Profile, sync, noise margin, throughput: - dependencies

armadillo · Oct 14, 2010, 20:55:47

I am asking this question separately from my post on surge protection to avoid complicating that thread even further. Rik said "fire away" so I am firing away here.

I can see that sync speed, measured noise margin, target noise margin, profile and data speed are interdependent but it is not clear to me which is cause and which is effect. i.e. what is driving what.

Whenever I reconnect (and I have tried to avoid frequent deliberate disconnects and reconnects), I seem to get whatever sync speed will give me a noise margin of 9dB. So it looks like I have target noise margin of 9dB. I am not sure if this is set by BT or the router. And if by BT, then what determines the value they choose?

I am currently synced at 7616 Kbps. Past experience tells me that it is likely to stay connected and synced at that value with no line drops for days on end.

If I were to disconnect and reconnect while my noise margin is 7dB, I am fairly certain I would sync at a lower rate so as to get a noise margin of 9dB.

What determines the profile? Mine is 5000 despite long periods of sync at 7616. And data speed varies between about 4Mbps and 6.5Mbps, without any obvious time of day pattern.

What does the profile determine? The BT exchange seems perfectly happy to sync me at 7616 despite a 5000 profile, provided it can achieve a noise margin of 9dB.

Can one of you kind and knowledgeable folk explain what algorithm ADSL max is using to hang all these parameters together and which of them actually matter to throughput?

The FAQs are great but I could not sort out the dependencies from them. I can only see that all the values are inter-related.

Steve · Oct 14, 2010, 21:17:01

Have read this page? http://www.kitz.co.uk/adsl/linestats.htm

armadillo · Oct 15, 2010, 00:48:36

Quote from: Steve on Oct 14, 2010, 21:17:01
Have read this page? http://www.kitz.co.uk/adsl/linestats.htm

Thanks Steve. Yes, I had already read that. It does not explain what is taking priority over what. For example, it does not explain what causes a given target noise margin to be set. (Says a longer line needs a higher margin but does not indicate whether the target margin is cast in stone at the exchange or adapts according to how the connection is performing, and if so, how).

It does not explain relationship between sync speed and profile, which one determines the other. Nor how a profile gets raised. e.g what would it take to shift a profile from 5000 to 6500, and what difference that would make to anything that affects the user.

e.g. my sync is 7616, profile 5000 and target noise margin 9dB. ( I deduce target noise margin from the fact that I reconnect at different sync speeds but always at slightly over 9dB noise margin).

I do not know what would cause any of those to change, which would change first and which would follow as a result. Nor whether any such change affects maximum data speed (subject to congestion).

Rik · Oct 20, 2010, 17:02:45

Target noise margin will be raised if the line is noisy and experiences frequent resyncs. At the time of sync, the router negotiates a speed based on both target NM and actual noise on the line, if that's high, speed will be lower. Profile is just a BT annoyance, but is determined by sync speed. A profile will fall immediately, but will increase slowly, 3-5 days, if things improve.

armadillo · Oct 20, 2010, 21:39:01

Quote from: Rik on Oct 20, 2010, 17:02:45
Profile is just a BT annoyance, but is determined by sync speed. A profile will fall immediately, but will increase slowly, 3-5 days, if things improve.

Thanks for coming back on this Rik. What you say about target noise margin, actual noise margin and negotiated sync speed is pretty much what I had worked out. But the part I have quoted is the bit that still foxes me.

It does indeed seem to me that profile is a mere consequence. I see no evidence that it actually determines anything. Sync speed is determined by target and actual noise margin. Profile is determined by what happens to sync speed (both in value and in time). But I cannot see anything which is determined by the profile. If something is, then what and how?

Steve · Oct 20, 2010, 21:41:20

If I understand the question, max download throughput is determined by the IP profile.

armadillo · Oct 20, 2010, 22:57:45

Thank you Steve. Yes, you have understood my question. I thought I had download throughput of over 6Mbps even when my profile was 5000 and then when it rose to 5500. So I thought my download throughput exceeded the profile and I was surprised.

Though now, looking back through my screenshots and text file records of BT speedtester tests, I cannot find evidence of this! So I must have been mistaken.

Perhaps what happened was that the last time I did a BT speedtest before getting a 6.3 Mbps throughput, my profile was 5500 and sync was 7616. Profile possibly then rose to 6500 (as it is now) without a resync and, because I had not done another BT speedtest, I did not know that the profile had gone up. I probably wrongly assumed that the profile was still 5500 and my download was exceeding that.

If profile does indeed restrict throughput, it would be sensible. It would certainly explain why my download speed suddenly increased without a resync. Stable sync at 7616 must gradually have increased my profile. Thanks Steve, that was the missing link in the system which you have cleared up for me.

Here was my last BT speedtest

What you say is consistent with it. Actually, BT's wording is rather misleading. It says "max achievable speed....7150". That makes sense (given overheads) for a sync of 7616. What it fails to say is that the max achievable speed is limited by the profile.

And, a few minutes later, and rather faster than the BT test:

If my download speed ever goes above 6500Kbps, I shall do another BT speedtest and see if my profile has gone above 6500. As Scott said, I may be some time! Once I have my Cat5e router cable, I shall leave the router undisturbed.

Steve · Oct 20, 2010, 23:04:31

I think 600-7150 is the range for any adslmax connection,obviously to get the full IP profile of 7150 you'll need a full sync of 8128.

armadillo · Oct 20, 2010, 23:29:17

Ah, I see, Steve. To get a sync of 8128, I will probably need to stay connected long enough for my target noise margin to drop from 9dB to 6dB. Actual noise margin is usually just over 9dB (occasionally dropping to about 7dB) at 7616. Maybe that Cat5e cable will take the margin up enough to allow a 500 higher sync at 9dB target but I doubt it. We shall see. Once I have the cable, I shall leave everything connected for at least 30 days and then try a resync. May even wait 60 days!