PSDN

PSDN or Public Switched Data Network is simply the technical name for the telephone system in use today.

ISDN

Stands for "Integrated Services Digital Network."  ISDN dates back to 1984, and allows much faster transfer rates than when using modems. Using ISDN, you can transfer 64K or 128K of data per second.

Satellites

Satellite communications makes it possible for you to make a telephone call or access the internet from almost anywhere on the planet.

Fax

A fax machine allows you to transmit printed material over the telephone system.  The sending fax machine scans the page and converts the information into sound.  The receiving fax converts the sound back into an image of the page for printing.  Most modern computers contain a modem that is capable of sending/receiving faxes without the need for a separate, dedicated fax machine.

Telex

Telex is an old system used for sending typed messages via the Telex network, which has largely been replaced by the fax and email systems.

Modem

Modem stands for “MODulate/DEModulate”.  The modem sends information from your computer across the telephone system.  The modem at the other end of the phone line, converts the signal back into a format that can be used by the receiving computer.

Digital vs. analogue

A digital system uses 1 or 0 to transmit data or to represent data.  Thus, a digital clock will display whole seconds, whole minutes and whole hours.

An analogue system, such as a traditional clock, does not use multiples of 1 or 0, but rather uses the full range of numbers, including fractions.  In this way, an analogue clock can display fractions of a second.

Baud rate

The baud rate tells you how fast a modem can send/receive data.  Most modern modems have a maximum baud rate of 56 Kilobits per second (Kb/sec).