On September 10, 1996, Rockwell Semiconductor Systems Shocked The Communications

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Term Paper Title	On September 10, 1996, Rockwell Semiconductor Systems Shocked The Communications
# of Words	5208
# of Pages (250 words per page double spaced)	20.83

On September 10, 1996, Rockwell Semiconductor Systems shocked the communications industry with its announcement of a revolutionary new dial up modem technology for communicating across the Public Switched Telephone Network (PSTN) at rates up to 56 Kbps*. Since that time, Rockwell’s 56Kbps technology has been supported and adopted by most of the communications related companies in the world as the way to communicate at rates up to 56Kbps.

Prior to this announcement, the modem communications industry had convinced itself that communications across the PSTN were limited by Shannon’s Theorem to rates below 35Kbps. Although Rockwell published a white paper describing its 56Kbps technology shortly after the public announcement, and submitted its technology for standardization to both the ITU and the ANSI TR30 committees, there have been continual requests for an in-depth discussion of how
the technology works, with particular emphasis on how this technology gets around the so-called Shannon Limit. This paper
attempts to address these issues.

Modem Fundamentals

Before describing how Rockwell’s 56Kbps technology works, let me first discuss how a traditional analog voice band modem works.

The voice band telephone channel is a bandpass channel, traditionally thought of as operating from about 300 Hz to 3,000 Hz.
Modem modulations, therefore, had to operate within this band. Early modems used tones (e.g., FSK) which fell within this frequency band for communicating data but the information density was not very high (the number of bits per hertz was significantly less than one).
Quadrature amplitude modulation (QAM) was a significant improvement, offering information densities of multiple bits per hertz.

Figure 1: Approximate frequency response of the filters associated with the codec in the line card.

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An ordinary telephone call will go through at least two of these filters, causing significant high frequency rolloff, usually 35 dB or more at 4Khz.

QAM operates by modulating a carrier sine wave signal in both amplitude and phase. Each unique combination of amplitude and phase is known as a "symbol". In the general case, a symbol is defined as an information carrying token which is sent from the transmitter to the receiver.

In the early days of modems, these tokens were called "baud" in honor of the French inventor Emile Baudot who, in 1875, invented a 5 bit code for representing the alphabet. Each 5 bits were a token communicating...

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