The State of UK Broadband — Not So Fast

Barence writes "The deplorable speed of British broadband connections has been revealed in the latest figures from the Office of National Statistics, which show that 42.3% of broadband connections are slower than 2Mb/sec. More worryingly, the ONS statistics are based on the connection's headline speed, not actual throughput, which means that many more British broadband connections are effectively below the 2Mb/sec barrier. Better still, a separate report issued yesterday by Ofcom revealed that the majority of broadband users had no idea about the speed of their connection anyway."

Comments

I've tried wrangling with BT over this

[OneSmartFellow]

I'm supposed to have a 8MB connection. I've checked the distance to my DSLAM, and I'm well within the distance that 8M should be possible.

I've got a good modem/router - Alcatel Speedtouch - which lets me run diagnostics on the line. The diagnostics report that my signal to noise ratio is just within the limits to establish an aDSL session (from memory it's 9dB), and certainly nowhere close to being able to run at max speed (which would need a S/N of something like 50+dB).

I've contacted BT about the poor state of my line, and they basically ignore me. Actually, it's worse than that, they lied to me claiming that they have tried to contact me by phone, but I provided only my cell phone number and my e-mail, and there is no record of any missed calls from BT, just an e-mail claiming they tried to call. (not to mention that I always have it switched on and within easy ear-shot during working hours).

I guess they just suck !

Re:I've tried wrangling with BT over this

[iangoldby]

The 9 dB figure quoted is the signal to noise margin. With adaptive rate ADSL (maxDSL) the DSLAM and modem negotiate a target noise margin, and sync at whatever speed is necessary to achieve this.

The target noise margin starts off at 6 dB. If this results in an unstable connection then the target gets increased, first to 9 dB, and then to 12 dB, and finally to 15 dB.

So 9 dB is really not that bad. It means that the quality of your line varies a bit, but not too much. The more your line quality varies, the higher the target noise margin that is automatically set.

The point of having a higher target noise margin is that when the line quality deteriorates after the modems have synced and the noise margin drops, it wont drop as far - that is - it starts from a higher value.

As the noise margin drops, first of all error correction kicks in. That can correct a certain amount of data corruption. As the noise margin drops further the error correction becomes inadequate and some packets get dropped because they contain uncorrectable errors.

Once the line starts dropping packets then the data transfer speed plummets because those packets have to be requested again. Soon you reach the point where even a simple text-only web page takes several minutes to load, or just times out.

Eventually as the noise margin approaches zero, the modem loses sync. At this point it will probably resync automatically, but this time at a much lower rate in order to re-establish the original target noise margin. If this happens regularly then BT's systems will automatically increase your target noise margin to try to prevent this happening as often.

The final insult is that when the modem resyncs at a slower speed, BT's systems reset your 'IP Profile' to match your new sync speed. The IP Profile is effectively a cap on the data rate (not the sync speed).

Note that this adjustment to your IP Profile happens immediately when your modem resyncs slower, but not when your modem resyncs faster. In the latter case your line has to remain stable at the higher speed for five days before BT will put your IP Profile back up.

With fixed-rate ADSL it is a little different. There is no target noise margin - the modem just connects at the fixed speed and the noise margin you get is just whatever it happens to be. Fixed rate doesn't do error connection so it generally needs a higher noise margin than adaptive rate to avoid retransmissions. But the good news is that there is no IP profile rate cap, so when a period of poor line performance ends your download speeds will recover immediately.

Now, what was the question?...