As I understand it, ADSL signals are passed along the twisted pair (is that right?) telephone cables which are the same used for voice calls. At a much higher frequency, of course. I am also under the impression that the length of this cable is critical for service quality and overall data rate (apart from junction / joint quality).
Would using coax for the last stretch to the subscriber's premises provide any better quality of service?
I only ask as my cable service uses coax and I wondered about the difference.
Only if "the last stretch" was a significant proportion of the total distance to the exchange, and even then you'd have to have impedance balancing devices between the twisted pair and the co-ax.
That's co-ax all the way - or at least until it reaches fibre optic or some other very fast 'highway', so it's going to be able in principle to provide a much faster connection.
Both ADSL and cable are adaptive technologies. In other words, they take the bit of wet string that is your line, analyse it, see where the frequency nulls are, and send the data in frequency bands to avoid them. As things change (atmospherics, day/night, water, interference, whatever), they can adjust the bands to cope.
In cable's case the network quality is much better so there are many fewer nulls and you can get much higher bandwidth out of it. However you're also sharing that coax with your neighbours - and it carries all the TV signals too[1]. Plus the cable operator needs to agree with each modem what bands it gets so they don't conflict.
So you could route ADSL over coax, but it's still a fundamentally different technology. Plus you'd have to impedance-match the interfaces between twisted pair and coax which would increase the losses. So a short coax run might make it worse not better.
Theo
[1] back when analogue cable was still alive, you could stuff the cable in the back of your TV and get FTA TV directly, no STB required. Same goes for FM radio.
The whole point of ADSL is that it was invented as a means to send high speed data over twisted pairs several km long that were only originally intended for base band voice. The system is highly adaptive, using digital line management.
ADSL also needs a modem at each end of each subscribers line, where co-ax distribution systems like Virgin's can be largely passive.
The UK cable network does not quite do that. There are splitter boxes in the footpath holes and a cable runs from the street cabinet and is split off at the nearest hole in the ground. There can be dozens of houses hanging off each cable coming from the street cabinet.
On a d-i-y point - put Coax just on last part and most likely you will have impedance mismatch, and degraded signal. If coax were used to exchange or to fibre cabinet then yes that would be better than twisted pair.
Yup pretty much. There are a few different flavours of DSL technology, and give a relatively short decent line some will get noticeably better performance than others. However once you get to a few km of wire they all become similar in performance.
See the graph for the tradeoff:
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Depends on if you mean "could the system be re-engineered to do this and would it help" (yes, and probably not noticeably)[1], or could you just "lashup a bit of coax in the place of twisted pair, and would that help" (yes, and absolutely not).
So the short answer no.
You can use co-ax or twisted pair for data transmission - each have pros and cons but the mechanisms at play for keeping the signal in and the noise out are different - so the electronics that drives and receives these signals needs to be specific to the cable type.
[1] The other way round could work better; replace the miles of damp string with co-ax and then use the damp string for the last few hundred yards ;-)
Well, I'd guess that there would be less crosstalk in a couple of coax cables running near each other. It really is the rest of the route that is the issue though, the twisted pair from your house to wherever the multiplexer is. If you are proposing coax all the way, then you are suggesting a very big job in many cases. The small bit from the pole or junction box t to your house is only part of it. I always thought adsl was ab bit of a bodge in any case, trying to use cables probably installed before the internet was even launched, for such purposes. Brute force in both directions comes to mind! Brian
Pretty much correct. People underestimate the bandwidth that can be carried over a twisted pair. 40 odd years ago broadcast video (up to
5+ MHz) was sent over half a dozen bonded twisted pairs, not far only a mile or so but shows what could be done with the twisted pair and the technology back then. This is ten years before the home computer and 20 odd before cheap general access to the internet.
It might but not without changing the kit each end. You couldn't just join the two together, twisted pair is balanced, coax unbalanced. Making a passive balun (balanced to unbalanced) convertor of suffcient bandwidth (30 odd MHz) wouldn't be easy.
AFAIR in the local cable system here which is and has been right from the word go fibre to the "cabinet" and then co-ax to the sub of varying lengths, there was or is an upper limit on the co-ax of 1 Ghz odd and at that end the losses are well on the go. I do believe they have equalisers to render the whole co-ax channel a lot flatter then it otherwise would be.....
But I do know that on our 30 Meg service each and every time I do a speed test its just a shade over the 30 mark which is were it should be supposed to be getting 60 'ere long:)...
Broadly, no. Twisted pair is surprisingly good as cabling for high bit rat e digital.
Coax developed for analogue signals. It's good at avoiding lots of problems that affect high bandwidth analogue. When digital telephony began over the local loop (ISDN, early '80s) it was discovered that pairs worked surprisi ngly well and also also that the problems affecting pairs (dispersion for o ne) weren't problems that were quite so important to this type of signal. C losely spaced twisted pair (with terminations designed to work with it) is even better. As it's also far cheaper, this is one reason for 10baseT repla cing both thick & thin coax for Ethernet.
Sasy rather that advanced adaptive signal processing and modulation schemas made ADSL over existing copper possible in a way it wasn't before.
Not quite the story for Ethernet as that was always possible but the cost of having a switch held it back a bit, until the cost of all that coax and the unreliability in large networks made todays arrangement vastly cheaper...
That's not really fair. Cable has a multi-channel VHF modem at each end - effectively something like 8 modems running in parallel on different RF frequencies on the same cable.
Its much more fun to add a tee and a few meters of coax onto a PC. If you get it right you can make it so some PCs are invisible to others while they work to the rest of the network. ;-)
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