Prioritization the medium at a specific instant in

Prioritization alone is insufficient to address the needs of this kind of traffic; if a low priority frame is already being transmitted, then that transmission will complete before a higher priority frame can access the transmission medium, so there could be a delay of up to a maximum-sized frame before a high priority transmission can start. If such delays occur at every hop, then the accumulated latency could be unacceptably large.To ensure a high priority traffic has guaranteed access to the medium at a specific instant in time, a protection window can be considered allowing transmission of a specific traffic class over the medium for the duration of the time window. This implies that the transmission of other traffic classes has to be stopped in advance to avoid interference. This means the transmission of lower priority traffic should be completed prior to the time (T1) when time critical traffic is granted access to the medium.

The difference between T1 (time at which time critical traffic gains access to the medium) and T0 (completion time of transmission of the last non-time critical traffic frame) should be equal to maximum-frame size in the worst case. This difference can be guaranteed by introduction of guard bands. Guard bands ensure non time critical traffic is not transmitted between the start of guard band (T0) and beginning of transmission of time critical traffic (T1) as can be seen in fig 3. The size of the guard band can be fixed to the size of one maximum-sized frame or be flexible if the implementation can determine from the size of the queued frame, that there is

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