Man-wai Chang ToDie (33.6k) wrote:
>
> Is there such a beast? I wonder whether I could go
> higher than 56k over regular voice phone line...
>
It doesn't work like that. The characteristics between
you and the Central Office, are different than the characteristics
through the digital telephony network. Digital telephony
was designed to transport a 4KHz analog signal, from
end to end. It does that by using 8KHz analog to digital
conversion, 8 bit sized samples, and then sending it as
64Kbit/sec digital channels through the voice network. At the
other end, the digital data is converted back to a
4KHz analog signal.
ADSL works, by taking advantage of the characteristics
of the wire that runs from you to the Central Office.
A box at the Central Office, converts the ADSL signal
into digital form, at much higher speeds. The DSLAM
grooms off the data stream, and sends it to the
data network part of the phone company. (The description
here, allows the DSLAM to be connected at a location
other than the Central Office, but the principle remains
the same.)
http://en.wikipedia.org/wiki/Dslam
Your proposal, is to "dial up" an ADSL stream. You're
trying to pass something like an 8 megabit/sec digital
stream, through the 64 kilobit/sec telephone network.
That won't work. ADSL and the voice telephone network,
are two parallel and separate networks.
phone
phone ----- Filter ---------+----- ADC 64kbit/sec voice ---- voice_network
0-4KHz | line |
| |
ADSL --------+ +----- DSLAM ---- IP networking --- data_network
>4KHz
The above diagram is a logical representation. I don't know
if the universal line card in the telephone switch, has
ADSL modem capability, or the signals actually are connected
to two separate boxes. Architecturally, the phone people like
to cram as much functionality as possible onto the universal
line card, but the phone switch (voice network) could not
handle the aggregate data rates involved. So it might require
the DSLAM to handle both normal voice services and ADSL
at the same time. The DSLAM might send a 64kbit/sec stream
to the telephone switch, for integration into the
voice network. That would mean running T-1 or bandwidth
equivalents, from the DSLAM to the telephone switch.
This diagram might make more sense.
8 megabit/sec data
phone ----- Filter ---- DSLAM --------------------- IP networking ---- data_network
0-4KHz | |
| |
ADSL --------+ +--- 64kbit/sec voice --- telephone_switch --- voice_network
>4KHz in digital form
To get ADSL, would mean connecting the voice line, to the DSLAM,
instead of to a universal line card on a telephone switch. The DSLAM
could already have high speed data connections to the telephone
switch, so as customers are moved to the DSLAM, no extra work
needs to be done between the DSLAM and the telephone switch.
(It doesn't make sense to move all the customers over to the
DSLAM, because that would cost too much.)
It's going to be something along those lines. The copper line
between you and the phone company, has better possibilities, than
the rest of the voice network, in terms of carrying things. And
that means connecting a special box to the line (DSLAM), to
make ADSL possible.
Paul
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