DLM on fibre?

talktalk are adamant that there is none but my speed has dropped over the week by 20%.

In reply to a post by ashmo:

talktalk are adamant that there is none but my speed has dropped over the week by 20%.

What do you mean by 'fibre'? If it's FTTP (ie; fibre to your property) then I can see no reason why there'd be DLM. Fibre either works or it doesn't so DLM would be daft. I thought only BT was doing FTTP though at the moment.

If it's the more normal FTTC (Fibre to your cabinet) then, yes, there's DLM. Everyone has to have it - even LLU operators like TalkTalk and Sky. I'd be with you on this - there's going to be DLM.

Edited by Andrue (Thu 13-Jun-13 15:33:33)

In reply to a post by Andrue:

In reply to a post by ashmo:

talktalk are adamant that there is none but my speed has dropped over the week by 20%.

What do you mean by 'fibre'? If it's FTTP (ie; fibre to your property) then I can see no reason why there'd be DLM. Fibre either works or it doesn't so DLM would daft.

However if it's the more normal FTTC (Fibre to your cabinet) then, yes, there's DLM. Everyone has to have it - even LLU operators like TalkTalk and Sky.

Yes FTTC talktalk lies again

As the others have said

And the first intervention of DLM is usually a drop of speed of 15-20%, unless you had plenty of capacity to be over 80Mbps anyway, and an increase in latency of 8ms.

Not lying as such, just ignorant.

Openreach run a DLM at the cabinet, so there is no way TT and Sky can avoid it. Where they are correct is that they don't have another one at their end, unlike BT Wholesale (so all other FTTC ISPs).

The BT Wholesale one doesn't control the line in the way their ADSLx one did/does, but it does set the IP Profile that you can see from a BT Wholesale speed test - Further Diagnostics, if not on TT/Sky. TT/Sky don't have a (published) IP Profile.

In reply to a post by WWWombat:

As the others have said

And the first intervention of DLM is usually a drop of speed of 15-20%, unless you had plenty of capacity to be over 80Mbps anyway, and an increase in latency of 8ms.

Just annoyed with TT support really they are basically saying your getting above the "BT Wholesale" estimate [censored] off were not going to help you :/

Estimate was 54mb Down 15mb Up

Engineer came and with his tool not sure what is called he said I should get 70mb Down and 20mb Up with no issues.

First week I was getting a solid connection between 70mb - 76mb Down and 17mb Up and a 15ms ping.

Then from yesterday I now get 58mb Down and 15mb Up with a 22ms ping.

I thought FTTC could in theory handle speeds way above 80/20.

I doubt there is much TalkTalk can do as their is no fault - you are indeed getting more than the estimate. As the line settles in it will find the programmed best combination of speed/stability. That balances the 2 - and with FTTC being early days there are limits to what ISPs or BT will do to squeeze the last ounce out of a connection (and even with the mature ADSL many ISPs will do little to maximise connection speed).

Also, it is possible that since you connected a lot of others may have connected (especially if it was a newly enabled cabinet) so you may now be seeing effects of crosstalk.

The engineer will likely have based what he told you on what he saw at that moment in time; in reality the line needs to adapt over time the the conditions that are present on your line and in your area.

The estimates BT provide will factor that in (and will be working with a larger comparable data set than an engineer would) - and quite often we see customers getting faster than the estimate for an initial period, before it gradually settles closer to the estimate. As another poster has mentioned, cross talk is a key factor - especially given a new cab won't see many for a little while - until uptake of FTTC services increases after activation.

Maximum speeds on VDSL2 can exceed 80Mbit/s - but potential speeds drop rapidly over distance and with crosstalk, so very few people could benefit from faster speeds if BT did choose to push it further. Vectoring will likely lead to improvements in crosstalk, but distance will always remain a factor with FTTC.

regards,
Phil.

In reply to a post by ashmo:

Just annoyed with TT support really they are basically saying your getting above the "BT Wholesale" estimate [censored] off were not going to help you :/

There is a very fine line to be drawn here.

In FTTC, most people do not suffer from physical line problems with the service, so there is little work needed to rectify such faults. It does happen to some people, but support staff don't get too much practice at dealing with problems like that.

In truth, we're all largely "early adopters" still, so we're getting put onto cabinets that don't yet have much take-up. The combination of early-adoption, and largely good quality lines means that most people are seeing speeds higher than the estimates. For now.

However, the biggest factor that will affect our speeds isn't a physical problem with the line, but is crosstalk - the signal for other subscribers that appears as noise to our modem. As take-up increases, the noise increases, and the effect on our modem increases too.

Unfortunately, the effect is random, and can occur at any time - totally out of control of the ISPs or Openreach. It depends on your neighbours, and what they do.

The BT estimate seems to take into account a large amount of crosstalk, though not quite the theoretical worst-case amount.

So... we all need to expect that crosstalk will arrive, and increase, as take-up increases (and there's room for a lot more yet). We need to expect that the high speeds we initially see, the above-estimate speeds, will not stay. We need to expect that FEC and interleaving will become the norm, and could be added at any time.

Hence the fine line. Your slowdown could be due to a fully-expected increase in crosstalk, for which there is no remedy as yet. Or it could be caused by a physical fault.

In one case, it is pointless sending an engineer, and a waste of resources. In the other case, it might be worthwhile.

How else can we judge which case it is? The estimate is one reasonable way to do so.

I also heard that BT will investigate if there has been a 25% drop in speed within a 7 day period. I'm not sure if this is a current threhold, or one that only applied sometime in the past.

I thought FTTC could in theory handle speeds way above 80/20.

Yes, but it is still distance-limited.

The top downstream speed for 17MHz VDSL2 is theoretically around 170Mbps very close to the cabinet, without crosstalk. 100Mbps is possible at 400-500 metres, again without crosstalk. Unfortunately, crosstalk can take away half of those theoretical speeds.

Vectoring is a big solution to these, which is being trialled later this summer. It holds good promise, but we should expect it to take 2-4 more years to upgrade the BT network, if it gets the go-ahead.

Why this reliance on what bt estimates say,?they are estimates based on little more than theory ,and not real life, just because someone has a speed that is higher or lower than what BT spew out should mean very little,
for instance they should not be used to determine if there is a fault or not as a way for an isp to avoid sending out an engineer

Edited by tommy45 (Sat 15-Jun-13 14:31:42)