In reply to a post by Chrysalis:

like it fixed (not) robertos current fault?

Phone went dead on Tue 27th. FTTC still worked erratically. Fixed today 16:10 but FTTC not up to speed.

Felt like one wire of the pair disconnected, so if it was making a bad contact for a while that could explain my problem. Too early to tell if my sporadic packet loss is cured, as it seemed to be at TV times.

DLM seems not to have been interested except for my base latency stepping up and down by 3ms or so at times. I'm quite happy with OR DLM.

In reply to a post by jchamier:

[
[quote](FWIW, C&W is now Vodafone Networks).

Yeah I know, sadly they have also gone for this DLM, but at least users do have a choice to disable it.

The DLM might be bad for a few people, but I suspect the majority of users (whom have never heard of this forum) are better off with it. Most people prioritise stability over ultimate speed, as streaming and downloading and internet banking don't work too well with disconnections.

Most people who don't use these have no idea what DLM is or what it does. Sky DLM is not bad to be honest, it seems to work ok. It is BT version that is a load of rubbish.
No doubt it works better on FTTC, but that is only because copper runs are pretty short from the cabinet. Have a long copper run and it will be as bad as ADSL.

In reply to a post by timl:

In my opinion DLM is used by BT mostly to avoid having to fix minor faults with their old copper lines which ultimately saves them money.

For ADSL this worked quite well... though there were a lot of people who love being without it... Talktalk and Sky, O2 and Be.

DLM on FTTC punishes good lines that are ok apart from a bit of occasional noise or crosstalk. We need an option to have it turned off.

Is it that bad on FTTC? Iin the few FTTC installs I have seen, I have yet to see one work at anywhere near the speed it is suppose to work at.

When I went onto the system I am now, I done so because I thought FTTc would not be here for a couple of years. when it did come here a month or so after, I thought maybe I made the wrong move, but I am sure now that I made the right move and not go to FTTc.

In reply to a post by zyborg47:

It is BT version that is a load of rubbish.
No doubt it works better on FTTC, but that is only because copper runs are pretty short from the cabinet. Have a long copper run and it will be as bad as ADSL.

It's clear that you utterly fail to understand that the two DLMs are completely different.

The ADSL one is owned and operated by BT Wholesale, FTTC is Openreach's and they have very little in common.

But don't let me confuse you with facts if your mind is made up.

Edited by billford (Thu 29-Aug-13 22:40:59)

For example, one major adjustment used by the BTW ADSLx DLM is raising the sync-time noise margin.

That function is completely absent from the Openreach FTTC one.

In reply to a post by billford:

It's clear that you utterly fail to understand that the two DLMs are completely different.

The ADSL one is owned and operated by BT Wholesale, FTTC is Openreach's and they have very little in common.

But don't let me confuse you with facts if your mind is made up.

I did not realise they was different, so wind your neck in a bit. even so it is still BT,, still the same company and don't give the rubbish they are separate.


i can understand why people went against you when you was a mod.

In reply to a post by zyborg47:

even so it is still BT,, still the same company and don't give the rubbish they are separate.

Like I said...

In reply to a post by RobertoS:

For example, one major adjustment used by the BTW ADSLx DLM is raising the sync-time noise margin.

That function is completely absent from the Openreach FTTC one.

Which means? Sorry a bit slow today.

In reply to a post by zyborg47:

In reply to a post by RobertoS:

For example, one major adjustment used by the BTW ADSLx DLM is raising the sync-time noise margin.

That function is completely absent from the Openreach FTTC one.

Which means? Sorry a bit slow today.

What it really means is that DLM is a bit of a catch-all name.

There are *many* different schemes that can be used under a broad title of "DLM" whose job is to react to differing line conditions, and attempt to bring stability to the digital comms going on over the line.

Some schemes are more dynamic than other, and can react faster to quick changes to conditions. Some are slower, or even static. Some schemes work well with other lines, while some schemes have a detrimental effect on other lines (I've seen one described as "zap your neighbour first"). Others have a green agenda, and attempt to reduce power usage.

Our understanding of various DLM schemes is advancing all the time, and comparisons made by the likes of Ofcom. An Ofcom subgroup (NICC) has a report on DLM techniques in the UK, but it only reports on a subset of the possible schemes. The Broadband forum has a newer report on DSL Quality Management techniques that gives a broader overview.

The major scheme used by BT Wholesale for ADSL Max and ADSL2+ is one that adjusts the target noise margin after errors (from the 6dB norm to 9, 12 or 15 dB). This scheme (which is the one mentioned by RobertoS) is known as AMA, or Automatic Margin Adjustment.

NICC prefers the newer scheme, known as TRA or Tiered Rate Adjustment, which aims for stability by reducing the sync speed targets into tiers. This is the DLM scheme used by BT Openreach in FTTC.

While BT Wholesale's ADSL Max products only use AMA, their ADSL2+ products start by using AMA but for intractable problem lines, they also have a form of TRA - but this is very much used as the last resort. Openreach FTTC *only* employs TRA.

Both TRA and AMA schemes utilise forward error correction as part of their strategies. The FEC and interleaving changes bring more stability, but the overheads reduce speed and add latency.

For FTTC, however, the specification requirements for the VDSL2 modems bring an alternative mechanism into play - the use of "PHYR", or retransmission at the physical layer. This will do away with the "static" overheads of FEC and interleaving, and replace them with a retransmission overhead that only happens when errors actually occur.

It seems likely (to me) that PHYR will be introduced either alongside vectoring, or afterwards.

Alike? Not even close.

Is it that bad on FTTC? Iin the few FTTC installs I have seen, I have yet to see one work at anywhere near the speed it is suppose to work at.


By way of balance, apart from a blip in Feb, I get 75.3 Down, 16.9 Up with a Latency of 21 at all times of the day so I am very happy with FTTC. That said, DLM does have a marked effect when line conditions go awry. My ISP Line Data shows that it took 16 days from interleaving to drop off after a fault before I got back the full 80/20 profile. It was many days later before interleaving fell away fully and I got back my current max attainable rates of 93.5 Down and 28 Up. I am fortunate as I live c.300M from FTTC cabinet. That said, when people live much further way from their cabinet and the max attainable line speed is less than 80, I can well see why the effects of DLM can be so frustrating.