CCNC/CCNC Module 1/Networks/Subsection Level CCNC Template18

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Communications between computers rely heavily on the public telephone system. Newer telephone line technologies have improved the standard of communications between networks considerably. The following is a brief description of some of the technologies that are available.


The PSTN or Public Switched Telephone Network refers to the originally telephone network. From a communications perspective it was slow and unreliable. Some of the exchanges on a PSTN may still make use of mechanical switches to route telephone calls. These add additional noise to the line. When lines are noisy, signals have to be resent repeatedly between the source and the destination. The PSTN makes use of analogue technology. Analogue technology uses continuously variable signals. An example of an analogue signal is ordinary speech. Newer digital technologies make use of pulses of fixed magnitude and duration.

In order to improve connections, it is possible to have an analogue leased line. This is a dedicated permanent telephone connection between two computers using the PSTN.

In order to connect a computer to a telephone network, you need a modem. This is an abbreviation for modulator-demodulator. The function of the modem is to convert the digital signals from the computer into an analogue form suitable for transmission on the PSTN.


ISDN or Integrated Services Digital Network is a technological development that is able to make use of the existing PSTN cabling to transmit digital signals.

Technically ISDN is an international standard for the transmission of data, voice and video or normal voice or digital telephone lines. ISDN supports rates of up to 64Kbps. An ISDN connection consists of two lines which can be used independently or together to give a combined rate of 128Kbps.

If you wish to connect a computer to an ISDN line you need a special ISDN modem. This is a different type of modem to the one used with an analogue line. Its purpose, however, is the same, to convert the digital signals of the computer into a form suitable for transmission on an ISDN line.

It is possible to get a dedicated connection between two computers using ISDN. This is called a diginet connection.

The older telephone systems make use of electrical currents transmitted through copper cabling. As electric signals are subject to interference, they are not the ideal method of transmitting data. Newer telephone systems make use of fibre optic cable. In fibre optic technology, light is transmitted along the cable. As light signals are not subject to the same interference problems as electrical signals, fibre optic is a far more efficient and reliable system.


ADSL or Asymmetric Digital Subscriber Lines allow the transmission of high speed digital data over ordinary telephone lines using a modulation technology called DMT or Discrete MultiTone. Ideally, fibre-optic is the ideal medium for high speed digital transmission. As the installation of fibre-optic is expensive, ADSL provides solution until copper cable is replaced by fibre-optic.