Communications

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User often want to send messages from one network to another or to computers on the other side of the world. This can be done using a variety of communication links. Normal copper telephone wires can be used over short distances but shielded cable called coaxial cable or coax can be used for longer distances. However, coax has largely been replaced by even faster cable known as fibre optics or fibre optic cable. (Note that the American spelling of fibre is fiber.) Fibre optic cable uses glass fibres to conduct a beam of laser light. To transmit signals long distances around the world satellites are often used. The signal is transmitted and received by earth-satellite stations positioned at suitable locations over the earth’s surface. Microwave transmissions are another means of transmitting signals from one microwave station to another.

An increasingly popular way of communicating using computers is voicemail. In this system, spoken messages are sent to a server computer where they are stored in areas called voice mailboxes. When the user connects to their mailbox they can listen to the stored messages.

Video conferencing is a more advanced form of communication by computer, which enables meetings to take place over long distances. Video cameras are used at each end of the communications link so that the participants can see and hear each other. The end of the link nearest a user is called Near End and the end of the link furthest from the user is called Far End. Each user has a keypad that enables them to control features of the system, for example to make or end a call, adjust the sound volume, and zoom the camera in to get close-up views, or zoom it back for a broader perspective. An additional display feature, called picture-in-picture, allows one image to be displayed inside another image.

When a piece of text is scanned using a scanner input device, an image of the text is input to the computer and displayed on the screen. It is not possible to change or edit the displayed text using a word processor, however, because the image is graphical, not digital. An optical character recognition (OCR) program must be used to convert the text image into digital text characters that can be edited. To identify a scanned text character, the software compares the caracter image to stored data about the shape of standard characters. It is easier for the OCR software to recognize the scanned characters if they are printed using block capitals and are not too small. If a text is written in ‘joined-up’ writing, it is more difficult for the OCR to identify because there are extra lines and loops between the letters.

3 Answer the following questions:

1 In what way can users send messages from one network to another or to computers on the other side of the world?

2 What means are usually used to send messages over short and long distances?

3 What has coax largely been replaced by?

4 What are the world satellites used for?

5 In what way does the voice system operate?

6 What is the most advanced form of communication?

Дата добавления: 2014-12-23