While it has been known for this behaviour (turning modem power off) to have an adverse effect on ADSL sync speed, it hasn't been seen yet on FTTC in a provable way - most people have just continued with the old advice to leave the modem turned on (it is OK to turn off the router, if it is a separate box).

My experience is that powering off/on of the modem actually has no effect - and I did this a lot on my old FTTC line (on 2 or 3 different days), where I was initially affected by error rates that caused interleaving and FEC to be turned on.

In fact, most times that people's speeds have dropped (on FTTC) seems to be because of error rates on the line - and the fact that DLM has turned interleaving on and FEC on. Further speed drops can then be seen as the FEC intervention is turned through low, medium and high settings.

When interleaving is first turned on, you will usually see around an 8ms delay added to ping times, and perhaps a 10% drop in speed. This seems pretty standard. Further intervention could cause a further 10% reduction in speed or so, but isn't usually associated with further increases in latency.

To see what has happened requires you to get the statistics data out of the modem. This can be done on the Huawei modem, but I'm not sure about the current state for the ECI modem. Those stats will include date about the level of both interleaving and FEC.

To see if turning off the modem*has* had an effect, you are best leaving it turned on for the next month.

In reply to a post by jchamier:

The DSLAM at the green box watches every time you power off and its convinced there is a problem, so it will increase your ping and worse it will slow your speeds down.

The DSLAM is monitoring for unexpected re-syncs, but it *ought* to be able to distinguish a "loss of power" from a "resync due to line quality".

The VDSL2 specification includes an "indicator bit" for the modem to report a "loss of power" back to the DSLAM (aka "dying gasp"). The DSLAM *should* see this as a "far-end loss of power", and make this available (with O&M protocols) to the DLM process.

Of course, no one knows the intimate details of BT's DLM process, and whether it takes the "dying gasp" indication into account... and I have seen no specification for the Openreach modems to say that they *send* the dying gasp.

Even if the dying gasp isn't used, this kind of "power-off on leaving home" event is easy for DLM to spot - a loss of sync followed by no attempt to sync for a long period.

My best guess is that this kind of power-down is *not* the cause of the problem.

In reply to a post by WWWombat:

The DSLAM is monitoring for unexpected re-syncs, but it *ought* to be able to distinguish a "loss of power" from a "resync due to line quality".

Agree technically it shouldn't have caused an issue - but other people have reported DLM interfering after lots of power downs.

DLM seems to like me.

even with my line at 3.8 snrm holding my 79999 sync I am still on fast path. I resynced when a crosstalking line was temporarily offline.

To begin with I thought I had found gaming nirvana with zen and fttc with my initial ping of 15ms and no jitter.
Now with my pings sitting at around 36ms and jitter of around 6ms I'm at a loss with what I should do. can I complain and would anybody even listen. At the moment my pings are worse than my old adsl connection was 9 odd years ago.

If you have high jitter that probably isnt caused by DLM.

Incidently too much stuff is going on with my line now and I am now also interleaved.

RL issues tho I wont be raising this with BT yet as I have higher priority stuff to deal with but I may raise it at a later date.

Long story short DLM has been fiddling with my US power at least twice a week dropping the sync in the process, so in my view its much more intrusive than adsl DLM. The most previous time it did this it lost my 79999 sync which was 3 days ago. Well 2 days ago I had a short period of noise on my line for about 1 hour and half from about 2am to about 3.30am on the morning of 3 march. I cant remember the exact amount of errors but it was either 5 or 6 digits in a 2 hour period, I guessed DLM reacting was inevitable. Then on the morning of the 4th my interleaving was turned on, not some small amount that has been suggested happens but a whopping 10ms has been added to my base latency and 14mbit removed of my sync for FEC purposes. I am now quite a bit below my estimated sync speed, and with some voice issues on my line I found also I will be doing a fault report maybe at some point in the future but not now. Since interleaving has been turned on I have only had1 crc error, so it may well turn itself off again before I report it anyway.

When I first got FTTC installed, my pings were a steady 20ms to BBC, which is good for Aberdeen and equal to Bethere.

But they gradually increased as my line couldn't hold sync and they've settled for the past several months at 32ms.

Goes to show how much they can be increased by and that if latency is a high priority, you better make sure your line/wiring is good before moving to FTTC.

That holds true when you reboot the modem, I'm not sure it does when you flick the mains switch, I think and I may be wrong - but the DLM will see it as a dropped sync.

The indicator bits are known as "lpr" - for loss of power - with an extra (local) indicator known as "flpr" - for far-end loss of power - set when the modem or DSLAM detects the far end has transmitted an lpr indicator.

They are distinctly loss of power, not an indicator of a reboot.

to be fair I think FTTC is far more resistant to line problems than adsl.

However the DLM is much more agressive. On adsl no matter how many errors you had as long as the line held sync you would be ok. Whilst on FTTC DLM will force a resync to do what it wants, so if it decides you are to be interleaved there is no escaping it. I also dont like all these upstream power adjustments its doing, if crosstalk is that bad so DLM has to be tweaking every week then really BT should be doing something physically about it or get into gear on vectoring.

Here is what my last 5 DLM changes have done.

latency down attainable up attainable
fast path 71 24
fast path 71(85 briefly) 32
fast path 71 21
fast path 71 26
interleaved 75 23
and today (edit)
fast path 71 23

like on adsl a interleaved line gets more overall snrm so higher attainable speed but that of course doesnt translate to actual speed as FEC uses a chunk of that up.

By the way DLM has kicked in again, I am already back on fast path. Have I got the fastest publicly known recovery to fast path? I think I was interleaved for 2 days maybe.

My PPP configuration on my router is working well now tho, all these DLM changes havent dropped my ppp, ssh sessions stayed up etc. and still on the 77.43 ip profile from my last 79999 sync. Its just been lucky I haven't been using the net right when its made the changes.

Edited by Chrysalis (Wed 06-Mar-13 23:03:32)