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analog signals to digital signals, and vice versa. The term modem is short for modulator/demodulator, a description of transforming the signals. Telephone wires transfer data via analog signals that continuously change voltages on a wire. Computers hate analog signals. Instead, they need digital signals, voltages that are either on or off, meaning the wire has voltage present or it does not. Computers, being binary by nature, use only two states of voltage: zero volts and positive volts. Modems take analog signals from telephone lines and turn them into digital signals that the PC can understand (Figure 25-7). Modems also take digital signals from the PC and convert them into analog signals for the outgoing telephone line.

Figure 25-7 Modem converting analog signal to digital signal

A modem does what is called serial communication: It transmits data as a series of individual ones and zeroes. The CPU can’t process data this way. It needs parallel communication, transmitting and receiving data in discrete 8-bit chunks (Figure 25-8). The individual serial bits of data are converted into 8-bit parallel data that the PC can understand through the universal asynchronous receiver/transmitter (UART) chip (Figure 25-9).

Figure 25-8 CPUs can’t read serial data.

Figure 25-9 The UART chip converts serial data to parallel data that the CPU can read.

There are many types of UARTs, each with different functions. All serial communication devices are really little more than UARTs. External modems can convert analog signals to digital ones and vice versa, but they must rely on the serial ports to which they’re connected for the job of converting between serial and parallel data (Figure 25-10). Internal modems can handle both jobs because they have their own UART built in (Figure 25-11).

Phone lines have a speed based on a unit called a baud, which is one cycle per second. The fastest rate that a phone line can achieve is 2,400 baud. Modems can pack multiple bits of data into each baud; a 33.6 kilobits per second (Kbps) modem, for example, packs 14 bits into every baud: 2,400 × 14 = 33.6 Kbps. Thus, it is technically incorrect to say, “I have a 56 K baud modem.” The correct statement is, “I have a 56 Kbps modem.” But don’t bother; people have used the term “baud” instead of bits per second (bps) so often for so long that the terms have become functionally synonymous.

Modern Modem Standards: V.90 versus V.92 The fastest data transfer speed a modem can handle is based on its implementation of one of the international standards for modem technology: the V standards. Set by the International Telecommunication Union (ITU), the current top standards are V.90 and V.92. Both standards offer download speeds of just a hair under 56 Kbps, but they differ in upload speeds: up to 33.6 Kbps for V.90, and up to 48 Kbps for V.92 modems. To get anywhere near the top speeds of a V.90 or V.92 modem requires a comparable modem installed on the other line and connecting telephone lines in excellent condition. In practice, you’ll rarely get faster throughput than about 48 Kbps for downloads and 28 Kbps for uploads.

Figure 25-10 An external modem uses the PC’s serial port.

Figure 25-11 An internal modem has UART built in.

Flow Control (Handshaking) Flow control, also known as handshaking, is the process by which two serial devices verify a conversation. Imagine people talking on a CB radio. When one finishes speaking, he will say “over.” That way the person listening can be sure that the sender is finished speaking before she starts. Each side of the conversation is verified. During a file transfer, two distinct conversations take place that require flow control: local (between modem and COM port) and end-to-end (between modems).

The modems themselves handle end-to-end flow control. PCs can do local flow control between the modem and COM port in two ways: hardware and software. Hardware flow control employs extra wires in the serial connection between the modem and the COM port to let one device tell the other that it is ready to send