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Cisco 837 Noise Margin bug?

Whenever my 837 ADSL router re-trains to a lower speed (tends to happen in the evenings), the reported "Noise Margin" goes negative:

router>show dsl int atm 0 ATU-R (DS) ATU-C (US) Modem Status: Showtime (DMTDSL_SHOWTIME) DSL Mode: ITU G.992.1 (G.DMT) ITU STD NUM: 0x01 0x01 Vendor ID: 'ALCB' 'TSTC' Vendor Specific: 0x0000 0x0000 Vendor Country: 0x00 0xB5 Capacity Used: 100% 54% Noise Margin: -51.0 dB 22.0 dB Output Power: 20.0 dBm 5.0 dBm Attenuation: 29.0 dB 14.0 dB Defect Status: LOM None Last Fail Code: None Selftest Result: 0x00 Subfunction: 0x15 Interrupts: 19714 (1 spurious) Activations: 3 Init FW: embedded Operartion FW: embedded SW Version: 3.9.220 FW Version: 0x1A04

IOS version is Version 12.4(7).

SNMP also reports a negative margin for the download.

ADSL-LINE-MIB::adslAturCurrSnrMgn.15 = Gauge32: 327170 ADSL-LINE-MIB::adslAtucCurrSnrMgn.15 = Gauge32: 220

The line is perfectly stable when the margin does negative so this doesn't seem to affect operation. The line typically re-trains from approx. 8000 to 6500 to cause this set of figures.

Am I missing something about Cisco's margin calculation or is this just a simple bug? If the modem later re-trains to any faster speed the negative margin is cleared (along with the Defect Status). I would expect the margin figure to reflect the margin with the current line speed.

Separately, is there an easy way to force a more conservative maximum connection speed to avoid the re-trains that each cause a short outage? I've tried altering the "dsl noise-margin" offset without success.

This is a GB ADSL line on the new BT "Max" service.

Cheers,

Reply to
Mark Knight
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try

dsl linerate

kbps - line rate, in kilobits per second, for the DSL ATM interface. Allowable entries are 72, 136, 200, 264, 392, 520, 776, 1032, 1160,

1544, 2056, and 2312.
Reply to
Merv

In message , Merv writes

"dsl linerate" does not appear to be a supported parameter on the 837. Trying it fails and dsl ? only shows:

Cheers,

Reply to
Mark Knight

have you tried playing with the 'dsl power-cutback" command ?

Reply to
Merv

Hi,

I have similar issues with stability on the new BT Max services. Though this 837 doesn't re-sync, it just suffers packet loss and then disappears off the internet.

By my reconing this in the interface atm0 config might help:-

dsl noise-margin 3

Which looks like it will force the router to train with a margin of 9. It may also be possible to use the dsl gain-setting tx-offset command to further improve things.

Will let you know how I get on.

Regards,

Reply to
Mark Balaam

I don't think that it's a bug. I have the same issue and when my Noise Margin goes into negative figures the ADSL kit at the ISP shows negative SNR to tie up with what I'm seeing. I've just been trialing the 8Mbps service but my router would only train up to about 4.9Mbps and then after a few hours I would see bucket loads of errors. A hard reset would make it okay again for a while.

So, a negative Noise Margin would suggest a line issue. Most likey you are pushing the bandwidth to more that the line can handle.

Chris.

Reply to
chris

In message , Mark Balaam writes

I tried this; it certainly helps. However I think I could do with it being set a little higher (still getting approx. 1-2 disconnects a day). Sadly the upper limit seems to be 3 :(

As for the negative Noise Margin I mentioned, this only seems to occur when "dsl enable-training-log" is in the running-config.

Cheers,

Reply to
Mark Knight

In message , Merv writes

Doesn't that only affect the upstream and default to the most conservative setting?

Reply to
Mark Knight

Just in case this is not obvious.

-ve dB is not less than zero.

It is less than one and greater than zero

i.e. fractional. Think of it as referring to the position of the decimal point. (or octal point, or...)

3 -> 1000 2 -> 100

-2 -> 0.01

-51 -> well, quite small.

-51dB would seem to me to be a _very_ small number if the value is sometimes positive.

Hoping I recall correctly;-)

Reply to
anybody43

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