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TEXT A

COMMUNICATIONS NEEDS

Today, computer communications serve a variety of needs, but most often we use communications to access data that is stored on another computer or to send data to another computer. For example, you may want to access data that is stored as a database of information on a mainframe computer at company headquarters. When you establish communications with another computer, you download data (transfer data from a remote computer to your computer), or you can upload data (transfer data from your computer to a remote computer).

Computer-to-Computer Communications

One of the most common communications uses is the exchange of data between two computer systems. The communication can be between two microcomputers.

Terminal-to Host Communications

Often a communications channel will be permanently established to access data that is stored on a host computer. In such a case, you might communicate with the host computer using a microcomputer or using a terminal (which includes a keyboard and a display monitor, but does not include any data processing or storage capability). Although a terminal can be purchased at low cost, with the price of microcomputers constantly decreasing, many terminals are being replaced with these small personal computers that can be used for other tasks in addition to communications.

13. Ответьте на вопросы:

1. What do we use communications for?

2. What does a terminal includes?

3. What can replace a terminal?

4. How can we access data that is stored on another computer?

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TEXT B

COMMUNICATIONS CHANNELS

If data is to be transmitted, a pathway or channel is required. The communications channel is the link or line through which the data is transmitted. There are three primary types of communications channels in use today— wire and cable, microwave, and satellite.

Wire and Cable

A channel makes use of a variety of wire and cable media types including twisted pair wire, coaxial cable, or fiber-optic cables. Twisted-pair wire is simply two strands of copper wire and is widely used in communications systems, including the phone system. Communications channels that require higher speeds of transmission use coaxial cable or fiber-optic cable.

Microwave

Unlike transmissions that use the various types of wire media, microwave is sent through the air in the form of electromagnetic waves. One problem with microwave transmissions is that they must be sent in a straight line. Therefore, microwave transmission positions must be established in direct line-of-sight configurations. Because the surface of the earth is curved, microwave relay stations must be atop high towers a few dozen miles apart.

Satellite

When data is transmitted via a satellite communications system, the transmission is relayed by a satellite that is placed in orbit 22,300 miles above the earth’s surface. At that altitude, the satellite can rotate at the same speed as the earth, maintaining a constant position above one point on the earth’s surface. Equipment aboard the satellite receives the transmission, amplifies it, and retransmits it back to earth. Although these systems are very expensive to put in place, the amount of information that can be transmitted and the fact that a set of properly placed satellites can almost instantly transmit to any place on earth, is resulting in constant expansion of satellite-based communications.