[time-nuts] Possibly off topic - Jitter on Ethernet over power adapters

[David]

On Sun, 10 Feb 2013 12:44:57 -0600, "Bill Hawkins" <bill at iaxs.net>
wrote:

>Rob,
>
>One of the common characteristics of power lines is noise.
>
>Seems to me that bursts of noise could interrupt the Ethernet signal,
>causing retries of the transmission.
>
>Now, I'm only familiar with SNTP, which uses UDP messages (User Datagram
>Protocol). The more familiar TCP will retry messages within the protocol,
>but UDP does not. It relies on the application to retry. Perhaps the 
>Ethernet to power line boxes are smart enough to retry after noise bursts.

The power line is definitely a hostile environment for data
transmission.  The recent ethernet over power line standards use ARQ
(Automatic Repeat Query) as a backup for the FEC (forward error
correction) so when data cannot be recovered at the receiver, it will
be resent.

While the adapters will certainly resent corrupted TCP data, I do not
know if they do the same for UDP data.  The marketing makes noise
about special support for streaming content but who knows what that
means.

The standards also include both TDMA and CSMA in some combination
because they are half duplex.  Those by themselves will add latency
and jitter.