User's Manual

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
professional data transfer purposes. Due to the limitation this equipment
allows you to receive data with up to 54kbps from your Internet Service
Provider (ISP). The maximum transmission speed is 33.6kbps. Fax
transmission and reception is working up to 14.4kbps.
What is a Modem?
The word “Modem” is derived from the terms “MOD-ulator” and “DE-
modulator”. 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 receives, 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
transmissions are made in both directions simultaneously and the analogue
signals which are transmitted are in the voice-band of the telephone network.
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
transmission or receiving line. At this stage, a word of explanation regarding
synchronous and asynchronous data transmission. In the synchronous mode,
additional synchronization signals are required, to synchronize the
transmission and reception signals. In the asynchronous mode
synchronization 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.
INTRODUCTION 1-1