Are high CRC errors normal?

I'm getting a high CRC error rate on my line - an almost constant 3000 per hour. I'm having a lot of discussion with BT as the noise margin keeps going from 6 to 9 and then 12 db. I understand this is happening automatically due to errors detected. I'm not seeing any disconnections other than when the margin changes up, or I call BT and they "make some changes at the Exchange" and they reset it. Doesn't seem to matter what the noise margin is set at, the CRC error rate always seems to be at this level. And when the margin goes up it's usually during the day.

I'm using a BT Home Hub 2.0 and these are typical stats after the margin changed earlier. Are they normal for this length of line?

Connection time 0 days, 5:35:18
Downstream 3,712 Kbps
Upstream 448 Kbps
VPI/VCI 0/38
Type PPPoA
Modulation ITU-T G.992.1
Latency type Interleaved
Noise margin (Down/Up) 10.2 dB / 23.0 dB
Line attenuation (Down/Up) 50.0 dB / 28.0 dB
Output power (Down/Up) 18.9 dBm / 11.9 dBm
Loss of Framing (Local) 11
Loss of Signal (Local) 1
Loss of Power (Local) 0
FEC Errors (Down/Up) 5646 / 0
CRC Errors (Down/Up) 17266 / 2147480000
HEC Errors (Down/Up) nil / 0
Error Seconds (Local) 8

Ignore the very high CRC error number - it's a bug, they all do that guv.

The error numbers seem a bit high to me - any audible noise on the voice side of things?

Yes I was referring to the downstream only. I know the upstream CRC number is a bug. But I seem to be getting 3000 CRC downstream errors per hour regardless of the use of the internet. It's up to 30,000 now after 10 hours. BT have checked the line several times but they can't find a problem although they acknowledge there's a high error rate. The problem is that the noise margin migrates quickly from 6db to 12 db meaning a significant loss of sync speed.

In reply to a post by smiths:

Loss of Framing (Local) 11
Loss of Signal (Local) 1

CRC errors are relatively unimportant. Most lines will have a few. On the other hand Loss of.. errors are of more concern. LOF indicates instances where the connection between DSLAM and modem became confused. LOS indicates times when it got so bad that it was dropped and a resync occurred.

Most lines can run without any of those but it is often a compromise with headline speed. For a casual user trading off a little stability for speed and getting a couple of LOS per 24 hour period might make sense as long as you're not on BT's DLM (Dynamic Line Management).

For people that play online games, use VoIP or stream video you really want none at all. As a BT subscriber you are also subject to their DLM and too many LOSes will impact performance.

You could have a look here:

http://www.kitz.co.uk/adsl/connectionprobs.htm

..but tbh for your line attenuation you are doing quite well. If they can peg the margin at 12db I'd have thought you could get total stability.

Edited by Andrue (Mon 31-Aug-09 09:00:53)

CRC errors are cause by corruption of the data. One of the major causes is interference getting into the router. Now while 3000 errors an hour are not excessive, it leads me to wonder if your internal wiring is fully up to scratch. If you have more than one phone socket have you removed the ring wires in all the sockets?

No problem with the house wiring as I don't have any. The connection to both phone and modem is via a filtered ADSL faceplate which has an integrated microfilter and was fitted by BT when they did a line check a few months ago. The phone is a cordless one as are all the extensions.

In that case unless you are using a lenthy cable from the filter to the router there is not much you can do with the internal wiring.

I wonder what your MTU is set to, for the BT IPstream produce the recommended is 1458. Is both the router and computer set to 1458?

for the BT IPstream produce the recommended is 1458

BT IPstream will handle the standard 1500 MTU without any problems.

I'm using the latest Home Hub 2.0 which AFAIK doesn't provide any means of viewing or changing the MTU. However a ping test shows it to be 1460 or thereabouts. The connection from the phone socket to the router is the standard 2m cable supplied by BT.

I would look at your errored seconds, only 8 so seems fine.

the crc errors do seem high especially for an interleaved line but like the others I would be inclined to believe it is due to a bug.

Edited by Chrysalis (Sat 05-Sep-09 14:28:18)