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It's not just a matter of the Shannon-Hartley limit, although that's a big one. You will also run into real world constraints (which you have to take seriously, as an engineer, instead of a theoretician) with the existing telephone infrastructure, the so-called "plain old telephone system". Because of how the DS-0 line cards work down at your telco central office and their use of the Nyquist sampling criteria, the most you can get out of an --unmodified-- analog line that gets converted into a digital signal at the phone company is 64000 bits per second (64 Kbps). Because of things like bit stealing, and an actual audio bandwidth closer to 3.7 KHz instead of 4 KHz, realistically, the best you can hope for is 56 Kbps, which has been accomplished. The next step is to change the existing equipment at the customer end, and at the central office end, to use xDSL, and then, you can modulate a broadband, high speed carrier on the existing copper wiring. You should consult technical documents that the telephone
industry uses, such as Bellcore SR-TSV-002275, "BOC Notes on the LEC Networks", 1994, which goes into hundreds of pages of technical detail about how the phone system actually works between the central office and the customer, and all the limitations. This stuff is a lot more complex than the "How Sparky makes the telephone ring" comic book that your understanding of the telephone system is based on. |
