Too good to be true

"Hello this is the engineer from BT I'm coming to install your broadband."
Me: "But I'm moving to Sky."
Engineer: "I know. Sky have no engineers so they pay BT to do their
installations."

So he turns up right at the appointed time, installs a BT Openreach wall
plate, plugs in a BT Openreach modem and away it goes. The only Sky part
of the system is the WiFi unit which also worked straight out of the box.

I told the engineer that Sky's line test had shown I would get 13Mbps. He said that with my distance from the cabinet I would be lucky to get 10Mbps, but when he
did the speed test he got 23Mbps over his WiFi connection. He seemed
amazed but assured me it should remain at that level and would be even
faster on my PC hard-wired to the modem. He was right. The speed test I
did showed an average download speed of 25.83Mbps and upload of 4.3Mbps.

Bit of a change from 0.9Mbps and even lower upload speed on previous
non-fibre connection.

My questions: Is he correct about the speed staying at approximately that level or will Sky quietly throttle me back to 13? I've never heard of anyone getting double their predicted speed, it's usually less than. Anyone any idea how this might have happened?

In reply to a post by deeor2013:

My questions: Is he correct about the speed staying at approximately that level or will Sky quietly throttle me back to 13? I've never heard of anyone getting double their predicted speed, it's usually less than. Anyone any idea how this might have happened?

Sky will not throttle you. The original 13 was an estimate of the best your line can do and so far it appears to have been pessimistic. Everyone will be pleased for you.

However..

If yours is a recently installed cabinet you can expect a slight speed drop over the next six months. This is because as more and more of your neighbours sign up you will begin to be affected by crosstalk (other lines interfering with yours). There is strong evidence that BT's estimates take this into consideration and therefore you might end up closer to the 13Mb/s estimate. It seems unlikely though - 23 to 13 is a big drop and I wouldn't think crosstalk would be that bad.

So...

Your speed will probably drop a little over the next six months but hopefully not by much and it won't be because Sky are throttling you.

Thanks, that's comforting . The cabinet is a rural one installed a couple of years ago but does appear to serve a wide area.

You may also find your speeds drop over the next couple of days. If there are many errors on the line then the DLM (dynamic line management) will apply varying levels of interleaving to correct those errors. The end result is you'll have less errors but a slower connection.

No, I've read people are predicted 40/50Mbps but some of them achieve max (80Mbps) syncs and some around 70Mbps.

I know people who have got well under their predicted speeds so it's really hit and a miss. Though I doubt it will stay at the same speed, as crosstalk increases (more people subscribe to FTTC) you will slowly see your speed degrade.

In reply to a post by ryant704:

as crosstalk increases (more people subscribe to FTTC) you will slowly see your speed degrade.

That's not necessarily how crosstalk works. You can have many connections that do nothing to your speed, then one further line is connected that interferes badly with yours and drops your speed significantly. There was a recent thread on here where someone lost 20% of his downstream sync speed in one go.

BT Openreach are experimenting with vectoring, which will help to eliminate the effect of crosstalk. Unfortunately, deployment still looks to be some way off.

In reply to a post by ryant704:

No, I've read people are predicted 40/50Mbps but some of them achieve max (80Mbps) syncs and some around 70Mbps

Depend on your high quality of modem/router

Sky will never throttle you down however that 13Mbps estimate is based on crosstalk being in place. Right now you may not have much crosstalk hence you are syncing much faster.

In time a neighbour or somebody may get fibre like you, BAM, this one person may introduce crosstalk onto your line. Crosstalk isn't usually gradual, it's normally very sudden. An example is you may get 23Mbps now and then an hour later it's down to 13Mbps. It wouldn't be a fault if it was crosstalk that had caused it.

So yes that speeds nice for now but don't be shocked if it suddenly drops down nearer the estimate. It has a chance to do this on any ISP. Sky will not throttle you down though. Like any ISP sky uses openreach which although retaining the BT logo etc is entirely different to the BT retail devision which sells BT broadband.

So Sky is at no disadvantage over BT here. All ISPs are treated equally by openreach, the best way to imagine openreach is as a company which maintains the infrastructure & to treat it in your mind as seperate to BT broadband etc.

Depend on your high quality of modem/router

??????????? This is just incorrect ?????????

In reply to a post by adslmax:

Depend on your high quality of modem/router

When are you going to stop posting such utter rubbish? You've been on these forums long enough now to know the basics, yet you constantly post totally wrong information, utter rubbish!

People come on here looking for answers, they may find yours and take it as the truth, and then they go away misinformed!

That means someone else then has to come along and correct what you've posted, wasting their time!

I should think the regulars are all getting pretty much fed up with it, I know I am.

To set the record straight the biggest thing which affects the speeds anybody will get is line length, followed by quality of line, cross talk and other interference, I should think the quality of your modem/router has very little to do with it.

Edited by R0NSKI (Sun 20-Oct-13 10:31:43)

I think hes just trolling tbh.

I have always guessed he was some very young lad, just keen to be posting ......

When are you going to stop posting such utter rubbish? You've been on these forums long enough now to know the basics, yet you constantly post totally wrong information, utter rubbish!

People come on here looking for answers, they may find yours and take it as the truth, and then they go away misinformed!

Completely agree.

All this talk regarding cross talk: say one syncs at ~16Mbps on ADSL2+ then one moves to FTTC and all 288 lines eventually are in use; at a distance of ~250 metres from the cabinet would a one see significantly higher speeds, i.e. 32Mbps or more, than the previous ~16Mbps - in other words would it really be worthwhile moving to fibre except for the increased upstream speed?

But in your scenario, the only place where all 288 lines are in the same cable, is the copper tail between the DSLAM and the PCP, and even then that will be spread over three separate 100pr tails.

Thanks Zarjaz - I keep seeing a new cabinet at the end of the street and wonder just how satisfactory the speeds might be be once it's capacity is full

Have often wondered about pulling in 500 odd meters of cable between the cab and me, and then doing a before and after between the two pairs. Then I think, sod it, pull in BFT, blow my own fibre in and sit and wait .......................

It's just that I get an estimate of up to 68.2Mbps downstream and up to 20Mbps upstream for WBC FTTC from my cabinet, and it seems that with "cross talk" those "up to" figures could be significantly reduced.

I'm not particularly interested in vdsl myself at the moment (until I wish to stream video at 20Mbps or more!) but for a typical neighbour who have several machines tapped into the same line, requiring plenty of bandwidth, cross talk could be significant?

Our fttc estimated speed table in the fibre guide does take into account crosstalk

In reply to a post by MrSaffron:

Our fttc estimated speed table in the fibre guide does take into account crosstalk

Thanks - your estimate does appear to be less than the "up to" estimate taken from the https://www.btwholesale.com/includes/adsl/main.html checked based on the distance to the cabinet

In reply to a post by David_W:

<snip>
There was a recent thread on here where someone lost 20% of his downstream sync speed in one go.
<snip>

Will you post the link, please?

In reply to a post by burakkucat:

In reply to a post by David_W:

<snip>There was a recent thread on here where someone lost 20% of his downstream sync speed in one go.
<snip>

Will you post the link, please?

Sure. I was thinking of this thread.

Thank you. Much appreciated.

In reply to a post by David_W:

In reply to a post by ryant704:

as crosstalk increases (more people subscribe to FTTC) you will slowly see your speed degrade.

That's not necessarily how crosstalk works. You can have many connections that do nothing to your speed, then one further line is connected that interferes badly with yours and drops your speed significantly. There was a recent thread on here where someone lost 20% of his downstream sync speed in one go.

BT Openreach are experimenting with vectoring, which will help to eliminate the effect of crosstalk. Unfortunately, deployment still looks to be some way off.

agreed but given his cabinet is 2 years old, hopefully any crosstalk is already in place so his speed stays as is.

Folks,
Thanks for all the advice.
David-W I found that link very helpful too.

I'll just have to tour the neighbourhood discouraging any further take-up of fibre

In reply to a post by Andrue:

It seems unlikely though - 23 to 13 is a big drop and I wouldn't think crosstalk would be that bad.

My speed has dropped from 69 Mbps to around 56 Mbps and there's at least 10 fibre connections coming from the same pole as my line so whether this could be caused by crosstalk, I have no idea.

In reply to a post by simon194:

In reply to a post by Andrue:

It seems unlikely though - 23 to 13 is a big drop and I wouldn't think crosstalk would be that bad.

My speed has dropped from 69 Mbps to around 56 Mbps and there's at least 10 fibre connections coming from the same pole as my line so whether this could be caused by crosstalk, I have no idea.

I must admit I was thinking any drops would be a percentage rather than in terms of absolute figures but I could be wrong.

My own connection dropped from 80 to 72, then interleaving kicked on and now it's 65. So that's a total drop in 18 months of either 18.5% or an absolute drop of 15Mb/s. 18.5% off the OP's current 23 ain't so bad but a drop of 15Mb/s would be painful. Then there's banding to consider.

So I don't know. I'd rather hope as Mr Saffron wrote that it being an established cabinet means he's already dealing with crosstalk but we've no way of knowing. Hopefully they'll come back with a status report in a few months

My max achievable has went from 110Mbps to around 89Mbps in 10 months.

When I got infinity installed my estimate was 41.6 down 6.5 up in reality I getting about 53.24 down and 13 up it's still good!

As others have said, your speed will probably drop.

BT line here (via the excellent Plusnet), predicted speed: 59/20

When the engineer came, his test equipment said 72/20. This translated to 62/15 when the modem was installed (direct on the master socket), and dropped to 50/15 within a couple of days. Currently the cabinet is 1 week in service so probably near-empty aside from us, so we have crosstalk to look forward to as well.

It's no longer playing rollercoaster during busy periods, so that's something.

Edited by deleted (Tue 22-Oct-13 13:16:20)

In reply to a post by David_W:

That's not necessarily how crosstalk works. You can have many connections that do nothing to your speed, then one further line is connected that interferes badly with yours and drops your speed significantly. There was a recent thread on here where someone lost 20% of his downstream sync speed in one go.

That is close, but not quite right - it leaves out the "threshold" nature of DLM.

I'd re-write your answer as this:

You can have many VDSL connections that do not interfere with your line, some connections that interfere a little, and a few that have the potential to interfere a lot - the effect appears random, so the line that causes most interference might not be one of your direct neighbours.

Each small bit of interference can cause a small loss of speed, or can cause additional errors on your line.

BUT... when the total interference adds up, and causes the errors to hit a threshold, DLM will intervene, and add protection mechanisms to your data to reduce the error rate.

When DLM does intervene, you can expect it to drop your speed anywhere between 10% and 20%.

In reply to a post by Andrue:

I must admit I was thinking any drops would be a percentage rather than in terms of absolute figures but I could be wrong.

It does tend to be in percentage terms - especially when DLM steals some bandwidth to turn on FEC. Interleaving alone doesn't drop the speed, and has been seen to happen when you have extra capacity to spare (especially upstream).

I'd rather hope as Mr Saffron wrote that it being an established cabinet means he's already dealing with crosstalk but we've no way of knowing. Hopefully they'll come back with a status report in a few months

That's the problem. An established cabinet means that crosstalk is already happening, but you will never know if you are already suffering, or whether your biggest disturber is going to sign up next week.

So if one can only "see" the IP Profile then the IP Profile could drop proportionately to a 20% drop in sync speed?

In reply to a post by 4M2:

All this talk regarding cross talk: say one syncs at ~16Mbps on ADSL2+ then one moves to FTTC and all 288 lines eventually are in use; at a distance of ~250 metres from the cabinet would a one see significantly higher speeds, i.e. 32Mbps or more, than the previous ~16Mbps - in other words would it really be worthwhile moving to fibre except for the increased upstream speed?

There is a document from the Broadband Forum that describes Vectoring, and includes a graph of behaviour at different distances.

That graph shows the optimal behaviour (no crosstalk), plus average and a "99% worst case" figure too. It then also shows the values when vectoring is turned on.

For a 250 metre line, it shows the following:
- Theoretical top speed: 160Mbps
- Range of speeds with crosstalk present: 60-95Mbps
- 99% worst case: 50Mbps
- Range of speeds with vectoring present: 140-160Mbps

So yes, you would see significantly higher speeds even if you encountered the (almost) worst possible case, but you are highly likely to get into the 70-80Mbps region.

The graph is on page 12 here.

In reply to a post by 4M2:

So if one can only "see" the IP Profile then the IP Profile could drop proportionately to a 20% drop in sync speed?

Yes.

IIRC, It is quite an aggressive FEC setting to get a 20% drop.

Thanks for the link - I will have to study it in more detail later

Interesting though that with a 99% worst case, with cross talk present, one has to be ~1000 metres from the cabinet before the speeds approximate to the max 24Mbps sync speed possible with ADSL2+ - that's if I'm reading the graph correctly?

Edited by 4M2 (Tue 22-Oct-13 15:50:40)

well DLM is an artificial change added by BT thats not part of VDSL spec.

Thats why we didnt mention it.

So DLM is a possible indirect affect of crosstalk. But not direct.

In reply to a post by 4M2:

Thanks for the link - I will have to study it in more detail later

Remember that it really describes a modelled situation (ie based on what wiring ought to be out there), so doesn't really take account for the cases where faults & problems do exist, such as corrosion.

It is also based on 0.4mm wires, so the UK ought to be in a slightly better situation.

Interesting though that with a 99% worst case, with cross talk present, one has to be ~1000 metres from the cabinet before the speeds approximate to the max 24Mbps sync speed possible with ADSL2+ - that's if I'm reading the graph correctly?

That's about the right reading of the graph - I'd say somewhere in the 900-1000 metre region.

But we hit the same proviso as above, that we use 0.5mm wires, so the distance ought to be longer - perhaps by as much as 20-30%.

True, it is an indirect effect.

However, the existence of a DLM is built into the VDSL2 spec, as all the facilities necessary for that DLM to work are specified. VDSL2 is written with an understanding that some form of DLM is going to exist and drive it.

But the specifics of BT's implementation of DLM isn't directly specified by VDSL2.

With luck, DLM will change alongside vectoring - and replace the use of FEC/interleaving with PHYR.

Perhaps that also really emphasises the need for 24SWG (0.5mm), solid core, 100% copper CAT5e for anybody wishing to use a vdsl "data extension", of up to 30 metres in length, for their modem particularly if they are 500 metres or more from the cabinet?

Edited by 4M2 (Wed 23-Oct-13 01:16:45)

isn't the wires BT use wholely inconsistent?

0.7 copper in one area, 0.2 in another, ali in another?

I got told my area is a mix of 0.6 ali and 0.2 copper by more then one local engineer.

If going to worry about copper gauge and the graphs, have people considered the varying power masks (3 are used based on distance of cab from exchange) and what effect this has compared to the theoretical situations.

In reply to a post by Chrysalis:

isn't the wires BT use wholely inconsistent?

0.7 copper in one area, 0.2 in another, ali in another?

I got told my area is a mix of 0.6 ali and 0.2 copper by more then one local engineer.

0.2 copper? Perhaps 0.020" = ~0.5mm?

In reply to a post by MrSaffron:

If going to worry about copper gauge and the graphs, have people considered the varying power masks (3 are used based on distance of cab from exchange) and what effect this has compared to the theoretical situations.

Sorry all this FTTC stuff is new to me - we've just had a cabinet enabled and I'm just trying to learn a little about it - preferably in layman's terms

I cant say for sure, but we were talking mm so as far as I now its 0.2mm.

In reply to a post by 4M2:

Sorry all this FTTC stuff is new to me - we've just had a cabinet enabled and I'm just trying to learn a little about it - preferably in layman's terms

It's faster

And may cost you more bags of gold each month

In reply to a post by MrSaffron:

And may cost you more bags of gold each month

I thought it was "too good to be true."