I can't find anything that says, Ofcom requires isp's/wholesale, to use DLM on broadband lines.

More to the point have you found anything from Ofcom to say they can't?

DLM is an operator's choice to deliver the balance of speed and reliability they feel appropriate. In their spec for active line access (ALA) OFCOM say...

"Ethernet ALA shall not preclude the ALA-user from being able to define and agree,
with the ALA-provider, the specific parameters of the VDSL2 line profiles to best
support the ALA-user�s product sets. This philosophy extends to the area of
Dynamic Line Management (DLM) and Dynamic Spectrum Management (DSM)."

In other words Sky or Talk Talk should be able to arrange DLM regimes that suit them in discussion with Openreach. DLM does of course already have more than one setting in terms of how stable it tries to be. Sky seemed happy with the three currently on offer.

yeah roberto and wombat are right.

On my own connection I did dozens of router ppp disconnects in a single day as was configuring it and finding out some settings dropped ppp. This had no affect on DLM or the sync whatsoever.

I have also disconnected my modem deliberatly a few times for things such as unlocking it and swapping to an ECI modem, and always made sure I never did it more than once a day, and no affect on DLM.

I suspect the prime trigger for DLM is error counts and they need to be consistent as well, eg. if you have a burst of crc errors but it happens just once DLM wont do anything but if its an issue for many hours then it may react the next morning, excessive disconnections may also trigger things tho but one a day I know is ok.

In reply to a post by superspeed:

Yes that is correct and DLM is a pain but that doesn't answer my question.

I can't find anything that says, Ofcom requires isp's/wholesale, to use DLM on broadband lines.

DLM may be perceived to be a pain to an individual user, but the real point is that it balances usage between many users, if done properly. Full DLM doesn't just employ interleaving & error correction, but also power masking, power backoff and other techniques. They're getting a lot of research study, and are getting a lot more complicated. Try not to be so dismissive of the newer techniques based on criticism of the original ones!

The full title, Dynamic Spectrum Management, is a toolbox of techniques which work to varying effect - one of the better known "future" ones is called "vectoring" which people expect to use to counter crosstalk on FTTC lines, gaining some 20-30 Mbps for a lot of lines. *That* kind of DSM I most certainly approve of.

Anyway...

Ofcom delegates the rules for connecting equipment to BT's metallic access network to a group known as NICC. One aspect they control is the ANFP, which can be found here. The ANFP defines the power masks, which are a part of DSM techniques (but not DLM).

They haven't specified that DLM *must* be used, but they have certainly studied DLM, and specified that it *should* be used as one technique of many.

DLM as we knew it in ADSL-Max (in BT) was a variant known as AMA (changing SNR targets). DLM as it is employed in FTTC is better known as TRA (restricting speeds). The technique used in ADSL2+ (in BT) is a cross between the two.

NICC have a report on the use of DSM in the UK, with this as part of the executive summary:

In reference to www.niccstandards.org.uk/files/current/NICC%20ND%201513%20(2010-01)...:

1) DLM or DSM Level 1 is undoubtedly a useful capability for UK DSL network operators.

2) The use of the TRA approach should be considered for any DSM Level 1/DLM deployment because of its potential to lower the impact of crosstalk as well as improving overall performance (speed versus stability versus transmit power trade-off) compared with AMA. AMA was evaluated with or without the use of VN, while TRA was evaluated only without VN. Improved politeness and consequential reduction in transmit power and crosstalk levels may bring benefits to UK DSL operators and their end-users.

3) Techniques such as DSM, AL-FEC (Application Layer � Forward Error Correction) and retransmission continue to develop and DSL service providers should consider DSM Level 1/DLM as one technique within a �toolbox� of techniques available to improve DSL transmission performance and stability.

...

7) At the time of publication DSM Level 2 and 3 techniques are at early stages of implementation and analysis of their benefits in the UK access network is left for further study.

The various techniques for DLM and DSM are discussed by "The Broadband Forum" in document TR-197, "DQS: DSL Quality Management Techniques". These specification govern the protocols that the equipment manufacturers build into their equipment. The editors of that document come from Ericsson, ECI and Huawei.

In reply to a post by yarwell:

Sky seemed happy with the three currently on offer.

Nice find.

With that sort of co-operative intent, we may find that vectoring can be made to work well in the UK, rather than the kind of problems they seem to be seeing in Germany.

In reply to a post by shalom2005:

Just to a thought - Surely the whole idea of DLM to keep the line stable.

That is very much the intention - and the DLM employed under FTTC seems to aim to make the line *very* error-free.

The intent would appear to match up with delivery of triple-play, high-quality video over FTTC. That hasn't been confirmed by anything from BT, but does match up with the NICC report I just posted the link to.

interesting power back off is part of DLM now?

on my own line I am curious if power was backed off. I do know my upstream transmit power is now lower after that sync dropped.

I believe downstream transmit power is always reported the same on the cpe so the true value isnt known to the user just the upstream power.

I believe downstream transmit power is always reported the same on the cpe so the true value isnt known to the user just the upstream power.

Nah, the DSLAM signals the downstream power to the modem. If the modem didn't know the downstream power it couldn't know the attenuation.

If less tones are in use (lower sync) there's less power, as the power per tone seems to be a constant.

Only if the 3% to 4% of UK lines with sub loop unbundling agree too

In reply to a post by MrSaffron:

Only if the 3% to 4% of UK lines with sub loop unbundling agree too

Are there any cases of SLU with more than one operator on a cabinet ? BT haven't duplicated Rutland Telecom's two cabinets that I know, how about SYDR ?