Advanced ADSL

A while back, I devoted an article to basic ADSL trouble­shooting, what to do if you couldn't connect, and how to possibly rectify that problem. Now I'm returning to that subject, but going a stage further. This time around, I'll be looking at more subtle ADSL problems, specifically those to do with your phone line.

I have to be honest, this month's column was actually prompted by a customer of mine, called James. He had a PCI ADSL modem and the resulting Internet connection was shared between two Windows 2000 PCs - courtesy of good old Internet connection sharing and a crossover cable. He also had a notebook, but it was a fiddle to hook this up to the shared Internet connection as there was no hub. My solution to his needs was the installation of a wireless ADSL router plus a wireless card for the notebook. I chose the venerable Netgear DG834G, a 54G wireless router I'm very familiar with; one that works well with remote access tasks.

Ups and downs

The installation went well, but after a couple of weeks James phoned me to report an intermittent Internet connection - could I come down and investigate? Well, I could certainly do some of the initial diagnosis remotely.

Firstly, the DG834G incorporates a decent set of diagnostic tools which I used to analyse the problem. Not all routers have these features - my old DG814 doesn't, for example - but most modern models do. Secondly, I had deliberately set up remote management of the router before I left, a task made much simpler by James having a static IP address.

Logging in to his Netgear router revealed that, yes indeed, James' Netgear router was rapidly connecting and disconnecting from the Internet. In fact, his Internet connection was going up and down like a yo-yo. Little wonder he was having trouble Web surfing and sending e-mails. I confirmed this fact by simply looking at the log feature on the DG834G.

If you look at Figure 1, you can see that, often in the space of just a few seconds, the LCP (Link Control Protocol) is establishing a connection only to be disconnected a few seconds later. A word of explanation here: ADSL in Australia uses the PPP (Point-to-Point Protocol). The LCP is used to establish, configure and test data-link Internet connections. It checks the telephone connection to see whether it is good enough to sustain data transmission at the intended rate.

Once the LCP packet accepts the link, network traffic can flow; if the LCP packet determines the link isn't up to it, the link is terminated. Which is clearly what has been happening here, but with alarming regularity.

Luckily for me the DG834G also has the capacity to measure line quality. A quick gander at his readings revealed a serious problem with the phone line. I have the same DG834G - see the Connection Statistics page of my Netgear router (see Figure 2).

Pay close attention to the Down­-stream Line Attenuation and Noise Margin figures - on my own ADSL line I'm losing 27dB of signal and the signal itself is 20dB above the noise floor. While these measurements won't be super-precise, they're accurate enough for our purposes. Note that different routers may have different names for these two values - the Draytek Vigor 2600w calls Noise Margin the SNR (signal-to-noise ratio) Margin and the Line Attenuation, Loop Attenuation. Check through your router's documentation for more info on this (see Figure 3).

Now look at the same Web page on James' router (Figure 4). Here you can see he has much worse figures - he's losing 52dB (a lot of signal) and his signal is a mere 6dB above the background noise.

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Roger Gann

PC Advisor (UK)
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