Ncast N-Way Server Manuel d'utilisateur télécharger pdf (Page 7)

NCast N-Way Reference Manual

remote units (but the reverse of that flow is not supported). Also, multicast traffic from the local LAN of each

remote unit maybe forwarded to the bridge. Contact NCast for details on the uses and potential limitations of

network architectures beyond a simple hub-and-spoke arrangement.

In each case, multicast traffic (the media streams) generated by the remote Telepresenter is encapsulated into a

packet which is sent via unicast to the N-Way bridge, where it is unpacked and mixed with all other multicast

streams present at the bridge. The server looks at all the multicast traffic in the mix, and forwards, via the reverse

unicast connection, the media streams desired by a remote unit. Multicast group traffic not requested by a remote

unit is discarded and not forwarded to any remote site.

1.5.4.One-to-Many Streaming using Multicast Bridging and RTSP

In this mode of operation one of the Telepresenters designated as the “Sender” is generating a media stream and

forwarding this stream via a multicast tunnel to the N-Way Server. At the server the media stream packets are de-

encapsulated and placed onto the local network as a multicast source. A streaming server process running in the

N-Way Server can then accept RTSP protocol requests from remote desktops and forward media streams in the

local multicast mix to the remote clients.

This mode of operation allows many hundreds of viewers (who have only unicast connections) to watch the

encoded output of a single Telepresenter. The N-Way Server must be at a network point where the aggregation

of many unicast streams will not impact functioning of the network.

There is, within each Telepresenter, a similar one-to-many RTSP connection function, and for small groups and

low bandwidth connections the use of an N-Way Server is not required. However, the number of viewers is limited

and it is possible that the sending (encoding) Telepresenter is at a network location where the aggregation of

multiple unicast streams will choke the network connection. In this case, the encoded stream should be tunneled

to a central network point where mass distribution to a large number of clients is workable.

1.5.5.Collaboration Mode Streaming to the Desktop using RTSP

Telepresenters can be used in a special mode called “Collaboration Mode” where each Telepresenter in the

conference can be given floor control in turn, and the other participating units will be viewing the graphics or video

output of the unit, which currently “has the floor”. Thus, the conference coordinator can switch presentations from

a central site to some remote site to allow a remote presenter (a student presenting a report, a foreign workgroup,

a remote knowledge expert, etc.) to speak and be seen by all the participants in the conference. When the remote

presenter is finished, the coordinator continues with his/her presentation from the central site and all the units are

redirected to view the proceedings emanating from the central location.

If this collaborative event is to be viewed by remote desktops, two important technical issues must be resolved.

First, as the source of the graphical or video presentation switches from Telepresenter to Telepresenter, the RTP

stream-id changes with each switch, and most desktop client players are unable to re-sync onto a media stream

with a new and different stream-id. They typically “lock-on” to the first stream-id they find, and ignore media with

all other stream-id’s. The net result is that the viewer sees only media from one presenter, and the picture is

frozen while other presenters have the floor.

Also, in a collaborative conference, each Telepresenter is continuously generating an audio stream, and these

multiple audio streams must be mixed into one sound source somewhere for playback. This function is normally

NCast Corporation

Revision 1.3

Page 7

1 2 3 4 5 6 7 8 9 10 11 12 ... 39 40

Pas de commentaire

Ncast N-Way Server Manuel d'utilisateur Page 7