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lukpac

join:2007-10-28
Madison, WI

1 edit

[ActionTec] DSL downstream SNR margin dropping

I seem to have a strange issue with my DSL. When my modem trains up, the downstream SNR margin is always fairly good - usually 13. However, over time this drops to 0 or below (apparently - the modem displays something like "2147483647") and errors (far end RS FEC or far end CRC) start going way up. What's strange is that even if the SNR margin is way down, resetting the connection (either rebooting the modem or unplugging the phone line) causes it to go right back up to 13 or so, which seems to suggest there isn't an inherent issue with noise on the line. Sometimes the connection drops, while sometimes the SNR margin goes back up (temporarily).

My ISP (TDS Metrocom) can't seem to figure it out, and the line vendor (AT&T) can't find any problems on the line. There are supposedly no bridge taps, and the distance has been measured at about 13.1 kft, which seems to correspond with the attenuation the modem is reporting (54 or 55). I don't recall the exact number they gave, but AT&T said stress was good - maybe around 15, with 30 being the upper limit.

Inside wiring has been ruled out, as I've had the modem plugged into the test port in the NID and the same thing happens. I've tried 3 modems (2 Actiontec GT-704WG's and 1 Actiontec GT-724WG) and TDS has had me on at least two different ports. The only difference I've seen is when I was on a test port I was seeing CRC errors instead of RS FEC.

The upstream SNR margin has always been good and steady. At 576 kbps it is steady around 7, at 480 steady around 11, and at 256 (they just dropped it again) steady around 18. They currently have me limited at 1216 downstream, where (as previously mentioned) the margin is initially 13 or 14.

Incidentally, at one time (before they had me limited to 1216) I was training up at 2304/768 with a margin of 6/6. Unfortunately I wasn't keeping track of any fluctuations then so I don't know steady it was then.

Has anyone ever seen something like this before? Is there something the telcos are missing? Some type of configuration to the DSLAM? I could understand fluctuations of a few dB's, but not 13, especially when the line itself doesn't seem to be getting any noisier.


lukpac

join:2007-10-28
Madison, WI

said by lukpac:

Has anyone ever seen something like this before? Is there something the telcos are missing? Some type of configuration to the DSLAM? I could understand fluctuations of a few dB's, but not 13, especially when the line itself doesn't seem to be getting any noisier.
Anyone? I've seen a handful of posts elsewhere of what seems to be similar behavior, but no answers.

Also, is there something unique about the telco infrastructure in other countries that enables higher speeds? Even when I didn't have a cap, the fastest I could train up at was 2300-2400 kbps. I'm seeing posts from folks in the UK and Australia with similar attenuation that are training up around 4000 kbps, which seems to be what the DSL standard should allow.