• Types of Redundancy Checks
o There are 4 types of redundancy checks
used in data communication:
-Vertical
Redundancy Check (VRC)
-Longitudinal
Redundancy Check (LRC)
-Cyclic
Redundancy Check (CRC)
-Checksum
o First 3 are normally implemented in the
physical layer for use in data link
layer
o Fourth is used by Upper layers
 |
Vertical Redundancy Check (VRC)
o Most common and least expensive
o Also called Parity Check
o A
redundant bit called
parity bit is
appended to every
data unit so
that total
number
of 1's in the unit becomes even including the parity bit
 |
ŠWe want to TX
the binary data unit 1100001
ŠAdding
together the number of 1's gives us 3, an odd number
o Before TX, we pass the data unit through
a parity generator, which counts the 1's
and
appends the parity bit (1) to the end
o The total number of 1's is now 4, an even
number
o The system now transfers the entire
expanded across the network link
o When
it reaches its
destination, the RX
puts all 8 bits through
an even parity
checking
function
o If
the RX sees
11100001, it counts
four ones, an
even number and
the data unit
passes
o But what if the data unit has been
damaged in transit?
o What if instead of 11100001, receiver
sees 11100101?
o Then when the parity checker counts the
1's, it gets 5 an odd number
o The receiver knows that an error has
occurred somewhere and therefore rejects the
whole
unit
o Some systems may also use ODD parity
checking
o The principal is the same as even parity
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