Tandberg | Video on Frame Relay |
3. Technical issues
There are two potential technical issues, which may affect the quality of packetised, digitised video. One is delay, or more properly jitter. Jitter is the variation in delay from one frame to the next. This is critical for video, as video requires a constant bit stream in order to maintain an image. The second is dropped frames. If a video frame is lost, it may cause a click or pop in the audio and some pixelation on the video. Too many lost frames and the video quality is impaired.
In leased line applications using TDM (Time Division Multiplexing) jitter is not an issue, as video frames arrive at known, predictable intervals. Concurrently, there is little likelihood of dropped frames unless the line itself malfunctions. However, public frame relay networks introduce issues that do not occur when running the frame relay protocol over private leased lines. Customers who wish to run digitised video over public frame relay services need to understand these issues.
Jitter can occur in public frame networks when an intermediate switch is processing someone else's frame when your frame arrives.
Jitter is created by differences in packet size
The second incoming frame is held in a buffer at the switch until the transmission of the first frame is completed. The delay that results is dependent on the length of the first frame. Since frame relay allows variable length frames, this delay is variable and unpredictable, resulting in jitter. If this jitter exceeds the ability of the receiving device to compensate by buffering, video quality will be degraded.
However, for the majority of public frame relay networks, jitter is more a theoretical problem than a real problem. Public services run on
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