured lecture material in many ways. After missing a lecture, they may listen to the audio or watch the video for a class in its entirety. If they nd a particular topic 5

File Type

html zip slide gif postscript audio AV other Total

Number Files Percent of Total Storage for Percent of Average Size of of This Type All Files File Type (KBytes) Total Storage File Type (KBytes) 10005 1 2612 4393 1506 198 181 1318 20214

49.49% 0.00% 12.91% 21.72% 7.50% 0.97% 0.90% 6.51% 100.00

192,055 9,328 79,686 39,890 97,535 804,484 1,842,782 1,269 3,067,029

6.26% 0.30% 2.60% 1.30% 3.18% 26.23% 60.08% 0.05% 100.00

19 9,328 31 9 65 4,063 10,181 1 152

Table 3: File size distribution according to le type for all classes using C2000 server in Spring 1998. Access Type

Total Discrete Media (Audio and AV) Audio AV

Number

100% 95.95% 4.05% 2.54% 1.51%

3,916 2,871 1,045

100% 73.31% 26.69%

Explicit User Accesses Access Type Number

Total Discrete File Accesses Media File Accesses

%

25,786 24,741 1,045 656 389

%

Table 4: Breakdown of Accesses from Eastnet/Resnet networks. confusing, they may review a small segment of a lecture. Before a midterm or a nal examination, they may access several lectures for review. Each type of access creates dierent retrieval and data delivery requirements for the classroom server. We restrict the data that is presented below in two ways. First, we lter the accesses presented to eliminate \unsuccessful" accesses, or accesses to the Apache web server that reported an error code and returned no useful data. Second, although students can access Classroom 2000 documents via the World Wide Web over a variety of networks, we only present statistics for accesses from two dormitory networks. The Resnet dormitory network included 137 machines that made 21241 accesses to the C2000 server in Spring 1998. The smaller Eastnet network included 17 machines that made 4545 accesses. Together, accesses on the Resnet and Eastnet networks account for approximately 21% of all accesses to the C2000 server in Spring 1998. Although this is a relatively small proportion of total accesses, we believe the access patterns are fairly representative of current and future use of the C2000 server. We focus on these dormitory networks for several reasons. Most importantly, we are able to assume that particular IP addresses correspond to accesses by a single student. This allows us to model the behavior of individual users and to estimate the length of study sessions. In addition, since students in these dormitories have relatively high-bandwidth Ethernet connections, we expect to see a somewhat higher rate of access to continuous media les (audio and video) than accesses over regular modem connections. This gives us some insight into how students will access continuous media streams in future environments that oer higher bandwidth connections.

4.1 Characterizing Types of Accesses

Table 4 characterizes accesses from 672 study sessions on the Eastnet/Resnet networks by access type. (We discuss how we de ne the start and end of a study session below.) These access types include discrete les, such as slide materials with pen strokes, and continuous media les, including Real Audio and Real Media les. The top part of Table 4 characterizes all accesses to the server, while the bottom part of the graph focuses on explicit user accesses. We dierentiate between explicit and implicit accesses here in Table 4 and also in Table 5. Explicit accesses are user-generated requests to view a slide or a portion of a media stream. Implicit accesses are requests made 6

Accesses 38.38

Average Accesses Per Session

Explicit Implicit 5.95

Discrete Media

32.43

36.82

1.56

Media Accesses Audio 0.98

Video 0.58

Table 5: Breakdown of access types per session. Type of transfer Total Bytes Transferred % Bytes Transferred AV Audio Discrete

745.9 megabytes 261.8 megabytes 265.8 megabytes

58.6% 20.6% 20.9%

Table 6: Bytes transferred by access type for Eastnet/Resnet sessions. automatically by the Classroom 2000 system. These accesses re ect the Classroom 2000 user interface, which fetches all discrete materials associated with a lecture (including slides and penstroke les) the rst time a user requests any portion of that lecture. The result is a bursty request pattern, with many implicit requests automatically issued when a user rst accesses a lecture. Table 5 characterizes the average number of accesses of each type in a study session. It shows that Classroom 2000 generates approximately 32 implicit accesses per study session, compared to approximately 6 explicit user accesses per session. Besides the large ratio of implicit to explicit accesses (over 5:1), Table 5 also shows that accesses to discrete les far outnumber accesses to media les. The average number of accesses to continuous media streams (averaged over all 672 sessions) is 1.56. This corresponds to approximately one audio access per session, on average, and a video access in about 60% of sessions. Although media accesses account for a small number of the access requests, they account for a large percentage of bytes transferred during a study session. Table 6 shows that continuous media transfers are responsible for over 79% of all bytes transferred by the C2000 server. Finally, we note that on average, students access 1.47 dierent lectures during a study session.

4.2 Use of Continuous Media Streams

Next, we characterize the proportion of all study sessions that access multimedia streams. Table 7 shows the fraction of study sessions that access Real Audio or Real Media les. 44% of all sessions on the Eastnet/Resnet networks accessed some type of continuous media stream. Of these, 27% accessed audio, nearly 24% accessed AV streams, and 7% of sessions accessed both media types. So, although we noted in the last section that on average, a study session contains 1.56 accesses to media streams, Table 7 shows that in the majority of study sessions, students accessed only discrete les such as slides and lecture annotations and did not access media les.

4.3 Session Length

Estimating session length was dicult in Classroom 2000 in Spring 1998. If a student accessing a continuous media stream did not explicitly terminate the access, it was dicult to determine a session end point. The MANIC system at the University of Massachusetts at Amherst encountered a similar diculty in determining session length [4]; we followed their example and used a cuto threshold of 30 minutes to determine session length. If a student has not made an explicit request for a period longer than the speci ed threshold, the session is considered to be ended. Any future accesses from the same network address are considered to be part of a new session. Because of this very rough estimate of session length, we do not present an \average session length" metric in this paper. Classroom 2000 has recently been modi ed to include more explicit end-of-session information, so our future estimates of session length will improve.

4.4 Media Accesses of Known Length

As just described, some accesses to media streams are of unknown length. However, about 71% of all media accesses are explicitly terminated. For these accesses, Table 8 shows the distribution of media access lengths. 7

Over half of these accesses are shorter than one minute, suggesting that students may be \browsing" short segments of a lecture. Over 85% of accesses of known length are shorter than ve minutes, and nearly 98% are shorter than 20 minutes. Accesses spanning several minutes likely correspond to students viewing short portions of lectures to review particular topics. Unfortunately, we are not able to present statistics on how often students access longer portions of continuous media streams, for example, to review an entire lecture from start to nish. If a student in a dorm room requests an AV stream and then watches an entire lecture without sending any more requests to the server, the server will not record a \termination" event for this stream access. Thus, we have no way of determining how long the student watched the AV stream. As mentioned, this corresponds to 29% of all media stream accesses. The current version of C2000 uses RealMedia logs to get better statistics on access termination, but these logs were not recorded for Spring 1998.

4.5 Jumping Behavior

Next, we characterize how students \jump" or \seek" to dierent positions within a media stream. In practice, jumping within continuous streams was fairly rare. Approximately 50% of audio and video sessions made a single stream access (no jumps); 20% included one jump operation, and 10% included two jumps. Few sessions included large numbers of jump operations. Table 9 characterizes jumps of known length made through the C2000 interface. (Students may also jump within a media stream using the native interface provided by the Real Networks viewer. Information on such jumps is not currently captured, and thus is not represented here.) Jumps were divided fairly evenly between forward and backward jumps. Average jump lengths were 7 5 minutes for forward jumps and 8 6 minutes for backward jumps. One interesting note is that the MANIC workload study [4] found relatively few backward jumps, while in our workload, we saw approximately 50% backward jumps. This discrepancy may be an artifact of the dierent user interfaces of MANIC and Classroom 2000. The Classroom 2000 server fetches all slide and penstroke images associated with a lecture the rst time the lecture is accessed. Then C2000 presents a timeline with a list of topics the professor will cover. By contrast, MANIC users fetch lecture materials slide by slide, and the user interface provides no insight into the length of an audio stream related to a particular slide. Providing more information about lecture topics and the time devoted to each topic appears to result in a more even distribution of forward and backward jumps. If veri ed by future study, this observation would have important implications for caching and prefetching strategies used by classroom server. :

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4.6 Distribution of Accesses Over Length of a Course

Next, we demonstrate how access rates vary over the course of a quarter. First we study a single course, cs3302a. As we would expect, we can detect distinct peaks in access behavior associated with midterm examinations. (There was no nal examination for this course.) Figures 3 and 4 show the number of simultaneous accesses and sessions for each day of the quarter. These graphs show two large peaks on April 23rd and May 17th corresponding to midterm exams. The session graph also shows a small peak at the beginning of the quarter most likely corresponding to students trying the classroom system for the rst time; there is not a corresponding peak in number of accesses. Periodic dips in both graphs indicate decreased activity on weekends. Figure 5 shows the number of sessions per day for all courses in Spring 1998. Since dierent courses have distinct peaks associated with dierent dates for midterms, this aggregate graph shows higher activity over a period of a week or so around midterm and nal exam periods.

4.7 Distribution of Accesses Over One Day

Next, we studied how students access the C2000 server over the course of a day. The results match our intuition about college student behavior, with the peak period of usage between noon and midnight. Figure 6 shows the number of simultaneous sessions for all courses with respect to time of day. The graph shows the number of active study sessions at 15-minute intervals over a 24-hour day. The y-axis shows the fraction of total accesses on a day that occur at a particular time on the x-axis. The graph begins at 6a.m. and ends at 6a.m. the following day. 8

Overall Access Characteristics

Session Type Number % Total Sessions 672 100% Sessions with Audio Accesses 185 27.41% Sessions with Video Accesses 160 23.81% Sessions with Audio and Video Accesses 47 6.99% Sessions with Any Media Accesses 298 44.35%

Table 7: Media Stream Access Characteristics for for study sessions on Resnet/Eastnet networks (Spring 1998)

Distribution of Known Access Lengths Media Accesses Shorter Than Maximum Maximum Percentage of Length Number Known Accesses 10 sec 20 sec 30 sec 1 min 2 min 3 min 4 min 5 min 10 min 20 min 30 min

168 259 325 437 538 585 618 637 697 731 745

22.56% 34.76% 43.62% 58.66% 72.21% 78.52% 82.96% 85.51% 93.56% 98.12% 100.00%

Intra-Lecture Jumps of Known Length

Total Jumps 276 100% Forward Jumps 134 48.55% Backward Jumps 142 51.45% Mean Length of Forward Jumps 453 seconds Mean Length of Backward Jumps 521 seconds

Table 9: Jump characterization for Eastnet/Resnet accesses, Spring 1998.

Table 8: Distribution of access lengths when length is

known. Access lengths are generally short. Over half of media accesses are shorter than 2 minutes, while nearly 70% are shorter than 15 minutes.

Number of Sessions − CS3302a Spring 98

Number of Accesses − CS3302a Spring 98

35

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1800

30 1600

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Number of Accesses

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20

15

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400

5 200

3/31

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4/20

4/30

5/10 5/20 Day of the Quarter

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6/9

3/31

Figure 3: Number of accesses per day for one course

4/10

4/20

4/30 5/10 Day of the Quarter

5/20

5/30

6/9

Figure 4: Number of sessions per day for one course

(cs3302a) over the Spring 1998 quarter.

(cs3302a) over the Spring 1998 quarter.

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Variation in the number of sessions over a day 70

60

60 50

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Number of sessions

Number of Sessions

40

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3/31

10

4/10

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5/10 5/20 Day of the Quarter

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0 6:00

6/29

Figure 5: Number of sessions per day for all courses over the Spring 1998 quarter.

9:30

11:00

13:30

16:00 18:30 Time of Day

21:00

23:30

2:00

4:30

Figure 6: Shows sessions over a 24-hour period for all courses over Spring 1998 quarter, presented at 15-minute intervals.

The number of active study sessions rises slowly from 6am to noon. From noon to around midnight, the number of active sessions remains fairly high. Between midnight and 3:00 a.m., sessions drop steadily, with few accesses between 3:00 a.m. and 6:00 a.m.

5 Related Work There are several eorts to put multimedia display and capture equipment into classrooms. However, few of these eorts have attempted a workload characterization similar to the one presented in this paper. Most closely related to our modeling eorts is the characterization of the MANIC project at the University of Massachusetts at Amherst [4]. Like C2000, MANIC is a classroom content delivery system that uses the RealNetworks media streams. The study focuses on a one-semester senior-level course, in which 200 \users" (identi ed by unique log-in IDs) accessed the audio and text content associated with the course. Padhye, et al., create a model of user access behavior within a study session. They characterize session length and accesses to audio streams, including start and stop positions and jumps between dierent points in the continuous stream. Our workload study is more general than the MANIC study, since it characterizes advance preparation, capture, and postprocessing as well as the access workload. In addition, Classroom 2000 has supported many more classes and students than the MANIC system. Finally, C2000 provides video as well as audio streams of captured lectures. There are several other media-enhanced classroom projects. The DEPEND project [6] includes four electronic classrooms, each with an electronic whiteboard, audio, and video sources. The authors measured throughput, delay and delay jitter for video streams. They surveyed students to obtain a notion of \acceptability" of audio and video stream quality with varying degrees of packet loss. As part of the CHITRA project at Virginia Polytechnic and State University, Abrams et al. [1] analyzed and modeled multimedia trac generated by students accessing course materials from the Web on both local and remote servers. The study focuses mainly on repeat accesses to the same World Wide Web locations, proxy cache hit rates, and le size and type distributions. The \Lecture Browser" project [7] at Cornell involves a multicamera classroom capture environment. Like C2000, this system merges video and audio streams in the RealNetworks formats with lecture slides during postprocessing. Performance measurements and models of this system are not yet available.

6 Conclusions Media-enhanced classrooms represent a rst step in using technology in innovative ways in the classroom. Such classrooms use display technologies to enhance the classroom experience as well as microphones, cameras and 10

computers to capture an enriched record of a lecture for later study. We have presented a careful workload characterization of the Classroom 2000 educational server, which has been used in a university environment over a period of three years to support dozens of graduate and undergraduate classes. We presented measurements of the C2000 server for a period of one academic quarter in Spring 1998. During this period, ten courses that included over 300 students used the system. We described the type and size distributions of les stored on the C2000 server during the advance preparation, capture, and postprocessing phases of use. We showed that continuous media les dominate the bytes stored on the classroom server. Such les account for less than 2% of les stored on the server, but for over 86% of the bytes stored. We also characterized the other le types stored on the C2000 server, including prepared slide images, pen stroke les captured during lecture, les containing links to World Wide Web pages accessed during class, and post-processed les that combine slide images with pen stroke data. Next, we characterized how users access data on the C2000 server. We characterized user study sessions, showing that they contain a mixture of accesses to discrete les such as slide images and pen stroke les as well as accesses to media streams. On average, a study session contained over 32 accesses to discrete les, one access to an audio le, and an access to a video le about 60% of the time. Although accesses to media les are a small portion of total accesses, they account for over 79% of all bytes transferred from the server. We also characterize length of media stream accesses and how students \jump" or change their viewing position within a media stream. Finally, our workload study includes access patterns over the length of an academic quarter and over the course of one day. Quarter-long access studies show distinct peaks associated with midterm and nal examinations. Focusing on a single day, student accesses reach peak levels between the hours of noon and midnight, with the number of simultaneous study sessions rising in the late morning hours and dropping o between midnight and 3:00 a.m. Based on the lessons learned from the Classroom 2000 workload, our current work involves designing a general, scalable educational server infrastructure. In particular, we are focusing on such issues as scaling the classroom system to support hundreds of classrooms with thousands of students and on the eectiveness of caching class data at various locations.

References

[1] M. Abrams, S. Williams, G. Abdulla, S. Patel, R. Ribler, and E.A. Fox. Multimedia Trac Analysis Using Chitra95. In Proceedings of ACM Multimedia 1995. ACM, ACM, November 1995. also TR-95-05, Dept. of Computer Science, Virginia Tech, April 1995. [2] Averill M. Law and W. David Kelton. Simulation Modeling and Analysis. McGraw-Hill, Inc., 1991. [3] Real Networks. Home Page. http://www.real.com. [4] Jitendra Padhye and Jim Kurose. An Empirical Study of Client Interactions with a Continuous-Media Courseware Server. Technical Report UM-CS-1997-056, University of Massachusetts, 1997. [5] Jitendra Padhye and Jim Kurose. An Empirical Study of Client Interactions with a Continuous-Media Courseware Server. IEEE Internet Computing, April, 1999. [6] T. Plagemann and V. Goebel. Experiences with the Electronic Classroom - QoS Issues in an Advanced Teaching and Research Facility. In Proceedings of the 5th IEEE Workshop on Future Trends of Distributed Computing Systems FTDCS'97. IEEE, IEEE, October 1997. [7] Cornell University "Project Zeno". Lecture Browser project. http://www2.cs.cornell.edu/zeno.

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