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

[Rob Kimberley]

All,

Strangely today the jitter numbers seem to be behaving themselves! Nothing
has been done to the set up.

Thanks to everyone for their comments. I'm looking at putting a direct
Ethernet cable in here to see what the difference might make.

Attached is a picture of latest NTP Monitor readout. Bottom two devices are
the Meinberg LanTimes. The others are pool servers.

Rob


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of David
Sent: 10 February 2013 23:49
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Possibly off topic - Jitter on Ethernet over power
adapters

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.
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