For months now, I’ve been complaining about my ADSL connection and I’m pleased to say that I think I’ve found the issue: it looks as though I had too many TCP/IP sessions open (multiple computers, browser tabs, etc.) and so some connections were getting dropped, so I increased the maximum on my router from the default 192 to the maximum 512.

In the course of my many months of investigation, I did find a lot of useful information though, so I thought it was worth sharing here, for future reference.

The first port of call is often the BT speedtester.  This requires Java to run but it’s likely your ISP will require you to run three separate tests at least one hour apart on this (which can be difficult when the tests don’t complete properly, as in my case) before they will even consider reporting an issue to BT.

Your ISP may have some tools that can help too – for example, mine (PlusNet) has a gateway checker (to see where my connection leaves their network and hits the Internet) and an exchange status checker. Using the exchange status checker, I could see there was an outstanding fault in the exchange and the ISP chased BT to investigate.  Sadly that fault was not related to my problem but it did at least let me know that BT’s infrastructure could have been part of the issue.

It does help to think about things logically – are the issues really speed related, or are you getting lots of disconnections but quite rapid response (as I was)? Does the issue occur on more than one operating system (e.g. Windows, Mac OS X, iOS, Android)? If you’re only having problems on one device, consider whether that device might have a configuration issue (e.g. incorrect DNS settings).

Get ready to put on your propeller head. There are a lot of factors that affect your ADSL connection speed and quality.  PlusNet has a support article to help explain some of the issues involved in diagnosing broadband speed faults and a broadband terminology guide (although it’s hard enough for me to follow, let alone the average consumer).  I found some really helpful people on my ISP’s support forums and one of the best resources they pointed me at was the Kitz ADSL information index.  Some of the information there was extremely technical but it helped me to understand some of the statistics on my line and to work out what speed my line should be able to support (around 6.5Mbps) in my case. It also gave me a much more detailed line check/exchange status check.

Some of the pointers that could have been useful, had I followed them up, included:

• “Pairing” my ADSL router with the BT exchange equipment (i.e. making sure they both use the same chipset). For this you will need to know the local and remote vendor identifiers.  My router didn’t expose the remote end, so it was difficult for me to test that but, for example, if you know that the exchange equipment uses a Broadcom chipset then you can try and get hold of an ADSL router that uses that chipset too.
• Some consumer ADSL routers can be modified with unofficial firmware to expose more settings.  Examples include the Linksys WRT54G (OpenWRT) and the Netgear DG834 (DGteam). Other routers cannot and have fairly basic user interfaces – even my ISP told me to avoid the Thomson router that they currently ship (mine is an elderly Solwise SAR110 and I can even configure it from a command line if necessary).
• Signal to noise (SNR) ratio can affect a lot of lines and changing this might help.  I found that my SNR margin (basically the amount of tolerance to noise on my line) was quite low at 5.5 dB (I’ve seen as low as 3.5, although the actual noise is pretty constant at 32.5dB) and, had I worked out how to increase this, I could potentially have found a more stable connection (albeit at a slightly lower speed) and then worked back to find the optimum setting (thanks to Garry Martin for that advice).
• Improving the physical connection by removing the ring/bell wire is a possibility, but probably not advisable since it requires interfering with the linebox (technically this is BT’s property). Another option (not suitable for all connections though – you need an NTE5 linebox although BT says 70% of homes have one of these) is to install a BT Broadband Accelerator/Iplate.  BT Total Broadband customers can get one of these for just the cost of postage (although PlusNet is a BT subsidiary, they don’t have the same offer) but they don’t cost much from the BT Shop.

So you can see that diagnosing ADSL connection issues is far from straightforward; however there are a few things to try that can yield significant results. I’m still trying to work out why my IP Profile has dropped in recent months to 5500Kbps (from 6500, which should be achievable based on the attenuation figure of 32.5), and why my line has interleaving enabled (again, based on the attenuation/loop loss, I’m only about 2.3km from the Exchange). I’ve also ordered an Iplate and it will be interesting to see what (if anything) the resulting speed increase is.