I've been with my ISP (IdNet) for a few months and have been getting some good speeds, but in the last week the speed has dramatically decreased (from 15 Mbps+ to around 0.2 Mbps). This happens at all times of day, not just peak periods.

Obviously I've done all I can to isolate problems my end - only one PC is connected to the router (via ethernet cable) and no other background programs are using the network etc.

I've raised the issue with the ISP and they've suggested trying a new ADSL filter to see if that is casuing the problem, but I thought it would also be good to get the opinion of superuser on possible causes or other troubleshooting I can do.

Here are the juicy stats :)

My router (Netgear DGN1000) reports:

                   Downstream Upstream 
Connection Speed   17602 kbps 1062 kbps 
Line Attenuation    17.9 db    8.6 db 
Noise Margin         6.0 db    6.1 db 

I used RouterStats and it seems to show those figures stay fairly consistent all the time

I ran the BT speedtest and it reported:

  • download speed of 164 kbps, out of a max achievable of 21000 kbps
  • upload speed of 859 kbps, out of 1048 kbps
  • DSL connection rate 17719 kbps down and 1048 kbps up
  • IP Profile of 15000 kbps

Is there any more troubleshooting I can do? Does this look like a problem with my equipment / wiring or with BT's line?

Any advice would be great :)

  • Have you added any phone equipment or made any other changes to your internal phone lines recently? I'd try disconnecting all the phone and other equipment from the phone lines, including their DSL filters and run your tests again. Can you make the same test with another computer in place of yours, to rule out issues on your system? It does look suspicious that the modem/router reports a nearly 18Mbps connection while you're getting such low speeds. – JRobert Nov 27 '10 at 15:56

That noise margin isn't such a big deal. When troubleshooting DSL lines, the basic rule of thumb is that when getting a signal-to-noise margin of under 6.0, you start seeing a drastic increase in bit errors between the DSLAM and the modem.

Since the sync speed is shown as 17 Mbps, and you're only getting 0,16, I would suggest asking your ISP if they can see any packetloss or bit errors (FEC, CRC, etc) coming your way. They have tools for checking that. If the modem reports hardly any errors to the ISP, then basically it's either a routing problem on their end giving you these problems, or it's a problem with your computer. Get another computer and plug it directly into the modem will all firewalls turned off. You should get better rates in that case.

Oh, and trying another modem never hurts.

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