Kitz Forum
Broadband Related => FTTC and FTTP Issues => Topic started by: JGO on April 16, 2013, 07:59:49 AM
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Came across this the other day.
A very clear description of the problems, (for people who think 2 wires = end of problems ?! )
http://www.analog.com/library/analogDialogue/archives/34-05/vdsl/VDSL.pdf
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I think that's actually quite an optimistic document but there's no getting away from the fact that FTTC is a short-term bodge to save money.
Unfortunately I think its a bodge which will delay real FTTP rollout for a decade or more :(
There's a limit to what you can do with even the most perfect unshielded twisted pair and that limit is way way WAY below what you can do with coax, never mind fibre.
I'm probably preaching to the converted here anyway, the unwashed masses actually believe Virgin uses fibre into the premises and BT's FTTC advertising is highly misleading in exactly the same way.
I'm hoping FTTP on demand is well established once my local PCP gets FTTC, ducts around here (1990 estate) are apparently all in good condition around here - mainly because all the phone wiring for the estate (300-400 houses) is underground, there's BT pavement covers every 20m or so on all the streets. I know where the duct runs from pavement to property as well and that is clear too.
I genuinely don't see the point of me upgrading to FTTC from a 20Mbps ADSL2+ connection, yeah sure the upstream is much better but I'm not sure I want to play the crosstalk game which will undoubtably occur with mass takeup, even with vectoring. I know someone in the city who has had 3 pair shifts now because the takeup has been high and the lines are ancient (1950s) so crosstalk is pretty bad. He's going to run out of options (pairs) soon but every 2-3 months the speed drops off by 50%+ and its simply crosstalk, all that sorts it is a pair swap away from the (new) FTTC customer who is now a noise source
Doesn't sound like a plan to me, so I'll wait and see how much FTTP install is likely to cost in a year or so.
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Rizzla, I think in your understandable enthusiasm for FTTP you are being a little harsh in calling it a bodge. In an ideal world, of course we would all have FTTP. Sadly the world is not at all perfect. Not every house was built in the 1990s, or in some quiet village. VDSL2 is IMHO a very good solution for urban areas, particularly large towns or cities. It would be harder to achieve the successes seen elsewhere, when there is restricted land access, properties with flatted dwellings, conservation and planning restictions etc.
I do think that since you are fortunate to have a 20Mb/s ADSL2+ connection, then as long as you are happy with your provider and the cost, then, for you, it probably doesn't make sense to move to Fibre at this point.
I have myself moved to FTTC recently. I had been thinking about it for quite a long time before, but was in no rush either as I had a reasonably respectible 14.5Mb/s on ADSL2+. In the end it was the SKY takeover of BE that gave me the final push, but moving from a 16-24Mb/s ADSL2+ BB+landline with BE to a 80Mb FTTC BB+landline with Plusnet has actually saved me money too!
IMHO, there was a significant advantage to me (and probably to most people except those on EO lines) in eliminating the effects of the E-side copper, which in my case was ~2.35Km. My PCP is only 150m away, on probably the 2nd if not the first DP, and via 5 UG joint boxes. As a consequence, my VDSL2 attainable at 91-94Mb/s DS exceeds the current product caps for profile 17b, but is probably capable of more than that with profile 30, and even more so with vectoring, which only requires firmware upgrades at both ends of the copper. All of this achieved with no changes to the internal wiring. It would be a significant hassle to me to accomodate an OTU, as the current entry point is in the bathroom at the rear of the building 2 floors up.
So, in the end, I think it's a case of horses for courses. Clearly FTTC doesn't appeal to you, but your circumstances are entirely different. But frankly, the state of the Virgin Coax pillars and distribution 'conduits' around here are horrific - a rats' nest open to the elements, corroding, shorting, rubbing through on the sharp edges of the cheap 'tin' casings. How anyone gets a good service out of that mess is beyond me. No thanks Virgin.
Just putting an alternative point of view for the sake of balance. There is no 'one size fits all' solution, just a number of different solutions which may be more or less appropriate in a given set of circumstances. :)
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Oh I do understand but within a decade we're going to have to do this all again because even with vectoring VDSL (FTTC) is crosstalk hell as its much much harder to prevent radiated emissions.
Bad enough in a cable bundle underground but overhead ancient TPs with HF on them? I shudder to think how far that's going to radiate. I can drive under overhead telephone cabling now and if I'm listening to MF radio then I get a "bzzzzt" tone within 20m of the cabling. Never used to happen until maybe 4-5 years ago as ADSL became ubiquitous.
VDSL works on much shorter wavelengths so you're going to see lengths of cable with faults becoming essentially half-wave dipoles where with the longer wavelengths of ADSL2+ there wouldn't have been an issue in terms of the radiated power. BTO engineers are going to have to become RF engineers to diagnose issues.
We'll see what we see but I think BT will ultimately look back on FTTC as a monumental waste of money.
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"We'll see what we see but I think BT will ultimately look back on FTTC as a monumental waste of money."
YES but it fits in with the common attitude of "That won't work; we've never done it before !" plus politicians wanting to show a result before the next election.
Most people seem to miss a vital point, fibre is immune to interference, short of a nearby atomic bomb where telephone lines would fry ! I did some work on a military trial in the 70's to verrify EMC. Since then fibre atteuation had reduced from 1 db/km - cable is 13dB/km ?!
As to "bodge being unfair" the 19th centuarl telegraph network was first bodged about 1880 to reject tram interference sufficient to receive hand morse (but a human operator is 4dB better at tolerating interference look up Piccolo). It was bodged again for telephones since channels are independent, so the group delay variation could be sidestepped. For high speed data it is a worse bodge; in my view we should regard ADSL as the final stop gap solution and go straight to a 100% fibre solution.
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Each to their own view of life.
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Each to their own view of life.
I'm not saying that FTTC is wrong ColinS - it clearly is appropriate in some circumstances but for dense urban areas it is the wrong solution.
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Rizzla, that was a general observation, not specific to anything you had said. :)
but for dense urban areas it is the wrong solution
Well, not in my experience is all I can say. :) And I live in an area that already has an extremely high penetration of broadband adoption of all kinds. Whatever FEXT results is already here. In fact, I am also already awash in other people's wifi too. Nevertheless VDSL technology delivers a very respectible solution - to me at least. One of the obvious 'benefits' (that offsets a number of the disadvantages) of being in a 'dense urban area' is that the loop lengths from the PCP are considerably shorter on average.
We appear to be in agreement that at least there are potentially different solutions which may be applicable in different circumstances, even if we do not necessarily agree on which ones those are. ;D
[philosophy] I am a little wary myself of evangelising a single answer to anything in life, as some other people are given to do on occasion. [/philosophy] :angel:
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I think if you're in an area with reasonably modern infrastructure (1990 onwards) then VDSL can perform well. It will still degrade as more connections are made - and with the greatest of respect you clearly don't have adjacent pairs running VDSL because if you did you wouldn't have the attainable rates you do. Not without vectoring anyway.
As mentioned before I know a guy in the area of central Leicester where I lived 20 years ago. Its student central - literally. The Virgin cable service is screwed at every head, Sky's service is appalling, BT Infinity was his only option to avoid the 24/7 downloaders and he's on the third pair switch which is 100% down to crosstalk on new VDSL services on adjacent pairs.
So I'm not at all convinced you are in the "high penetration area" for VDSL. I suspect you'd know all about it if you were ;)
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" Each to their own view of life. "
This is only true when their view of life doesn't impact other people's lives. An extreme example was the Holocaust but there are others which affect others lives. I don't see how anyone can defend FFTC against FFTP on TECHNICAL grounds but taking the figures bandied about for Australia, a 20% cost reduction may be ECONOMICALLY desirable - but then maybe they aren't so lackadasical about interference there.
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" Each to their own view of life. "
This is only true when their view of life doesn't impact other people's lives. An extreme example was the Holocaust but there are others which affect others lives. I don't see how anyone can defend FFTC against FFTP on TECHNICAL grounds but taking the figures bandied about for Australia, a 20% cost reduction may be ECONOMICALLY desirable - but then maybe they aren't so lackadasical about interference there.
Oh, I'm sorry. :( It's quite obvious to me now that you're the one with the balanced view of life, not me. ::) You've already made it quite clear that you don't see how anyone can hold a different view from your own.
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Ermm I'll leave you boys to argue amongst yourselves I think.....
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Ermm I'll leave you boys to argue amongst yourselves I think.....
Actually, I'm leaving it all to JGO, as I don't feel the need to push my own personal point of view.
You may be right about it Rizzla. As you have already observed: We shall (all) see ...
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Can I add fuel to the fire by suggesting that if I had FTTC available now I almost certainly would not be taking it.
As it is, until BDUK puts it in, I won't have the option anyway.
But so far as I use the web an achieved download speed of 12Mbps on ADSL2+ is quite adequate here.
Quite what I would do with a FTTC supply of 80Mbps as I'm 100 yards from the cabinet I really don't know.
I'm sure this will change in the future - but that time is some way off.
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I started this topic to expose some useful facts, because I make my mind up on the basis of facts, and may change them for new ones.
Over half the engineers I've met make decisions based on "We've never done it before, so it won't work ! " I'm thinking of one who didn't apply his brain just applied first order theory and shouted down any disagreements . He lost the firm £20,000,000 !
So what are the arguments for wire not fibre ? The only one I've seen is the economic one, and is copper theft weighed into that ? No one seems to be saying it is a better system.
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Can I add fuel to the fire by suggesting that if I had FTTC available now I almost certainly would not be taking it.
As it is, until BDUK puts it in, I won't have the option anyway.
But so far as I use the web an achieved download speed of 12Mbps on ADSL2+ is quite adequate here.
Quite what I would do with a FTTC supply of 80Mbps as I'm 100 yards from the cabinet I really don't know.
I'm sure this will change in the future - but that time is some way off.
I think the change will come when you don't even think about the speed your connection is running at (as a techie), and I'm sure that isn't going to happen with VDSL.
To some extent I have that on downstream anyway, upstream is a lot more of a pain in the backside due to Sky's asinine profiles which (because they are profiles) don't degrade gracefully - you come close to the profile speed and pings go through the roof etc instead of the (slightly) more graceful way TCP would work on a line not artificially limited. For those of you who remember how ADSL worked years ago (before DLM/profiles) then you could run upstream at close to max (-10kbps or so) and latency would be normal. Try that these days :D
Upstream would be the main driver for me but both of the kids are likely to be off to uni (poor sods) inside 3 years so not much point in that. If FTTP is £1k or less for this house next year then I think we'll just do it as all the houses here are much of a muchness so FTTP might swing the sale.
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From my viewpoint the article is useful because it points up the damaging effects of extension spurs.
I suspect a lot of connections are not running at full capability because their owners can't understand that just because it works OK at 300-3000 Hz, the extension it isn't good for ADSL, and they will be disapointed on going to FFTC/VDSL where the effect is worse.
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Correct me if I'm wrong but I believe that the article in question was written when the original VDSL specification was current.
Now, with VDSL2, the situation will be actually worse due to the even higher frequencies used. :(
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"Correct me if I'm wrong but I believe that the article in question was written when the original VDSL specification was current.
Now, with VDSL2, the situation will be actually worse due to the even higher frequencies used. "
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As the article is dated 2000 you are probably correct, don't know for sure. I find it is quite difficult to find system parameters.
Yes obviously the situation will be even worse on a wider bandwidth system !!
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Correct me if I'm wrong but I believe that the article in question was written when the original VDSL specification was current.
Now, with VDSL2, the situation will be actually worse due to the even higher frequencies used. :(
As you increase frequency then the current will flow in a decreasingly small cross-section of a conductor - known as skin-effect. As that happens the signal is more likely to radiate even in really good TP bundles.
The other problem is that as the frequency increases then the wavelength shortens. This means that (for example) a 15m pair run with one leg disconnected will become a pretty efficient noise emitter as it'll look much like a half-wave dipole to some of the frequencies.
Vectoring will help but only up to a point and faults are going to cause much more havoc than now.
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And those faults, will they just affect the FTTC community, or also muck it up for the plebeians like me on ADSL+ and normal phone users?
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And those faults, will they just affect the FTTC community, or also muck it up for the plebeians like me on ADSL+ and normal phone users?
Everyone, to a greater or lesser extent. :-X
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And those faults, will they just affect the FTTC community, or also muck it up for the plebeians like me on ADSL+ and normal phone users?
FTTC really as most of this stuff will be happening above 3MHz - it starts getting serious around 10MHz which is 30m wavelength (15 for half-wave dipole) as you can reasonably expect those sort of lengths on DP runs - which is where most faults are.
The significance of that is when you get to full, half (or even quarter in some instances) wavelength on the conductor it will start to become very efficient at "broadcasting" the VDSL signal so it won't just be affecting the local cable bundle. It will to all intents and purposes become an aerial if one of the TP legs is disconnected.
ADSL2+ for example would have a minimum wavelength of around 130m so if something similar happened then it'd be conductors in very close proximity which would be affected as it wouldn't make a good aerial.
Make any sense?
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There is another factor which no one has mentioned. The Short wave bands are NOT empty bandwidth just waiting for VDSL !! They are heavily used so interference is possible from both radio and electrical - I seem to remeber reading that car ignition could clobber TV in Band 1 at 40- 70 MHz if not supressed.
Also, in response to renlulop, not quite clear what you mean by "faults" but I think the variation of noise level on my ADSL, till I interupted the bell wire, was due to received electrical noise?
As to it happening to 'phones, it was commonplace in Rugby till GBR on 16kHz shut down a year or so ago !