8 The signal indicates a suitable frequency channel for the call.
9 The cellphone tunes to the allocated channel.
The cellphone user hears the ringing tone.
The call is answered.
The user can speak and listen using the cellphone, as with a normal telephone.
The call is finished.
The cellphone signals to the base station.
The cellphone sends a short burst of signal at 8 kHz.
If the code is not received,...
... the cellphone abandons the call.
... the cellphone tries again later.
Tuning-in
Task 1 Study this diagram, which shows how a document can be sent from one fax
machine to another.
Now try to answer these questions.
How are fax machines linked?
How is the image transferred from the document to a microprocessor within the fax?
In what form is information sent down the lines?
In what form is information fed to the thermal head in the receiving fax?
How does the thermal head create images on paper?
Task 2 Read Text 1 to check your answers to Task 1 and to find the answers to any
questions you were unable to answer from the diagram.
Text 1
A document is fed into the fax machine, face down.
It passes over a fluorescent tube which bounces light off the paper, reflecting the image on to a lens.
The lens passes the light on to a microprocessor which breaks the image down to a series of horizontal lines (7.7 lines to a millimetre).
Another microprocessor converts each line to a series of black and white dots, which are then coded, usually as '0' for black and '1' for white (binary code). This is in turn converted into digital information.
A third microprocessor (or modem) converts this information into signals, called analogue tones, which can be sent down telephone lines.
At the receiving machine, the analogue tones are converted back into binary signals and fed into a thermal head.
The thermal head is a mechanism containing a line of dots which heat up or cool down (in blocks of eight) depending on the electrical current supplied to them by the binary code.
The chemically treated paper reacts to heat along this line by forming black dots. As it passes across the thermal head, an image is formed corresponding to the information supplied by the binary code.
Reading Comparing sources
When trying to understand a difficult explanation, it is often useful to look at more than one source. There are a number of reasons for this:
Some sources are easier to understand than others.
By combining information from several sources, we can obtain a more complete explanation.
By reading about the same topic described in different ways, we can improve our understanding of written English and widen our vocabulary.
In this example, we are interested in how fax machines operate.
Task 3 Work in groups, A and B.
Group A: Compare Text 1 on page 154 and Text 2 below. Underline any
points which help you to understand how a fax operates which are only found in Text 1. Compare your answers within your group.
Group B: Compare Text 1 on page 154 and Text 2 below. Underline any
points which help you to understand how a fax operates which are only found in Text 2. Compare your answers within your group.
Text 2
FAX
para
Facsimile machines only came into widespread use in the late 1
1970s when international standards were set by the Comite Consultatif International Telegraphique et Telephonique (CCITT), a body based in France. Before this, machines could only 5 communicate with those made by the same manufacturer.
Since then, facsimile technology has become increasingly 2
sophisticated. The latest machines, which must be linked to a special digital phone line, can send a document to several places at once for the price of one phone call.
10 Facsimile transmission involves sending a document along а з
telephone line and converting the received signals into a reproduction of the original. 'Fax' machines can now send an A4 document, containing images as well as words, in less than a minute.
15 When you feed a document into the machine, a fluorescent lamp 4 reflects the image on to a series of mirrors which reduce its size so that the whole document can be reflected on to a camera lens. The lens can only read the image in black and white. This information is converted, via a microprocessor, into binary information. The
20 machine records black as 0 and white as 1.
Another microprocessor then converts the binary data into digital 5 information, which allows more data to be stored on the microchip. But, because most telephone systems cannot read digital information, this is again changed, via another microprocessor 25 (modem), into analogue tones, or pitches of noise. The first machine transmits these tones to the second.
The receiving machine converts the analogue tones back into 6
digital and then binary information. It sends a signal (in binary code) to the thermal head, or printer. This turns heated elements on 30 or off according to the pattern of 0s and 1s contained in the signal. The pattern of black and white is then printed on to heat-sensitive paper.
Fax machines send information at the rate of 9,600 baud, or bits of i information per second. A few seconds' interference on the phone 35 line can make several lines of a document illegible. If the line is noisy, the sending machine will slow down to reduce the amount of information lost.
Task 4
Which lines in Text 2 contain similar information to the paragraphs in Text 1 ?
Text 1 paragraphs Text 2 lines
1
5
8
Task 5
Work in pairs. Discuss which text contains the best explanation. Which is the easier to understand?
Task 6 Find the references in Text 2 for each of the following:
a body based in France (lines 3-4)
those made by the same manufacturer (line 5)
The latest machines (line 7)
This information is converted (lines 18-19)