.

1. 10.1
Chapter 10
Error Detection
and
Correction
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. 10.2
Position of the data-link layer

3. 10.3
Data link layer duties
◼ Packetizing: move data packets or frames from one hop to the next within the
same LAN or WAN
◼ Addressing: uses physical addresses (MAC Addresses) that are assigned for
and used to find the next hop in the hop-to-hop delivery
◼ Error Control: uses error detection and correction to reduce the severity of
error occurrence
◼ Flow Control: controls how much data a sender can transmit before it must
wait for an acknowledgement from the receiver in order not to overwhelm it
◼ Medium Access Control (MAC): controls how the nodes should access a
shared media in a way to prevent packet collisions

4. 10.4
Data can be corrupted
during transmission.
Some applications require that
errors be detected and corrected.
Note

5. 10.5
10-1 INTRODUCTION
Let us first discuss some issues related, directly or
indirectly, to error detection and correction.
• Types of Errors
• Redundancy
• Detection Versus Correction
• Forward Error Correction Versus Retransmission
• Coding
• Modular Arithmetic
Topics discussed in this section:

6. 10.6
In a single-bit error, only 1 bit in the data
unit has changed.
Note

7. 10.7
Figure 10.1 Single-bit error

8. 10.8
A burst error means that 2 or more bits
in the data unit have changed.
Note

9. 10.9
Figure 10.2 Burst error of length 8
◼ The length of the burst is measured from the 1st to the last corrupted bits
◼ The burst error doesn’t necessarily mean that errors occur in consecutive bits
◼ Burst errors are most likely to occur in serial transmission
◼ The noise duration is usually longer than the duration of one bit
◼ The severity of burst errors are dependent on the data rate & the noise duration
◼ For example: a noise duration of 0.01 sec would corrupt 10 bits of a 1Kbps
transmission and 10,000 bits of a 1Mbps source

10. 10.10
Error Detection & Correction Concepts
◼ Error detection: find out whether an errors has occurred
◼ Error correction: find the exact number of corrupted bits
and their exact location within the message
◼ Error detection is the first step towards error correction
◼ Error detection and correction use the concept of
redundancy, which is adding extra bits to the data unit
◼ The extra bits carry information about the data unit that
help find out whether any change may have happened to
the data unit at the receiver side
◼ The extra bits are discarded as soon as the accuracy of
the transmission has been determined

11. 10.11
To detect or correct errors, we need to
send extra (redundant) bits with data.
Note

12. 10.12
Forward Error Correction vs. Retransmission
◼ Error correction by retransmission
◼ If error is detected, the sender is asked to retransmit
◼ Forward Error Correction (FEC):
◼ The receiver uses an error correcting code, which automatically
corrects certain types of errors
◼ Theoretically, any error can be automatically corrected
◼ Error correction is harder than error detection and needs more
redundant bits
◼ The secret of error correction is to locate the invalid bits
◼ To locate a single-bit error in a data unit, you need enough
redundant bits to cover all states of change in the data bits as
well as the redundant bits and the no-change case

13. 10.13
Coding
◼ Redundancy is achieved via coding schemes as follows:
◼ Sender adds extra bits to the data bits based on a certain relationship (e.g.
CRC circuit)
◼ Receiver checks the relationship in order to detect and/or correct errors
◼ Two factors of coding schemes to observe:
◼ The ratio of the redundant (r) bits to the data bits (d)→ (r/d)
◼ The robustness of the whole process (does not give false info)
◼ Categories of coding schemes:
1. Block coding ( we cover it in this chapter)
2. Convolution coding (not covered here)

14. 10.14
Modular Arithmetic (X mod-N)
◼ Use a limited number of integers
◼ Define an upper limit called modulus, N
◼ Use only the integers from 0 to N-1, inclusive
◼ This is called modulo-N arithmetic
◼ If a number is greater than N, it is divided by N and
the remainder is the result
◼ If a number is negative, add as many N’s as needed to
make it positive
◼ There is no carry out when numbers are added or
subtracted
◼ In modulo-2 arithmetic, the addition and subtraction
are similar and are performed using the bit-wise XOR
function

15. 10.15
Figure 10.4 XORing of two single bits or two words

16. 10.16
10-2 BLOCK CODING
In block coding,
• we divide our message into blocks, each of k bits,
called datawords.
• Then, we add r redundant bits to each block to
make the length n:
• n = k + r. The resulting n-bit blocks are called
codewords.
• Error Detection
• Error Correction
• Hamming Distance
• Minimum Hamming Distance
Topics discussed in this section:
k: dataword;
r: redundant bits
n: codeword

17. 10.17
Figure 10.5 Datawords and codewords in block coding

18. 10.18
The 4B/5B block coding discussed in Chapter 4 is a good
example of this type of coding.
In this coding scheme, k = 4 and n = 5,
• No. of datawords=2k = 16
• No. of codewords= 2n = 32.
We saw that 16 out of 32 codewords are used for message
transfer and the rest are either used for other purposes or unused.
Example 10.1

19. 10.19
Figure 10.6 Process of error detection in block coding
e.g. X2+X+1

20. 10.20
Let us assume that k = 2 and n = 3. Table 10.1 shows the list of datawords and
codewords. Later, we will see how to derive a codeword from a dataword.
Assume the sender encodes the dataword 01 as 011 and sends it to the receiver.
Consider the following cases:
1. The receiver receives 011. →It is a valid codeword→ The receiver extracts the
dataword 01 from it.
2. The codeword is corrupted during transmission, and 111 is received. This is
not a valid codeword and is discarded.
3. The codeword is corrupted during transmission, and 000 is received. This is a
valid codeword. The receiver incorrectly extracts the dataword 00.
Two corrupted bits have made the error undetectable.
Example 10.2
Table 10.1 A code for error detection (Example 10.2)

21. 10.21
• An error-detecting code can detect
only the types of errors for which it is
designed;
• other types of errors may remain
undetected.
Note

22. 10.22
Figure 10.6 Process of error detection in block coding
e.g. X2+X+1

23. 10.23
• Let us add more redundant bits to Example 10.2 to see if the
receiver can correct an error without knowing what was actually
sent.
• We add 3 redundant bits to the 2-bit dataword to make 5-bit
codewords. Table 10.2 shows the datawords and codewords.
• So, k=2, r=3,→n=2+3=5 bits
Example 10.3

24. 10.24
• Assume the dataword is d= 01. The sender creates the codeword
01011.
• The codeword is corrupted during transmission, and 01001 is
received.
• First, the receiver finds that the received codeword is not in the
table.
• This means an error has occurred. The receiver, assuming that
there is only 1 bit corrupted, uses the following strategy to guess
the correct dataword.
Example 10.3

25. 10.25
◼ Codeword sent = 01011, Codeword received = 01001
◼ 01001 vs. 00000 ➔ 2 bit difference 
◼ 01001 vs. 10101 ➔ 3 bit difference 
◼ 01001 vs. 11110 ➔ 4 bit difference 
◼ 01001 vs. 01011 ➔ 1 bit difference ☺
1. The receiver will compare received code word with all the valid
codewords looking number of difference in bits.
2. It will choose the least difference
Compare 01001
with all
codewords
difference=
Hamming
distance

26. 10.26
The Hamming distance between two
words is the number of differences
between the corresponding bits.
Note
d=Hamming
distance

27. 10.27
Let us find the Hamming distance between two pairs of
words.
1. The Hamming distance d(000, 011) is 2 because
Example 10.4
2. The Hamming distance d(10101, 11110) is 3 because
d=Hamming
distance

28. 10.28
The minimum Hamming distance (dmin) is
the smallest Hamming distance between
all possible pairs in a set of words.
Note

29. 10.29
Find the minimum Hamming distance of the coding
scheme in Table 10.1.
Solution
We first find all Hamming distances.
Example 10.5
The dmin in this case is 2.

30. 10.30
Find the minimum Hamming distance of the coding
scheme in Table 10.2.
Solution
We first find all the Hamming distances.
The dmin in this case is 3.
Example 10.6

31. 10.31
To guarantee the detection of up to s
errors in all cases, the minimum
Hamming distance in a block
code must be dmin = s + 1.
Note

32. 10.32
The minimum Hamming distance for our first code scheme (Table
10.1) is 2. This code guarantees detection of only a single error.
• For example, if the third codeword (101) is sent and one error
occurs, the received codeword does not match any valid
codeword.
• e.g. send 101→received 111 (detected →because not a valid
code),
• If two errors occur, however, the received codeword may match a
valid codeword and the errors are not detected.
→e.g: send 101→received 000 (considered valid→ not detetcted)
Example 10.7
dmin = s + 1.
s=1

33. 10.33
Our second block code scheme (Table 10.2) has dmin = 3. This code
can detect up to two errors. Again, we see that when any of the
valid codewords is sent, two errors create a codeword which is not
in the table of valid codewords. The receiver cannot be fooled.
However, some combinations of three errors change a valid
codeword to another valid codeword. The receiver accepts the
received codeword and the errors are undetected.
Example 10.8
dmin = s + 1.
s=2

34. 10.34
To guarantee correction of up to t errors
in all cases, the minimum Hamming
distance in a block code
must be dmin = 2t + 1.
Note

35. 10.35
A code scheme has a Hamming distance dmin = 4. What
is the error detection and correction capability of this
scheme?
Solution
This code guarantees the detection of up to three errors
(s = 3), but it can correct up to one error. In other words,
if this code is used for error correction, part of its
capability is wasted.
• Error correction codes need to have an odd
minimum distance (3, 5, 7, . . . ).
Example 10.9
dmin = s + 1.
s=3
dmin = 2t + 1.
t=?

36. 10.36
10-3 LINEAR BLOCK CODES
• Almost all block codes used today belong to a
subset called linear block codes.
• A linear block code is a code in which the
exclusive OR (addition modulo-2) of two valid
codewords creates another valid codeword.
• Codeword1 (xor) codeword2➔codeword3
1. Minimum Distance for Linear Block Code
2. Example of Linear Block Codes
Topics discussed in this section:

37. 10.37
In a linear block code, the exclusive OR
(XOR) of any two valid codewords
creates another valid codeword.
Note

38. 10.38
1. The scheme in Table 10.1 is a linear block code because
• the result of XORing any codeword with any other
codeword is a valid codeword.
• For example, the XORing of the second and third codewords
creates the fourth one.
• 101 + 011 = 110 (cod #4 in the table)
2. The scheme in Table 10.2 is also a linear block code. We can
create all four codewords by XORing two other codewords.
Example 10.10

39. 10.39
Note: The minimum Hamming distance for a linear block code is the
smallest number of 1’s in the nonzero codewords
In (Table 10.1), the numbers of 1s in the nonzero codewords are 2, 2,
and 2. So the minimum Hamming distance is dmin = 2.
In (Table 10.2), the numbers of 1s in the nonzero codewords are 3, 3,
and 4. So in this code we have dmin = 3.
Example 10.11

40. 10.40
A simple parity-check code is a single-bit
error-detecting code in which n = k + 1
with dmin = 2.
→i.e. #. of r bits= 1 bit extra
→ Even parity→ add 1’s so total numbers of 1’s is even
parity-check code

41. 10.41
Table 10.3 Simple even parity-check code C(5, 4) parity bit

42. 10.42
Figure 10.10 Encoder and decoder for simple parity-check code

43. 10.43
How it works?
To send (a3 a2 a1 a0 : 1011)→ p=1→codeword = (a3 a2 a1 a0 r0 10111)-----> sent to
the receiver. We examine five cases:
1. No error occurs;
→the received codeword is (b3 b2 b1 b0 q0: 10111. →syndrome S= 0 (no
error) →extracted dataword 1011.
2. One error a1 is changed.
→ received codeword is 10011 → S= 1 (error)→ discard data.
3. One error: r0 is changed:
→received codeword is 10110→S=1 (error)→ discard data.
4. Two errors: r0 & a3 are changed:
→received codeword is 00110→ S=0 (no error: false!) → dataword 0011 is
created at the receiver.
→Note that here the dataword is wrongly created due to the syndrome
value.
5. Three errors: a3, a2, a1 are changed →The received codeword is 01011.
→S=1 (error) →The dataword is not created.
****This shows that the simple parity check, guaranteed to detect one single
error, can also find any odd number of errors.
Example 10.12

44. 10.44
A simple parity-check code can detect
an odd number of errors.
Note

45. 10.45
Figure 10.11 Two-dimensional parity-check code
◼ Data words are organized in a table (rows & columns)
◼ For each row and each column, a parity-check bit is
added
Parity bits

46. 10.46
Figure 10.11 Two-dimensional parity-check code
Original
data

47. 10.47
Performance of the Two-Dimensional
Parity Check
Two-Dimensional Parity Check can do the following
◼ Guarantee to detect single error
◼ Increases the likelihood of detecting burst errors
◼ It fails if the error bits are in exactly the same positions
in two different data units
→because the changes will cancel each other in this case

48. 10.48
The Hamming Error Correction Codes
Hamming codes can be used in Error Detection and correction
◼ The category of Hamming codes discussed here is with
dmin= 3:
→2-bit detection & 1-bit correction
◼ We need to choose an integer m≥3 such that:
◼ n=2m-1 & k=n-m (i.e. n=k+m)
◼ The number of check bits r=m
◼ For example:
◼ If m=3, then n=7 & k=4
◼ This is Hamming Code C(7,4) with dmin= 3
◼ Each code word = a3 a2 a1 a0 r2 r1 r0

49. 10.49
Table 10.4 Hamming code C(7, 4)

50. 10.50
Figure 10.12 The structure of the encoder and decoder for a Hamming code
generator creates 3-bit even-parity check
◼ r0 = a2 + a1 + a0 (modulo-2)
◼ r1 = a3 + a2 + a1 (modulo-2)
◼ r2 = a1 + a0 + a3 (modulo-2)
checker creates 3-bit syndrome
◼ s0 = b2 + b1 + b0 + q0 (modulo-2)
◼ s1 = b3 + b2 + b1 + q1 (modulo-2)
◼ s2 = b1 + b0 + b3 + q2 (modulo-2)

51. 10.51
Table 10.5 Logical decision made by the correction logic analyzer
◼ The 3-bit syndrome represent 1 out 8 different error conditions
as shown in the table
◼ The decoder is only concerned with the conditions in the table
that are not shaded (i.e. data:b3b2b1b0), in which a data bit has
been flipped
s2 s1 s0
s2 s1 s0 =001→q0
q0 in 1st equation
s2 s1 s0 =111→b1
b1 in all equations

52. 10.52
Let us check some cases:
1. No error: d=dataword 0100 → c=codeword= 0100011.
→received: b=0100011. S=000 (no error)→ dataword = 0100.
2. One error: d=0111 → c=0111001
→received b=0011001, S=011 (error in b2)→correct(flip b2)
→the final dataword is 0111 (correct).
3. Two errors: d=1101→ c= 1101000.
→received b= 0001000, S=101 (error b0), correct(flip b0)
→the final dataword is 0000 (false data)
→This shows that our code detects 2 errors, but cannot correct
Example 10.13

53. 10.53
We need a dataword of at least k>=7 bits. Calculate values
of k and n that satisfy this requirement.
Solution
We need to make k = n − m >=7, or (2m − 1) − m ≥ 7.
1. If we set m = 3, →n = 23 − 1, and k = 7 − 3=4, which is
not acceptable.
2. If we set m = 4,→n = 24 − 1 = 15 and k = 15 − 4 = 11,
which satisfies the condition. So the code is
Example 10.14
C(15, 11)
C(n, k)

54. 10.54
Performance of the Hamming Code
◼ It can detect a 2-bit error
◼ It can correct a 1-bit error
◼ It can be modified in order to detect burst
errors of size N:
◼ It is based on splitting the burst error between N
codewords
◼ Convert the frame into N codewords
◼ Rearrange the codewords in the frame from row-wise
to column-wise (see the example)

55. 10.55
Figure 10.13 Burst error correction using Hamming code

56. 10.56
10-4 CYCLIC CODES
Cyclic codes
• are special linear block codes with one extra
property.
• In a cyclic code, if a codeword is cyclically shifted
(rotated), the result is another codeword.
• Cyclic Redundancy Check (CRC)
• Hardware Implementation
• Polynomials
• Cyclic Code Analysis
• Advantages of Cyclic Codes
• Other Cyclic Codes
Topics discussed in this section:

57. 10.57
Table 10.6 A CRC (Cyclic Redundancy Check) code with C(7, 4)

58. 10.58
Figure 10.14 CRC encoder and decoder

59. 10.59
Figure 10.14 CRC encoder and decoder

60. 10.60
Cyclic Redundancy Check (CRC)
◼ At the encoder of the sender:
◼ The k-bit dataword is augmented by
appending (n-k) zeros to the right
hand side of it
◼ The n-bit result is fed into the
generator
◼ The generator uses an agreed-upon
divisor (checker) of size (n-k+1)
◼ The generator divides the
augmented dataword by the divisor
using modulo-2 division
◼ The quotient of the division is
discarded
◼ The remainder is appended to the
dataword to create the codeword

61. 10.61
Cyclic Redundancy Check (CRC)
◼ At the decoder of the receiver:
◼ The received codeword is divided by the same divisor to create the
syndrome, which has a size of (n-k) bits
◼ If all the syndrome bits are zeros, then no error is detected
◼ Otherwise, an error is detected and the received dataword is
discarded

62. 10.62
Figure 10.21 A polynomial to represent a binary word
◼ The 1’s and 0’s represent the coefficients of the polynomial
◼ The power of each term represents the position of the bit
◼ The degree of the polynomial is the highest power of it
◼ Adding and subtracting the polynomials are done by adding
and subtracting the coefficients of the same-power terms

63. 10.63

64. 10.64

65. 10.65
Figure 10.22 CRC division using polynomials

66. 10.66
The divisor in a cyclic code is normally
called the generator polynomial
or simply the generator.
Note

67. 10.67

68. 10.68
Figure 10.15 Division in CRC encoder
sender

69. 10.69
Figure 10.16 Division in the CRC decoder for two cases
receiver

70. 10.70
examples

71. 10.71
examples

72. 10.72
Cyclic Code Analysis
◼ Define the following cyclic code parameters as
polynomials:
◼ Dataword: d(x)
◼ Codeword: c(x)
◼ Generator: g(x)
◼ Syndrome: s(x)
◼ Error: e(x)
◼ Received codeword = c(x) + e(x)
◼ Received codeword/g(x) = c(x)/g(x) + e(x)/g(x)
◼ c(x)/g(x) does not have a remainder by definition
◼ s(x) is the remainder of e(x)/g(x)
◼ If s(x)≠0, then an error is detected
◼ If s(x)=0, then either no error has occurred or the error has not
been detected
◼ If e(x) is divisible by g(x), then e(x) can’t be detected

73. 10.73
In a cyclic code,
If s(x) ≠ 0, one or more bits is corrupted.
If s(x) = 0, either
a. No bit is corrupted. or
b. Some bits are corrupted, but the
decoder failed to detect them.
Note

74. 10.74
In a cyclic code, those e(x) errors that
are divisible by g(x) are not caught.
Note

75. 10.75
Single-Bit Error
◼ A single-bit error is e(x)=xi, where i is the
position of the bit
◼ If a single-bit error is caught, then xi is not
divisible by g(x) (i.e.; there is a
remainder)
◼ If g(x) has, at least, two terms and the
coefficient of x0 is not zero, then e(x) can’t
be divided by g(x). That is, all single-bit
errors will be caught

76. 10.76
If the generator has more than one term
and the coefficient of x0 is 1,
all single errors can be caught.
Note

77. 10.77
Which of the following g(x) values guarantees that a
single-bit error is caught? For each case, what is the error
that cannot be caught?
a. x + 1 b. x3 c. 1
Solution
a. No xi can be divisible by x + 1. Any single-bit error can
be caught.
b. If i is equal to or greater than 3, xi is divisible by g(x).
All single-bit errors in positions 1 to 3 are caught.
c. All values of i make xi divisible by g(x). No single-bit
error can be caught. This g(x) is useless.
Example 10.15

78. 10.78
Figure 10.23 Representation of two isolated single-bit errors using polynomials
◼ Two isolated single-bit errors are represented as
◼ e(x)= xj + xi = xi (xj-i + 1) = xi (xt + 1)
◼ If g(x) has, at least, two terms and the coefficient of x0 is
not zero, then xi can’t be divided by g(x)
◼ Therefore, if g(x) is to detect e(x), it must not divide (xt + 1)
where t is the distance between the two error bits and t
should be between 2 and n-1 (n is the degree of g(x))

79. 10.79
If a generator cannot divide xt + 1
(t between 2 and n – 1),
then all isolated double errors
can be detected.
Note

80. 10.80
Find the status of the following generators related to two
isolated, single-bit errors.
a. x + 1 b. x4 + 1 c. x7 + x6 + 1 d. x15 + x14 + 1
Solution
a. This is a very poor choice for a generator. Any two
errors next to each other cannot be detected.
b. This generator cannot detect two errors that are four
positions apart.
c. This is a good choice for this purpose.
d. This polynomial cannot divide (xt + 1) if t is less than
32,768]
15. A codeword with two isolated errors up to
32,768 15 bits apart can be detected by this generator.
Example 10.16

81. 10.81
A generator that contains a factor of
(x + 1) can detect all odd-numbered
errors.
Note

82. 10.82
❏ All burst errors with L ≤ r will be
detected. (r is the degree of g(x))
❏ All burst errors with L = r + 1 will be
detected with probability 1 – (1/2)r–1.
❏ All burst errors with L > r + 1 will be
detected with probability 1 – (1/2)r.
Note

83. 10.83
Find the suitability of the following generators in relation
to burst errors of different lengths.
a. x6 + 1 b. x18 + x7 + x + 1 c. x32 + x23 + x7 + 1
Solution
a. This generator can detect all burst errors with a length
less than or equal to 6 bits;
3 out of 100 burst errors with length 7 will slip by;
1 – (1/2)6–1 = 1 - 1/32 = 0.97
16 out of 1000 burst errors of length 8 or more will slip by. 1 –
(1/2)6 = 1 – 1/64 = 0.984
Example 10.17

84. 10.84
b. This generator can detect all burst errors with a length
less than or equal to 18 bits;
8 out of 1 million burst errors with length 19 will slip by;
4 out of 1 million burst errors of length 20 or more will
slip by.
c. This generator can detect all burst errors with a length
less than or equal to 32 bits;
5 out of 10 billion burst errors with length 33 will slip by;
3 out of 10 billion burst errors of length 34 or more will
slip by.
Example 10.17 (continued) Reading
only

85. 10.85
A good polynomial generator needs to
have the following characteristics:
1. It should have at least two terms.
2. The coefficient of the term x0 should
be 1.
3. It should not divide xt + 1, for t
between 2 and n − 1.
4. It should have the factor x + 1.
Note

86. 10.86
Performance of Cyclic Codes
◼ Detect single-bit errors
◼ Detect double-bit isolated errors
◼ Detect odd number of errors
◼ Detect burst errors of length less than or
equal to the degree of the generator
◼ Detect, with a very high probability, burst
errors of length greater than the degree of
the generator
◼ Can be implemented in either SW or HW

87. 10.87
Table 10.7 Standard polynomials

88. 10.88
10-5 CHECKSUM
• The last error detection method we discuss here is
called the checksum.
• The checksum is used in the Internet by several
protocols although
• not used at the data link layer.
• we briefly discuss it .
• Idea
• One’s Complement
• Internet Checksum
Topics discussed in this section:

89. 10.89
Examples : protocols uses CheckSum:
UDP & TCP
TCP
UDP

90. 10.90
The concept of check sum as follows:
• Suppose our data is a list of five 4-bit numbers that we want
to send to a destination.
• In addition to sending these numbers, we send the sum of the
numbers.
• Send : We want to sedn : (7, 11, 12, 0, 6),
→send (7, 11, 12, 0, 6, -36),
note : -36= -Sum (7, 11, 12, 0, 6), . T
• The receiver adds (7, 11, 12, 0, 6, -36)=result,
→ If result=0, No error→ accept data
→ else if result ≠ 0 → error occurs
→data are not accepted.
Example 10.18: Concept

91. 10.91
If we we use 4-bit numbers, how can we represent the
number 21 in one’s complement arithmetic using
only four bits?
Solution
The number 21 in binary is 10101 (it needs five bits).
We can wrap the leftmost bit and add it to the four
rightmost bits.
We have (0101 + 1) = 0110 or 6.
Example 10.20

92. 10.92
How can we represent the number −6 in one’s
complement arithmetic using only four bits?
Solution
• one’s complement (x)= inverting all bits(x).
• Positive 6 is 0110; negative 6 is 1001.
• If we consider only unsigned numbers, this is 9.
• In other words, the complement of 6 is 9.
• Another way to find the complement of a number in
one’s complement arithmetic is to subtract the
number from 2n − 1 (16 − 1 in this case n=4).
Example 10.21
Checksum = INV (data 1 ⊕ data 2 ⊕ ... ⊕ data n)

93. 10.93
0 1 1 0 6
1 0 0 1 9
1 1 1 1 15
0 0 0 0 0
Figure 10.24 Example 10.22

94. 10.94
1’s complement example:

95. 10.95

96. 10.96
The Internet Checksum
◼ The checksum generator subdivides the data unit into
equal-length data units of 16-bit each
◼ The data units are added using the 1’s complement
arithmetic such that the total is always n-bit long
◼ The data units are binary added normally to give the partial sum
◼ The outer carry out is added to the partial sum to give the sum
◼ The result is 1’s complemented to give the checksum
◼ The checksum is appended to the data unit
◼ The checksum checker performs the same operation
including the checksum:
◼ If the result is zero, then the data unit has not been altered

97. 10.97
Sender site: (Internet Checksum)
1. The message is divided into 16-bit words.
2. The value of the checksum word is set to 0.
3. All words including the checksum are
added using one’s complement addition.
4. The sum is complemented and becomes the
checksum.
5. The checksum is sent with the data.
Note

98. 10.98
Receiver site: (Internet Checksum)
1. The message (including checksum) is
divided into 16-bit words.
2. All words are added using one’s
complement addition.
3. The sum is complemented and becomes the
new checksum.
4. If the value of checksum is 0, the message
is accepted; otherwise, it is rejected.
Note

99. 10.99
C+1=D—>
D+1=E→
E+C (mod 16) =A→
A+5=F→
F+6 (mod 16)→5
5+9=E
1sr column
addition

100. 10.100

101. 10.101
✓ Let us calculate the checksum for a text of 8 characters
(“Forouzan”).
✓ The text needs to be divided into 2-byte (16-bit) words. We use
ASCII (see Appendix A) to change each byte to a 2-digit
hexadecimal number.
✓ E.g. F = 0x46 ASCII, o= 0x6F.
✓ Figure 10.25 shows how the checksum is calculated at the sender
and receiver sites.
✓ In part a of the figure, the value of partial sum for the first
column is 0x36.
✓ We keep the rightmost digit (6) and insert the leftmost digit (3) as
the carry in the second column.
✓ The process is repeated for each column.
Example 10.23

102. 10.102
Figure 10.25 Example 10.23
F F F F
F F

103. 10.103
Performance of the Checksum
◼ Detect all odd number of errors
◼ Detect most even number of errors
◼ If corresponding bits in different data units of
opposite values are affected, the error will not
be detected
◼ Generally weaker than the CRC