TANDBERG D11624 manual Technical issues

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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 high-speed backbones. Since delay is inversely proportional to speed, this means that delay at intermediate nodes is highly unlikely. Also, many of today's Public frame relay networks use a cell (fixed frame length) based architecture between nodes, which also reduces the likelihood of jitter.

D11624 rev.01

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Contents Video on Frame Relay Table of Contents Introduction Background on transmission of video over frame relay Technical issues Cell based systems cut packet into fixed sizes Framing the Picture Equipment for Video over Frame RelayAbout the Product VFX-250S Application Overview Technical Specification of VFX-250S Mechanical/Environmental Power SupplyFCC Class a Digital Devices and Peripherals ApprovalsCable VFX-250S Economics of Video over Frame Relay Standard Console Cable Appendix a Cables and PinoutsStandard EIA-449/RS449 Cables Standard EIA-530/RS530 Cables Standard V.35 Cables AVB Appendix C Glossary and abbreviationsUSA Appendix D Information about the manufacturer