Why is broadband advertised as upto 10x dial up?

i regularly got 5.5kb/sec on my old phone line.

It depends on the modem and the quality of the phone line. 56kbit/second is
10.28 * faster than the 576kbit that dsl modems on a half meg connection
will connect at.

analogue modems are a very old technology. bear in mind that it's possible
to get way in excess of a half meg connection through your phone line. even
with a relatively crap line, half meg is doable.

 

Well I get upto about 4k/s on a 56k modem on dialup and upto 64k/s on
broadband. That's about 15 times the speed which actually fits with my
download times. This is the first time I have seen advertising claims sell
a product short.

Anyway how come a 56k modem never ever reaches anywhere near 7k/s and yet
broadand often reaches its limit?

 

You are unlikely to be getting 64k/s via a 512 ADSL link without using some
form of compression.
Check your true speed via the speed test here :-
http://www.adslguide.org.uk/

 

Well I get upto about 4k/s on a 56k modem on dialup and upto 64k/s on

I agree - with my 56k, I used to get about 4k/sec max, but broadband gets a
steady 60 kb/sec. It really is 15 times the speed, not "up to 10 times".

 

Modem connections support compression so I was quite used to getting
downloads of text newsgroups at anywhere between 7 and 9 KB/s on a
dialup. In that context to say ADSL is 10 times faster is not selling
the product short - in fact they should only be saying up to 7 times
faster for downloading text.

Maybe you had a crap modem - I've regularly max'ed out a 115.2k serial
port with a V.90 modem with compression enabled. In fact with a modem
with a 230.4k serial port I've gone over 170kbps on a V.90 modem - not
often, but with a download that is compressible, it can happen.

 

I agree! Exactly my experiences.

 

Analogue modems can get that "fast" if you're downloading something easily
compressible, ie text. But most of us all download already compressed files,
zips, jpegs, mp3s, divx video so few of us ever enjoyed the dubious
pleasures of on-the-fly modem compression to it's full extent.

Although they work over the same lines I believe analogue modems have to
operate in a "noisier" medium, there's less signal to noise so error
correction kicks in and that incurs an overhead in your available bandwidth.
That was one of the big selling points of ISDN, it was a pure digital medium
so you got max possible bandwidth.

I'm not sure if xDSL counts as a digital transport though... We do still use
modems on the services so that suggests to me there is still some form of
analogue <> digital conversion going on. If anyone knows I'd be interested
in learning if DSL is "pure" digital.

 

Hi,
I'm no expert on this, but as I uderstand it ADSL is not pure digital.
The DSLAM in the exchange is a special kind of modem, just like your normal
analogue modem. The signal for ADSL is split into a number of channels each
occupying a particular frequency range. The channels occur at frequencies
between about 300 KHz and 2.2 MHz (I'm not sure of the exact figures, but
its about that).

So whereas with a normal modem, you only have a single channel, and it
transfers data sequentially, with ADSL, the multiple channels allow you to
transfer many streams of data in parallel.

However, all of this data is encoded into analogue signals for transmission
accross the phone line using Frequency Modulation. So it's a bit like
connecting 10 standard modems to the phone line, but with each one operating
in its own frequency band, then distributing the data accross the modems.

 

^^^^^^^^^^^^
you mean
Data-rate=Bandwidth*(log2(1+signal-to-noise ratio))??