Quiz 1 Content (Ch. 3) Flashcards Preview

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

What does the data link layer do?

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


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


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


What are the two possible errors that need error control?

- frame contains faulty information
- frame vanish completely


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


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


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


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


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


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


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


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


What is the hamming code?

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


List the 4 error correction codes

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


List the 3 error detection codes

1. Parity
2. Checksum
3. Cyclic Redundancy


What is used to handle burst errors?



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


What is selective repeat?

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


How does sliding window allow two-way communication?

uses piggybacking acknowledgements with data


What must sliding window protocols handle?

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