exercises

19. groupware

EXERCISE 19.19 [extra - not in book]

Site X on a network has connections to 4 other sites A, B, C and D
The connections have the following characteristics:
            site X – site A            high bandwidth            low latency            low jitter
            site X – site B            high bandwidth            high latency            low jitter
            site X – site C            low bandwidth            low latency            low jitter
            site X – site D            high bandwidth            low latency            high jitter

  1. Comment on the suitability of these connections for the following media types:

    (a) audio-conference (e.g. telephone style chatting)
    (b) live interactive video-only connection (e.g. web cam)
    (c) streaming HI-FI audio (e.g. Radio 3)
    (d) video-on-demand (e.g. hotel pay-per-view video service)

    For this part assume that there is not enough memory available to provide any buffering.  Make sure your answer clearly states the effects on the users, not just technical effects.

  2. Describe how buffering might improve/alter any of the above?
  3. An Internet-based collaborative graphics application includes a shared electronic whiteboard where participants can each annotate and manipulate a high-resolution photograph .

    Two potential architectural solutions have been proposed

    • Sending each user's freehand drawing actions (individual mouse movements) to a central server
    • Download an applet and do drawing actions on a local copy of the whiteboard contents

    Describe advantages and disadvantages of these solutions with regard to potential deployment on:

    • network connections similar to site X–B
    • network connections similar to site X-C

answer available for tutors only

  1. This answer may be organised as media within sites (as here) the other way round, or a mix.

    X–A – high bandwidth, low delays and low jitter – good for everything!

    X–B – high latency will behave badly with both audio and video for interactive conversations (a) & (b).  In particular, normal conversation has very small gaps (fraction of a second) which are used by participants to hand-over to one another.  The participants will have problems similar to trans-continental satellite links. The high latency will not hinder streaming video/audio which can simply be slightly delayed.

    X–C – low bandwidth – will mean high compression and/or loss of quality.  This will rule it out for Hi-Fi audio and high quality video.  Interactive audio may cope (depending on how low the bandwidth) and web cam can simply have reduced frame rate and resuloution.

    X–D – high jitter – (without buffering) will mean that both interactive and streaming audio will be badly affected (we can not stand delays/missing parts of speech).  The interactive video will simply occasionally have broken frames which is acceptable.  Depending on the level of jitter this will probably mean the quality is too bad for hotel video application.

  2. Buffering can be used to change jitter into delay (there are various diagrams of this in the course notes, or they can simply describe why).  This effectively means that X–D can then be used for streaming audio/video, but still not suitable for interactive audio except at 'trans-Atlantic' style quality
  3. In case X–B, the applet based solution would be necessary to achieve good interactive feedback for free-hand drawing.  However, applet security would mean that the server would need to sit on the same machine as the web server (or need signed applet).  Also the software will need to be more complicated as there will be the possibilities of race conditions when updates are done simultaneously by different participants.  Feedthrough will also be poor.

    In case X–C, the central server solution may be chosen as the low latency and jitter mean that it can give fully interactive feedback even for mouse movements (ideally students may quote figures of around 200 ms maximum round trip feedback time for this).  This will have an easier implementation.  There may still be problems however in updating photo images on the individual displays, perhaps a delay when the application is first launched as the photos are downloaded over the low bandwidth connection.  Feedthrough better.

Other exercises in this chapter

ex.19.1 (ans), ex.19.2 (ans), ex.19.3 (tut), ex.19.4 (tut), ex.19.5 (tut), ex.19.6 (tut), ex.19.7 (tut), ex.19.8 (tut), ex.19.9 (tut), ex.19.10 (tut), ex.19.11 (tut), ex.19.12 (tut), ex.19.13 (tut), ex.19.14 (tut), ex.19.15 (tut), ex.19.16 (tut), ex.19.17 (tut), ex.19.18 (tut), ex.19.19 (tut), ex.19.20 (tut)

all exercises for this chapter