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On Sat, 14 Apr 2012, Philip Hudson wrote:
Can't believe nobody's confirmed for Tony that he's spot on about the term "modem" being wrong for *DSL. It's pure marketing-speak, a holdover from pre-broadband days, something the punters understood and was close enough in function. Less sure of my ground now: I think it's correct to call it a codec, which we usually think of as a software library these days, but can be hardware. Gordon would know.
Oh, I don't know... maybe... :) It's a modem (as far as I'm aware).CODEC is enCODed DECoded or COmpressor DE-Compressor... These days essentially a data compression algorithm, (often patented, but that's another matter) used to convert streaming data (typically audio or video) into something using less bits... Often "lossy", but not always.
You can transmist data over copper in several ways - the obvious one is baseband - you put 5V on the wire and it's a digital "1", remove the 5V and it's a digital 0. Ethernet works this way. (sort of). One reason theres a distance limitation. The more wires you have, the more data bits you can send and receive in parallel. 10/100Mb Ethernet use 2 pairs, one in each direction. Gb use 4 pairs, 2 pairs in each direction (Although Gb isn't strictly baseband - google PAM encoding if you want to know the gorey internals)
The opposite of baseband is broadband - a misused name in this terminology (IMO), but essentially a technology that modulates an analogue signal over the copper. In *DSL there are many different frequencies, each frequency carrying a portion of the data in the same way that multiple copper wires can be used to carry more data. The more different frequencies you can squeeze down the wires the more data you can carry. The limitation on the frequencies you can put down the line is dependant of the power output level, the sensitivity of the reciever, the length of the line (and thus the resistance and data loss) and other sources of loss.
1MHz is in the middle of what we call "medium wave" radio - so the output power can't be too high or it will raidate and cause interference not just with other wires running next to yours, but with other equipment.
1MHz is 300m and that's right in the middle of "medium wave" radio transmissions. (those old enough will remember Radio 1 on 247m...)
Which is why a de-tuned "AM" radio is a good way to detect interference on your ADSL ...
Essentially ADSL sits on-top of your telephone wires at frequencies too high to hear so it doesn't interfere with voice. It's also not unknown for ADSL to work when voice doesn't, as the radio frequencies can jump the gap of a bad connection whereas voice often can't.
You'll want to know the difference between baud rate and bit rate next... Gordon -- The Mailing List for the Devon & Cornwall LUG http://mailman.dclug.org.uk/listinfo/list FAQ: http://www.dcglug.org.uk/listfaq