Cellular techniques

Speaking of cellular technique it’s well to begin with the first wireless system called IMTS (Improved Mobile Telephone Service). It was put into service for cars in 1946. This system was unsophisticated: there was a tall transmitter tower. It was set up near the center of a metropolitan area. The tower had an antenna. Several channels were transmitted & received from the antenna. Unfortunately, the number of channels did not satisfy the need. The metropolitan area grew, and it was a problem to transmit and receive signals.

The solution to this problem was cellular radio. Metropolitan areas were divided into cells. Each cell had a few miles in diameter. Each cell operated on its own set of frequencies (send and receive). Two characteristics of cellular systems were important:

• first, the systems controlled handoff

• second, the systems were able to locate subscribers by paging them in each of cells.

The first transmission technology used between cars and cells was analog by nature. The analog scheme was called Frequency Division Multiple Access (FDMA). Then a digital transmission scheme appeared named Time Division Multiple Access (TDMA). Now a special spread-spectrum scheme is working in the world. It is Code Division Multiple Access (CDMA).

Computer systems

There are many different computers in the world today. Computers are operating at the bank, at home, at school. In principle there are two general types of computers: general-purpose computers and special-purpose computers.

But what is a computer? Simply put, a computer is a device that processes raw data into useful information. A computer has three basic characteristics:

• a computer is completely electronic

• a computer can remember information

• a computer is programmable.

General-purpose computers can be subdivided into four classes: micro-, mini-computers, mainframes and supercomputers. Microcomputers (or personal computers) are the smallest and are designed for a single user.

Minicomputers are larger than microcomputers (up to the size of a refrigerator). Mainframes can take up a whole room. They are often used in large organizations and institutions. Supercomputers are the most sophisticated computers. They process the huge amounts of data with a high speed.

The first micros were sold to computer hobbyists in 1975. In 1977, Apple entered the market with the Apple II, and in 1981 IBM joined the race.

Modern computers can be used for all kinds of tasks. We can use them to write papers, perform mathematical computations, communicate with friends, play games, buy tickets.

All computers consist of hardware and software. Hardware is the computer itself plus other physical devices: a keyboard, a mouse, a scanner, a modem and so on. Software is a program, data and you, the user. A computer manipulates data forming the computing cycle. It has got four processes that can be described by the verbs:

• to input

• to process

• to output

• to store.

All microcomputers use discs to store data magnetically.

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