I was back at my parents’ over Christmas, like usual. Before I got back my Dad had mentioned they’d been having ADSL stability issues. Previously I’d noticed some issues with keeping a connection up for more than a couple of days, but it had got so bad he was noticing problems during the day. The eventual resolution isn’t going to surprise anyone who’s dealt with these things before, but I went through a number of steps to try and improve things.
Firstly, I arranged for a new router to be delivered before I got back. My old Netgear DG834G was still in use and while it didn’t seem to have been the problem I’d been meaning to get something with 802.11n instead of the 802.11g it supports for a while. I ended up with a TP-Link TD-W8980, which has dual band wifi, ADSL2+, GigE switch and looked to have some basic OpenWRT support in case I want to play with that in the future. Switching over was relatively simple and as part of that procedure I also switched the ADSL microfilter in use (I’ve seen these fail before with no apparent cause).
Once the new router was up I looked at trying to get some line statistics from it. Unfortunately although it supports SNMP I found it didn’t provide the ADSL MIB, meaning I ended up doing some web scraping to get the upstream/downstream sync rates/SNR/attenuation details. Examination of these over the first day indicated an excessive amount of noise on the line. The ISP offer the ability in their web interface to change the target SNR for the line. I increased this from 6db to 9db in the hope of some extra stability. This resulted in a 2Mb/s drop in the sync speed for the line, but as this brought it down to 18Mb/s I wasn’t too worried about that.
Watching the stats for a further few days indicated that there were still regular periods of excessive noise, so I removed the faceplate from the NTE5 master BT socket, removing all extensions from the line. This resulted in regaining the 2Mb/s that had been lost from increasing the SNR target, and after watching the line for a few days confirmed that it had significantly decreased the noise levels. It turned out that the old external ringer that was already present on the line when my parents’ moved in was still connected, although it had stopped working some time previously. Also there was an unused and much spliced extension in place. Removed both of these and replacing the NTE5 faceplate led to a line that was still stable. At the time of writing the connection has been up since before the new year, significantly longer than it had managed for some time.
As I said at the start I doubt this comes as a surprise to anyone who’s dealt with this sort of line issue before. It wasn’t particularly surprising to me (other than the level of the noise present), but I went through each of the steps to try and be sure that I had isolated the root cause and could be sure things were actually better. It turned out that doing the screen scraping and graphing the results was a good way to verify this. Observe:
The blue/red lines indicate the SNR for the upstream and downstream links - the initial lower area is when this was set to a 6db target, then later is a 9db target. Green are the forward error correction errors divided by 100 (to make everything fit better on the same graph). These are correctable, but still indicate issues. Yellow are CRC errors, indicating something that actually caused a problem. They can be clearly seen to correlate with the FEC errors, which makes sense. Notice the huge difference removing the extensions makes to both of these numbers. Also notice just how clear graphing the data makes things - it was easy to show my parents’ the graph and indicate how things had been improved and should thus be better.