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.