User manual

INTRODUCTION 1-1

Chapter 1 INTRODUCTION

The Data Modem you have purchased represents the latest state of the art in data

communication; its comprehensive facilities provide all you will need for profes-

sional data transfer purposes.

In data modem mode, you can achieve active transfer speeds of up to 57 600 bps

and up to 14 400 bps in fax mode.

This handbook, together with the descriptive information provided with your

communications software, gives all the information you need to install and operate

the equipment.

What is a Modem?

The word “Modem” is derived from the terms “MOD-ulator” and “DE-modula-

tor”. Putting it more simply, it is a device which modulates digital information into

an analogue carrier signal (tones) and demodulates the carrier signals which it re-

ceives, changing them back into digital data. This permits the transmission of data

along wires, between data terminal equipment (computers, terminals, etc…).

About this Modem

This modem operates as a full duplex, voice-band modem, where signal transmis-

sions are made in both directions simultaneously and the analogue signals which

are transmitted are in the voice-band of the telephone network - between 300 and

3000 Hz.

Data transmission between modem and terminal unit is in serial form - in other

words, the individual data bits are sent, one after another, along a single transmis-

sion or receiving line. At this stage, a word of explanation regarding synchronous

and asynchronous data transmission. In the synchronous mode, additional syn-

chronisation signals are required, to synchronize the transmission and reception

signals. In the asynchronous mode synchronisation is by means of “start-bits” and

“stop-bits” which mark the beginning and end of each data word. The modem can

dial by itself and also react automatically to incoming calls. The information it

needs in order to dial a telephone number, together with the various conﬁguration

commands, are provided by the respective data terminal equipment via the same

serial interface which is used to send the data. In this mode, the system operates

with the so-called “AT” command set or to V.25bis.

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