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 ???