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
Yes and no.
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. 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.
 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
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
On Sat, 8 Feb 2014 09:24:37 -0800 (PST), David Paste
You have sort of answered your own question.
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.
On Mon, 10 Feb 2014 11:07:09 +0000, John Rumm wrote:
It could be some of the higher speed fibre connections use more than
one laser frequency (aka carrier), but then you are probably talking
silly data rates like a few Tbps... B-)
GPON uses two down a single fibre but in opposite directions.
Of course the vast majority saying that they have "fibre broadband"
haven't they have VDSL with the local head end fed by fibre. I reckon
all this crappy FTTC will come home to roost in ten years or so. When
streaming multiple channels of HDTV at sensible bit rates (10 Mbps or
more) becomes the norm, rather than the barely better than SD 2.5
Mbps or so that is used at present.
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.
UK SelfBuild: http://uk.groups.yahoo.com/group/UK_Selfbuild/
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:
Depends on if you mean "could the system be re-engineered to do this and
would it help" (yes, and probably not noticeably), 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.
 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
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!
From the Sofa of Brian Gaff Reply address is active
"David Paste" < email@example.com> wrote in message
On Sat, 8 Feb 2014 09:24:37 -0800 (PST), David Paste wrote:
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:)...
On Saturday, 8 February 2014 17:24:37 UTC, David Paste wrote:
Broadly, no. Twisted pair is surprisingly good as cabling for high bit rat
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
(in-ep-toc’-ra-cy) – a system of government where the least capable to
Unreliable it is if you've ever had to make a tap on thick ethernet -
or had smart-alec physicists think they can just add 50m of their own
2mm thick 50 ohm coax on the end of a segment of thin ethernet.
"That excessive bail ought not to be required, nor excessive fines imposed,
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