3.1.5.3 Signaling of Simulcast

Multiple simulcast streams MAY be encoded using the same camera feed at the same time. Each simulcast stream MAY be decoded by itself and might contain multiple layers, including one base layer and 0 or more enhancement layers.

The sender signals the simulcast by using stream layout PACSI NAL units<6>. The format of a stream layout PACSI is defined in section 2.2.6 of this protocol.

A full stream layout (section 2.2.5.1) PACSI NAL unit MUST be sent as the very first NAL unit.

Whenever one or more layers are removed, an update stream layout (section 2.2.5.1) or a full stream layout PACSI NAL unit MUST be sent. The update stream layout PACSI NAL unit SHOULD be used if no layer information has changed since the previous full stream layout and there is no need for the layer presence byte fields to be present.

Whenever one or more layers are added and the layer descriptions are not changed, an update stream layout or a full stream layout PACSI NAL unit MUST be sent. For the same reason noted in the previous paragraph, the update stream layout PACSI NAL unit is preferred.

For other changes in the simulcast streams, a full stream layout PACSI NAL unit MUST be sent. This includes a new layer that is never present before it is added or any Layer Description field changes.

A stream layout PACSI MAY be sent in any layer. There SHOULD NOT be any VCL data NAL units following it in the same access unit. A stream layout PACSI MUST be sent prior to the change it signals. This means that when a layer is added into the simulcast streams, the stream layout PACSI NAL unit MUST be sent before any other NAL unit of the new layer is sent. After a stream layout PACSI NAL unit is sent with a layer removed from the simulcast streams, no NAL unit of the removed layer is allowed to be sent. This is needed so that the receiver has timely, accurate information about the simulcast streams.