1. BABA GHULAM SHAH BADSHAH
UNIVERSITY RAJOURI
Department of computer sciences
Presentation on Computer Networks
Presentation Topic:- Error detection, types and
detection methods.
Presented to :-Dr. Sanjay Jamwal
Presented by Suhail Ahamd Khanday
Course MCA, Roll No:- 02-Mca-2015
3. Basic conceptsBasic concepts
Networks must be able to transfer data from one device to
another with complete accuracy.
Data can be corrupted during transmission.
For reliable communication, errors must be detected and
corrected.
Error detection and correction are
implemented either at the data link layer or the
transport layer of the OSI model.
8. • The term burst errorburst error means that two or more bits in
the data unit have changed from 1 to 0 or from 0 to
1.
• Burst errors does not necessarily mean that the
errors occur in consecutive bits.
• The length of the burst is measured from the first
corrupted bit to the last corrupted bit.
• Some bits in between may not have been
corrupted.
9. Burst error is most likely to happen in serial transmission
since the duration of noise is normally longer than the
duration of a bit.
The number of bits affected depends on the data rate and
duration of noise.
Example:Example:
If data is sent at rate = 1Kbps then a noise of 1/100 sec
can affect 10 bits.(1/100*1000)
If same data is sent at rate = 1Mbps then a noise of 1/100
sec can affect 10,000 bits.(1/100*106
)
10. Error detectionError detection
Errors in the received frames are detected by
means of Parity Check and Cyclic Redundancy
Check (CRC).
In both cases, few extra bits are sent along with
actual data to confirm that bits received at
other end are same as they were sent.
If the counter-check at receiver’ end fails, the
bits are considered corrupted
11. Parity check
One extra bit is sent along with the original bits to make
number of 1s either even in case of even parity, or odd in
case of odd parity.
The sender while creating a frame counts the number of 1s
in it.
For example, if even parity is used and number of 1s is even
then one bit with value 0 is added. This way number of 1s
remains even. If the number of 1s is odd, to make it even a
bit with value 1 is added.
12. •The receiver simply counts the number of 1s in a frame.
• If the count of 1s is even and even parity is used, the frame
is considered to be not-corrupted and is accepted.
• If the count of 1s is odd and odd parity is used, the frame is
still not corrupted
13. •CRC is a error detection mechanism.
• It is based on binary division
•The divisor is generated using polynomials.
•The sender performs a division operation on the bits being
sent and calculates the remainder.
•Before sending the actual bits, the sender adds the remainder
at the end of the actual bits.
• Actual data bits plus the remainder is called a code word.
The sender transmits data bits as code word.
14.
15. •At the other end, the receiver performs division operation on
code words using the same CRC divisor.
•If the remainder contains all zeros the data bits are accepted,
otherwise it is considered as there some data corruption
occurred in transit.
16. Check sum
A checksum is a count of the number of bits in a transmission unit that is
included with the unit so that the receiver can check to see whether the
same number of bits arrived.
If the counts match, it's assumed that the complete
transmission was received.
Both TCP and UDP communication layers provide a checksum
count and verification as one of their services.
17. The sender follows these steps
•The unit is divided into k sections, each unit of n bits
•All sections are added using one’s complement to get
the sum
•The sum is complemented and becomes the
checksum
•The checksum is sent with the data
18. The receiver follows these steps
•The unit is divided in k sections each of n bits
•All sections are added using one’s complement to get
the sum
•The sum is complemented
•If the result is zero, the data are accepted otherwise,
rejected
