One document matched: draft-burger-xcon-mmodels-00.txt
XCON E. Burger
Internet-Draft SnowShore Networks, Inc.
Expires: August 8, 2004 February 8, 2004
Centralized Conferencing (XCON) Media Models
draft-burger-xcon-mmodels-00
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes various models for endpoint control of media
policy for centralized conferencing services. The models include
detailed mixer control, as in H.248, individual end-point
negotiation, and participant roles, as in MSCML.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Low-Level Stream Control . . . . . . . . . . . . . . . . . . . 3
2.1 Decription . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.3 Drawbacks . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Individual End-Point Negotiation . . . . . . . . . . . . . . . 4
3.1 Decription . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2 Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.3 Drawbacks . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Role-Directed Control . . . . . . . . . . . . . . . . . . . . 5
4.1 Decription . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2 Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3 Drawbacks . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Other Thoughts . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
Informative References . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 6
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . 7
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1. Introduction
There is debate about how one should control the centralized mixing
service in the XCON framework. We term this control media policy.
Some offer low-level manipulation of media streams and resources on a
per-user basis. Others offer models based on end-device negotiation
of streams of interest and doing local manipulation. Yet others
offer models based on fixed frameworks and user roles to indirectly
determine the media policay.
In one sense, the different models are isomorphic. One can construct
the end-device-centric model from the low-level model. Likewise, one
can construct the frameworks-and-roles model from the
end-device-centric or the low-level model. Given this, it is
important to consider the use cases for media policy manipulation.
As this document will assert, the different models impose quite
different requirements on the endpoints.
Another factor this document will examine is the ease of
implementation, both of the server (mixer) and client (endpoint).
Everything needs to loop back to and have examples for how to meet
the Conferencing Scenarios draft [1].
2. Low-Level Stream Control
2.1 Decription
Low-level stream control is where the endpoint manipulates a direct
representation of the media processing resources of the mixer. This
representation can be physical, as in terms of directly plumbing DSP
resources in the manner of H.248.1. The representation can be
logical, as in terms of XML mappings of DSP topologies.
2.2 Benefits
This is the most flexible model available. Just as one can construct
all logic circuits from an OR gate and a NOT gate, one can construct
all possible media mixing scenarios from a two-input mixer and an
attinuator.
2.3 Drawbacks
Unless the mixer physically maps to the primitives of the device
control model, it is very conceivable for a legal mixing request to
be unrenderable by the mixing device. For example, an 8-input mixer
with the loudest three talkers with four outputs being the full mix,
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the mix less the loudest talker, the mix less the second loudest
talker, and the mix less the third loudest talker is a 13-element
composition (7x 2-input mixers, 3x loudest talker selectors, and 3x
2-input mixers with "not loudest talkers"). The mixing device must
be able to map the representation into primitives it understands,
such as an 8-input mixer.
Another drawback is the mixer may not be able to globally optimize
the mixing resources, as there are many ways of representing the same
mix result. For small mixers, this is a denial of service
opportunity. For large mixers, properly scaling the system requires
minimizing the resource utilization for the average conference.
Finally, this model requires the endpoint developer to be an expert
in DSP technology. This is because the developer has to manage and
manipulate the DSP resources at a fairly low level.
3. Individual End-Point Negotiation
3.1 Decription
The individual end-point negotiation model is where the endpoint
negotiates directly with other conference endpoints and does any
mixing locally. One often finds this model for small audio or
multimedia conferences. If the endpoint wants a big video window, it
will ask the remote endpoint for a big video transmission. If the
endpoint wants a thumbnail, it asks for a thumbnail.
With the streams, the endpoint locally composes whatever layout or
mixing policy it feels like.
This model often relies on the us of silence-suppression codecs to
reduce bandwidth to the endpoint and make active talker
identification easier. For the most part, only two people will try
to talk for more than 500ms. Human protocol usually results on one
of the participants to backoff.
3.2 Benefits
The main benefit of this model is the user is in complete control of
their experience. There are no questions about whether a mixing
service can support the user interface, whether because of resource
availability, computational complexity, or forsight into the possible
media manipulation needs.
3.3 Drawbacks
All media must traverse the network from all endpoints to the given
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endpoint. This is not realistic for Internet-scale protocols.
There is no centralized control of policy such as floor control. For
example, the moderator may wish to mute a particular speaker; the
endpoint may still allow that speaker into the mix, ignoring the
moderators mute request.
The endpoint developer must be able to put together the media
manipulation on their own. For example, mixing; video demodulation,
mixing, and remodulation; and stream selection are all now endpoint
functions.
4. Role-Directed Control
4.1 Decription
In role-directed control, the mixing service provides a set of
templates for which different users have different roles. The roles
in the template dictates the media policy. For example, in a lecture
template, there is a lecturer and possibly listeners and questioners.
The moderator is the only stream in the mix, unless a user becomes a
questioner. As a questioner, their media is added appropriately to
the mix. While straightforward for audio, the mix for video is more
involved. A likely scenario is to have multiple templates for
different preferences, such as video switching to the current
speaker, split pane with the lecturer and current questioner, and so
on.
4.2 Benefits
The endpoint operates at the level of a conference. It only needs to
know what role the user wants to be. It does not need to know
anything about DSP primitives or plumbing.
4.3 Drawbacks
There is unquestionably a loss of generality with role-directed
control. However, that loss is made up with an increase in
usability.
5. Other Thoughts
BOBW: Role-Directed Control with a Framework Markup language?
6. Security Considerations
While this document is entirely informative, it is worthwhile to note
the above mentioned denial of service opportunities some of the
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methods outlined in the paper present.
Informative References
[1] Even, R., "Conferencing Scenarios",
draft-ietf-xcon-conference-scenarios-00 (work in progress),
December 2003.
Author's Address
Eric Burger
SnowShore Networks, Inc.
285 Billerica Rd.
Chelmsford, MA 01824-4120
USA
EMail: e.burger@ieee.org
Appendix A. Acknowledgements
If I do this paper for real, there are boatloads of references to do,
like Rohan's XML media representation, Jonathan's conferencing
framework, Brian's extreamly cool Viper product, and all of Alan and
Henry's work in the conferencing space. Please forgive me this -00
done in 00 time at all!
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