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2DatalinkLayer
Fichas del conjunto (43)
- 1Link Layerhow to transfer messages = frames over one or more links
- 2Framing method: Byte countStart each frame with length field
but difficult to re sync after error - 3Byte stuffinghave special FLAG byte that menas start/end of frame
- 4Bit stuffingcan stuff at bit level too
6 consecutive 1's is a flag - 5PPPpoint to point protocol, widely used for link framing
E.g. used to frame IP packets sent over SONET links - 6SONETbit stream
- 7Error CodesData bits D + Check bits R = fn(D)
Sender sends D & R receiver calculates R' nd checks if same as R - 8Hamming Distancemin number of bit flips to change D+R to D+R'
- 9Error detectionfor code of HD d+1 up to d errors will be detected
- 10Error correctionfor code of HD 2d+1 up to d errors can always be corrected
- 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
- 12Internet Checksum(sender)1. Arrange data in 16 bit words
2.Add 0000
3.Add any carry over
4. Negate to get checksum - 13Internet Checksum(receiver)1. Arrange data in 16 bit words
2. Add checksum
3.Add carry over
4.Negate and check if zero - 14Error D in practiceCRC's widely used in links
Internet Checksum used in internet - 15Why is Error correction hard?error could be in check bits as well as in data bits
- 16Hamming CodeSee example
- 17ARQautomatic repeat request
often used when errors common or must be corrected - 18ARQ retransafter timeout
easy on lan not over internet - 19sequence numberto distinguish current frame from next one single bit is sufficient
called stop and wait - 20sliding windowallows W frames to be outstanding/ sent in RTT(=2D)
- 21time division multiplexingusers take turns on a fixed schedule
- 22frequency division multiplexingdifferent users on different frequency bands
- 23tdm vs fdmtdm: users sends at high rate a fraction of time
fdm:low rate all the time - 24multiplexing network traffictwo users each need R -> together need R'< 2R
- 25randomized multiple accessmultiple access control or medium access control = MAC
- 26ALOHA protocolnode just sends when it has traffic
if there was collision(no ack) wait a random time and resend - 27CSMAcarrier sense multiple access
improve aloha by listening for activity before sending - 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 - 29CSMA persistenceif there are N queued senders want each to send next with probability of 1/N
- 30BEBbinary exponential backoff
nth collision wait 2 frame times: 0 to 7 for 3rd collision - 31classic ethernet or IEEE802.3crc-32 for error detecion no acks or retrans
- 32hidden terminalscan't hear each other yet collide at a node
- 33exposed terminalshear each other but dont collide
- 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 - 35contention free multiple accessbased on turns not randomization
- 36issues with random multiple accesscsma good under low load but not high load due to high overhead & access time varies
- 37token ringarrange nodes in a ring token rotatoes permission to send
- 38turn taking advantagesfixed overhead: more efficient under load
regular chance to send: predictable service - 39switched ethernethosts are wired to ethernet switches
- 40inside hub & inside switchhub: ports wired together more convenient and reliabe
switch: uses frame addresses to connect right input port with output port - 41switch multiple input for one outputneed buffer-> sustained load will fill buffer and lead to frame loss
- 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 - 43forwarding loop solutionsspanning tree
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- Descripción
- ETH osnet phys Fichas sobre 2DatalinkLayer, creado por Francis Du el 25/11/2017.
- Creado por
- Francis Du
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