L4

Question 1

draw basic diagram of communication channel (L4 2)

Answer 1

write on paper

Question 2

define entropy and write equation (L4 4)

Answer 2

• measure of uncertainty of a discrete random variable X
• entropy is measured in bits
• example: entropy of a fair coin toss is 1 bit
• more uncertainty means more new information is transmitted (my opinion)

Question 3

calculate entropy of a binary symmetric channel (BSC) (L4 5)

Answer 3

write on paper

Question 4

define joint entropy and write equation (L4 7)

Answer 4

measures how much uncertainty there is in the two random variables taken together

Question 5

write equations of mutual information and channel capacity (L4 8)

Answer 5

write on paper

Question 6

Main ideas of automatic repeat request (ARQ)

Answer 6

• technique necessary at receiver side to detect erroneous frames based on error detections at the DLC layer
• received frame from network layer is appended with control bits at header and trailer to identify erroneous frames
• receiver side request retransmission of erroneous frames

Question 7

draw frame structure at DLC layer (L4 12)

Answer 7

write on paper

Question 8

list types of ARQ

Answer 8

• stop-and-wait
• modified stop-and wait
• Go back n
• selective repeat ARQ

Question 9

Main ideas of stop-and-wat ARQ

Answer 9

• assures each packet has been received correctly and then acknowledges the transmitter to transmit the next one
• if frame is received erroneously, receiver sends negative acknowledgement to retransmit it once more
• transmitter might receive ACK too late causing problems or to retransmit a wrong frame
• receiver cannot tell wheter second frame is a retransmission of packet 0 or first transmission of packet 1 (major drawback)
• solution is to identify each frame and use a counter in the receiver

Question 10

draw schematic of major problem of stop-and-wait ARQ (L4 15)

Answer 10

write on paper

Question 11

main problem of modified stop-and-wait ARQ

Answer 11

channel is not used optimally and there will be repeated frames

Question 12

main ideas of Go back n ARQ (L4 20)

Answer 12

• avoids transmitting ACKs or NAKs but rather numbering transmission sequence at transmitter side
• packets are sent successively without waiting for ACKs/NAKs
• number n determines how many successive packets can be sent before requesting a new packet

Question 13

main properties of selective repeat ARQ (L4 23)

Answer 13

• different of Go BacK n in the sense that it can retransmit only erroneous or lost packets
• when a frame is lost receiver keeps requesting that frame, but sender continues sending frames that are kept in the buffer of receiver
• requires more storage memory

Question 14

functions of DLC layer

Answer 14

• error detection based on parity check and cyclic redundancy check (CRC) techniques
• to provide error-free packets to higher layers
• received frame is appended with control bits at header and trailer to identify erroneous frames

Question 15

give an example of even single parity check (L4 28)

Answer 15

write on paper

Question 16

main ideas of cyclic redundancy check

Answer 16

• parity checks codes used for error detection are CRC
• CRC codes are based on polynomial arithmetic

Question 17

Example of CRC (division of polynomials over GF(2)) (L4 35)

Answer 17

write on paper