Modems （Modulator/Demodulator） are widely used to transmit and receive digital

　　data using analog modulation over the Plain Old Telephone Service （POTS） network

　　as well as private lines. Although the data to be transmitted is digital， the telephone

　　channel is designed to carry voice signals having a bandwidth of approximately 300

　　to 3300Hz. The telephone transmission channel suffers from delay distortion， noise，

　　crosstalk， impedance mismatches， near-end and far-end echoes， and other

　　imperfections. While certain levels of these signal degradations are perfectly

　　acceptable for voice communication， they can cause high error rates in digital data

　　transmission. The fundamental purpose of the transmitter portion of the modem is

　　to prepare the digital data for transmission over the analog voice line. The purpose

　　of the receiver portion of the modem is to receive the signal which contains the

　　analog representation of the data ， and reconstruct the original digital data at an

　　acceptable error rate. High performance modems make use of digital techniques to

　　perform such functions as modulation， demodulation， error detection and correction，

　　equalization， and echo cancellation.

　　A block diagram of an ordinary telephone channel （often referred to as plain old

　　telephone service – or POTS） is shown in Figure 9.1. Most voiceband telephone

　　connections involve several connections through the telephone network. The 2-wire

　　twisted pair subscriber line available at most sites is generally converted to a 4-wire

　　signal at the telephone central office： two wires for transmit， and two wires for

　　receive. The signal is converted back to a 2-wire signal at the far-end subscriber

　　line. The 2- to 4-wire interface is implemented with a circuit called a hybrid. The

　　hybrid intentionally inserts impedance mismatches to prevent oscillations on the 4-

　　wire trunk line. The mismatch forces a portion of the transmitted signal to be

　　reflected or echoed back to the transmitter. This echo can corrupt data the

　　transmitter receives from the far-end modem.

　　Half-duplex modems are capable of passing signals in either direction on a 2-wire

　　line， but not simultaneously. Full-duplex modems operate on a 2-wire line and can

　　transmit and receive data simultaneously. Full-duplex operation requires the ability

　　to separate a receive signal from the reflection （echo） of the transmitted signal. This

　　is accomplished by assigning the signals in the two directions different frequency

　　bands separated by filtering， or by echo cancelling in which a locally synthesized

　　replica of the reflected transmitted signal is subtracted from the composite receive

　　signa