Filter
Top Products
Tandberg Video on Frame Relay
D11624 rev.01 5
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.