Canon fax basic 2000 Manuale Di Servizio

2–3

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GENERAL DESCRIPTION OF A FACSIMILE

1.3

Facsimile Groups

ITU-T recommendations classify facsimiles into groups called “G3” and

“G4.” G3 facsimiles enable the transmission of

A4-size type docu-

ments in about one minute on a public telephone network. G4 facsimiles,

as a general rule, enable the error-free reception of documents on a public

digital line. Both G3 and G4 facsimiles handle image data as digital sig-

nals. Previously, there also existed the G1 and G2 facsimile groups that

handled picture data as analog signals. However, these two groups have

currently been deleted from the ITU-T recommendations.

These days, mention the word “facsimile” and you are generally referring

to a G3 facsimile whose use has spread to households. G4 facsimiles are

capable of transmitting pictures faster and at higher quality than a G3 fac-

simile. Though they are still expensive, their popularity is expected to gain

in the future.

1.4

Telephone Lines

A facsimile is a machine that uses telephone lines to transmit images. So,

we must understand telephone lines in order to understand facsimiles.

There are various kinds of telephone lines: analog lines, digital lines and

Facsimile communication network services.

The telephone line that we use most of the time is an analog line called a

PSTN (Public Switched Telephone Network) which has been designed for

transmitting speech. The information that a PSTN is capable of transmit-

ting are analog signals that are obtained by converting changes in current

or voltage as they are to waveforms. When image data is transmitted on a

G3 facsimile, it is converted to analog signals before it is transmitted.

Analog lines have spread throughout the world to the point that any simple

mention of telephone lines generally refers to analog lines.

ISDN (Integrated Services Digital Network) is a digital line capable of

sending all various media such as speech, documents, video and data as 0

and 1 digital signals. G4 facsimiles are capable of transmitting imae data

as it is. When speech is transmitted, the analog signals are converted once

to 0 and 1 digital signals before they are transmitted.

As digital lines are capable of transmitting digital signals as they are, trans-

mission speeds are high and there are no transmission errors. For this rea-

son, digital lines are ideal for personal computer-based communications

and the Internet, and are expected to expand further in the future.