Multicast streaming is a one-to-many relationship between a Windows Media server and the clients receiving the stream. With a multicast stream, the server streams to a multicast IP address on the network, and clients receive the stream by subscribing to the IP address. All clients receive the same stream and do not have control of content playback. Because there is only one stream from the server regardless of the number of clients receiving the stream, a multicast stream requires the same amount of bandwidth as a single unicast stream containing the same content. Using a multicast stream preserves network bandwidth and can be useful for low bandwidth local area networks.

You can deliver content as a multicast stream only from a broadcast publishing point. In addition, your network routers must be multicast-enabled, meaning that they can transmit class-D IP addresses. If your network routers are not multicast-enabled, you can still deliver content as a multicast stream over the local segment of your local area network.

All multicast content must have a specified time-to-live (TTL) value that limits the number of routers that the multicast can traverse before it expires.

The WMS Multicast Data Writer plug-in controls how the server uses multicast delivery with your publishing point. This plug-in is disabled by default and can only be enabled on broadcast publishing points.

The following figure shows an example of content being distributed from an encoder as a multicast stream by using a broadcast publishing point.

Multicast streams are much less demanding on the server and network but may require network modification for multicasts and regular network traffic to coexist effectively. Consider using multicast streaming if:

• You are broadcasting content to a large audience, and network bandwidth and server capacity are limited.
  
• Your network is multicast-enabled.
  

Note
Multicast streaming and the WMS Multicast Data Writer plug-in are not available if Windows Media Services is running on Microsoft® Windows Server® 2003, Standard Edition, Windows Server 2008 Standard, or Windows Web Server 2008. For more information, see Decide which version of Windows Server is right for you.

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