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Understanding Advanced FF/RW

Advanced FF/RW improves upon the standard fast-forward and rewind "trick mode" functionality for the video portion of encoded files.

When clients use standard trick mode to fast-forward or rewind an on-demand video file, the Windows Media server receives a request that includes the following information:

The server responds by rapidly searching the source content file for all available key frames after the presentation time reported by the client so that it can send the data quickly at the requested rate. The server divides the total presentation time by the rate request value to calculate the correct trick mode speed, and then delivers the content, beginning at the first available key frame after the presentation time reported by the client.

Clients that use standard trick mode to fast-forward or rewind an on-demand video file can create enormous demands for network resources because identical amounts of presentation data must be processed and sent at faster speeds to fulfill the client requests. Servers must process additional presentation data from source content disks, or from storage devices in Storage Area Networks (SANs), at accelerated rates, potentially creating server performance bottlenecks caused by higher disk-read rates.

The key frames that are sent to clients during standard trick mode consume additional bandwidth, especially at high speeds. This can exceed the original bandwidth of the content. If the content has low key-frame distances, more network bandwidth is consumed at high speeds because the server must send more key frames in a specific time period. If the content has high key-frame distances, less network bandwidth is consumed at high speeds because the number of key frames sent in the specific time period is lower. However, the motion viewed in the client may not be smooth while it fast-forwards or rewinds the file.

Advanced FF/RW addresses the performance issues that accompany standard trick mode functionality by smoothing the rate at which data is sent from servers to clients and from back-end network devices to servers. For example, when a client requests that content be rewound at five times normal speed (5x), the client is delivered a copy of the original source file that contains only 20 percent of the frames that were originally present (a file created from the original such that for every 5 frames, 4 frames are dropped). When the server delivers this file to the client in place of the original source file, it reads the presentation data without incurring a high disk-read rate from the source content disk.

Because key frames contain all of the data that is required to reconstruct the video image in the client, without reference to previous frames, the fast-forward or rewind presentation in the client remains smooth, alleviating spikes in network bandwidth.

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