CLUE WG A. Romanow
Internet-Draft Cisco Systems
Intended status: Informational M. Duckworth, Ed.
Expires: August 05, 2012 Polycom
A. Pepperell
B. Baldino
Cisco Systems
February 4, 2012

Framework for Telepresence Multi-Streams


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.

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This Internet-Draft will expire on August 05, 2012.

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Table of Contents

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 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: the scene that is captured by a collection of Capture Devices. A Capture Scene may be represented by more than one type of Media. A Capture Scene may include more than one Media Capture of the same type. An example of a Capture Scene is 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. A middle box may also express Capture Scenes that it constructs from Media streams it receives.

*Capture Set: A Capture Set includes media captures that are arranged by the provider to help the consumer choose which captures it wants. The entries in a Capture Set represent different alternatives for representing the same 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.

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].

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:

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. This constitutes a significant change from previous video conferencing systems in which transmission of content was determined primarily by the sender.

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 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 sets. A consumer chooses which media captures it wants to receive according to the capture sets 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 producers 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:

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 Sets

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:

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 that can be used by the consumer to help decide which Media Captures should be requested. Attributes are defined by a variable and its value. The currently defined attributes and their values are:

Purpose: {main, presentation}

A field with enumerated values which describes the role of the Media Capture and can be applied to any media type.

A value of 'main' describes the primary content of the room (such as participant media).

A value of 'presentation' describes the secondary content of the room (such as media coming from a laptop).

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 not intended to describe the layout used when compositing video streams.

Audio Channel Format: {mono, stereo} A field with enumerated values 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.

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 producer 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.

Area of Capture:

{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 set 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.

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 Set

In order for a provider's individual media captures to be used effectively by a consumer, the provider organizes the media captures into capture sets, with the structure and contents of these sets being sent from the provider to the consumer.

A provider may advertise multiple capture sets or just a single capture set. A capture set can be said to correspond to a provided "scene", and a media provider might typically use one capture set for main participant media and another capture set for a computer generated presentation. Capture sets will commonly include media captures of different types, for instance, audio captures and video captures.

A provider can express spatial relationships between media captures that are included in the same capture set. But there is no spatial relationship between media captures that are in different capture sets.

A capture set is most usefully thought of as being a collection of entries, with each entry being a list of media captures. In grouping multiple media captures together within a capture set entry, the provider is signaling that those captures together form a representation of that capture set's scene. Media captures within the same capture set entry must be of the same media type - it is not possible to mix audio and video captures in the same capture set entry, for instance. The provider must be capable of encoding and sending all media captures in a single entry simultaneously.

When a provider advertises a capture set 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 set entries for an endpoint-style media provider:

  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 set 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 set 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 set (for instance, the consumer should attempt to perform lip sync between all audio and video captures from the same capture set).

6.2.1. Capture set attributes

Attributes can be applied to capture sets as well as to individual media captures. Attributes specified at this level apply to all constituent media captures.

Area of Scene attribute

The area of scene attribute for a capture set 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 set entries.

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:

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.

Two Simultaneous Transmission Sets
Simultaneous Sets
{VC0, VC1, VC2}
{VC0, VC3, VC2}

The Simultaneous Transmission Sets MUST allow all the Media Captures in a particular capture set 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.

Individual Video Encoding Parameters
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
Individual Audio Encoding Parameters
Name Description
maxBandwidth Maximum number of bits per second

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.

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)

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||   |
|  `.| | |           | |         | `---------'|   |
|    `.| `-----------' `---------'            |   |
|      `--------------------------------------'   |

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.

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 set 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 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:

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 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 set it advertises to be tailored to the consumer.

TBD - the content of this message needs to be better defined. The authors believe there is a need for this message, but have not worked out the details yet.

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:

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:

The audio and video captures for this endpoint can be described as follows.

Video Captures:

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.

   ,-. 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 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.

Audio Captures:

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:

{VC0, VC1, VC2, VC3, VC4, VC6}
{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 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 set. 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.

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

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

Capture Sets:

The following table represents the capture sets for this provider. Recall that a capture set is composed of alternative captures covering the same scene. Capture Set #1 is for the main people captures, and Capture Set #2 is for presentation.

Each row in the table is a separate entry in the capture set

Capture Set #1
VC0, VC1, VC2
AC0, AC1, AC2
Capture Set #2

Different capture sets are unique to each other, non-overlapping. A consumer can choose an entry from each capture set. 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 Set #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 set 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 Set #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 Set #2 (VC6).

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

11.3. The MCU Case

This section shows how an MCU might express its Capture Sets, 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 Set #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 Set #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 set advertised by the provider.

A sane, basic, algorithm might be for the consumer to go through each capture set 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 sets based either on hard-coded 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 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


14. Security Considerations


15. 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.
[RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117, January 2008.

Appendix A. Open Issues

Appendix A.1. Video layout arrangements and centralized composition

In the context of a conference with a central MCU, there has been discussion about a consumer requesting the provider to provide a certain type of layout arrangement or perform a certain composition algorithm, such as combining some number of most recent talkers, or producing a video layout using a 2x2 grid or 1 large cell with 5 smaller cells around it. The current framework does not address this. It isn't clear if this topic should be included in this framework, or maybe a different part of CLUE, or maybe outside of CLUE altogether.

Appendix A.2. Source is selectable

A Boolean variable. True indicates the media consumer can request a particular media source be mapped to a media capture. Default is false.

TBD - how does the consumer make the request for a particular source? How does the consumer know what is available? Need to explain better how multiple media captures are different from a single media capture with choices for the source, and when each concept should be used.

Appendix A.3. Media Source Selection

The use cases include a case where the person at a receiving endpoint can request to receive media from a particular other endpoint, for example in a multipoint call to request to receive the video from a certain section of a certain room, whether or not people there are talking.

TBD - this framework should address this case. Maybe need a roster list of rooms or people in the conference, with a mechanism to select from the roster and associate it with media captures. This is different from selecting a particular media capture from a capture set. The mechanism to do this will probably need to be different than selecting media captures based on capture sets and attributes.

Appendix A.4. Endpoint requesting many streams from MCU

TBD - how to do VC selection for a system where the endpoint media consumers want to receive lots of streams and do their own composition, rather than MCU doing transcoding and composing. Example is 3 screen consumer that wants 3 large loudest speaker streams, and a bunch of small ones to render as PiP. How the small ones are chosen, which could potentially be chosen by either the endpoint or MCU. There are other more complicated examples also. Is the current framework adequate to support this?

Appendix A.5. VAD (voice activity detection) tagging of audio streams

TBD - do we want to have VAD be mandatory? All audio streams originating from a media provider must be tagged with VAD information. This tagging would include an overall energy value for the stream plus information on which sections of the capture scene are "active".

Each audio stream which forms a constituent of an entry within a capture set should include this tagging, and the energy value within it calculated using a fixed, consistent algorithm.

When a system determines the most active area of a capture scene (either "loudest", or determined by other means such as a button press) it should convey that information to the corresponding media stream consumer via any audio streams being sent within that capture set. Specifically, there should be a list of active coordinates and their VAD characteristics within the audio stream in addition to the overall VAD information for the capture set. This is to ensure all media stream consumers receive the same, consistent, audio energy information whichever audio capture or captures they choose to receive for a capture set. Additionally, coordinate information can be mapped to video captures by a media stream consumer in order that it can perform "panel switching" if required.

Appendix A.6. Private Information

Do we want a way to include private information?

Authors' Addresses

Allyn Romanow Cisco Systems San Jose, CA 95134 USA EMail:
Mark Duckworth editor Polycom Andover, MA 01810 US EMail:
Andrew Pepperell Langley, England UK EMail:
Brian Baldino Cisco Systems San Jose, CA 95134 US EMail: