ERRORS IN DATA TRANSMISSION
and other code elements, such as "start" and "stop" elements, parity bits, and other
"overhead" bits, were removed from the transmitted bit stream. In other words, the channel
would be 100% efficient if all bits transmitted were information bits. Obviously, there is
a tradeoff of cost and benefits somewhere between maximum efficiency on a data circuit
and systematically added redundancy.
There is an important concept here that should not be missed. An entirely error-free
channel does not exist. It is against the laws of nature. We may have a channel with an
excellent error performance, but some few errors will persist. A performance parameter
in these circumstances would be residual error rate.
Nature of Errors
In binary transmission, an error is a bit that is incorrectly received. For instance, suppose
a 1 is transmitted in a particular bit location and at the receiver the bit in that same
location is interpreted as a 0. Bit errors occur either as single random errors or as bursts
Random errors occur when the signal-to-noise ratio deteriorates. This assumes, of
course, that the noise is thermal noise. In this case noise peaks, at certain moments of
time, are of sufficient level as to confuse the receiver's decision, whether a 1 or a 0.
Burst errors are commonly caused by fading on radio circuits. Impulse noise can
also cause error bursts. Impulse noise can derive from lightning, car ignitions, electrical
machinery, and certain electronic power supplies, to name a few sources.
Error Detection and Error Correction
Error detection just identifies that a bit (or bits) has been received in error. Error correction
corrects errors at a far-end receiver. Both require a certain amount of redundancy to
carry out the respective function. Redundancy, in this context, means those added bits
or symbols that carry out no other function than as an aid in the error-detection or
One of the earliest methods of error detection was the parity check. With the 7-bit
ASCII code, a bit was added for parity, making it an 8-bit code. This is character parity.
It is also referred to as vertical redundancy checking (VRC).
We speak of even parity and odd parity. One system or the other may be used. Either
system is based on the number of marks or 1s in a 7-bit character, and the eighth bit is
appended accordingly, either a 0 or a 1. Let us assume even parity and we transmit the
ASCII bit sequence 1010010. There are three 1s, an odd number. Thus a 1 is appended
as the eighth bit to make it an even number.
Suppose we use odd parity and transmit the same character. There is an odd number
of 1s (marks), so we append a 0 to leave the total number of 1s an odd number. With
odd parity, try 1000111. If you added a 1 as the eighth bit, you'd be correct.
Character parity has the weakness that a lot of errors can go undetected. Suppose two
bits are changed in various combinations and locations. Suppose a 10 became a 01; a
0 became a 1, and a 1 became a 0; and two 1s became two 0s. All would get by the
To strengthen this type of parity checking, the longitudinal redundancy check (LRC)
was included as well as the VRC. This is a summing of the 1s in a vertical column of all
characters, including the 1s or 0s in each eighth bit location. The sum is now appended at
the end of a message frame or packet. Earlier this bit sequence representing the sum was
called the block check count (BCC). Today it may consist of two or four 8-bit sequences