Quiz 1 Content (Ch. 3)

Question 1

What does the data link layer do?

Answer 1

Uses the services of the physical layer to send and receive bits over communication channels

Question 2

What does the Data link layer do in regards to framing?

Answer 2

Use the service provided to it by the physical layer, break up the bit stream into discrete frames, compute a short checksum for each frame, and include the checksum in the frame when it is transmitted

Question 3

What are the Data link layer’s functions?

Answer 3

1. provide interface to network layer above
2. deals with transmission errors
3. regulates the flow of data dynamically as needed

Question 4

What are the two possible errors that need error control?

Answer 4

• frame contains faulty information

- frame vanish completely

Question 5

What are the two kinds of flow control?

Answer 5

1. Feedback-based flow control: Reciever sends back information to the sender giving it permission to send more data
2. Rate-based flow control: The protocol has a built-in mechanism that limits the rate at which senders may transmit data

Question 6

What are error-correction codes?

Answer 6

Include enough redundant information to enable the receiver to deduce what the transmitted data must have been
Works better on channels that may make many errors
-> ie wireless links

Question 7

What are Error-detection codes?

Answer 7

Include only enough redundancy to allow the reciever to deduce that an error has occurred but not which (error), and have it request a retransmission
Works better on channels that are highly reliable
-> ie fiber

Question 8

Suppose that the block size was 1000 bits and the error rate was 0.0001 per bit. What is the difference between a single bit error and a burst error?

Answer 8

single bit error: most blocks would contain a one bit error

burst error: 1 block out of 1000 block with have an error of 100 bits

For burst error, less blocks will be affected, but harder to correct than single bits

Question 9

What are other errors (besides single bit error and a burst error)?

Answer 9

Bit erasure: Physical layer recieved an analog signal that is far from expected value for a 0 or 1, so it declared the bit to be lose

Bit flip

Question 10

What is a binary convolutional code?

Answer 10

Not a block code, specified in terms of rate and constraint length
Popular because it is easy to factor the uncertainty of a bit being a 0 or a 1 into the decoding

Question 11

What is Reed-Solomon

Answer 11

Linear block codes
Defined as polynomials that operate over finite fields
Performs on m bit symbols:
Codewords are (2^m)-1 long, popular choice is m=8, then a codeword is 255 bytes long

Question 12

What is LPDC? and what is it good for

Answer 12

Linear block codes

• good for large blocks
• each output bit is formed from only a fraction of the input bits, leads to a matric representation of the code that has a low density of 1s. The recieved codewords are decdoded with an approximation algorithm. That iteratively improves on a best fit of the recieved data to a legal codeword, that corrects error

Question 13

What is the hamming code?

Answer 13

An error correction code.

• linear block code
• performs 1 bit error

Question 14

List the 4 error correction codes

Answer 14

1. Hamming codes
2. Binary convolutional codes
3. Reed-Solomon codes
4. LDPC

Question 15

List the 3 error detection codes

Answer 15

1. Parity
2. Checksum
3. Cyclic Redundancy

Question 16

What is used to handle burst errors?

Answer 16

Interleaving

Question 17

What is Go-back-N?

Answer 17

discard all frames recieved thus fay except forthe next expected one in sequence eventually forcing sender to retransmit lost one

Question 18

What is selective repeat?

Answer 18

Buffer all out-of-order frames, send negative achnowledgements for lost ones

Question 19

How does sliding window allow two-way communication?

Answer 19

uses piggybacking acknowledgements with data

Question 20

What must sliding window protocols handle?

Answer 20

loss of frames due to error and consider bandwidth vs memory requirements