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- Path: sparky!uunet!pipex!unipalm!uknet!dcl-cs!nigel
- From: nigel@comp.lancs.ac.uk (Nigel Davies)
- Newsgroups: comp.sys.isis
- Subject: Groups in a mixed media setting
- Message-ID: <1993Jan26.165131.28041@comp.lancs.ac.uk>
- Date: 26 Jan 93 16:51:31 GMT
- Organization: Department of Computing at Lancaster University, UK.
- Lines: 78
-
-
- In response to Ken's message suggesting that more general distributed systems/
- group issues be discussed in this group I thought people might be interested in
- some experiences we've had of using groups in a mixed media environment.
- I'll follow these with a few questions I hope someone might be able to help
- me with.
-
- For the last few years we have been building a platform to support distributed
- multimedia applications. The platform has been based on the ANSA computational
- model (essentially a client-server object model) and on the ANSAware
- implementation of this model.
-
- The question which we had to deal with was how to model the transmission of
- media types such as audio and video. The solution we adopted was to introduce
- new objects called stream objects which represent the transmission of continuous
- media. Application developers simply plumb together sources and sinks of
- continuous media data using these stream objects.
-
- The next question was how to model multicast of continuous media info. We had
- already done a _very_ simplistic implementation of object groups for
- invocation, so it seemed natural to try and exploit this for continuous media
- transmission (the model not the implementation). In fact, we simply allowed
- programmers to group together sources and sinks, and specified that streams
- connected together source and sink groups rather than individual devices.
-
- O O
- \ /
- sources 0- |group interface ===stream======group interface| -O sinks
- /
- O
-
- information flow ---->
-
- This way we could exploit the group management functionality to look after
- control of new objects joining the groups.
-
- The programmer can also use the groups for control purposes, e.g. sending
- start/stop messages to the devices which have been grouped together.
-
- Finally, in order to allow us to use groups for management purposes we
- implemented a form of group hierarchies which allowed us to provide equivalent
- functionality to those systems supporting domains (I can point people in the
- direction of references if anyone wants more details on any of this work).
-
- We are now entering a period of 'reflection' as far as this model goes and
- I'd welcome any thoughts, specifically with reference to the following:-
-
- 1) I seem to remember that there was some talk of collaboration between APM
- (the developers of ANSAware) and Cornell a year or so ago. Did this ever
- come to anything ? (The most recent version of ANSAware still does not
- provide support for groups so I am guessing not ...)
-
- 2) It is some time since I have seen any ISIS documentation. Does ISIS support
- hierarchies of groups, and if so what sort of event ordering guarantees
- does it provide ?
-
- 3) One of the reasons for using the group abstraction for describing multicast
- /multidrop continuous media configurations was to provide the programmer
- with similar tools for carrying out both invocation and continuous media
- transmission. However, judging by the recent postings there may not be a
- requirement for multidrop invocation groups.
-
- 4) We are now considering an implementation of the platform using mobile
- radio as the communications medium (supporting a _very_ limited range of
- media types). Does anyone have any experiences in implementing group
- protocols over a radio link ? More generally, does anyone know of any
- distributed systems work aimed at such an environment ?
-
-
- Cheers,
-
- Nigel Davies.
-
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