Analog Modems

Analog modems allow two computers to send and receive a serial stream of bits, with no physical changes required on the typical analog local loop between a residence and the telco CO. Because the switch in the CO expects to send and receive analog voice signals over the local loop, modems simply send an analog signal to the PSTN and expect to receive an analog signal from the PSTN. However, that analog signal represents some bits that the computer needs to send to another computer, instead of voice created by a human speaker. Similar in concept to a phone converting sound waves into an analog electrical signal, a modem converts a string of binary digits on a computer into a representative analog electrical signal.

Modems encode a binary 0 or 1 onto the analog signal by varying the frequency, amplitude, or phase. Changing the analog signal is referred to as modulation. For instance, one of the earliest standards called for a modem to send an analog signal of 2250 Hz for a binary 1, and 2100 Hz for binary 0. A modem would modulate, or change, between the two frequency levels to imply a binary 1 or 0.

To achieve a particular bit rate, the sending modem would modulate the signal at that rate. For instance, to send 9600 bps, the sending modem would change the signal (as necessary) every 1/9600th of a second. Similarly, the receiving modem would sample the incoming analog signal every 1/9600th of a second, interpreting the signal as a binary 1 or 0. (The process of the receiving end is called demodulation. The term modem is a shortened version of the combination of the two words modulation and demodulation.)

Modems must work over the existing PSTN. Figure 15-5 outlines the basic process.

Figure 15-5 Basic Operation of Modems over PSTN

PSTN

Figure 15-5 Basic Operation of Modems over PSTN

PCM Codec Converts Analog-> Digital

PCM Codec Converts Analog-> Digital

Barney's

Local Loop pc

Barney's

Local Loop pc

Modem Converts Analog-> Digital

Modem Converts Analog-> Digital

First, a circuit (call) must be established. One modem signals the phone number for the call in the same way that a telephone does today, by sending the tones associated with the keys on a telephone keypad. The CO switch interprets these tones, called dual-tone multifrequency (DTMF) tones, just like it would for a voice call.

When the circuit has been established by the telco, the two modems must agree to what modem standard they will use. As long as the two modems use the same rules for how they perform modulation and demodulation, the modems can communicate. Many modem standards exist, and many modems support several standards. Modems can probe and negotiate to find the best modem standard that both endpoint modems support. These standards are explained briefly and listed later in the chapter.

Note that the PSTN still converts the analog signals to and from PCM using a codec. In effect, the data ping-pongs between different states as it passes through the network:

1. The bits start out stored in digital form on a computer.

2. The bits are converted to an analog signal by the modem.

3. The analog signal is converted into a different digital format by a switch in the PSTN, using a PCM codec.

4. The CO switch near the receiving end using a PCM codec to convert back to an analog signal.

5. The receiving modem converts the incoming analog signal to the correct set of bits.

Modems work well and have been around for a long time, so the conversion steps do not pose a problem.

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