redundancy

1. parallel The provision of multiple interchangeable

components to perform a single function in order to cope with

failures and errors. Redundancy normally applies primarily to

hardware. For example, one might install two or even three

computers to do the same job. There are several ways these

could be used. They could all be active all the time thus

giving extra performance through parallel processing as well

as extra availability; one could be active and the others

simply monitoring its activity so as to be ready to take over

if it failed ("warm standby"); the "spares" could be kept

turned off and only switched on when needed ("cold standby").

Another common form of hardware redundancy is diskmirroring.

Redundancy can also be used to detect and recover from errors,

either in hardware or software. A well known example of this

is the cyclic redundancy check which adds redundant data to

a block in order to detect corruption during storage or

transmission. If the cost of errors is high enough, e.g. in a

safety-critical system, redundancy may be used in both

hardware AND software with three separate computers programmed

by three separate teams and some system to check that they all

produce the same answer, or some kind of majority voting

system.

2. communications The proportion of a message's gross

information content that can be eliminated without losing

essential information.

Technically, redundancy is one minus the ratio of the actual

uncertainty to the maximum uncertainty. This is the fraction

of the structure of the message which is determined not by the

choice of the sender, but rather by the accepted statistical

rules governing the choice of the symbols in question.

[Shannon and Weaver, 1948, p. l3]

[Better explanation?]

(1995-05-09)