Connection rate vs IP profile

[stevenrw]

Dummy question but please indulge me.
Can somebody please explain in words of one syllable the difference between IP Profile, DSL Connection rate and actual download/upload speed.
My stats according to BT tester this evening are
DSL Connection Rate :6655 Kbps(DOWN-STREAM), 1007 Kbps(UP-STREAM)
IP Profile for your line is - 5500 Kbps

I am getting measured d/l speed of 5790, upload 831

To a dummy like me all these seem to conflict and I'd really like it explained for once and for all.

Also how the stability of the line leads to a reduction/increase in ip profile.

I'm on IDNet ADSL2+ (and happy with it of course!)

[armadillo]

I shall have a go at this because I also struggled with it. If I say anything wrong, one of the experts will jump in (I hope not on me) and correct it.

Connection rate (downstream) depends on how noisy your line is when you connect to the exchange and the target signal to noise ratio (SNR) margin which has been set at the exchange for your connection.

SNR margin decreases with increasing connection speed.
When you connect, the kit at the exchange will connect you at the highest connection rate such that your actual SNR margin is not below the target SNR margin.

The IP profile is the maximum download rate that your connection is allowed to achieve. It is never higher than your connection speed and is usually lower. I believe that it excludes overheads too. So if you measure your actual download speed with a tester that includes overheads, it may be slightly higher than your IP profile but not much. Actual throughput is often lower than the profile due to congestion anywhere between the host server and you.

The IP profile is always lower than the connection speed.

If you disconnect your line and resync, or if your line drops by itself and resyncs, the speed at which you connect again depends on your actual SNR margin at that time. If you reconnect when the line is noisy, you may therefore get a lower connection speed.

If your connection speed is much less than it was before and you do not quickly reconnect at a higher speed, your profile will drop fairly quickly (as it is always lower than the connection speed). That will limit your download speed. Someone will say how quickly it drops. I think it is 75 minutes.

If you disconnect and then reconnect at a higher speed, your IP profile will remain reduced until you have stayed connected at the higher speed for a length of time which depends on how much faster you have connected. Large increases of connection speed increase the profile in a few hours. Small increases take up to a week.

If you connect at a higher speed before your profile has had a chance to reduce, you may not lose profile at all.

But if you disconnect and reconnect too many times in a short while, your target SNR may increase and you will get a reduced connection speed again!

So poor stability reduces both your sync and your IP profile. Poor stability can be caused by disconnects beyond your control or by repeated attempts to disconnect and reconnect at a higher speed.

Good stability gradually increases your profile up to the maximum allowed by your connection speed.

This is BT's version of Russian Roulette.

The figures you quote suggest that your throughput speeds were measured including overheads but the IP profile excludes overheads.

OK experts - jump in now  

[Rik]

Perfect answer, Dill. 

The BT tester is unreliable of late and often returns throughout figures higher than the profile, which is theoretically impossible.

For the record, profiles are completely unnecessary and there for the convenience of BT.

[stevenrw]

For the record, profiles are completely unnecessary and there for the convenience of BT.

Thanks Dill & Rik for the response. Interesting that IP profile has no real meaning as it was that figure that was compounding the confusion.

So, if I understand Dill's (excellent) explanation correctly, the best I can reasonably expect from my line is the DSL connection rate quoted as that's presumably goverened for the most part by my geographic location and the fact that my measured D/L speed is as close to that as makes no difference, means that my Billion 7800N router/modem is doing a pretty good job making the best of what's available.

The interesting bit for me is Dill's explanation about resynching and potentially then getting a lower speed due to noise. If that does happen I know now to try again later. Presumably line noise will be lower at off peak times like v.early am or very late pm.

[Rik]

Line noise is lowest in the morning, it gets higher at night due to the increased propagation of radio waves (MW band).