21.2 56k Modems (V.90, V.92)
The "modulation" method used for speeds above 33.6k is entirely different than the common phase-amplitude modulation used at 33.6k and below. Since ordinary telephone calls are converted to digital signals at the local offices of the telephone company, the fastest speed that you can send digital data by an ordinary telephone call is the same speed that the telephone company uses over its digital portion of its network (for a phone call). What is this speed? Well, it's close to 64kbps. It's sometimes 64k and sometimes less if bits are "stolen" for signalling purposes. If the phone Co. knows that the link is not for voice, bits may not get stolen. The case of 64k will be presented and then it will be explained why the actual speed is lower (56k or less --often significantly less).
Thus 64k is the absolute top speed possible (not counting date compression) for an ordinary telephone call using the digital portion of the circuit that was designed to send digital encodings of the human voice. In order to use 64k, the modems need to either have direct access to the digital portion of the circuit or be able to determine the exact digital signal that generated a received analog signal (and conversely). This task is far too error prone if both sides of a telephone call have only an analog interface to the telephone company. But if one side has a digital interface, then it's possible (in one direction for V.90 and in both directions for V.92). Thus if your ISP has a digital interface to the phone company, the ISP may send out a certain digital signal over the phone lines toward your PC. The digital signal from the ISP gets converted to analog at the local telephone office near your PC's location (perhaps near your home). Then it's your modem's task to try to figure out exactly what that digital signal was. If it could do this, then transmission at 64k (the speed of the telephone company's digital signal) is possible in this direction.
What method does the telephone company use to digitally encode analog signals? It uses a method of sampling the amplitude of the analog signal at a rate of 8000 samples per second. Each sample amplitude is encoded as a 8-bit byte. (Note: 8 x 8000 = 64k) This is called "Pulse Code Modulation" = PCM. These bytes are then sent digitally on the telephone company's digital circuits where many calls share a single circuit using a time-sharing scheme known as "time division multiplexing". Then finally at a local telephone office near your home, the digital signal is de-multiplexed resulting in the same digital signal as was originally created by PCM. Then this signal is converted back to analog and sent to your home. This analog to digital conversion (and conversely) is done by telephone company hardware called a "codec" (coder/decoder). Each PCM 8-bit byte creates a certain amplitude of the analog signal. Your modem's task is to determine just what that PCM 8-bit byte was, based on the analog amplitude it detects.
This was originally called is called "modulus conversion". It's now often called "PCM"-something since its just like encoding/decoding PCM but with the added problem of sampling at the precise time that the codec generated the analog voltage from the digital PCM code.
In order to determine the digital codes the telephone Co. used to create the analog signal, the modem must sample this analog signal amplitude at exactly the same points in time the phone Co. did when it created the analog signal. To do this an 8kHz clock timing signal is generated with help from a residual 4kHz signal on the analog phone line. The creation of amplitudes to go out to your home/office at 8k amplitudes/sec sort of creates a 4kHz signal. Suppose every other amplitude was of opposite polarity. Then there would be a 4kHz sine-like wave created. Each amplitude is in a sense a 8-bit symbol and when to sample amplitudes is known as "symbol timing". The modem's task is to insure that it's 8kHz clock runs at precisely twice the speed of the 4kHz signal (which could drift slightly off 4kHz) and that the modem's clock is synchronized with that used by the telephone company's codec. The actual
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