Pushing an ADSL line!

So my mums ADSL line is finally working properly after a E & D side pair change!

Now it's been stable for a few days at 6db SNR and I have been logging into the router remotely to check the stats a few times a day and have noticed the SNR doesn't dip below 6 in the evenings.

I've asked Plusnet to put the line on fastpath and reduce the target SNR for both downstream and upstream to 3db to pull the most bandwidth and get the best performance possible.

The line length is around 3.5-4km long, currently syncs at 3776kbps with a target of 6db and the upstream is at 640kbps with an SNR of 12db

so do you think it will be OK dropping it too 3db.

If it fails I can always get it returned to 6db after 10 days!

Edited by mlmclaren (Wed 07-Jan-15 12:40:52)

I am assuming you mean ADSL2+ as te upload is over 448kbps.

My line is around 3.2 km and when I had ADSL2+, I would achieve around 4500kbps downstream with a 6dB margin so with your line you are not too different although the inherent noise level may be quite high.

When my modem sync'd a with 3dB margin, I managed to get around 5200kbps if I timed it right.

Get a copy or RouterStatsLite and run it for a day or two. You will see a change in SNR throughout the day with it dropping in the evening and overnight.

Identify when the SNR is at a maximum - day to day it will be around the same time however as dawn gets earlier it will move too. Then, power off the modem followed by disconnecting the cable a couple of minutes later. Leave it off for 5 to 10 minutes and repower the modem (disconnected), wait until it is fully rebooted and then reconnect it.

It should reconnect at 6dB and a higher speed, the margin will drop below 6dB during the day however the modem should not resync.

Run RSL to monitoir it again for a couple more days.


My 2700 modem would hold the line with an SNR of below 0dB with the lowest I saw being just under -1.0dB and I found the best time to sync was around 30 minutes after dawn although it was a rarity as the line would stay up for weeks and months without problems.

I would personally be concerned about running a line that long at 3dB - but if it is really stable it could be ok. You want ideally to keep an eye on the errors as whilst the connection may hold it might produce masses of errors at a lower margin. This is especially with fastpath as the errors won't be corrected as they may be on interleaved.

I used to run a 10Mb/s ADSL2+ line at 3dB fine but not sure if it will be ok on your Mum's.

Personally, with a line that length I'd more more concerned over stability and drop-outs than squeezing the last few hundred kbps unless it's causing problems in use. A long line is rather more sensitive to noise changes than a shorter one. That said, personal choice, but tuning everything to the n'th degree might not be the wisest thing to do.

I'm on a 21CN exchange but my connection is limted to ADSL1 (g.dmt) and engineer mentioned I was on an up to 8mbps product and not up to 24mbps product.

Currently my mums router is getting 9db SNR as I have restarted too many times and its been picked up as drops on the line.

It might not be so easy to run that tool you mentioned but I will keep it in mind, I have logged a ticket with Plusnet to get these changes done and have also mentioned it on the Plusnet forum though the guy dealing with it don't seem to be about today.

In reply to a post by TheEulerID:

Personally, with a line that length I'd more more concerned over stability and drop-outs than squeezing the last few hundred kbps unless it's causing problems in use. A long line is rather more sensitive to noise changes than a shorter one. That said, personal choice, but tuning everything to the n'th degree might not be the wisest thing to do.

While I agree, the engineer seemed pretty confident that the line could out perform its current sync and limits and personally I would rather push everything to the limit and monritor it before deciding on its state (whether it needs to be interleaved or SNR raised)

Are you also monitoring the error rates? Higher sync would increase them, and on Fast Path a large number of retransmissions may be caused, giving a lower effective throughput than interleaved.

Whilst on FTTC interleaving significantly affects the speed, on ADSLx it normally merely increase latency a bit, with no effect on sync speeds but with improved throughput.

I'm not saying your ideas are wrong. Just that I would look at the error rates over a period of a few days, not just the total numbers, before requesting Plusnet intervention to reset the DLM. It may just swap back to interleaved within a short time. It will lower the margin to 3dB itself these days if it deems it appropriate.

Edit - drafted and posted before your two previous posts had appeared.

Edited by RobertoS (Wed 07-Jan-15 13:33:06)

In reply to a post by RobertoS:

I'm not saying your ideas are wrong. Just that I would look at the error rates over a period of a few days, not just the total numbers

I have been keeping an eye on them, My idea was to start from the lowest point and increase interleave depths and SNR as required.

I think it is going to be a good idea to have interleaving on to give it a smoother running, but in at the deep end I say!

Same applies - to basic ADSL.

Once you have te changes made, try the resync at 8:00 to 9:00 AM or thereabouts.

A typical SNR graph will look like THIS you can see where the rises towards the maximum for the day.

If you are doing a reboot or resync - power the modem OFF before anything else. The DSLAM should then get a last dying breath packet or two from the modem saying it has lost power which should not have any adverse DLM affects. Disconnecting the lead for example is seen as loss of sync and treated differently.

On BT Wholesale ADSLx there is no direct control of interleaving depth by the ISP. As far as I know the connections are all ordered on the fastest of the three options, whatever they are called these days, and the DLM takes over from there.

IIRC the options are Standard/Stable/Super stable. The latter two can be requested by the ISP and result in raised noise margins and possibly interleaving - I don't know the detail, and higher fixed (sync-time) noise margins can be requested.