If you’re deploying business grade DSL solutions then you need a router that provides plenty of troubleshooting commands in order to diagnose potential problems. The Cisco “show DSL interface atm” command is excellent at troubleshooting DSL issues, but frustratingly doesn’t seem to be very well documented.
Here is my breakdown of the key parts of the output:
Router#show dsl interface atm 0
ATU-R (DS) ATU-C (US)
“DS” denotes Downstream and “US” denotes Upstream.
Modem Status: Showtime (DMTDSL_SHOWTIME)
SHOWTIME indicates the line is in sync – it’s showtime baby!
DSL Mode: ITU G.992.1 (G.DMT) Annex A
DSL Mode denotes the type of DSL. Here we have G.992.1 Annex A – for a complete list of different flavours of DSL see http://en.wikipedia.org/wiki/Digital_Subscriber_Line
Noise Margin: 4.5 dB 6.0 dB
Also known as signal-to-noise ratio, noise margin shows how strong the signal is compared to the noise on the line. There is also a relationship between noise margin, line speed, and stability to be considered – generally the higher the noise ratio the more stable the line, but the lower the sync rate. In the UK the optimal noise ratio is normally 6.0dB. Anything less is likely to result in re-sync’s. If you experience a high frequency of re-sync’s it is worth asking your ISP to increase the noise-margin on the DSLAM.
Attenuation: 65.0 dB 31.5 dB
Attenuation is the signal strength lost over the line, not due to noise, but due to distance from the exchange and/or the cable quality. The lower the better. 20-30dB Excellent, 30-40dB Good, 40-60db Average. There’s little you can do about attenuation short of plugging your router directly into the master socket, or moving house!
Speed (kbps): 480 0 416 0
Speed shows the sync rate. Here we see 480kbps downstream and 416kbps upstream
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