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 disk mirroring.
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)
Nearby terms:
Reduced Instruction Set Computer « reduction «
reduction strategy « redundancy » Redundant
Array of Inexpensive Disks » Redundant Arrays of
Independent Disks » Redundant Arrays of Inexpensive
Disks
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