draw basic diagram of communication channel (L4 2)
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define entropy and write equation (L4 4)
- 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)
calculate entropy of a binary symmetric channel (BSC) (L4 5)
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define joint entropy and write equation (L4 7)
measures how much uncertainty there is in the two random variables taken together
write equations of mutual information and channel capacity (L4 8)
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Main ideas of automatic repeat request (ARQ)
- 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
draw frame structure at DLC layer (L4 12)
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list types of ARQ
- stop-and-wait
- modified stop-and wait
- Go back n
- selective repeat ARQ
Main ideas of stop-and-wat ARQ
- 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
draw schematic of major problem of stop-and-wait ARQ (L4 15)
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main problem of modified stop-and-wait ARQ
channel is not used optimally and there will be repeated frames
main ideas of Go back n ARQ (L4 20)
- 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
main properties of selective repeat ARQ (L4 23)
- 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
functions of DLC layer
- 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
give an example of even single parity check (L4 28)
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main ideas of cyclic redundancy check
- parity checks codes used for error detection are CRC
- CRC codes are based on polynomial arithmetic
Example of CRC (division of polynomials over GF(2)) (L4 35)
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