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Flashcards

2DatalinkLayer

Francis Du ETH osnet phys Alemão 43 flashcards

Flashcards do conjunto (43)

  1. 1Link Layerhow to transfer messages = frames over one or more links
  2. 2Framing method: Byte countStart each frame with length field
    but difficult to re sync after error
  3. 3Byte stuffinghave special FLAG byte that menas start/end of frame
  4. 4Bit stuffingcan stuff at bit level too
    6 consecutive 1's is a flag
  5. 5PPPpoint to point protocol, widely used for link framing
    E.g. used to frame IP packets sent over SONET links
  6. 6SONETbit stream
  7. 7Error CodesData bits D + Check bits R = fn(D)
    Sender sends D & R receiver calculates R' nd checks if same as R
  8. 8Hamming Distancemin number of bit flips to change D+R to D+R'
  9. 9Error detectionfor code of HD d+1 up to d errors will be detected
  10. 10Error correctionfor code of HD 2d+1 up to d errors can always be corrected
  11. 11Simple Error detection: parity bittake D data bits, add 1 checkbit that is the sum of D bits, sum is modulo 2 or XOR of D
  12. 12Internet Checksum(sender)1. Arrange data in 16 bit words
    2.Add 0000
    3.Add any carry over
    4. Negate to get checksum
  13. 13Internet Checksum(receiver)1. Arrange data in 16 bit words
    2. Add checksum
    3.Add carry over
    4.Negate and check if zero
  14. 14Error D in practiceCRC's widely used in links
    Internet Checksum used in internet
  15. 15Why is Error correction hard?error could be in check bits as well as in data bits
  16. 16Hamming CodeSee example
  17. 17ARQautomatic repeat request
    often used when errors common or must be corrected
  18. 18ARQ retransafter timeout
    easy on lan not over internet
  19. 19sequence numberto distinguish current frame from next one single bit is sufficient
    called stop and wait
  20. 20sliding windowallows W frames to be outstanding/ sent in RTT(=2D)
  21. 21time division multiplexingusers take turns on a fixed schedule
  22. 22frequency division multiplexingdifferent users on different frequency bands
  23. 23tdm vs fdmtdm: users sends at high rate a fraction of time
    fdm:low rate all the time
  24. 24multiplexing network traffictwo users each need R -> together need R'< 2R
  25. 25randomized multiple accessmultiple access control or medium access control = MAC
  26. 26ALOHA protocolnode just sends when it has traffic
    if there was collision(no ack) wait a random time and resend
  27. 27CSMAcarrier sense multiple access
    improve aloha by listening for activity before sending
  28. 28CSMA/CDcollision detection
    time window of hearing a collision is 2D
    therefore impose min frame size of 2D seconds so node cant finish before collision
  29. 29CSMA persistenceif there are N queued senders want each to send next with probability of 1/N
  30. 30BEBbinary exponential backoff
    nth collision wait 2 frame times: 0 to 7 for 3rd collision
  31. 31classic ethernet or IEEE802.3crc-32 for error detecion no acks or retrans
  32. 32hidden terminalscan't hear each other yet collide at a node
  33. 33exposed terminalshear each other but dont collide
  34. 34nodes can't hear while sending solution?MACA
    1.sender node transmits a RTS
    2.receiver replies with a CTS
    3. sender transmits the frame while nodes hearing the cts stay silent
  35. 35contention free multiple accessbased on turns not randomization
  36. 36issues with random multiple accesscsma good under low load but not high load due to high overhead & access time varies
  37. 37token ringarrange nodes in a ring token rotatoes permission to send
  38. 38turn taking advantagesfixed overhead: more efficient under load
    regular chance to send: predictable service
  39. 39switched ethernethosts are wired to ethernet switches
  40. 40inside hub & inside switchhub: ports wired together more convenient and reliabe
    switch: uses frame addresses to connect right input port with output port
  41. 41switch multiple input for one outputneed buffer-> sustained load will fill buffer and lead to frame loss
  42. 42switch forwarding how?backward learning: forward frames with a port/address table as follows:
    1.to fill table looks at source address of input frames
    2.to forward it sends to the port or broadcasts to all ports
  43. 43forwarding loop solutionsspanning tree

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ETH osnet phys FlashCards sobre 2DatalinkLayer, criado por Francis Du em 25-11-2017.
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Francis Du
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