Allied Data Technologies TRON SECURE 33.6, Руководство пользователя

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Sending computer programs instead of text is a little more complicated. After all, in a program
each combination of data-bits can occur and therefore also a combination that represents a
control character. For each (type of) computer, the meanings of these combinations can be
different. Most of the time, sending programs between different kinds of computers therefore
requires a conversion at the sender and the receiver. If a conversion is used, it is even possible
to send programs as text. Among others, this is done with Prestel and Teletel.
1.3 Function of the modem
We have seen that computers can exchange information through the serial port if both sides stick
to a number of agreements. With a direct connection of two serial ports, it is only possible to
bridge a limited distance (about 50 feet or 15 meters).
The telephone network is used to bridge a greater distance. It is an existing infrastructure by
which connections can be established all over the world.
However, the telephone network is not suitable to process digital signals. Only analog signals
(voice) can be processed cor rectly.
Technically this can be solved as follows: the sender converts (modulates) the digital signals of
the serial port into analog signals that are suitable for the telephone line. The receiver converts
(demodulates) these analog signals again into digital ones and sends these to its serial port.
We call the device that can carry out these conversions a modem (MOdulator / DEModulator).
A modem’s function is therefore the conversion of digital (RS232C) into analog signals and vice
versa.
However, with communication through a telephone line, a number of additional agreements are
necessary:
1. It must be possible to establish a connection automatically (see paragraph 1.4)
2. The analog signal has to meet international agreements (see paragraph 1.5)
1.4 Setting up a connection
Establishing a data connection through the telephone line can be done in two different ways.
1. The easiest way is when first a normal (voice) connection is established and
subsequently is switched to a data connection (communication through the modems)
by hand.
2. A faster way is when the modem itself is capable to establish the connection (dialling a
number) and the other modem can answer independently. This is called auto dial and
auto answer respectively.
Through a communication program, the auto dial modem receives a command to dial a telephone
number. When the other modem is in the auto answer mode, this one detects that the bell rings
and it picks up the receiver, after which a data connection is established.
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