Line attenuation and SNR

signal to noise

After our move the adsl internet connection has been less than stable. Sometimes it’s up for a week, sometimes it disconnects 20 times a day. According to the isp this is due to us being 4km from the exchange, which is kinda weird as we only dropped a floor and never had problems before. So now I’m reading into things like line attenuation and dB signal to noise ratio so I actually measure what the hell is going on and if any improvements I make to the indoor cabling have any effect whatsoever. Now if only the cable isp didn’t block certain server ports I’d switch to cable in a heartbeat.

A high SNR (signal to noise ratio) simply means how loud the signal is over background noise. The higher the SNR margin the more stable the connection. You have a strong signal and have plenty of head room to receive faster speeds. Generally you would have a high SNR if you’re on a restricted speed plan eg. 256/64, 512/128 or 1500/256. The faster your connection speed the lower your SNR will be. Generally on unrestricted speed plans like Adsl2+ up to 24mbit the isp will set the SNR margin at which your modem connects generally a range between 6dB to 14dB. This will give you the fastest speed while maintaining a relatively stable connection.
Attenuation on the other hand is a measurement of the resistance to the signal on the line and should never change regardless of speed.

There are many factors which affect both SNR and attenuation. A few are line distance, gauge or thickness of line, quality or age of line, number of bridge taps on line etc.

So now I can start focusing on the latter, one step at a time, as the inhouse cabling is the only thing I can influence myself. And if that doesn’t help… time to start thinking about cable after all.

4 Responses to Line attenuation and SNR

  1. Good luck man. The part of my brain that took networking all those years ago was trying to flicker back into action around the attenuation part of the post, but, that’s long gone. It was a pretty informative read. I managed to learn something new today. :)

    • Thanks, I always like reading up on stuff like this whenever I encounter a technical problem, because let’s face it, if you want something done right…
      But yeah, turns out my SNR is hovering between 5 and 7dB, so now I can try and cut out as many wires at home as possible this weekend and then hope I see an increase. Especially the 5dB at night would explain the drops.
      I’m still looking for a good conversion table that would translate my attenuation reading into the distance I’m supposed to be from the exchange, because as far as I can tell that’s a guesstimate at best because it’s influence by all the noise on the line, meaning if I can cut out some crappy wire at home my attenuation could drop due to a signal improvement, making it appear that I’m closer to the exchange.

  2. Wow, X you did a thorough research. Like the info, what tools did you use to measure your connection quality? At my home the main telephone connection was very old. I took a centimeter of the main cable and solder it to a new cable, connection quality went up a lot (according my modem stats). I’m looking forward to your conclusions.

    • I have a new Fritz!Box 7170 modem which, once you enable the expert settings, gives you more stats than you can shake a stick at, including all the data you need like the line attenuation, SNR and how you’re connected to the exchange. So once you have the data the rest is a question of interpretation and checking your logs. I now consistently see that during off peak my SNR keeps hanging around 7dB which is low but high enough to maintain the connection, but then during peak hours I see it drop to 5dB, which is too low for the modem to maintain the sync causing the connection to drop. Most drops are between 21:00 and 01:00 which makes sense as most people are at home then, so they use their lines as well as other equipment which adds noise. So this weekend I hope to cut out the splitters and extension wires and connect the modem directly to the socket via a very short cable. If I then see a sustained SNR of 7dB or higher during peak hours I should be safe in assuming I’ll have solved the problem. If I still get 5dB I’m pretty much screwed as the only way to improve the main line is for KPN to step in. Which will probably happen somewhere between when pigs fly and the apocalypse.

Work in progress... not home!
Trying to get all/most of the new code working before I start on the eyecandy.