[time-nuts] Low-long-term-drift clock for board level integration?
Hal Murray
hmurray at megapathdsl.net
Tue Feb 21 01:24:15 UTC 2012
brooke at pacific.net said:
> It was my understanding that the receiving station knows the path taken, is
> that the case? Or are you saying even when it's the same path the time
> delay has large variations?
Even if the path is stable, the delays vary due to queuing delays in routers.
The simple example is a router with 3 cables. Call them A, B, and C. Assume
they are all the same speed.
If B is idle, packets that come in on A can go out C right away.
But suppose A and B are both sending traffic out C. If packets arrive at
close to the same time, the second one will have to wait on a queue until the
first one is finished. If a clump of packets arrives the queue can get
longer.
There is another source of queuing. That's when the link speed changes.
Suppose the server has a 100 megabit connection, the backbone has gigabit
links, and the last hop is 1 megabit. (just to pick some round numbers) If
the server sends a clump of packets, they go out at 100 megabits, probably
won't hit much queuing delay in the backbone, but then they pile up on the
last hop.
--
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