Modems : Transmission Speed

The speed at which data can be transmitted over a telephone line has increased steadily. Most modern modems can receive and transmit data at up to 14.4KBps (kilobits per second), 28.8KBps or 33.6KBps. 33.6KBps is approximately equivalent to 3 kilobytes per second or 3400 character per seconds.

The very latest modems can receive data at speeds up to 56.6KBps. Such speeds are only possible over very short distances. These modems are most useful for connecting a home computer to the Internet. This is because the telephone number that you have to dial to connect to the Internet via an Internet Service Provider is usually a local one.

Faster transmission speeds are desirable because they :

• Reduce the time a user must wait whilst data is being transmitted
• Reduce the cost of transmitting data. This is because most telephone companies charges depend upon how long a telephone line is used for.

Transmission speeds over a telephone line are much slower than those over a Local Area Network (LAN). A modem operating at 33.6KBps will transmit and receive data over 300 times slower than a slow LAN.

The actual speed at which a particular communication takes place between two computers will depend upon :

• The maximum speed of the transmitting computer's modem.
• The maximum speed of the receiving computer's modem.
• The quality of the telephone line. A poor quality line will result in errors which may mean data has to be resent.

GCSE ICT Companion 04 - (C) P Meakin 2004