Munin monitoring your SB6120 Comcast Cable Modem

For those who have spent time debugging their Comcast Internet connection, we all know the frustration of trying to explain to Comcast that something on their end is the problem. In this case, more data is better: latency history, ping times, traceroutes, etc. You can run Smokeping to monitor latency between your home connection and a remote Internet IP address for example. You can also print out traceroute examples and email them if you have an astute support contact. But if you want to monitor the data your cable modem is seeing, you need to look at the signal to noise ratio of your connection. This ratio refers to how much of your signal has been disturbed by noise on the physical line (Thanks Wikipedia). Newer cable modems will use multiple channels along the same line to increase your download and upload speed, and each channel can be disturbed independently.

Enter Munin, an RRD-graph based tool that creates easy to understand (for the most part) graphs of data over time. Munin plugins can be written to scrape any data you are able to programatically retrieve and whip it into a pretty graph. The plugin I wrote for Munin scrapes the data from a Motorola Surfboard SB6120 modem’s status page, and presents it in an easy-to-digest format for Munin. Previous Munin plugins handled the old SB4120 and SB5120 modems. These are old DOCSIS 2.0 products, and therefore the status page has evolved, and displays the connection data in a way where the old plugin does not work on the SB6120 modem.

The graph shows the various downstream and upstream channels, graphing the signal-to-noise ratio dB (decibel) of each respective channel.  Suggested values for Comcast in particular can be found in a FAQ entry from Broadband Reports. These are just suggestions, and different values will have different effects on your Internet connection. A severe dip in the graph could correspond to your modem being unable to stay sycned with Comcast’s infrastructure, which translates to an unstable Internet connection.

Installation instructions follow common Munin plugin practices, by creating the plugin (or a symlink to the plugin) in the /etc/munin/plugins/ directory to the plugin code. Mine is written in Python, so a basic Python install is required on your machine. Source can be found can be found in my munin-surfboard6120 repo on Github.

Enjoy! (Comments/criticisms are welcomed to improve the usefulness of this plugin)

Jeffrey Forman
Jeffrey Forman

I do things that make the Internet work at work, and I play around with things that make the Internet work at home.