Quiz 1 Content (Ch. 3) Flashcards Preview

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Flashcards in Quiz 1 Content (Ch. 3) Deck (20):
1

What does the data link layer do?

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

2

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

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

3

What are the Data link layer's functions?

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

4

What are the two possible errors that need error control?

- frame contains faulty information
- frame vanish completely

5

What are the two kinds of flow control?

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

6

What are error-correction codes?

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

7

What are Error-detection codes?

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

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?

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

9

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

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

10

What is a binary convolutional code?

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

11

What is Reed-Solomon

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

12

What is LPDC? and what is it good for

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

13

What is the hamming code?

An error correction code.
- linear block code
- performs 1 bit error

14

List the 4 error correction codes

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

15

List the 3 error detection codes

1. Parity
2. Checksum
3. Cyclic Redundancy

16

What is used to handle burst errors?

Interleaving

17

What is Go-back-N?

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

18

What is selective repeat?

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

19

How does sliding window allow two-way communication?

uses piggybacking acknowledgements with data

20

What must sliding window protocols handle?

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