Error Rate Definition/Meaning:
1. of a communication channel. The frequency with which errors or
noise are introduced into the channel. Error rate may be measured in terms of
erroneous bits received per bits transmitted. For example, one or two errors per
100000 bits might be a typical rate for a narrowband point-to-point line. The
distribution of errors is usually nonuniform: errors tend to come in bursts (see
burst error). Thus the
error rate of a channel may be specified in terms of percentage of error-free
seconds. Frequently an error rate is expressed as a negative power of ten: an
error rate of one bit per 100000 would be expressed as an error rate of 10-5.
Another method of presenting error rate is to consider the errors as the result
of adding the data signal to an underlying error signal. The ratio of the
strengths of the two signals - the signal-to-noise ratio -is expressed in
2. of a magnetic tape subsystem. A measurement of the proportion of
errors occurring in data transfers to or from magnetic tape. It is usually
expressed in terms of the average number of bytes or bits of data transferred
per error, e.g. 1 error per 109 bytes, although it can also be useful to express
the rate as the average time between errors for typical usage of the subsystem,
e.g. 1 undetected error in 6 weeks at 10% duty cycle.
The error rates most frequently specified relate to the following.
(or recoverable) read error occurs during reading and can be recovered by the
error recovery procedure prescribed for the magnetic tape subsystem (see error management).
A typical figure for 1/2² tape is 1 in 109 bytes. Where the tape
format provides sufficient redundancy to allow some error to be recovered
on-the-fly, i.e. without stopping the tape and re-reading the block, it is
necessary to define also the raw error rate, which is the rate that would be
perceived if on-the-fly error recovery was not applied.
A permanent (or irrecoverable) read error cannot be recovered by the
prescribed error recovery procedure. A typical figure is 1 in 1011 bytes.
A transient (or recoverable) write error occurs during writing and can be
recovered by the error recovery procedure prescribed. It is desirable, though
not easy, to distinguish two components of this error rate: errors attributable
to flaws in the media and to failings of the device (one reason for the
difficulty is that these tend to interact). A typical figure, excluding media
errors, is 1 in 108 bytes.
A permanent (or irrecoverable) write error cannot be recovered by the
prescribed procedure. Again it is necessary to distinguish between media flaws
and device errors: it is now usual not to give a figure for the latter but to
regard each occurrence as a fault to be accounted for in the failure rate of the
An undetected error is an error that is not detected by the magnetic tape
subsystem, presumably because of some inadequacy in the error check facilities
defined by the format or in their implementation, or because of errors occurring
outside the ambit of these facilities. A typical figure is
1 in 1013 bytes.