6 Make a brief summary of the text.

Text 3

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Coaxial cables, a high-frequency transmission cable, replace the multiple wires of telephone lines with a single solid copper core. In terms of number of telephone connections, a coaxial cable has 80 times the transmission capacity of twisted pair. Coaxial cable is often used to link parts of a computer system in one building.

In fiber optic cable data is transmitted as pulses of light through tubes of glass. Indeed, a fiber optic tube can be half the diameter of a human hair. Although limited in the distance they can carry information, fiber-optic cables have several advantages. Such cables are immune to electronic interference, which makes them more secure. They are also lighter and less expensive than coaxial cable and are more reliable at transmitting data. They transmit information using beams of light at light speed instead of pulses of electricity, making them faster than copper cable. Fiber-optic cable is rapidly replacing twisted-pair telephone lines.

In microwave communications channel the medium is not a solid substance but rather the air itself. Microwaves are high-frequency radio waves that travel in straight lines through the air. Because the waves cannot bend with the curvature of the earth, they can be transmitted only over short distances. Thus, microwave is a good medium for sending data between buildings in a city or on a large college campus. For longer distances, the waves must be relayed by means of ‘dishes’, or antennas. These can be installed on towers, high buildings, and mountaintops.

It is possible to transmit signals around the globe by bouncing them from a satellite to an earth station and then to another satellite.

Originally designed to carry voice traffic, they are able to carry hundreds of thousand of separate simultaneous calls. These systems are being increasingly adopted to provide for business communications, including the transmission of traffic for voice, facsimile, data and vision. Complex functions are being performed by people using advanced terminals which are intended to be as easy to use as the conventional telephone.

EXERCISES

1 Answer the following questions:

1 What is a coaxial cable?

2 What is the advantage of a coaxial cable over the twisted pair?

3 What is coaxial cable used for?

4 How is data transmitted in fiber optic cable?

5 Why are fiber optic cables replacing twisted pair telephone lines?

6 What is the transmitting medium in microwave communications channels?

7 What is the main disadvantage of microwaves?

8 What allows transmitting signals around the globe?

2 Read the following paragraph. Fill in each blank with a suitable word given below.

Computers, carry, make, like, to, on, as, for, which, who, provide, few, standards, transmitters, mobile, systems, where, with, different, receive.

The new global satellite-communications ___ will offer three kinds of service, ____ may overlap in many ____ kinds of receivers:

Voice. Satellite telephones will be able to ___ calls from anywhere on earth ___ anywhere else. Today’s ___ phones depend ___ earth-bound ____ , whose technical ____ vary from country to country.

Messaging. Satellite messagers have the same global coverage ___ satellite telephones, but ___ text alone, which could be useful ___ those with laptop ____ . Equipped ___ a small screen ___ today’s pagers, satellite messagers will also _____ short messages.

Tracking. Voice and messaging systems will also tell their users ___ they are within a ___ hundred meters. Satellite systems will ___ better positioning information to anyone ___ has a receiver for their signals.