Module 5

Question 1

Describe the general idea of error detection, using redundancy.

Answer 1

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

In order to make errorrs detectable we send extra bits along side the payload. This will add something to be checked for corruption.

Question 2

P10-3 Exclusive OR (XOR) is one of the most used operations in the calculation of codewords. Apply the exclusive OR operation on the following pairs of patterns. Interpret the results.

A) (10001) XOR (10001)
B) (11100) XOR (00000)
C) (10011) XOR (11111)

Answer 2

A) (10001) XOR (10001) = 00000
B) (11100) XOR (00000) = 11100
C) (10011) XOR (11111) = 01100

Question 3

P10-10 Assuming odd parity, find the parity bit for each of the following data units.

a. 1001011
b. 0001100
c. 1000000
d. 1110111

Answer 3

a. 1
b. 1
c. 0
d. 1

Question 4

P10-12 Given the dataword 10100111 the divisor 10111, show the generation of the CRC codeword at the sender site (using binary division).

Answer 4

Codeword = 101001111
Checksum = 00110

Question 5

P10-20 This problem shows a very special case in checksum handling. A sender has two data items to send: 0x4567 and 0xBA98. What is the value of the checksum?

Answer 5

4567
BA98
0000
---------
FFFF
0000

Question 6

What is the main role of the data link layer?

Answer 6

node to node delivery of packets called frames

Question 7

What are the duties of the data link layer?

Answer 7

Packetizing, addressing, error control, flow control, access control

Question 8

Explain the basic idea of error detection.

Answer 8

• Generator at the sender generates the redundancy from the message to send by using an algorithm.
• The message and the redundancy are transmitted together
• At the receiver, the checker generates the redundancy from the received message by using the same algorithm, and then the new redundancy and the the received redundancy is compared.
• Only after they are the same is the message accepted