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185 lines
5.6 KiB
Markdown
185 lines
5.6 KiB
Markdown
---
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title: "21-data-link-layer"
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aliases:
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tags:
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- cosc203
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- lecture
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sr-due: 2022-10-21
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sr-interval: 3
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sr-ease: 250
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---
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# data link layer services
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- transfer datgram from one node to **physically adjacent** node over a link
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- terminology
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- hosts and routers: node
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- communication channels between adjacent nodes: links
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- wired
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- wireless
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- LANs
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- layer-2 packet: frame, encapsulates datagram
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- 
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services
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- framing, link access:
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- encapsulate datgram into frame, add head, trailer
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- do not use IP address
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- use MAC address in frame headers
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- reliable delivery of data between adjacent nodes
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- flow control: pacing between adjacent sending and recieving nodes
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- error detection and correction
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- errors from: signal attenuation, noise
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- receiver detects errors and signals retransmission, or drops frame
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- reciever identifies and correct bit error(s) without retransmission
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implemented in network interface card: (NIC) or on a chip
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- ethernet, WiFi card or chip
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- implements link and physical layer
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each NIC has a unique ID - MAC (medium access control)
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- 48 bits: six groups of two hex digits
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- also called physical address, ethernet hardware address
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- manufacturer buys portion of MAC adress space (to assure uniqueness)
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## flow control
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- dont send to much data otherwise it will overflow
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different from transport layer flow controls
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- single link not end to end
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- frame oriented not byte oriented
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- als use sliding window scheme
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- go back N and selective repeat
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# Error detection
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- add bit(s) to detect and correct bit errors (i.e., redundancy)
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- bits are added before frame is send over link
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- when recieved if some bits are corrupted frame is dropped
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## parity checking
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single bit
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- even parity: add parity bit and set so there is an even number of 1s
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- can detect single bit errors
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- cannot detect two bit errors
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two dimensional bit parity
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- detect and correct single bit errors
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- detect two bit errors
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## CRC
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- more powerful error-checking coding
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- D: data bits
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- G: bit pattern (generator), of r+1 bits (given)
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- widely used
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# multiple access links and protocols
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- single shared broadcast linl/channel
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- shared wire or medium
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- interference between two or more simultaneous tranmissions
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- collision may occur if node recieves two or more signals at the same time
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## MAC protocols
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- coordinates nodes sharing a channel
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- communication about channel sharing must use channel itself
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- no out-of-band channel for coordination
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classes
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- channel partitinioning
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- divide channel into smaller pieces: (time slots, frequency)
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- allocate piece to node for exclusive use
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- e.g., TDMA, FDMA
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- random access: contention-based
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- channel not divided, allows for collisions
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- "recover" from collisions
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- e.g., ALOHA, CSMA
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- "taking turns"
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- node takes turns, but nodes with more to send can take longer turns
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- e.g., token-passing
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### TDMA time division mulitple access
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- acces to channel in "rounds"
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- each station gets a fixed length slot (length = packet transmission time) in each round
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- unused slots go idle
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### FDMA freqency division multiple access
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- channel spectrum divided into frequency bands
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- each station assigned fixed freqency band
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- unused bands go idle
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- 
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- can communicate in parrallel
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### slotted ALOHA
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assumptions
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- all frames same size
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- tiem divided into equal-size slots (time to transmit one frame)
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- start to transmit only slot beginning
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- nodes are synchronized
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- if two or more nodes transmit in a slot, all nodes detect collision
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operation
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- when node obtains fresh frame, transmits in next slot
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- if no collision: node can send new frame in next slot
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- if collision: node retransmits frame in each subsequent slot with probability p until success
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- simple
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- decentralized
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- single node transmit at full rate
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- wasting slots: collisions idle slots
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- requires synchronisation
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### pure ALOHA
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- unslotted aloha, no sync
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- when first frame arrives, transmit immediately
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- collision probability increases with no sync
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- less efficient that slotted aloha
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-
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### CMSA carrier sense multiple access
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simple CSMA: listen before transmit:
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- idle: transmit entire frame
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- busy: defer transmission
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CSMA/CD: CSMA with collision detection
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- collisions detected within short time
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- colliding transmissions aborted, reducing channel wastage
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collisions can still occur with carrier sensing
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- propagation delay means two nodes may not hear each others just-started transmissions
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- on a collision the entire packet transmission time is wasted
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- distance and propagation delay play role in determining collision probability
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### Ethernet CSMA/CD
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1. nic recieves datagram and created frame
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2. nic senses channel:
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1. if idle, start transmission
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2. else wait wait until idle
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3. if nic transmits entire frame without collision, nic is done
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4. if nic detect another transmission while sending, abort, send jam signal
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5. after aborting, nic enters binary (exponential) backoff:
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1. after mth collision, nic chooses K at random from {0, 1, 2, ..., 2^(m)-1}, nic waits K x 512 bit times, returns to step 2
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2. more collisions: longer backoff interval
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### taking turns
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polling:
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- master node invites other nodes to transmit in turn
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- typically used with "dumb" devices
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- concerns:
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- polling overhead
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- latency
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- single point of failure: master
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token passing:
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- control token passed from one node to next sequentially
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- a node can send data when it has the token
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- token message
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- concerns:
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- token overhead
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- latency
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- signle point of failure: token |