2 Complete the following simple process description of the function of a terminal repeater station:

- selection of modulation frequency;

- placing of signal on frequency axis;

- ______________________________;

- ______________________________;

- ______________________________;

- ______________________________.

3 Complete the following tree diagram:

Transmission systems

Wire & cable Microwave

Open-wire

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

For, to, that, which, plug, by, media, ways, kinds, both, equipment, transmission, copper, coaxial, telephone, optic, consist, cable, culminates.

The two ____ of connecting microcomputers with each other and with other ____ are through the ____ and through the air. Specifically, five ____ of technology are used ___ transmit data. These are ____ lines (twisted pair), ____ cable, fiber-____ cable, microwave and satellite.

Most telephone lines ___ you see strung on poles ____ of cables made up hundreds of _____ wires, called twisted pairs. A single twisted pair ____ in a wall jack into ___ you can ___ you phone. Telephone lines have been the standard _____ medium ___ years for ____ voice and data. However, they are now being phased out ___ more technically advanced and reliable____.

Text 2

Why optical fibers are useful and how they work

Fiber-optic communications, in which electrical signals are converted into pulses of light that are squirted along very thin glass pipes, have several advantages over conventional copper cables:

• The signal fades (attenuates) less so repeaters can be more widely spaced.

• Fibers can carry a lot of information.

• Most fibers are made from silica, which is very cheap. At present a kilometer costs £1,000 or more but when they are produced in bulk, this figure should come down to as little as £25.

• Fibers are immune to interference and crosstalk.

• The material is lightweight and flexible. A 500 m fiber weighs about 25 kg; a coaxial cable of the same length weighs 5 tons.

Although ordinary glass can only support effective light transmission for a few tens of meters, optical fibers, which are made of pure glass, can carry light signals for up to 50 km without amplification. Silica glass fiber has two components–the highly transparent 'core' at the centre and the opaque surrounding called 'cladding'. Three main types of fiber are possible: stepped-index monomode, stepped-index multimode and graded-index multimode. Stepped-index fibers proceed from transparency to opaqueness in straight, defined bands while graded-index fibers go progressively from transparency to opaqueness. Monomode fiber has a very narrow core–such a fiber can support just one 'guided electromagnetic mode'. In a multimode fiber, up to 500 light rays, each of slightly different wavelengths, pass through.

Telecommunication engineers want to keep reflections in the core to a minimum so that pulses of light do not overlap and make messages unintelligible. In the early days of fibers, engineers used graded-index multimode fiber, rather than stepped-index, to keep overlapping to a minimum. But it is more satisfactory to use monomode fiber, in which pulse spreading does not occur.

Light sources can be either light-emitting diodes (LEDs) or lasers. The first give less powerful signals but are considerably cheaper and last longer. Lasers, however, produce light of a closely-defined wavelength and so are suitable for monomode fibers.

There are also two types of photo detectors – the silicon pin photodiode and the silicon avalanche photodiode. The avalanche devices are generally more sensitive but they are also more expensive.

EXERCISES