Re: OT - internet speeds - a question.
- From: Joe <joe@xxxxxxxxxxxxxx>
- Date: Tue, 14 Aug 2007 23:06:31 +0100
Mel Rowing wrote:
On 14 Aug, 20:39, "onlyme" <onlyme_sitt...@xxxxxxxx> wrote:
Well, clearly this was bollox...
Clearly the 'fault' lay with the equipment in the exchanges....once
upgraded, we are now all capable of recieving high speed broadband
data.....and for most of us, the lines from the exchanges to our houses have
remained unchanged....
The rate at which data can be transmitted down a line is determined
by the bandwidth of a channel.
The old audio telephone channels had a bandwidth of 3kHz (I seem to
remember)
Upgrading broadened these channelsby simply enveloping a number
together.
There is no realistic limit on the amount of data that can be carried
on a domestic copper conductor as your TV set aerial will testify.
Broadband. as the name implies, involves the integration of a number
of narrow channels to form one broad one which is why telephone and
internet can be carried on the same channel.
And they now blame the drop-off from 8MB/s ....on 'your distance from thean
exchange'.
It's not just a question of bandwidth.
Data is sent down a line in pulses known as packets. As each packet is
received and successfully decoded your computer sends a response
packet which instructs a register in the exchange to sent the next
packet and so on.
It takes a discrete time for both the data packet and the response
packet to travel along the lines between your computer and the
exchange and of course this time will depend upon distance.
The only way that line speed can be improved short of building more
exchanges is by creating broader bandwidthes.
Eventually ultra high speed data connections will utilise fibre optic
cables all the way to the user. These have tremendous bandwidth
capabilites.
In the days of old telephones, the power that drove the earpiece had
to come down the cable. There was no amplification at the telephone
end. Not much loss of power could be tolerated.
The bandwidth of telephone cables is quite low, probably no more than 10KHz, which was adequate for the 3KHz which was all that the telephone
system delivered.
Different types of cable have different bandwidths. The TV coax
referred to can carry a few hundred MHz without much loss over about
ten metres, and twin flex cannot. Again, loss is important since the
signal isn't amplified until it reaches the TV. What gets to your TV
set has to be significantly greater than the thermal noise power
generated by the TV amplifier. If not, you see a snowy picture. Try
substituting some twin flex for the coax cable between your aerial and
TV to see the difference in bandwidth. Someone living very close to a
transmitter might actually see some kind of picture using the flex.
What is different about Internet connection signals is that loss is
not important, within limits. ADSL signals are more like radio than
audio, and suffer enormous losses along old telephone cables. But as
long as enough gets through to be amplified and decoded, the system
works. Probably less than a millionth of the power put in at one end
gets out of the other end. There's no way old fashioned telephones
could work with those kind of losses, but that's of a similar order of
magnitude to the level of TV signal arriving at your house compared to the transmitted level. If it's comfortably larger than the noise, it
will be decoded accurately.
Some ADSL modems are better than others at this task. Some types of
encoding can be more accurately decoded than others, which explains the
progression of modem speeds from about 4.8Kbaud, which the phone cable
could handle comfortably, up to 56K, which it couldn't. At the higher
rates, the signal was no longer a simple on-off but was varied in time
slightly to carry more information. It did require more sophisticated
decoding, but once chips were designed to do this, the price became
acceptable.
The old modem signals had to pass through a system strictly limited to
about 3.4KHz, while the ADSL hardware is attached to your phone line
before it enters BT filters at the exchange, and the signal never passes
through real telephone exchange equipment. This is why ADSL data rates
are very dependent on the characteristics of that last bit of cable, and
the old-style modems were not. Individual phone lines could always carry
more bandwidth than the exchange equipment.
Presumably, more complex encoding can increase the data rate carried
by DSL on phone wires, and lower noise electronics can make more sense
of lower level signals, so data rates can go higher. There will come a
point when the extra cost becomes comparable with installing some extra
hardware to make full use of the fibre-optic cables that run throughout
our cities, and which are grossly under-utilised at the moment.
.
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