FTTC IP Profile <> Modem max downstream rate?

From http://speedtest.btwholesale.com/
IP Profile for your line is - 39.73 Mbps

From HG612 stats
Max: Upstream rate = 10308 Kbps, Downstream rate = 41312 Kbps

What is the relationship between IP profile and max downstream rate?

I've just resynced the modem and I thought the IP profile should match the max downstream rate.

96.8%

The IP profile is a little less because of the way certain overheads are counted or not counted.
So they do 'match', but not as exact numbers.
I've seen 96.79% quoted ( http://forum.kitz.co.uk/index.php?topic=10514.0 ),
your is just under that at 96.17%

On the whole it is best not to resync the modem. That can be seen as errors by DLM and your speed can lower.
Resyncing the router (or disconnecting and reconnecting) is what reestablishes the PPPoE connection and updates the IP/BRAS profile to 'match' the sync speed.
Sometimes the modem resync will be quick enough that the higher level router->exchange connection is not broken; in that case you will get an out of date IP/BRAS profile.

Edited by StephenTodd (Fri 17-May-13 09:19:10)

Thanks. Hopefully the DLM will not be upset by a resync after 5 days.

From the HG612, my sync rate is 40793 which *.9679 = 39.48Mbps, close to my IP profile.

This connection seems to maintain an SNR of about 6.2. Another FTTC connect close by has a SNR of 4.4, sync rate of 46529 and IP Profile of 46.37. The SNR seems to be reducing over time. Is my SNR likely to reduce similarly over time, giving me an improved profile?

It might improve a little. They usually seem to want to make a connection at around 6db SNR, unless there is a history of problems on the connection.
Having said you don't want to fiddle with the modem too much, you could wait until you see a particularly high snr (eg the signal is particlarly good), and then force a resync.
That will probably give you 6db against the good signal, which will drop as the signal deteriorates.
However, doing that might increase the error rate and cause DLM to force extra error correction, so you would not necessarily win out long term.

My understanding from other posts (especially very helpful ones from WWWombat). Remember: my understanding is NOT 100% reliable ...
~~
The Max figures you get from the HG612 give you an indication of the very best you could possibly get accepting a significant amount of unreliability.
The main difference between Max and actual are partly due to SNR and partly due to the overheads of error correction.

The overheads of error correction are R/N (or R/I) (R,N,I from the modem figures).
If DLM dropped error correction on your line, you would see a speed of

Text
1	NOECCPath = Path/(1-R/I)

I think that generally to losses due to error correction are greater than the losses due to too high an SNR.

DLM does not gratuitously force higher SNR and ECC; it does it because it sees (or thinks it sees) errors.
So you certainly can't expect miraculous changes.

~~~~~~~~~
If SNR decreases without forcing a new connection, that is because the actual signal is deteriorating. Once a modem connection is made, the sync speed will not change during the life of that connection. If SNR gets too low, the errors will build up so much that DLM will force a new connection, and that will be at a lower sync speed.

Edited by StephenTodd (Fri 17-May-13 10:49:27)

Ah, I misread your question. The Modem max downstream rate is just a calculated figure and has little to do with reality - reality being the actual sync rate.

might it be the sync speed at 0 dB SNR margin ?

Mine is quite often below my sync speed

might it be the sync speed at 0 dB SNR margin ? That's my guess.
Mine is quite often below my sync speed What snr is it quoting when it is below sync speed?

I guess that at sync time max was above the sync; as time went on max wanders below sync, but the line quality is good enough it still manages to keep the connection.

In reply to a post by NormanB:

Max: Upstream rate = 10308 Kbps, Downstream rate = 41312 Kbps

What is the relationship between IP profile and max downstream rate?

There is no relationship between IP Profile and max downstream rate.

The relationship is with the sync speed.

In reply to a post by NormanB:

This connection seems to maintain an SNR of about 6.2. Another FTTC connect close by has a SNR of 4.4, sync rate of 46529 and IP Profile of 46.37. The SNR seems to be reducing over time. Is my SNR likely to reduce similarly over time, giving me an improved profile?

The settings for VDSL2 have a target SNR margin of 6dB, no matter what the DLM intervention level. When a re-sync happens, the resulting SNRM should always be at 6dB or above, and should only be above that level if there is more capacity on the line compared with the package speed (40 or 80Mbps) or a DLM-banding speed limit.

If the actual, current, SNR has reduced below this level, then noise levels have increased since the last sync.

A reduction in SNR on a live connection doesn't alter the IP Profile. When a resync happens later, the target will remain at 6, so the resync is likely to result in a lower sync speed and lower IP Profile.