0195136047.pdf

324 DIGITAL BUILDING BLOCKS AND COMPUTER SYSTEMS

Data Transmission and Modems

Data can be transferred between two stations in eitherserialorparalleltransmission. Parallel

data transmission, in which a group of bits moves over several lines at the same time, is used when

the two stations are close to each other (usually within a few meters), as in a computer–printer

configuration. Serial data transmission, in which a stream of bits moves one by one over a single

line, is used over a long distance. Serial data transfer can be eitherasynchronousorsynchronous.

Asynchronous data communication is most commonly applied in low-speed terminals and small

computers. Large-scale integration (LSI) devices known as UARTs (universal asynchronous re-

ceivers/transmitters) are commercially available for asynchronous data transfer. Synchronous data

communication is used for transferring large amounts of data at high speed. USARTs (universal

synchronous/asynchronous receivers/transmitters) are commercially available LSI devices.

Frequency-division multiplexing(FDM) is a technique for data transmission widely used

in telephone, radio, and cable TV systems in which the transmission frequency spectrum (i.e.,

bandwidth) is divided into smaller bands known as subchannels.

Data transmission between two stations can be achieved in eithersimplex, half-duplex,or

full-duplexmode. In a simplex mode, mainly used in radio and TV broadcasts, information travels

only in one direction. This mode is rarely used in data communications. In half-duplex mode, used

by radio communications, information may travel in both directions, but only in one direction at

a time. The transmitter becomes the receiver and vice versa. In a full-duplex mode, information

may travel in both directions simultaneously. This mode, used in telephone systems, adopts two

different carrier frequencies.

Amodem(modulator/demodulator) is an electronic device that takes digital data as a serial

stream of bits and produces a modulated carrier signal as an output. That is to say, the digital

signals are converted to an analog form with a relatively narrow bandwidth. The carrier signal is

then transmitted over the telephone line to a similar modem at the receiving end, where the carrier

signal is demodulated back into its original serial stream of bits, as shown in Figure 6.4.4. The

serial digital data to be transmitted are modulated, filtered, and amplified for analog transmission;

the analog data received at the receiving end are amplified, filtered, and demodulated to produce

serial digital signals.

There are four different types of modems:half-duplex, full-duplex, synchronous,andasyn-

chronous.With half-duplex modems data can be transmitted in only one direction at a time. Full-

duplex modems transmit data in both directions at the same time; one modem is designated as the

originating modem and the other as the answering modem, while transmitting and receiving data

are done at different frequencies. Asynchronous modems are low-data-rate modems transmitting

serial data at a rate of about 1800 bits per second (bps). Synchronous modems are high-data-rate

modems transmitting serial data at a rate of about 10,800 bps.

Modems can also be classified asvoice-bandorwide-bandmodems. Voice-band modems are

low-to-high speed modems designed for use on dial-up, voice-grade, standard telephone lines up to

Modulated signal

Modem Modem

Digital signal Source Destination Digital signal

Figure 6.4.4Input/output signals of a modem.