XVI. DSL Diagnostics

When ADSL lines were limited to 2Mb/s, many people's lines could reliably connect at that speed but where they couldn't (due to line length or quality), the ISP would permanently limit the line speed to what was believed to be prudent. Since the introduction of ADSLMax, it will try to connect at as high a speed as possible based on the current conditions, up to 8Mb/s (the top speed of ADSL). The speed achieved depends on the conditions of the line, which means that quality of lines has become more important. Line length is an important factor, but as you can't move your home/office nearer the exchange you can still try to improve your local connections (wiring). In addition, if your line is quite short but connection speeds or reliability are lower than expected, it may be that BT or your ISPs street wiring could be improved.

Detecting DSL Line faults from the Router's Diagnostics

An ATM Cell contains 53 bytes, consisting of 48 bytes of payload and 5 header bytes. Poor line quality, and particularly intermittent line faults will lead to increasing HEC Errors where part of the cell is lost or damaged. On the Vigor, these can be viewed from the telnet interface (comment 'show adsl') under 'LocalInterLeavedChannelHEC=' (or 'LocalFastChannelHec'). The value is '4' circled in the telnet screenshot below. Additionally, a packet can fail its CRC check.

Checking your line errors is only one method of diagnosis. You should also pay attention to the line Attenuation and S/N ratio. The attenuation will be higher, the further you are from the exchange (copper wire, although a good conductor does provide some resistance). Poor connections, water across terminals or corrosion along the line can also affect line quality considerably. You need to speak to your ISP or BT if you suspect that their wiring (between the exchange and your master socket) could be improved.

Testing and improving your own Wiring

Quite commonly, it is your own wiring that can be improved - i.e. the phone extensions around your house, fed from the master socket. Badly made connection or corrosion can all occur. You can diagnose any such local problems by comparing your connection speed, attenuation and s/n ratios when the extension sockets are connected and disconnected.

Modern BT Master sockets (NTE5) as pictured below, have a two part faceplate. The front part is specifically designed to isolate (disconnect) all other extensions on the line so that the line can be tested without any local interference/problems. In the first photo, you can see the master socket with the ADSL filter connected.

Remove the two screws as shown above and then carefully pull off the bottom part of the faceplate as shown below. Be careful not to pull of the wires on the back of the faceplate.

Once you have removed the faceplate, all extension sockets will be disconnected. You will find another phone socket now where the faceplate was; you can then plug your ADSL microfilter directly into this and then see how your router connects. See if it is faster, more reliable or gives better statistics.

Some users have reported considerable differences, for example 5Mb/s instead of a normal 3Mb/s. Such a difference indicates that you really do need to improve your local wiring. This cna sometimes mean opening up the extensions and remaking all connections better, ensuring that you are using proper telephone wire (not bell wire, speaker wire, mains flex etc..). Also ensure that you are using the 'pairs correctly' - for example the white/Blue and blue/white wires are a pair and should be on pins 2 and 5. Using the wrong colours won't affect performance as long as you're consistent, but not using pairs will be a problem. If your sockets use 'push down' connections, try to use a professional push down tool if you can borrow/buy one, rather than the cheap plastic ones.

Removing the 'Ring' Wire

A telephone line has only two wires (one pair) and these are carried on pins 2 and 5 of the BT sockets. A third wire is used to send the bell signal around the office/home (the current which drives the phone's ringer or bell). Many phones don't need this extra wire, and several users have reported increased ADSL performance by removing the pin 3 wire at the master socket. Doing this may prevent your extension phones from ringing, so test carefully and ensure you have a push down tool to reconnect the bell wire connection if you have a problem.

In the photo above, you can see that the Orange wire has been removed from its connection (No.3). Your own colouring may vary, but it's always Pin 3. Pins 2 & 5 must be connected. Pin 4 is not used normally, but is harmless and always left connected. Once you have finished your testing, re-assemble your master socket to reconnect all extensions.

IMPORTANT NOTE: If you are not completely sure what you are doing, seek professional advice as you are fully responsible for anything you do to your line or wiring.

How do you rate this article?