MaxDSL diagnostics

A logical approach to problems with your ADSL connection speed. 

You'll be needing some data to work with, so for a start you should find out the downstream speed that your modem or router is connecting at.  To discover this and other statistics look for your hardware on the Kitz ADSL site.  Make a note of modem sync speeds, attenuations, SNR or noise margins both upstream and downstream.


The first check is to see if you are actually on MaxDSL or not. If your downstream modem sync speed is 288, 576, 1152 or 2272 then it is highly likely that you are in fact on a fixed speed product. If the upstream sync speed is 288 then its a certainty. You need to sort your ISP out as you aren't on the package you think you are.


Next we need to measure the speed you are getting using speed testers such as ADSLguide, DSLzoneuk, BBmax, the Old BT speedtester or New BT speedtester . These will measure your downstream speed in kbits/s and some also measure the upstream.  Ideally you should take a speed measurement early one morning as well as at "normal" times, this will indicate if congestion is slowing you down - ADSL is a shared or contended service and at busy times a fast MaxDSL line may run at the same speed as a 2M service. The maximum speedtest you'll see on MaxDSL is about 6700 kbits/s.


 If the speedtests say that your downstream speed is 70% of more of the modem sync speed then you don't really have a problem, providing the modem sync speed is sensible. The BT Wholesale checker gives a prediction of MaxDSL modem sync speed for a phone line and usually this is conservative and you'll do a bit better.  Lines that are eligible for the 2M fixed speed service or with attenuation below 45 dB should manage at least 6M sync speed, at 35 dB or below the full 8128 is usually possible. 4M should be possible at 50 dB and 2M at 60 dB, approximately. If your modem sync speed is well below these figures or the checker prediction then read the next paragraph, otherwise you can skip over it.


Low sync speed, and unstable connections, are often due to  electrical noise from your internal extension wiring. To eliminate this you should plug your modem / router into the test socket behind the master BT socket. Note that the test socket is only connected to the outside line and is therefore not the same as just plugging into the master socket. If you find you get a much better sync speed in the test socket then consider using a filtered faceplate or disconnecting the ring wire from your extensions. I'm assuming here that everything plugged into your phone system is fed through a good quality microfilter (not the T shaped things). Note that everything includes Sky boxes, fax machines, 56k modems, CLID units, auto-diallers, burglar alarms and anything else you might have hooked up. Troublesome items like Sky boxes or DECT phones sometimes benefit from being fed through 2 filters in series - if you can hear audible noise on the line with ADSL present you have a filtering problem.


If your modem sync speed is respectable but the speedtest results are low there are a number of potential causes. One is the congestion that can occur at the telephone exchange or at the ISP. This will usually not be present at 05:30 but may persist throughout the working day and evening. If you can get good speeds early in the morning then the line etc is all OK. To determine if the congestion is specific to your ISP you can login in with a different username in your modem / router to speedtest@speedtest_domain (which will bypass the ISP) and run either of the BT speedtests listed above.  If the BT login gives good speeds and the ISP low speeds then you know where the problem lies.


If the speedtest is always low you should check that your TCP/IP settings are not impairing speed. In particular the size of the receive window buffer (RWIN) should be 64,240 or so as a minimum, check this with tweak testers at netmonitor or speedguide or dslreports. If its too small change it with DrTCP.exe by putting a new value in the top left box. Experiment with bigger values like 250,000 if you use a lot of US or Asian sites for downloads.


Another check is for errors. If your modem or router reports thousands of CRC errors, errored seconds or the like per hour then your speed will be impaired by lost packets and retransmissions. These usually show up on the  tweak testers above as well as in the error counters. Errors are most likely if the downstream noise margin or SNR margin is low (the default is 6 dB) and if the internal extension wiring is picking up noise. Note that it is common for the SNR margin to fall when it gets dark, often creating more errors and leading to resyncs to lower speeds.


The final speed limit is the IP Profile or BRAS profile which BT assign to every MaxDSL line. This controls the data rate to a value your modem / line can handle and has 0.5Mbit/s increments, so on slower lines it can make a big difference. A sync speed of 1120 kbits/s for example has an IP Profile of 512k whereas 1152 kbits/s get you a 1M profile.  This profile value follows your modem sync speed - if the sync speed falls the profile follows it within 75 minutes. If the sync speed goes up and stays up for 3 days the profile follows it up. Unfortunately the IP profile has a habit of sticking, especially when a line is first moved to MaxDSL - many people have found they got the same 2M speeds after the upgrade. Only your ISP can get BT to fix the IP profile, but at least with the new BT speedtester you can see what it is - it may take several goes or an early morning start but knowing what your profile is can be invaluable, so persevere with it if you have a speed problem.


 If you find your line is unstable and reconnecting frequently then BT's systems should after a while set the target SNR margin to a higher value. This starts at 6 dB then steps up to 9,12, 15 dB giving reduced speeds but hopefully better stability and less errors. If a line improves the target margin should come down after perhaps 2 weeks of stability. You can help yourself with a filtered faceplate or improved wiring and filtering, or using the features in some routers to limit the downstream speed or adjust the target SNR margin. If you need help on this or other issues the best place to start are the ADSLguide forums.


Finally remember that your ISP is responsible for your ADSL service and if you can't fix it or get a satisfactory solution then badger them to get a BT ADSL engineer out to look at it on site.