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

David davidwhess at gmail.com
Sun Feb 10 14:00:14 EST 2013

The poster is asking about ethernet over power line and not power over
ethernet.  As you point out, the later should have zero effect on
ethernet latency.

There are several ethernet over power standards.  Latency will include
a bridge in each adapter, the effects of a noisy uncontrolled AC power
line when ARQ (automatic repeat query) is used, and the TDMA or CSMA
media access control system.

I suspect varying power line conditions will have the largest effect
because any jitter from the media access control system will be
multiplied by ARQ.

On Sun, 10 Feb 2013 08:14:38 -0800, Chris Albertson
<albertson.chris at gmail.com> wrote:

>THose power over Ethernet devices work with analog signals and don't
>evn look at the data packets.   All they do is place a DC bias on the
>twisted pair.    Ethernet is always transformer coupled so your
>routers, switches and computers never see DC.
>What is your NTP server using for a reference clock?  I'd suspect that
>is the problem.   If the reference is an Internet pool server than a
>few mS is about what you should expect.   If using GPS then look to
>see if you have a good signal from enough satellites.
>But those POE boxes don't mess with the data packets, or at least the
>are not designed to do that.   If one is broken it could be adding
>noise to the line.  Broken hardware can do "anything".
>On Sun, Feb 10, 2013 at 3:18 AM, Rob Kimberley
><robkimberley at btinternet.com> wrote:
>> I'm not sure if this is the best place to ask the question, but does anyone
>> have experience of using Ethernet over power line adapters? I have an
>> outside office, and my router is in the house plugged into the phone master
>> socket. I have used two Ethernet over power adapters, one at the router and
>> one in the office here to get internet access. The output of the adapter
>> then goes to a multi-port hub to give me Ethernet to all my office devices
>> including two Meinberg NTP servers.
>>  I've noticed large jitter readings on Meinberg's NTP monitor program.  Can
>> be as low as 2ms, but much higher (50mS +), and at this point NTP goes
>> haywire.
>> Not sure if it is the physical set up or something else.
>> Any comments appreciated.
>> Thanks.
>> Rob

More information about the time-nuts mailing list