Wondering if anyone has ran PTP traffic along side high volume UDP traffic on the same switch running at 100 Mbps. Would it be expected to get massive (15 us) jitter on the PTP packets without setting QoS, especially if the link is not being saturated? The high volume traffic would be characterized as one packet every 125 us with a size of roughly 250 bytes.


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An IP(?) packet of 250 bytes translates to 23 μs link time on Fast Ethernet (=maximum additional latency with gapped packets). Half that somewhat matches the jitter you see, so it's to be expected.

For more precision you'll need dedicated or faster links. QoS wouldn't improve this as a frame isn't terminated in the middle of a transmission for the sake of a higher prioritized frame.

QoS helps with a (near) saturated link in that it can limit the additional latency to that of a frame's transmission time, independent of the queue length.

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