Information Technology Reference
In-Depth Information
This problem is overcome if LRC and VRC are used together. With VRC/LRC the only
time an error goes undetected is when an even number of bits, in an even number of charac-
ters, in the same bit positions of each character are in error. This is of course very unlikely.
On systems where only single-bit errors occur, the LRC/VRC method can be used to de-
tect and correct the single-bit error. For systems where more than one error can occur it is not
possible to locate the bits in error, so the receiver prompts the transmitter to retransmit the
message.
Example
A communications channel uses ASCII character coding and LRC/VRC bits are added to
each word sent. Encode the word 'Freddy' and, using odd parity for the VRC and even parity
for the LRC; determine the LRC character.
Answer
F
r
e
d
d
y
LRC
b0
0
0
1
0
0
1
0
b1
1
1
0
0
0
0
0
b2
1
0
1
1
1
0
0
b3
0
0
0
0
0
1
1
b4
0
1
0
0
0
1
0
b5
0
1
1
1
1
1
1
b6
1
1
1
1
1
1
0
VRC
0
1
1
0
0
0
1
