[ntp:questions] Win7: ntpd adjusting time backwards
nomail at example.com
Sat Dec 22 16:25:43 UTC 2012
Jan Ceuleers <jan.ceuleers at computer.org> wrote:
> Having said that: there can indeed be asymmetrical transmission delays
> that are linked to the technology being used. My VDSL2 modem tells me
> that the downstream delay is 14.1ms and the upstream delay is 4.4ms. The
> ratio of these numbers is not equal to the ratio of the downstream and
> upstream bitrates (which are 16544 kbit/s and 2056 kbit/s respectively).
> So note also that the downstream delay is greater than the upstream
> delay, although the downstream bitrate is higher.
This is caused by interleaving.
Packets are not sent back to back sequentially, but they are split into
smaller chunks that are interleaved serially.
This means a single packet is spread out over time, meaning that a hit
of interference takes out a smaller part of each packet. And that again
means that the error correction coding that is added to each packet has
more chance of recovering the packet.
(it is better to hit 8 packets each with a small amount of interference
than to hit a single packet with a big blow)
This improves the performance for streaming, but it increases the delay
because the receiving modem has to wait longer for the packet to be
complete. As the downstream with its higher bitrate is more affected
by interference, more interleaving is usually applied to it as well.
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