Re: POTS question

Andy wrote:

Andy writes:

I'd like to hear some discussion from people here on the following
question, which has been bothering me for some time.

In the normal POTS ( plain old telephone system ) , in the past it
was noted that the frequency response fell off after about 3 khz. It
was
used only for voice and low speed fax, and any attempts to do more
would
just roll off and phase distort and was doomed to failure,
Now, my dialup can handle 56 Kbaud. This means at least at 26 khz
bandwidth with low phase distortion, and probly more, depending on how
low the BER needs to be....

So, have the POTS lines been upgraded or was the system capable
of doing this all along ???????????


Now, I understand M-ary signalling, data modems, and multiphase
encoding.... However, none of that seems to account for a 3khz line
suddenly being able to handle a 26khz min bandwidth. I expect to
hear several different explanations, and I'm interested in all of them.
The gurus of data , and bandwidth, and Fourier series are all asked to
give the explanation their own particular spin on this..

Thank a lot , guys...... I am always interested in learning new ways

to account for things that I "thought" I understood....

Andy in Eureka, Texas (retired engineer )


The signal goes from your phone to a line card at the switch. The line card converts it to digital at an 8kHz sampling rate. Each digital sample is 8 bits, but the 8th bit gets stolen periodically for signaling, leaving 7 reliable bits.

AFAIK the 56kBaud is only downstream to you: upstream is at a lower rate. Your ISP talks to a T1 line with straight digital, and your modem is smart enough to figure out the inverse of the reconstruction filter used at the line card, as well as the effects of the line from the line card to you (get far enough out in the boonies and this won't work anymore, by the way). Your modem 'dealiases' it, decompresses it, synchronizes to the 8kHz, and happily samples straight binary.

And there you are. It's really magic, but they put it in a brushed aluminum case so you'd think it was technology.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html
.

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