Fundamentals of Communication and Computing Technology

The Telegraph: Distant Communication System

The telegraph is an electrically operated device or system for distant communication (the first ever invented) by means of visible or audible signals.

The method used throughout most of the world is based in large part on the mid-19th-century work of Samuel F.B. Morse. It utilizes an electric circuit set up customarily by using a single overhead wire and employing the earth as the other conductor to complete the circuit.

In the telegraph's simplest form, an electromagnet in the receiver is activated by alternately making and breaking the circuit. Reception by sound, with the Morse Code signals received as audible clicks, is the basis for a low-cost, reliable method of signaling.

Modern telegraph messages are sent not only via wires and cables, but also by means such as:

• Radio waves
• Microwaves
• Communication satellites

Telex is a telegraphy system that transmits and receives messages in printed form. Today, the telegraph is less widely used, having been supplanted by telephones, facsimile machines, and electronic mail.

Cellular Telephony and Radio Systems

A cellular telephone or cellular radio is a telecommunications system in which a portable or mobile radio transmitter and receiver, or “telephone,” is linked via microwave radio frequencies to base transmitter and receiver stations. These stations connect the user to a conventional telephone network.

How Cellular Systems Work

The geographic region served by a cellular system is subdivided into areas called cells. Key characteristics of a cell include:

• Each cell has a central base station.
• Each cell uses two sets of assigned transmission frequencies: one set for the base station and the other for mobile telephones.

To prevent radio interference, each cell uses frequencies different from those used by its surrounding cells. However, cells sufficiently distant from each other can reuse the same frequencies.

When a mobile telephone leaves one cell and enters another, the telephone call is transferred from one base station and set of transmission frequencies to the next using a computerized switching system.

The first cellular telephone system began operation in Tokyo in 1979, and the first U.S. system began operation in 1983 in Chicago.

The Computer: Arithmetic and Logical Operations

A computer is a device capable of performing a series of arithmetic or logical operations. It is distinguished from a calculating machine, such as an abacus or electronic calculator, by its ability to:

• Store a computer program (allowing it to repeat operations and make logical decisions).
• Store and retrieve data without human intervention.

Analog vs. Digital Computers

Computers are generally classed as analog or digital:

An analog computer operates on continuously varying data. It represents data as physical quantities and operates on the data by manipulating these quantities. In complex analog computers, continuously varying data are converted into varying electrical quantities, and the relationship of the data is determined by establishing an equivalent relationship, or analog, among the electrical quantities. Although analog computers are commonly found in forms such as speedometers and watt-hour meters, they have largely become obsolete for general-purpose mathematical computations and data storage, having been replaced by digital computers.

A digital computer performs operations on discrete data.

Data Representation in Digital Systems

Within a digital computer, data are expressed in binary notation (a series of “on-off” conditions that represent the digits “1” and “0”).

A series of eight consecutive binary digits, or bits, is called a byte. A byte allows 256 “on-off” combinations, meaning each byte can represent one of up to 256 alphanumeric characters.

Arithmetic and comparative operations can be performed on data represented in this way, and the result stored for later use. Digital computers are essential for:

• Reservation systems
• Scientific investigation
• Data-processing applications