DLM and impact on other users of the network

I apologize to readers of multiple forums that this is a cross-post from both BT and BE forums, but I haven't had a reliable answer to either of my posts there.

Do the issues that DLM prevents (higher error rates, unstable line, etc) seriously impact other users of the network.

e.g. from BT forum: http://community.bt.com/t5/BT-Infinity/Speed-increas...
its very irritating that openreach wont allow a user to force fastpath on but they want to protect their network. But this is typical of many posts on many forums about FTTC and DLM. I'm interested in technical fact that will permit a reliable comment and take the heat out of such questions.

Attempts to overegg a line with fastpath and low snr clearly give problems. Eventually the errors and retransmissions would mean the user got lower speeds, less predictable response, and more disconnections. But, up to a certain point many people (especially gamers) would be more than happy to accept these problems if they were allowed to.

Unfortunately, many people would jump at the risk, but then complain bitterly about the consequences.

So, back to my question, would allowing this impact the network overall and thus the other users? Maybe Openreach aren't protecting their network, but they are protecting their service people and the service people of the ISPs that use them, and also the reputation of FTTC. And maybe they are overdoing it, so that actually both service and reputation suffer?

Does anyone know the answer to the question?

Standard engineering practice

Don't push the network to its absolute limits until you have seen it running for millions and know how it actually performs.

FTTC roll-out is a massive experiment to learn what works and what does not, so safe settings are an engineers dream.

Alternatively why not allow them to enable FASTPATH but drop the sync by several Mbps to keep stability. Maybe some extra noise margin.

Give it time, took BT engineering a few years to get off the fixed speed ladder for ADSL

One problem is that errors cause retransmissions, which will reduce network performance for everyone.

Thanks everyone. In summary; I don't know, you don't know, and even the Openreach engineers (the real ones, not the installers) don't know yet; so they are playing it safe.

I agree about the retransmissions. I guess on the upload having to do lots of error correction could (not should) slow down the processor in the cabinet enough to impact others. I doubt either is a significant effect, but ...???

I have read (elsewhere on this forum?) that Openreach does have a choice of three DLM settings for FTTC, but that the ISPs don't generally pass even that limited choice on to the users.

In reply to a post by BatBoy:

One problem is that errors cause retransmissions, which will reduce network performance for everyone.

I can't really see how retransmissions reduce network performance for everyone.

There is no link layer retransmit, so it's just TCP that will do the work, and that work is on server/client not the network between.

Packet loss to signal congestion is how TCP was designed to work.

Non congestion loss will only really hurt the speed of the individual flow so it could be argued that it reduces burst load on the network - benefiting everyone else. Of course nothing is clear cut - it will take more data eventually than without the loss - but then loss is normal for TCP anyway.

In reply to a post by legume:

In reply to a post by BatBoy:

One problem is that errors cause retransmissions, which will reduce network performance for everyone.

I can't really see how retransmissions reduce network performance for everyone.

There is no link layer retransmit, so it's just TCP that will do the work, and that work is on server/client not the network between.

Packet loss to signal congestion is how TCP was designed to work.

Non congestion loss will only really hurt the speed of the individual flow so it could be argued that it reduces burst load on the network - benefiting everyone else. Of course nothing is clear cut - it will take more data eventually than without the loss - but then loss is normal for TCP anyway.

Thats what i thought too, as the claim of an individual line profile being set differently to that of others having a adverse effect on the network which in turn causes others to suffer, is total bull manure

One of the reasons behind the BT logic for Failure to offer the customer any choice of having DLM ON or OFF or Fastpath being an an available option, are just too much liker hard work for bt to implement, as they would have to actually train their support staff in india ,and as for providing these options within the customers account via a GUI (like be and some others do already) would be beyond their logic, they are still in the dark ages

Because it doubles the amount of packets transmitted thus halving network capacity.

In reply to a post by BatBoy:

Because it doubles the amount of packets transmitted thus halving network capacity.

If that was the case then the same would apply to dsl ???

Correct, DLM is present on BT DSL systems. I thought everyone knew that.

In reply to a post by tommy45:

In reply to a post by legume:

In reply to a post by BatBoy:

One problem is that errors cause retransmissions, which will reduce network performance for everyone.

I can't really see how retransmissions reduce network performance for everyone.

There is no link layer retransmit, so it's just TCP that will do the work, and that work is on server/client not the network between.

Packet loss to signal congestion is how TCP was designed to work.

Non congestion loss will only really hurt the speed of the individual flow so it could be argued that it reduces burst load on the network - benefiting everyone else. Of course nothing is clear cut - it will take more data eventually than without the loss - but then loss is normal for TCP anyway.

Thats what i thought too, as the claim of an individual line profile being set differently to that of others having a adverse effect on the network which in turn causes others to suffer, is total bull manure

One of the reasons behind the BT logic for Failure to offer the customer any choice of having DLM ON or OFF or Fastpath being an an available option, are just too much liker hard work for bt to implement, as they would have to actually train their support staff in india ,and as for providing these options within the customers account via a GUI (like be and some others do already) would be beyond their logic, they are still in the dark ages

In reply to a post by BatBoy:

Because it doubles the amount of packets transmitted thus halving network capacity.

A TCP connection with 50% loss would be totally unusable as TCP takes loss as a sign of congestion and slows down.

A DSL connection should train to a lower speed on fast for the same target margin, so shouldn't get that much extra loss.

Yes, that would be nice. Unfortunately DLM turns up interleaving as well.

In reply to a post by tommy45:

Thats what i thought too, as the claim of an individual line profile being set differently to that of others having a adverse effect on the network which in turn causes others to suffer, is total bull manure

There may be some room for confusion around the word profile here.

IIUC on openreach VDSL profile X can mean something different to that we are used to - vectoring which helps cancel cross talk between lines. In this sense of the word it may be the case that all lines should use the same vectoring profile.

I may be wrong - but I can't see how whether the line is fast or interleaved can have anything to with this as it's done at bitstream level, whereas vectoring is likely much lower level.

I don't think vectoring has been implemented on BT FTTC.

In reply to a post by BatBoy:

Yes, that would be nice. Unfortunately DLM turns up interleaving as well.

But you can request fast on ADSL ....

I assume if DLM detects too many errors on forced fast you will get a higher target margin - but that is a choice you have, a choice which is missing on FTTC.

In reply to a post by BatBoy:

I don't think vectoring has been implemented on BT FTTC.

Ahh, OK I don't know whether it has or not..

I think DLM on FTTC is controlled by Openreach. They give the ISP the choice of Stable, Standard and Speed to offer to their customers, but they don't offer the choice of Interleave or Fast. All 3 choices can apply varying levels of interleaving which I think, is to protect the network.

I imagine Fastpath can damage the network as I mentioned earlier.

In reply to a post by BatBoy:

I think DLM on FTTC is controlled by Openreach. They give the ISP the choice of Stable, Standard and Speed to offer to their customers, but they don't offer the choice of Interleave or Fast. All 3 choices can apply varying levels of interleaving which I think, is to protect the network.

I imagine Fastpath can damage the network as I mentioned earlier.

And HOW exactly do you come to that conclusion ???? where is any evidence that would suggest this to be the case, fact is quite probably you just are no wiser than anyone else honestly "I imagine Fastpath can damage the network Is correct You Imagine you do actually know this to be fact do you?

If that was the case then they would be the only ISP not to offer fastpath be it DSL or FTTC
The spreading of this manure like info is not a good thing is it (get some facts to back up your imagination before engaging fingers) seeing as you are always asking that of everyone else on this forum

In reply to a post by BatBoy:

I think DLM on FTTC is controlled by Openreach. They give the ISP the choice of Stable, Standard and Speed to offer to their customers, but they don't offer the choice of Interleave or Fast. All 3 choices can apply varying levels of interleaving which I think, is to protect the network.

I imagine Fastpath can damage the network as I mentioned earlier.

And how exactly if this was the case (in reality) would it affect the bt network when the end user is with sky or talktalk using their network for backhaul and peering ect ??

Sorry Anonymous, that discussion is the whole point of this thread. It's probably easier to participate if you're registered.

If Sky, TT, etc, want to be free of Openreach's protection, why don't they offer their own local loop solution? It's a handy get-out for them to simply blame Openreach as it stands.

In reply to a post by BatBoy:

If Sky, TT, etc, want to be free of Openreach's protection, why don't they offer their own local loop solution? It's a handy get-out for them to simply blame Openreach as it stands.

you could say the same thing for dsl solutions but they don't present any threat to the bt openwound network , bt has little to do with things in the case of any llu provider , as they are not handing the data be it upstream or downstream
Apart from the copper pair (that they don't really care about)and the fiber link back to the exchange,
Are you suggesting that they (bt ) do not install sufficient capacity for the number of lines that can be physically connected to any FTTC cab and the max amount of data flow by default ? as surely this should exceed this amount otherwise there would be little point in implementing FTTC in some areas where take up is high

I think they are protecting their network to ensure sufficient bandwidth is available for up coming new things such as the Premiership on BT Vision. I don't know if BT Fusion is dead or about to be relaunced.

In reply to a post by BatBoy:

I think DLM on FTTC is controlled by Openreach. They give the ISP the choice of Stable, Standard and Speed to offer to their customers, but they don't offer the choice of Interleave or Fast. All 3 choices can apply varying levels of interleaving which I think, is to protect the network.

It's a jungle out there trying to get pukka info. You are close in principle, but I think slightly incorrect in detail. There are at least four relevant BT documents, of which only three are in the public domain. I hope I can clarify things a little both for you, and for the repliers to you who in some cases seem a bit ill-informed or are just guessing.

First is the Openreach GEA public info.

Openreach run their DLM on the cabinet DSLAM. This determines the noise margins, and line profile - which is not the same thing as the IP Profile. The documentation of this DLM does not mention any facility for the CP to affect any of the stability or error correction levels, so far as I can see. So no CP options for fast path or interleaving.

The three CPs directly using GEA are BT Wholesale, TalkTalk and Sky. So far as I know the facilities Openreach make available to all three are identical.

Second is the public BT Wholesale info about WBC/WBMC, which is how all 21CN and fibre products are delivered to BTW's CP customers.

BT Wholesale have their DLM controlling the end user's traffic, on top of the Openreach one. This of course does not affect the "LLU" TT and Sky GEA users as it runs within the WBC system after the handover from Openreach in the exchange. Amongst other things, this introduces the IP Profile into the connection, affecting all the customers of all the ISP using BT Wholesale WBC/WBMC.

This documentation is self-contradictory, in that an old (non-rescinded) one document says that another applies in full except for section 3 in that other. Section 3 is to do with end user access and mentions stability options like you list, but they are Standard, Stable and Super Stable. (I think in ADSL2+ terms that means target margins of 6, 9 or 12dB. Further complicated by the fact that the BTW DLM can now lower that to 3dB).

However there is a non-public BT Wholesale FTTC Handbook. I believe from an impeccable source that the January 2011 version contains this same information about Standard, Stable and Super Stable. It seems from another source that there is a later version, but like the more recent FTTC rollout data spreadsheets I can't find out anyhting about it. So it may or may not be present in that.

Now whether this can somehow be achieved within the WBC system, or whether there is a way BTW, (and therefore TT and Sky), can have the related settings in the Openreach DLM adjusted, I don't know. It seems odd that if there is such a facility it isn't mentioned in the wealth of detail Openreach provide publicly.

A minefield unless you are a BT Wholesale and/or Openreach customer and have access to the non-public information.

In reply to a post by tommy45:

In reply to a post by BatBoy:

I think DLM on FTTC is controlled by Openreach. They give the ISP the choice of Stable, Standard and Speed to offer to their customers, but they don't offer the choice of Interleave or Fast. All 3 choices can apply varying levels of interleaving which I think, is to protect the network.

I imagine Fastpath can damage the network as I mentioned earlier.

And how exactly if this was the case (in reality) would it affect the bt network when the end user is with sky or talktalk using their network for backhaul and peering ect ??

Because he has to be talking about the Openreach GEA network and its DLM, which is the direct setter of interleaving and noise margin levels, and Sky and TT are subject to this in exactly the same way as BTW are.

In reply to a post by legume:

There may be some room for confusion around the word profile here.

OK ignore this - I can't find anything I've read about vectoring that mentions profile (I should not rely on vague memory), it's all vectored groups and non vectored groups - and as has been said I don't even know whether Openreach use it or not.

Correct, DLM is present on BT DSL systems. I thought everyone knew that.
We pretty much know that in ADSL systems, there is no significant impact on other users from users with overoptimistic SNRs and inappropriate fastpath. BE, and I am sure others, have been running ADSL systems with user configuration on SNR/fastpath for many years; and it is clear from posts on their forums that some customers try to get too much out of their lines, and suffer for it. Despite these issues I've never heard a BE user suggest BE should remove the options.

Their network has been very good, but not 100% for various reasons. One reason I've never seen proposed is that problems arise from others on the shared (exchange) equipment using over-hopeful settings.

I agree. There are at least three profiles. The lowest level vectoring; the DLM profile applied to the connection at the cabinet, and the BRAS profile applied at the exchange. We are talking about the DLM profile here; which (approximately) covers sync speed, target snr and interleaving.

I think the most likely place one overambitious user could affect others is in the DSLAM itself. This is doing a complex job of multiplexing/demultiplexing one fibre stream to multiple copper (we hope) streams. If one of those streams misbehaves, this could impact the the DSLAM enough to upset its ability to serve other customers to the best, even beyond normal contention for resources.

I don't have any reason to believe it does happen. But I have seen analogous impact from misbehaving clients having disproportionate impact on others in routers, and in large scale servers.

In reply to a post by tommy45:

In reply to a post by legume:

In reply to a post by BatBoy:

One problem is that errors cause retransmissions, which will reduce network performance for everyone.

I can't really see how retransmissions reduce network performance for everyone.

There is no link layer retransmit, so it's just TCP that will do the work, and that work is on server/client not the network between.

Packet loss to signal congestion is how TCP was designed to work.

Non congestion loss will only really hurt the speed of the individual flow so it could be argued that it reduces burst load on the network - benefiting everyone else. Of course nothing is clear cut - it will take more data eventually than without the loss - but then loss is normal for TCP anyway.

Thats what i thought too, as the claim of an individual line profile being set differently to that of others having a adverse effect on the network which in turn causes others to suffer, is total bull manure

It's still wasting 'network bandwidth' and that has an indirect knock-on effect on all of us. Without seeing diagnostic logs we can't know but perhaps during their development BT found that the wastage due to retransmitting lost packets was significant. We'd all suffer one way or another if BT (and ISPs) had to provide 20% more capacity just to cater for 20% 'wastage'.

Edited by Andrue (Fri 24-Aug-12 11:55:28)

1 they are being paid for that bandwidth by the customers

2, they cannot 100% accurately gage the needed amount of capacity from cab to exchange until those that want it subscribe to it, as we all know just how inaccurate their wholesale checker can be

3, people should be offered choice of if they require fast path or not , If by being given that choice and that does have a detrimental impact on things, then perhaps there network or equipment isn't up to the job

If that be the case then i will be waiting until their proposed Fibre on demand product is released as that will be fastpath and free of dlm ,
But that will be connected to the same infrastructure and not have this unfounded/proven negative affect on things,And would potentially have higher transfare speeds which again would suggest that there is some misinformation out there ,

Edited by tommy45 (Fri 24-Aug-12 12:23:02)

Those are good points. I dunno really. I've often wondered why they need any of that stuff given that LLUOs never have. I don't think anyone has ever come up with a good explanation. Presumably there is some logic behind it though. BTor are not technically inept. At least I don't think so. I'm reasonably happy to accept that it's there for a reason and it's just that no-one outside of BTor has worked out why.