A 5 6-Kbps Primer

The x2 and K56flex technologies use similar techniques to capitalize on the increasingly digital nature of the Public Switched Telephone Network (PSTN). Both also boost the speed limit for downloads to 56 Kbps.

For two decades, the phone companies have been gradually replacing portions of their analog networks with digital circuits. In a typical phone-network connection, the only analog portion remaining is from your home to the telephone company's central office. Your Internet service provider (ISP) probably has its servers connected digitally to the PSTN.

A V.34 mode m generates an analog signal that the PSTN digitizes by sampling 8000 times per second. At the receiving end, the data is converted back into an analog wave form. The difference between the original signal and the reconstructed signal is called quantization noise, a factor that limits the bandwidth of the line to around 35 Kbps.

Instead of generating an analog signal and allowing the PSTN to digitize it, a 56-Kbps modem bypasses initial sampling and generates samples directly into the PSTN. No A/D conversion means no quantization errors and, theoretically, a 64-Kbps communications channel.

However, other constraints conspire to limit bandwidth. The D/A conversion at your local telephone office, for example, is designed to reconstruct voice, not data. Some pairs of digital values produce analog tones that are too closely spaced to distinguish on noisy lines. As a result, a 56-Kbps modem encoder uses only a subset of the 256 available digital values -- 128 on the most robust connection. Finally, an FCC limitation on the amount of power that your ISP's digital modem can pump into the phone lines further limits the possible speed to about 53 Kbps.

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An x2 D/A Connection

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Transferring digital data directly to the PSTN allows x2 connection speeds.