Week 7 - Data-link error control and Data-Link Control (DLC)

Question 1

What causes errors to occur in the first place?

Answer 1

Interference

Question 2

What are the 2 types of error?

Answer 2

1. Single bit
2. Burst error

Question 3

If a codeword is valid, is it uncorrupted?

Answer 3

No, we cannot guarantee that the codeword is uncorrupted even if it’s valid.

Question 4

What is Block Coding?

Answer 4

It’s the basis of datawords and codewords.

A message is divided into blocks.

Each block has redundant bits added for error control which creates a codeword (ex: hamming code)

Question 5

What is hamming distance?

Answer 5

The number of bits that differ between 2 words.

(NOTE: min hamming distance for error detection refers to the min HD between VALID codewords.)

Question 6

What are linear block codes?

Answer 6

XOR of 2 valid codewords creates another valid codeword.

Question 7

What is linear block code - Parity-check code?

Answer 7

Add 1 bit to the dataword to create codeword with parity bit. Even parity considered

Question 8

What is linear block code - Cyclic codes?

Answer 8

Code word rotated to the left to create another valid codeword.
(Ex: 1011000 -> 0110001)
Performance-oriented, used in LAN/WAN.

Question 9

What is Checksum?

Answer 9

Error detection for message of any length, typically used in network layer.

Question 10

What is Forward error correction (FEC) for?

Answer 10

Used in real-time media, takes too long to retransmit so we add redundant bits that allow us to correct the error at the transport layer.

Question 11

What are the 3 FEC techniques?

Answer 11

1. Hamming distance
2. XOR operations
3. Chunk interleaving

Question 12

How does DLC frame data?

Answer 12

1. Fixed size frame
2. Variable sized (byte/bit oriented approach)

Question 13

What 2 ways does the data-link layer implement flow control?

Answer 13

1. Buffer overflow, drops packets
2. Receiver sends feedback to sender

Question 14

Data-link error control, what happens when error is detected? (2 outcomes)

Answer 14

1. frame dropped (udp?)
2. frame dropped + ACK sent (tcp?)

Question 15

What are the 2 DLC protocols for error and flow control?

Answer 15

1. Simple (no flow or error control)
2. Stop-and-wait (timer used by sender)

Question 16

What is byte stuffing?

Answer 16

Adding an ESC char whenever there is an ESC or FLAG in the text

Question 17

What is bit stuffing?

Answer 17

Adding an extra 0 after 5 consecutive 1s so that 01111110 in the text does not get mistaken for a flag.