CLUE WG A. Romanow
Internet-Draft Cisco Systems
Intended status: Informational M. Duckworth, Ed.
Expires: January 7, 2013 Polycom
A. Pepperell
B. Baldino
Cisco Systems
July 6, 2012
Framework for Telepresence Multi-Streams
draft-ietf-clue-framework-06.txt
Abstract
This memo offers a framework for a protocol that enables devices in a
telepresence conference to interoperate by specifying the
relationships between multiple media streams.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 7, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Overview of the Framework/Model . . . . . . . . . . . . . . . 6
5. Spatial Relationships . . . . . . . . . . . . . . . . . . . . 7
6. Media Captures and Capture Scenes . . . . . . . . . . . . . . 8
6.1. Media Captures . . . . . . . . . . . . . . . . . . . . . . 8
6.1.1. Media Capture Attributes . . . . . . . . . . . . . . . 9
6.2. Capture Scene . . . . . . . . . . . . . . . . . . . . . . 11
6.2.1. Capture scene attributes . . . . . . . . . . . . . . . 13
6.2.2. Capture scene entry attributes . . . . . . . . . . . . 14
6.3. Simultaneous Transmission Set Constraints . . . . . . . . 15
7. Encodings . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.1. Individual Encodings . . . . . . . . . . . . . . . . . . . 16
7.2. Encoding Group . . . . . . . . . . . . . . . . . . . . . . 17
8. Associating Media Captures with Encoding Groups . . . . . . . 19
9. Consumer's Choice of Streams to Receive from the Provider . . 19
9.1. Local preference . . . . . . . . . . . . . . . . . . . . . 20
9.2. Physical simultaneity restrictions . . . . . . . . . . . . 20
9.3. Encoding and encoding group limits . . . . . . . . . . . . 20
9.4. Message Flow . . . . . . . . . . . . . . . . . . . . . . . 21
10. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 22
11. Examples - Using the Framework . . . . . . . . . . . . . . . . 22
11.1. Three screen endpoint media provider . . . . . . . . . . . 22
11.2. Encoding Group Example . . . . . . . . . . . . . . . . . . 29
11.3. The MCU Case . . . . . . . . . . . . . . . . . . . . . . . 30
11.4. Media Consumer Behavior . . . . . . . . . . . . . . . . . 30
11.4.1. One screen consumer . . . . . . . . . . . . . . . . . 31
11.4.2. Two screen consumer configuring the example . . . . . 31
11.4.3. Three screen consumer configuring the example . . . . 32
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 32
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
14. Security Considerations . . . . . . . . . . . . . . . . . . . 32
15. Changes Since Last Version . . . . . . . . . . . . . . . . . . 32
16. Informative References . . . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34
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1. Introduction
Current telepresence systems, though based on open standards such as
RTP [RFC3550] and SIP [RFC3261], cannot easily interoperate with each
other. A major factor limiting the interoperability of telepresence
systems is the lack of a standardized way to describe and negotiate
the use of the multiple streams of audio and video comprising the
media flows. This draft provides a framework for a protocol to
enable interoperability by handling multiple streams in a
standardized way. It is intended to support the use cases described
in draft-ietf-clue-telepresence-use-cases-02 and to meet the
requirements in draft-ietf-clue-telepresence-requirements-01.
The solution described here is strongly focused on what is being done
today, rather than on a vision of future conferencing. At the same
time, the highest priority has been given to creating an extensible
framework to make it easy to accommodate future conferencing
functionality as it evolves.
The purpose of this effort is to make it possible to handle multiple
streams of media in such a way that a satisfactory user experience is
possible even when participants are using different vendor equipment,
and also when they are using devices with different types of
communication capabilities. Information about the relationship of
media streams at the provider's end must be communicated so that
streams can be chosen and audio/video rendering can be done in the
best possible manner.
There is no attempt here to dictate to the renderer what it should
do. What the renderer does is up to the renderer.
After the following Definitions, a short section introduces key
concepts. The body of the text comprises several sections about the
key elements of the framework, how a consumer chooses streams to
receive, and some examples. The appendix describe topics that are
under discussion for adding to the document.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Definitions
The definitions marked with an "*" are new; all the others are from
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draft-wenger-clue-definitions-00-01.txt.
*Audio Capture: Media Capture for audio. Denoted as ACn.
Camera-Left and Right: For media captures, camera-left and camera-
right are from the point of view of a person observing the rendered
media. They are the opposite of stage-left and stage-right.
Capture Device: A device that converts audio and video input into an
electrical signal, in most cases to be fed into a media encoder.
Cameras and microphones are examples for capture devices.
*Capture Scene: a structure representing the scene that is captured
by a collection of capture devices. A capture scene includes
attributes and one or more capture scene entries, with each entry
including one or more media captures.
*Capture Scene Entry: a list of media captures of the same media type
that together form one way to represent the capture scene.
Conference: used as defined in [RFC4353], A Framework for
Conferencing within the Session Initiation Protocol (SIP).
*Individual Encoding: A variable with a set of attributes that
describes the maximum values of a single audio or video capture
encoding. The attributes include: maximum bandwidth- and for video
maximum macroblocks (for H.264), maximum width, maximum height,
maximum frame rate.
*Encoding Group: A set of encoding parameters representing a media
provider's encoding capabilities. Media stream providers formed of
multiple physical units, in each of which resides some encoding
capability, would typically advertise themselves to the remote media
stream consumer using multiple encoding groups. Within each encoding
group, multiple potential encodings are possible, with the sum of the
chosen encodings' characteristics constrained to being less than or
equal to the group-wide constraints.
Endpoint: The logical point of final termination through receiving,
decoding and rendering, and/or initiation through capturing,
encoding, and sending of media streams. An endpoint consists of one
or more physical devices which source and sink media streams, and
exactly one [RFC4353] Participant (which, in turn, includes exactly
one SIP User Agent). In contrast to an endpoint, an MCU may also
send and receive media streams, but it is not the initiator nor the
final terminator in the sense that Media is Captured or Rendered.
Endpoints can be anything from multiscreen/multicamera rooms to
handheld devices.
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Front: the portion of the room closest to the cameras. In going
towards back you move away from the cameras.
MCU: Multipoint Control Unit (MCU) - a device that connects two or
more endpoints together into one single multimedia conference
[RFC5117]. An MCU includes an [RFC4353] Mixer. [Edt. RFC4353 is
tardy in requiring that media from the mixer be sent to EACH
participant. I think we have practical use cases where this is not
the case. But the bug (if it is one) is in 4353 and not herein.]
Media: Any data that, after suitable encoding, can be conveyed over
RTP, including audio, video or timed text.
*Media Capture: a source of Media, such as from one or more Capture
Devices. A Media Capture (MC) may be the source of one or more Media
streams. A Media Capture may also be constructed from other Media
streams. A middle box can express Media Captures that it constructs
from Media streams it receives.
*Media Consumer: an Endpoint or middle box that receives media
streams
*Media Provider: an Endpoint or middle box that sends Media streams
Model: a set of assumptions a telepresence system of a given vendor
adheres to and expects the remote telepresence system(s) also to
adhere to.
*Plane of Interest: The spatial plane containing the most relevant
subject matter.
Render: the process of generating a representation from a media, such
as displayed motion video or sound emitted from loudspeakers.
*Simultaneous Transmission Set: a set of media captures that can be
transmitted simultaneously from a Media Provider.
Spatial Relation: The arrangement in space of two objects, in
contrast to relation in time or other relationships. See also
Camera-Left and Right.
Stage-Left and Right: For media captures, stage-left and stage-right
are the opposite of camera-left and camera-right. For the case of a
person facing (and captured by) a camera, stage-left and stage-right
are from the point of view of that person.
*Stream: RTP stream as in [RFC3550].
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Stream Characteristics: the media stream attributes commonly used in
non-CLUE SIP/SDP environments (such as: media codec, bit rate,
resolution, profile/level etc.) as well as CLUE specific attributes,
such as the ID of a capture or a spatial location.
Telepresence: an environment that gives non co-located users or user
groups a feeling of (co-located) presence - the feeling that a Local
user is in the same room with other Local users and the Remote
parties. The inclusion of Remote parties is achieved through
multimedia communication including at least audio and video signals
of high fidelity.
*Video Capture: Media Capture for video. Denoted as VCn.
Video composite: A single image that is formed from combining visual
elements from separate sources.
4. Overview of the Framework/Model
The CLUE framework specifies how multiple media streams are to be
handled in a telepresence conference.
The main goals include:
o Interoperability
o Extensibility
o Flexibility
Interoperability is achieved by the media provider describing the
relationships between media streams in constructs that are understood
by the consumer, who can then render the media. Extensibility is
achieved through abstractions and the generality of the model, making
it easy to add new parameters. Flexibility is achieved largely by
having the consumer choose what content and format it wants to
receive from what the provider is capable of sending.
A transmitting endpoint or MCU describes specific aspects of the
content of the media and the formatting of the media streams it can
send (advertisement); and the receiving end responds to the provider
by specifying which content and media streams it wants to receive
(configuration). The provider then transmits the asked for content
in the specified streams.
This advertisement and configuration occurs at call initiation but
may also happen at any time throughout the conference, whenever there
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is a change in what the consumer wants or the provider can send.
An endpoint or MCU typically acts as both provider and consumer at
the same time, sending advertisements and sending configurations in
response to receiving advertisements. (It is possible to be just one
or the other.)
The data model is based around two main concepts: a capture and an
encoding. A media capture (MC), such as audio or video, describes
the content a provider can send. Media captures are described in
terms of CLUE-defined attributes, such as spatial relationships and
purpose of the capture. Providers tell consumers which media
captures they can provide, described in terms of the media capture
attributes.
A provider organizes its media captures that represent the same scene
into capture scenes. A consumer chooses which media captures it
wants to receive according to the capture scenes sent by the
provider.
In addition, the provider sends the consumer a description of the
streams it can send in terms of the media attributes of the stream,
in particular, well-known audio and video parameters such as
bandwidth, frame rate, macroblocks per second.
The provider also specifies constraints on its ability to provide
media, and the consumer must take these into account in choosing the
content and streams it wants. Some constraints are due to the
physical limitations of devices - for example, a camera may not be
able to provide zoom and non-zoom views simultaneously. Other
constraints are system based constraints, such as maximum bandwidth
and maximum macroblocks/second.
The following sections discuss these constructs and processes in
detail, followed by use cases showing how the framework specification
can be used.
5. Spatial Relationships
In order for a consumer to perform a proper rendering, it is often
necessary to provide spatial information about the streams it is
receiving. CLUE defines a coordinate system that allows media
providers to describe the spatial relationships of their media
captures to enable proper scaling and spatial rendering of their
streams. The coordinate system is based on a few principles:
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o Simple systems which do not have multiple Media Captures to
associate spatially need not use the coordinate model.
o Coordinates can either be in real, physical units (millimeters),
have an unknown scale or have no physical scale. Systems which
know their physical dimensions should always provide those real-
world measurements. Systems which don't know specific physical
dimensions but still know relative distances should use 'unknown
scale'. 'No scale' is intended to be used where Media Captures
from different devices (with potentially different scales) will be
forwarded alongside one another (e.g. in the case of a middle
box).
* "millimeters" means the scale is in millimeters
* "Unknown" means the scale is not necessarily millimeters, but
the scale is the same for every capture in the capture scene.
* "No Scale" means the scale could be different for each capture
- an MCU provider that advertises two adjacent captures and
picks sources (which can change quickly) from different
endpoints might use this value; the scale could be different
and changing for each capture. But the areas of capture still
represent a spatial relation between captures.
o The coordinate system is Cartesian X, Y, Z with the origin at a
spot of the provider's choosing. The provider must use the same
coordinate system with same scale and origin for all coordinates
within the same capture scene.
The direction of increasing coordinate values is:
X increases from camera left to camera right
Y increases from front to back
Z increases from low to high
6. Media Captures and Capture Scenes
This section describes how media providers can describe the content
of media to consumers.
6.1. Media Captures
Media captures are the fundamental representations of streams that a
device can transmit. What a Media Capture actually represents is
flexible:
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o It can represent the immediate output of a physical source (e.g.
camera, microphone) or 'synthetic' source (e.g. laptop computer,
DVD player).
o It can represent the output of an audio mixer or video composer
o It can represent a concept such as 'the loudest speaker'
o It can represent a conceptual position such as 'the leftmost
stream'
To distinguish between multiple instances, video and audio captures
are numbered such as: VC1, VC2 and AC1, AC2. VC1 and VC2 refer to
two different video captures and AC1 and AC2 refer to two different
audio captures.
Each Media Capture can be associated with attributes to describe what
it represents.
6.1.1. Media Capture Attributes
Media Capture Attributes describe static information about the
captures. A provider uses the media capture attributes to describe
the media captures to the consumer. The consumer will select the
captures it wants to receive. Attributes are defined by a variable
and its value. The currently defined attributes and their values
are:
Content: {slides, speaker, sl, main, alt}
A field with enumerated values which describes the role of the media
capture and can be applied to any media type. The enumerated values
are defined by [RFC4796]. The values for this attribute are the same
as the mediacnt values for the content attribute in [RFC4796]. This
attribute can have multiple values, for example content={main,
speaker}.
Composed: {true, false}
A field with a Boolean value which indicates whether or not the Media
Capture is a mix (audio) or composition (video) of streams.
This attribute is useful for a media consumer to avoid nesting a
composed video capture into another composed capture or rendering.
This attribute is not intended to describe the layout a media
provider uses when composing video streams.
Audio Channel Format: {mono, stereo} A field with enumerated values
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which describes the method of encoding used for audio.
A value of 'mono' means the Audio Capture has one channel.
A value of 'stereo' means the Audio Capture has two audio channels,
left and right.
This attribute applies only to Audio Captures. A single stereo
capture is different from two mono captures that have a left-right
spatial relationship. A stereo capture maps to a single RTP stream,
while each mono audio capture maps to a separate RTP stream.
Switched: {true, false}
A field with a Boolean value which indicates whether or not the Media
Capture represents the (dynamic) most appropriate subset of a
'whole'. What is 'most appropriate' is up to the provider and could
be the active speaker, a lecturer or a VIP.
Point of Capture: {(X, Y, Z)}
A field with a single Cartesian (X, Y, Z) point value which describes
the spatial location, virtual or physical, of the capturing device
(such as camera).
When the Point of Capture attribute is specified, it must include X,
Y and Z coordinates. If the point of capture is not specified, it
means the consumer should not assume anything about the spatial
location of the capturing device. Even if the provider specifies an
area of capture attribute, it does not need to specify the point of
capture.
Axis of Capture Point: {(X, Y, Z)}
A field with a single Cartesian (X, Y, Z) point value (virtual or
physical) which describes a position in space of a second point on
the axis of capture of the capturing device; the first point being
the Point of Capture (see above).
The axis of capture point MUST NOT be specified if the Point of
Capture is not present for this capture device. When the Axis of
Capture Point attribute is specified, it must include X, Y and Z
coordinates. These coordinates MUST NOT be identical to the Point of
Capture coordinates. If the Axis of Capture point is not specified,
it means the consumer should not assume anything about the axis of
Capture of the capturing device.
Area of Capture:
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{bottom left(X1, Y1, Z1), bottom right(X2, Y2, Z2), top left(X3, Y3,
Z3), top right(X4, Y4, Z4)}
A field with a set of four (X, Y, Z) points as a value which describe
the spatial location of what is being "captured". By comparing the
Area of Capture for different Media Captures within the same capture
scene a consumer can determine the spatial relationships between them
and render them correctly.
The four points should be co-planar. The four points form a
quadrilateral, not necessarily a rectangle.
The quadrilateral described by the four (X, Y, Z) points defines the
plane of interest for the particular media capture.
If the area of capture attribute is specified, it must include X, Y
and Z coordinates for all four points. If the area of capture is not
specified, it means the media capture is not spatially related to any
other media capture (but this can change in a subsequent provider
advertisement).
For a switched capture that switches between different sections
within a larger area, the area of capture should use coordinates for
the larger potential area.
EncodingGroup: {<encodeGroupID value>}
A field with a value equal to the encodeGroupID of the encoding group
associated with the media capture.
6.2. Capture Scene
In order for a provider's individual media captures to be used
effectively by a consumer, the provider organizes the media captures
into capture scenes, with the structure and contents of these capture
scenes being sent from the provider to the consumer.
A capture scene is a structure representing the scene that is
captured by a collection of capture devices. A capture scene
includes one or more capture scene entries, with each entry including
one or more media captures. A capture scene represents, for example,
the video image of a group of people seated next to each other, along
with the sound of their voices, which could be represented by some
number of VCs and ACs in the capture scene entries. A middle box may
also express capture scenes that it constructs from media streams it
receives.
A provider may advertise multiple capture scenes or just a single
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capture scene. A media provider might typically use one capture
scene for main participant media and another capture scene for a
computer generated presentation. A capture scene may include more
than one type of media. For example, a capture scene can include
several capture scene entries for video captures, and several capture
scene entries for audio captures.
A provider can express spatial relationships between media captures
that are included in the same capture scene. But there is no spatial
relationship between media captures that are in different capture
scenes.
A media provider arranges media captures in a capture scene to help
the media consumer choose which captures it wants. The capture scene
entries in a capture scene are different alternatives the provider is
suggesting for representing the capture scene. The media consumer
can choose to receive all media captures from one capture scene entry
for each media type (e.g. audio and video), or it can pick and choose
media captures regardless of how the provider arranges them in
capture scene entries.
Media captures within the same capture scene entry must be of the
same media type - it is not possible to mix audio and video captures
in the same capture scene entry, for instance. The provider must be
capable of encoding and sending all media captures in a single entry
simultaneously. A consumer may decide to receive all the media
captures in a single capture scene entry, but a consumer could also
decide to receive just a subset of those captures. A consumer can
also decide to receive media captures from different capture scene
entries.
When a provider advertises a capture scene with multiple entries, it
is essentially signaling that there are multiple representations of
the same scene available. In some cases, these multiple
representations would typically be used simultaneously (for instance
a "video entry" and an "audio entry"). In some cases the entries
would conceptually be alternatives (for instance an entry consisting
of 3 video captures versus an entry consisting of just a single video
capture). In this latter example, the provider would in the simple
case end up providing to the consumer the entry containing the number
of video captures that most closely matched the media consumer's
number of display devices.
The following is an example of 4 potential capture scene entries for
an endpoint-style media provider:
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1. (VC0, VC1, VC2) - left, center and right camera video captures
2. (VC3) - video capture associated with loudest room segment
3. (VC4) - video capture zoomed out view of all people in the room
4. (AC0) - main audio
The first entry in this capture scene example is a list of video
captures with a spatial relationship to each other. Determination of
the order of these captures (VC0, VC1 and VC2) for rendering purposes
is accomplished through use of their Area of Capture attributes. The
second entry (VC3) and the third entry (VC4) are additional
alternatives of how to capture the same room in different ways. The
inclusion of the audio capture in the same capture scene indicates
that AC0 is associated with those video captures, meaning it comes
from the same scene. The audio should be rendered in conjunction
with any rendered video captures from the same capture scene.
6.2.1. Capture scene attributes
Attributes can be applied to capture scenes as well as to individual
media captures. Attributes specified at this level apply to all
constituent media captures.
Description attribute - list of {<description text>, <language tag>}
The optional description attribute is a list of human readable text
strings which describe the capture scene. If there is more than one
string in the list, then each string in the list should contain the
same description, but in a different language. A provider that
advertises multiple capture scenes can provide descriptions for each
of them. This attribute can contain text in any number of languages.
The language tag identifies the language of the corresponding
description text. The possible values for a language tag are the
values of the 'Subtag' column for the "Type: language" entries in the
"Language Subtag Registry" at [IANA-Lan] originally defined in
[RFC5646]. A particular language tag value MUST NOT be used more
than once in the description attribute list.
Area of Scene attribute
The area of scene attribute for a capture scene has the same format
as the area of capture attribute for a media capture. The area of
scene is for the entire scene, which is captured by the one or more
media captures in the capture scene entries. If the provider does
not specify the area of scene, but does specify areas of capture,
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then the consumer may assume the area of scene is greater than or
equal to the outer extents of the individual areas of capture.
Scale attribute
An optional attribute indicating if the numbers used for area of
scene, area of capture and point of capture are in terms of
millimeters, unknown scale factor, or not any scale, as described in
Section 5. If any media captures have an area of capture attribute
or point of capture attribute, then this scale attribute must also be
defined. The possible values for this attribute are:
"millimeters"
"unknown"
"no scale"
6.2.2. Capture scene entry attributes
Attributes can be applied to capture scene entries. Attributes
specified at this level apply to the capture scene entry as a whole.
Scene-switch-policy: {site-switch, segment-switch}
A media provider uses this scene-switch-policy attribute to indicate
its support for different switching policies. In the provider's
advertisement, this attribute can have multiple values, which means
the provider supports each of the indicated policies. The consumer,
when it requests media captures from this capture scene entry, should
also include this attribute but with only the single value (from
among the values indicated by the provider) indicating the consumer's
choice for which policy it wants the provider to use. If the
provider does not support any of these policies, it should omit this
attribute.
The "site-switch" policy means all captures are switched at the same
time to keep captures from the same endpoint site together. Let's
say the speaker is at site A and everyone else is at a "remote" site.
When the room at site A shown, all the camera images from site A are
forwarded to the remote sites. Therefore at each receiving remote
site, all the screens display camera images from site A. This can be
used to preserve full size image display, and also provide full
visual context of the displayed far end, site A. In site switching,
there is a fixed relation between the cameras in each room and the
displays in remote rooms. The room or participants being shown is
switched from time to time based on who is speaking or by manual
control.
The "segment-switch" policy means different captures can switch at
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different times, and can be coming from different endpoints. Still
using site A as where the speaker is, and "remote" to refer to all
the other sites, in segment switching, rather than sending all the
images from site A, only the image containing the speaker at site A
is shown. The camera images of the current speaker and previous
speakers (if any) are forwarded to the other sites in the conference.
Therefore the screens in each site are usually displaying images from
different remote sites - the current speaker at site A and the
previous ones. This strategy can be used to preserve full size image
display, and also capture the non-verbal communication between the
speakers. In segment switching, the display depends on the activity
in the remote rooms - generally, but not necessarily based on audio /
speech detection.
6.3. Simultaneous Transmission Set Constraints
The provider may have constraints or limitations on its ability to
send media captures. One type is caused by the physical limitations
of capture mechanisms; these constraints are represented by a
simultaneous transmission set. The second type of limitation
reflects the encoding resources available - bandwidth and
macroblocks/second. This type of constraint is captured by encoding
groups, discussed below.
An endpoint or MCU can send multiple captures simultaneously, however
sometimes there are constraints that limit which captures can be sent
simultaneously with other captures. A device may not be able to be
used in different ways at the same time. Provider advertisements are
made so that the consumer will choose one of several possible
mutually exclusive usages of the device. This type of constraint is
expressed in a Simultaneous Transmission Set, which lists all the
media captures that can be sent at the same time. This is easier to
show in an example.
Consider the example of a room system where there are 3 cameras each
of which can send a separate capture covering 2 persons each- VC0,
VC1, VC2. The middle camera can also zoom out and show all 6
persons, VC3. But the middle camera cannot be used in both modes at
the same time - it has to either show the space where 2 participants
sit or the whole 6 seats, but not both at the same time.
Simultaneous transmission sets are expressed as sets of the MCs that
could physically be transmitted at the same time, (though it may not
make sense to do so). In this example the two simultaneous sets are
shown in Table 1. The consumer must make sure that it chooses one
and not more of the mutually exclusive sets. A consumer may choose
any subset of the media captures in a simultaneous set, it does not
have to choose all the captures in a simultaneous set if it does not
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want to receive all of them.
+-------------------+
| Simultaneous Sets |
+-------------------+
| {VC0, VC1, VC2} |
| {VC0, VC3, VC2} |
+-------------------+
Table 1: Two Simultaneous Transmission Sets
A media provider includes the simultaneous sets in its provider
advertisement. These simultaneous set constraints apply across all
the captures scenes in the advertisement. The simultaneous
transmission sets MUST allow all the media captures in a particular
capture scene entry to be used simultaneously.
7. Encodings
We have considered how providers can describe the content of media to
consumers. We will now consider how the providers communicate
information about their abilities to send streams. We introduce two
constructs - individual encodings and encoding groups. Consumers
will then map the media captures they want onto the encodings with
encoding parameters they want. This process is then described.
7.1. Individual Encodings
An individual encoding represents a way to encode a media capture to
become an encoded media stream sent from the media provider to the
media consumer. An individual encoding has a set of parameters
characterizing how the media is encoded. Different media types have
different parameters, and different encoding algorithms may have
different parameters. An individual encoding can be used for only
one actual encoded media stream at a time.
The parameters of an individual encoding represent the maximimum
values for certain aspects of the encoding. A particular
instantiation into an encoded stream might use lower values than
these maximums.
The following tables show the variables for audio and video encoding.
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+--------------+----------------------------------------------------+
| Name | Description |
+--------------+----------------------------------------------------+
| encodeID | A unique identifier for the individual encoding |
| maxBandwidth | Maximum number of bits per second |
| maxH264Mbps | Maximum number of macroblocks per second: ((width |
| | + 15) / 16) * ((height + 15) / 16) * |
| | framesPerSecond |
| maxWidth | Video resolution's maximum supported width, |
| | expressed in pixels |
| maxHeight | Video resolution's maximum supported height, |
| | expressed in pixels |
| maxFrameRate | Maximum supported frame rate |
+--------------+----------------------------------------------------+
Table 2: Individual Video Encoding Parameters
+--------------+-----------------------------------+
| Name | Description |
+--------------+-----------------------------------+
| maxBandwidth | Maximum number of bits per second |
+--------------+-----------------------------------+
Table 3: Individual Audio Encoding Parameters
7.2. Encoding Group
An encoding group includes a set of one or more individual encodings,
plus some parameters that apply to the group as a whole. By grouping
multiple individual encodings together, an encoding group describes
additional constraints on bandwidth and other parameters for the
group. Table 4 shows the parameters and individual encoding sets
that are part of an encoding group.
+-------------------+-----------------------------------------------+
| Name | Description |
+-------------------+-----------------------------------------------+
| encodeGroupID | A unique identifier for the encoding group |
| maxGroupBandwidth | Maximum number of bits per second relating to |
| | all encodings combined |
| maxGroupH264Mbps | Maximum number of macroblocks per second |
| | relating to all video encodings combined |
| videoEncodings[] | Set of potential encodings (list of |
| | encodeIDs) |
| audioEncodings[] | Set of potential encodings (list of |
| | encodeIDs) |
+-------------------+-----------------------------------------------+
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Table 4: Encoding Group
When the individual encodings in a group are instantiated into actual
encoded media streams, each stream has a bandwidth that must be less
than or equal to the maxBandwidth for the particular individual
encoding. The maxGroupBandwidth parameter gives the additional
restriction that the sum of all the individual instantiated
bandwidths must be less than or equal to the maxGroupBandwidth value.
Likewise, the sum of the macroblocks per second of each instantiated
encoding in the group must not exceed the maxGroupH264Mbps value.
The following diagram illustrates the structure of a media provider's
Encoding Groups and their contents.
,-------------------------------------------------.
| Media Provider |
| |
| ,--------------------------------------. |
| | ,--------------------------------------. |
| | | ,--------------------------------------. |
| | | | Encoding Group | |
| | | | ,-----------. | |
| | | | | | ,---------. | |
| | | | | | | | ,---------.| |
| | | | | Encoding1 | |Encoding2| |Encoding3|| |
| `.| | | | | | `---------'| |
| `.| `-----------' `---------' | |
| `--------------------------------------' |
`-------------------------------------------------'
Figure 1: Encoding Group Structure
A media provider advertises one or more encoding groups. Each
encoding group includes one or more individual encodings. Each
individual encoding can represent a different way of encoding media.
For example one individual encoding may be 1080p60 video, another
could be 720p30, with a third being CIF.
While a typical 3 codec/display system might have one encoding group
per "codec box", there are many possibilities for the number of
encoding groups a provider may be able to offer and for the encoding
values in each encoding group.
There is no requirement for all encodings within an encoding group to
be instantiated at once.
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8. Associating Media Captures with Encoding Groups
Every media capture is associated with an encoding group, which is
used to instantiate that media capture into one or more encoded
streams. Each media capture has an encoding group attribute. The
value of this attribute is the encodeGroupID for the encoding group
with which it is associated. More than one media capture may use the
same encoding group.
The maximum number of streams that can result from a particular
encoding group constraint is equal to the number of individual
encodings in the group. The actual number of streams used at any
time may be less than this maximum. Any of the media captures that
use a particular encoding group can be encoded according to any of
the individual encodings in the group. If there are multiple
individual encodings in the group, then a single media capture can be
encoded into multiple different streams at the same time, with each
stream following the constraints of a different individual encoding.
The Encoding Groups MUST allow all the media captures in a particular
capture scene entry to be used simultaneously.
9. Consumer's Choice of Streams to Receive from the Provider
After receiving the provider's advertised media captures and
associated constraints, the consumer must choose which media captures
it wishes to receive, and which individual encodings from the
provider it wants to use to encode the capture. Each media capture
has an encoding group ID attribute which specifies which individual
encodings are available to be used for that media capture.
For each media capture the consumer wants to receive, it configures
one or more of the encodings in that capture's encoding group. The
consumer does this by telling the provider the resolution, frame
rate, bandwidth, etc. when asking for streams for its chosen
captures. Upon receipt of this configuration command from the
consumer, the provider generates streams for each such configured
encoding and sends those streams to the consumer.
The consumer must have received at least one capture advertisement
from the provider to be able to configure the provider's generation
of media streams.
The consumer is able to change its configuration of the provider's
encodings any number of times during the call, either in response to
a new capture advertisement from the provider or autonomously. The
consumer need not send a new configure message to the provider when
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it receives a new capture advertisement from the provider unless the
contents of the new capture advertisement cause the consumer's
current configure message to become invalid.
When choosing which streams to receive from the provider, and the
encoding characteristics of those streams, the consumer needs to take
several things into account its local preference, simultaneity
restrictions, and encoding limits.
9.1. Local preference
A variety of local factors will influence the consumer's choice of
streams to be received from the provider:
o if the consumer is an endpoint, it is likely that it would choose,
where possible, to receive video and audio captures that match the
number of display devices and audio system it has
o if the consumer is a middle box such as an MCU, it may choose to
receive loudest speaker streams (in order to perform its own media
composition) and avoid pre-composed video captures
o user choice (for instance, selection of a new layout) may result
in a different set of media captures, or different encoding
characteristics, being required by the consumer
9.2. Physical simultaneity restrictions
There may be physical simultaneity constraints imposed by the
provider that affect the provider's ability to simultaneously send
all of the captures the consumer would wish to receive. For
instance, a middle box such as an MCU, when connected to a multi-
camera room system, might prefer to receive both individual camera
streams of the people present in the room and an overall view of the
room from a single camera. Some endpoint systems might be able to
provide both of these sets of streams simultaneously, whereas others
may not (if the overall room view were produced by changing the zoom
level on the center camera, for instance).
9.3. Encoding and encoding group limits
Each of the provider's encoding groups has limits on bandwidth and
macroblocks per second, and the constituent potential encodings have
limits on the bandwidth, macroblocks per second, video frame rate,
and resolution that can be provided. When choosing the media
captures to be received from a provider, a consumer device must
ensure that the encoding characteristics requested for each
individual media capture fits within the capability of the encoding
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it is being configured to use, as well as ensuring that the combined
encoding characteristics for media captures fit within the
capabilities of their associated encoding groups. In some cases,
this could cause an otherwise "preferred" choice of streams to be
passed over in favour of different streams - for instance, if a set
of 3 media captures could only be provided at a low resolution then a
3 screen device could switch to favoring a single, higher quality,
stream.
9.4. Message Flow
The following diagram shows the basic flow of messages between a
media provider and a media consumer. The usage of the "capture
advertisement" and "configure encodings" message is described above.
The consumer also sends its own capability message to the provider
which may contain information about its own capabilities or
restrictions.
Diagram for Message Flow
Media Consumer Media Provider
-------------- ------------
| |
|----- Consumer Capability ---------->|
| |
| |
|<---- Capture advertisement ---------|
| |
| |
|------ Configure encodings --------->|
| |
In order for a maximally-capable provider to be able to advertise a
manageable number of video captures to a consumer, there is a
potential use for the consumer, at the start of CLUE, to be able to
inform the provider of its capabilities. One example here would be
the video capture attribute set - a consumer could tell the provider
the complete set of video capture attributes it is able to understand
and so the provider would be able to reduce the capture scene it
advertises to be tailored to the consumer.
TBD - the content of the consumer capability message needs to be
better defined. The authors believe there is a need for this
message, but have not worked out the details yet.
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10. Extensibility
One of the most important characteristics of the Framework is its
extensibility. Telepresence is a relatively new industry and while
we can foresee certain directions, we also do not know everything
about how it will develop. The standard for interoperability and
handling multiple streams must be future-proof.
The framework itself is inherently extensible through expanding the
data model types. For example:
o Adding more types of media, such as telemetry, can done by
defining additional types of captures in addition to audio and
video.
o Adding new functionalities , such as 3-D, say, will require
additional attributes describing the captures.
o Adding a new codecs, such as H.265, can be accomplished by
defining new encoding variables.
The infrastructure is designed to be extended rather than requiring
new infrastructure elements. Extension comes through adding to
defined types.
Assuming the implementation is in something like XML, adding data
elements and attributes makes extensibility easy.
11. Examples - Using the Framework
This section shows some examples in more detail how to use the
framework to represent a typical case for telepresence rooms. First
an endpoint is illustrated, then an MCU case is shown.
11.1. Three screen endpoint media provider
Consider an endpoint with the following description:
o 3 cameras, 3 displays, a 6 person table
o Each video device can provide one capture for each 1/3 section of
the table
o A single capture representing the active speaker can be provided
o A single capture representing the active speaker with the other 2
captures shown picture in picture within the stream can be
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provided
o A capture showing a zoomed out view of all 6 seats in the room can
be provided
The audio and video captures for this endpoint can be described as
follows.
Video Captures:
o VC0- (the camera-left camera stream), encoding group=EG0,
content=main, switched=false
o VC1- (the center camera stream), encoding group=EG1, content=main,
switched=false
o VC2- (the camera-right camera stream), encoding group=EG2,
content=main, switched=false
o VC3- (the loudest panel stream), encoding group=EG1, content=main,
switched=true
o VC4- (the loudest panel stream with PiPs), encoding group=EG1,
content=main, composed=true, switched=true
o VC5- (the zoomed out view of all people in the room), encoding
group=EG1, content=main, composed=false, switched=false
o VC6- (presentation stream), encoding group=EG1, content=slides,
switched=false
The following diagram is a top view of the room with 3 cameras, 3
displays, and 6 seats. Each camera is capturing 2 people. The six
seats are not all in a straight line.
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,-. d
( )`--.__ +---+
`-' / `--.__ | |
,-. | `-.._ |_-+Camera 2 (VC2)
( ).' ___..-+-''`+-+
`-' |_...---'' | |
,-.c+-..__ +---+
( )| ``--..__ | |
`-' | ``+-..|_-+Camera 1 (VC1)
,-. | __..--'|+-+
( )| __..--' | |
`-'b|..--' +---+
,-. |``---..___ | |
( )\ ```--..._|_-+Camera 0 (VC0)
`-' \ _..-''`-+
,-. \ __.--'' | |
( ) |..-'' +---+
`-' a
The two points labeled b and c are intended to be at the midpoint
between the seating positions, and where the fields of view of the
cameras intersect.
The plane of interest for VC0 is a vertical plane that intersects
points 'a' and 'b'.
The plane of interest for VC1 intersects points 'b' and 'c'.
The plane of interest for VC2 intersects points 'c' and 'd'.
This example uses an area scale of millimeters.
Areas of capture:
bottom left bottom right top left top right
VC0 (-2011,2850,0) (-673,3000,0) (-2011,2850,757) (-673,3000,757)
VC1 ( -673,3000,0) ( 673,3000,0) ( -673,3000,757) ( 673,3000,757)
VC2 ( 673,3000,0) (2011,2850,0) ( 673,3000,757) (2011,3000,757)
VC3 (-2011,2850,0) (2011,2850,0) (-2011,2850,757) (2011,3000,757)
VC4 (-2011,2850,0) (2011,2850,0) (-2011,2850,757) (2011,3000,757)
VC5 (-2011,2850,0) (2011,2850,0) (-2011,2850,757) (2011,3000,757)
VC6 none
Points of capture:
VC0 (-1678,0,800)
VC1 (0,0,800)
VC2 (1678,0,800)
VC3 none
VC4 none
VC5 (0,0,800)
VC6 none
In this example, the right edge of the VC0 area lines up with the
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left edge of the VC1 area. It doesn't have to be this way. There
could be a gap or an overlap. One additional thing to note for this
example is the distance from a to b is equal to the distance from b
to c and the distance from c to d. All these distances are 1346 mm.
This is the planar width of each area of capture for VC0, VC1, and
VC2.
Note the text in parentheses (e.g. "the camera-left camera stream")
is not explicitly part of the model, it is just explanatory text for
this example, and is not included in the model with the media
captures and attributes. Also, the "composed" boolean attribute
doesn't say anything about how a capture is composed, so the media
consumer can't tell based on this attribute that VC4 is composed of a
"loudest panel with PiPs".
Audio Captures:
o AC0 (camera-left), encoding group=EG3, content=main, channel
format=mono
o AC1 (camera-right), encoding group=EG3, content=main, channel
format=mono
o AC2 (center) encoding group=EG3, content=main, channel format=mono
o AC3 being a simple pre-mixed audio stream from the room (mono),
encoding group=EG3, content=main, channel format=mono
o AC4 audio stream associated with the presentation video (mono)
encoding group=EG3, content=slides, channel format=mono
Areas of capture:
bottom left bottom right top left top right
AC0 (-2011,2850,0) (-673,3000,0) (-2011,2850,757) (-673,3000,757)
AC1 ( 673,3000,0) (2011,2850,0) ( 673,3000,757) (2011,3000,757)
AC2 ( -673,3000,0) ( 673,3000,0) ( -673,3000,757) ( 673,3000,757)
AC3 (-2011,2850,0) (2011,2850,0) (-2011,2850,757) (2011,3000,757)
AC4 none
The physical simultaneity information is:
Simultaneous transmission set #1 {VC0, VC1, VC2, VC3, VC4, VC6}
Simultaneous transmission set #2 {VC0, VC2, VC5, VC6}
This constraint indicates it is not possible to use all the VCs at
the same time. VC5 can not be used at the same time as VC1 or VC3 or
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VC4. Also, using every member in the set simultaneously may not make
sense - for example VC3(loudest) and VC4 (loudest with PIP). (In
addition, there are encoding constraints that make choosing all of
the VCs in a set impossible. VC1, VC3, VC4, VC5, VC6 all use EG1 and
EG1 has only 3 ENCs. This constraint shows up in the encoding
groups, not in the simultaneous transmission sets.)
In this example there are no restrictions on which audio captures can
be sent simultaneously.
Encoding Groups:
This example has three encoding groups associated with the video
captures. Each group can have 3 encodings, but with each potential
encoding having a progressively lower specification. In this
example, 1080p60 transmission is possible (as ENC0 has a maxMbps
value compatible with that) as long as it is the only active encoding
in the group(as maxMbps for the entire encoding group is also
489600). Significantly, as up to 3 encodings are available per
group, it is possible to transmit some video captures simultaneously
that are not in the same entry in the capture scene. For example VC1
and VC3 at the same time.
It is also possible to transmit multiple encodings of a single video
capture. For example VC0 can be encoded using ENC0 and ENC1 at the
same time, as long as the encoding parameters satisfy the constraints
of ENC0, ENC1, and EG0, such as one at 1080p30 and one at 720p30.
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encodeGroupID=EG0, maxGroupH264Mbps=489600, maxGroupBandwidth=6000000
encodeID=ENC0, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=489600, maxBandwidth=4000000
encodeID=ENC1, maxWidth=1280, maxHeight=720, maxFrameRate=30,
maxH264Mbps=108000, maxBandwidth=4000000
encodeID=ENC2, maxWidth=960, maxHeight=544, maxFrameRate=30,
maxH264Mbps=61200, maxBandwidth=4000000
encodeGroupID=EG1 maxGroupH264Mbps=489600 maxGroupBandwidth=6000000
encodeID=ENC3, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=489600, maxBandwidth=4000000
encodeID=ENC4, maxWidth=1280, maxHeight=720, maxFrameRate=30,
maxH264Mbps=108000, maxBandwidth=4000000
encodeID=ENC5, maxWidth=960, maxHeight=544, maxFrameRate=30,
maxH264Mbps=61200, maxBandwidth=4000000
encodeGroupID=EG2 maxGroupH264Mbps=489600 maxGroupBandwidth=6000000
encodeID=ENC6, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=489600, maxBandwidth=4000000
encodeID=ENC7, maxWidth=1280, maxHeight=720, maxFrameRate=30,
maxH264Mbps=108000, maxBandwidth=4000000
encodeID=ENC8, maxWidth=960, maxHeight=544, maxFrameRate=30,
maxH264Mbps=61200, maxBandwidth=4000000
Figure 2: Example Encoding Groups for Video
For audio, there are five potential encodings available, so all five
audio captures can be encoded at the same time.
encodeGroupID=EG3, maxGroupH264Mbps=0, maxGroupBandwidth=320000
encodeID=ENC9, maxBandwidth=64000
encodeID=ENC10, maxBandwidth=64000
encodeID=ENC11, maxBandwidth=64000
encodeID=ENC12, maxBandwidth=64000
encodeID=ENC13, maxBandwidth=64000
Figure 3: Example Encoding Group for Audio
Capture Scenes:
The following table represents the capture scenes for this provider.
Recall that a capture scene is composed of alternative capture scene
entries covering the same scene. Capture Scene #1 is for the main
people captures, and Capture Scene #2 is for presentation.
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Each row in the table is a separate entry in the capture scene
+------------------+
| Capture Scene #1 |
+------------------+
| VC0, VC1, VC2 |
| VC3 |
| VC4 |
| VC5 |
| AC0, AC1, AC2 |
| AC3 |
+------------------+
+------------------+
| Capture Scene #2 |
+------------------+
| VC6 |
| AC4 |
+------------------+
Different capture scenes are unique to each other, non-overlapping.
A consumer can choose an entry from each capture scene. In this case
the three captures VC0, VC1, and VC2 are one way of representing the
video from the endpoint. These three captures should appear adjacent
next to each other. Alternatively, another way of representing the
Capture Scene is with the capture VC3, which automatically shows the
person who is talking. Similarly for the VC4 and VC5 alternatives.
As in the video case, the different entries of audio in Capture Scene
#1 represent the "same thing", in that one way to receive the audio
is with the 3 audio captures (AC0, AC1, AC2), and another way is with
the mixed AC3. The Media Consumer can choose an audio capture entry
it is capable of receiving.
The spatial ordering is understood by the media capture attributes
area and point of capture.
A Media Consumer would likely want to choose a capture scene entry to
receive based in part on how many streams it can simultaneously
receive. A consumer that can receive three people streams would
probably prefer to receive the first entry of Capture Scene #1 (VC0,
VC1, VC2) and not receive the other entries. A consumer that can
receive only one people stream would probably choose one of the other
entries.
If the consumer can receive a presentation stream too, it would also
choose to receive the only entry from Capture Scene #2 (VC6).
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11.2. Encoding Group Example
This is an example of an encoding group to illustrate how it can
express dependencies between encodings.
encodeGroupID=EG0, maxGroupH264Mbps=489600, maxGroupBandwidth=6000000
encodeID=VIDENC0, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=244800, maxBandwidth=4000000
encodeID=VIDENC1, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=244800, maxBandwidth=4000000
encodeID=AUDENC0, maxBandwidth=96000
encodeID=AUDENC1, maxBandwidth=96000
encodeID=AUDENC2, maxBandwidth=96000
Here, the encoding group is EG0. It can transmit up to two 1080p30
encodings (Mbps for 1080p = 244800), but it is capable of
transmitting a maxFrameRate of 60 frames per second (fps). To
achieve the maximum resolution (1920 x 1088) the frame rate is
limited to 30 fps. However 60 fps can be achieved at a lower
resolution if required by the consumer. Although the encoding group
is capable of transmitting up to 6Mbit/s, no individual video
encoding can exceed 4Mbit/s.
This encoding group also allows up to 3 audio encodings, AUDENC<0-2>.
It is not required that audio and video encodings reside within the
same encoding group, but if so then the group's overall maxBandwidth
value is a limit on the sum of all audio and video encodings
configured by the consumer. A system that does not wish or need to
combine bandwidth limitations in this way should instead use separate
encoding groups for audio and video in order for the bandwidth
limitations on audio and video to not interact.
Audio and video can be expressed in separate encoding groups, as in
this illustration.
encodeGroupID=EG0, maxGroupH264Mbps=489600, maxGroupBandwidth=6000000
encodeID=VIDENC0, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=244800, maxBandwidth=4000000
encodeID=VIDENC1, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=244800, maxBandwidth=4000000
encodeGroupID=EG1, maxGroupH264Mbps=0, maxGroupBandwidth=500000
encodeID=AUDENC0, maxBandwidth=96000
encodeID=AUDENC1, maxBandwidth=96000
encodeID=AUDENC2, maxBandwidth=96000
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11.3. The MCU Case
This section shows how an MCU might express its Capture Scenes,
intending to offer different choices for consumers that can handle
different numbers of streams. A single audio capture stream is
provided for all single and multi-screen configurations that can be
associated (e.g. lip-synced) with any combination of video captures
at the consumer.
+--------------------+---------------------------------------------+
| Capture Scene #1 | note |
+--------------------+---------------------------------------------+
| VC0 | video capture for single screen consumer |
| VC1, VC2 | video capture for 2 screen consumer |
| VC3, VC4, VC5 | video capture for 3 screen consumer |
| VC6, VC7, VC8, VC9 | video capture for 4 screen consumer |
| AC0 | audio capture representing all participants |
+--------------------+---------------------------------------------+
If / when a presentation stream becomes active within the conference,
the MCU might re-advertise the available media as:
+------------------+--------------------------------------+
| Capture Scene #2 | note |
+------------------+--------------------------------------+
| VC10 | video capture for presentation |
| AC1 | presentation audio to accompany VC10 |
+------------------+--------------------------------------+
11.4. Media Consumer Behavior
This section gives an example of how a media consumer might behave
when deciding how to request streams from the three screen endpoint
described in the previous section.
The receive side of a call needs to balance its requirements, based
on number of screens and speakers, its decoding capabilities and
available bandwidth, and the provider's capabilities in order to
optimally configure the provider's streams. Typically it would want
to receive and decode media from each capture scene advertised by the
provider.
A sane, basic, algorithm might be for the consumer to go through each
capture scene in turn and find the collection of video captures that
best matches the number of screens it has (this might include
consideration of screens dedicated to presentation video display
rather than "people" video) and then decide between alternative
entries in the video capture scenes based either on hard-coded
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preferences or user choice. Once this choice has been made, the
consumer would then decide how to configure the provider's encoding
groups in order to make best use of the available network bandwidth
and its own decoding capabilities.
11.4.1. One screen consumer
VC3, VC4 and VC5 are all different entries by themselves, not grouped
together in a single entry, so the receiving device should choose
between one of those. The choice would come down to whether to see
the greatest number of participants simultaneously at roughly equal
precedence (VC5), a switched view of just the loudest region (VC3) or
a switched view with PiPs (VC4). An endpoint device with a small
amount of knowledge of these differences could offer a dynamic choice
of these options, in-call, to the user.
11.4.2. Two screen consumer configuring the example
Mixing systems with an even number of screens, "2n", and those with
"2n+1" cameras (and vice versa) is always likely to be the
problematic case. In this instance, the behavior is likely to be
determined by whether a "2 screen" system is really a "2 decoder"
system, i.e., whether only one received stream can be displayed per
screen or whether more than 2 streams can be received and spread
across the available screen area. To enumerate 3 possible behaviors
here for the 2 screen system when it learns that the far end is
"ideally" expressed via 3 capture streams:
1. Fall back to receiving just a single stream (VC3, VC4 or VC5 as
per the 1 screen consumer case above) and either leave one screen
blank or use it for presentation if / when a presentation becomes
active
2. Receive 3 streams (VC0, VC1 and VC2) and display across 2 screens
(either with each capture being scaled to 2/3 of a screen and the
centre capture being split across 2 screens) or, as would be
necessary if there were large bezels on the screens, with each
stream being scaled to 1/2 the screen width and height and there
being a 4th "blank" panel. This 4th panel could potentially be
used for any presentation that became active during the call.
3. Receive 3 streams, decode all 3, and use control information
indicating which was the most active to switch between showing
the left and centre streams (one per screen) and the centre and
right streams.
For an endpoint capable of all 3 methods of working described above,
again it might be appropriate to offer the user the choice of display
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mode.
11.4.3. Three screen consumer configuring the example
This is the most straightforward case - the consumer would look to
identify a set of streams to receive that best matched its available
screens and so the VC0 plus VC1 plus VC2 should match optimally. The
spatial ordering would give sufficient information for the correct
video capture to be shown on the correct screen, and the consumer
would either need to divide a single encoding group's capability by 3
to determine what resolution and frame rate to configure the provider
with or to configure the individual video captures' encoding groups
with what makes most sense (taking into account the receive side
decode capabilities, overall call bandwidth, the resolution of the
screens plus any user preferences such as motion vs sharpness).
12. Acknowledgements
Mark Gorzyinski contributed much to the approach. We want to thank
Stephen Botzko for helpful discussions on audio.
13. IANA Considerations
TBD
14. Security Considerations
TBD
15. Changes Since Last Version
NOTE TO THE RFC-Editor: Please remove this section prior to
publication as an RFC.
Changes from 05 to 06:
1. Capture scene description attribute is a list of text strings,
each in a different language, rather than just a single string.
2. Add new Axis of Capture Point attribute.
3. Remove appendices A.1 through A.6.
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4. Clarify that the provider must use the same coordinate system
with same scale and origin for all coordinates within the same
capture scene.
Changes from 04 to 05:
1. Clarify limitations of "composed" attribute.
2. Add new section "capture scene entry attributes" and add the
attribute "scene-switch-policy".
3. Add capture scene description attribute and description language
attribute.
4. Editorial changes to examples section for consistency with the
rest of the document.
Changes from 03 to 04:
1. Remove sentence from overview - "This constitutes a significant
change ..."
2. Clarify a consumer can choose a subset of captures from a
capture scene entry or a simultaneous set (in section "capture
scene" and "consumer's choice...").
3. Reword first paragraph of Media Capture Attributes section.
4. Clarify a stereo audio capture is different from two mono audio
captures (description of audio channel format attribute).
5. Clarify what it means when coordinate information is not
specified for area of capture, point of capture, area of scene.
6. Change the term "producer" to "provider" to be consistent (it
was just in two places).
7. Change name of "purpose" attribute to "content" and refer to
RFC4796 for values.
8. Clarify simultaneous sets are part of a provider advertisement,
and apply across all capture scenes in the advertisement.
9. Remove sentence about lip-sync between all media captures in a
capture scene.
10. Combine the concepts of "capture scene" and "capture set" into a
single concept, using the term "capture scene" to replace the
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previous term "capture set", and eliminating the original
separate capture scene concept.
16. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353,
February 2006.
[RFC4796] Hautakorpi, J. and G. Camarillo, "The Session Description
Protocol (SDP) Content Attribute", RFC 4796,
February 2007.
[RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
January 2008.
[RFC5646] Phillips, A. and M. Davis, "Tags for Identifying
Languages", BCP 47, RFC 5646, September 2009.
[IANA-Lan]
IANA, "Language Subtag Registry",
<http://www.iana.org/assignments/
language-subtag-registry>.
Authors' Addresses
Allyn Romanow
Cisco Systems
San Jose, CA 95134
USA
Email: allyn@cisco.com
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Mark Duckworth (editor)
Polycom
Andover, MA 01810
USA
Email: mark.duckworth@polycom.com
Andrew Pepperell
Langley, England
UK
Email: apeppere@gmail.com
Brian Baldino
Cisco Systems
San Jose, CA 95134
USA
Email: bbaldino@cisco.com
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