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

Bill Hawkins bill at iaxs.net
Sun Feb 10 18:44:57 UTC 2013


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.

Can you monitor your power line for noise? Do you see any pattern to the
50 millisecond delays?

Is there a way to tell NTP to discard the data and retry if the round trip
time is unreasonable?

Can you run a fiber optic pair to your office? How about internet access
with a cell phone?

Hope there was something useful in all that.

Bill Hawkins

P.S. The Meinberg article at their site says that NTP and SNTP both use
TCP/IP. I know that SNTP uses UDP/IP, so perhaps they are confused. TCP
(Transmission Control Protocol) requires a request/confirm / indication/
/response handshake using those protocol primitives. UDP simply sends a
message in the hope that it will get to the receiver. When it works, you
get good time data, but you get garbage when it doesn't. The TCP handshake
would be coumterproductive for NTP.


Here is the original from Rob Kimberly, for those with short memories:

"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"




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