If it were me, the first thing I;d be doing is asking Zen to get then target SNR set back to 6db, making the service a tad more stable may well mean these 'blips' aren't an issue.

Thanks RobertoS,

All the usual suspects were worked through and eliminated before posting here. Replaced filters, moved router to test socket etc. (No Sky box here!).

I will swap the Draytek router for an old Netgear when the problem reappears just to eliminate the Draytek - but I'm not convinced the Draytek router is the cause. It does look like noise from somewhere.

Also the telegraph pole is right outside my front door so the cable run is only about 8 metres.

The cable run goes right back to the exchange. At any point in that run it could pick up noise.

It is more likely the underground run from pole to cabinet to exchange could be picking up noise.

I had a case about 4 to 5 years back - friend of a friend. His ADSL would basically go down some evenings, but only when he was out. His neighbours suffered too although there were only a few with ADSL from a sparsely populated cabinet. It turned out to be him!! He ran a mobile burger van from a lay-by around 1 mile from his house and the cabinet was along side. He would put his generator on the verge - sometimes near the cabinet and sometimes not - when close up the interference was enough to affect the ADSL lines. At a guess the generator was sitting above te cable feeding him and his few neighbours down the lane.

ROFL
Nice one .

Thanks ian72

And from the pole it goes underground and along to the cabinet where it emerges briefly before its onward subterranean journey to the exchange (just over a mile away)

The most likely places where noise would get injected are

1) Cable from house to pole (acting like an aerial)
2) Junction box at top of pole
3) D side in the cabinet
4) Exchange JB

But wherever it's happening it's a pain

In reply to a post by Zarjaz:

If it were me, the first thing I;d be doing is asking Zen to get then target SNR set back to 6db, making the service a tad more stable may well mean these 'blips' aren't an issue.

Hello Zarjaz,

I spoke with Zen support today and they have set the noise margin back to 6db at the cost of about 1Mb sync speed (now 18.1 down and 12.1 up).

So far no ATM CRC errors - time will tell

Well I hope it continues. Speed is good, but worth nothing without stability.

Keep us posted.

OK so Zen have altered the noise margin and moved it back to 6db.

The only effect this has had is to reduce the routers sync speed from 19.7Mb to 18.0Mb.

The problem still exists as described.

Can anyone say why the router would receive these ATM cell CRC errors even when there is ZERO IP traffic to and from the internet?

Don't forget we are talking about hundreds of errors every few seconds (150,000 per hour)

There is a constant traffic of packets at the sync/connection speed between you and the exchange even when no data is passing. Think of empty envelopes.

When you use the connection, data goes into those packets.

Basically the CRC error rate is unlikely to change whether the connection is in use or not.