quartz/content/notes/21-data-link-layer-LAN.md

title	aliases	tags
21-data-link-layer-LAN		cosc203 lecture

data link layer services

• transfer datgram from one node to physically adjacent node over a link
• terminology
  • hosts and routers: node
  • communication channels between adjacent nodes: links
    • wired
    • wireless
    • LANs
  • layer-2 packet: frame, encapsulates datagram
    • 

services

• framing, link access:
  • encapsulate datgram into frame, add head, trailer
  • do not use IP address
    • use MAC address in frame headers
• reliable delivery of data between adjacent nodes
  • flow control: pacing between adjacent sending and recieving nodes
  • error detection and correction
    • errors from: signal attenuation, noise
    • receiver detects errors and signals retransmission, or drops frame
    • reciever identifies and correct bit error(s) without retransmission

implemented in network interface card: (NIC) or on a chip

• ethernet, WiFi card or chip
• implements link and physical layer

each NIC has a unique ID - MAC (medium access control)

• 48 bits: six groups of two hex digits
• also called physical address, ethernet hardware address
• manufacturer buys portion of MAC adress space (to assure uniqueness)

flow control

• dont send to much data otherwise it will overflow

different from transport layer flow controls

• single link not end to end
• frame oriented not byte oriented
• als use sliding window scheme
  • go back N and selective repeat

Error detection

• add bit(s) to detect and correct bit errors (i.e., redundancy)
• bits are added before frame is send over link
• when recieved if some bits are corrupted frame is dropped

parity checking

single bit

• even parity: add parity bit and set so there is an even number of 1s
• can detect single bit errors
• cannot detect two bit errors

two dimensional bit parity

• detect and correct single bit errors
• detect two bit errors

CRC

• more powerful error-checking coding
• D: data bits
• G: bit pattern (generator), of r+1 bits (given)

• widely used

multiple access links and protocols

• single shared broadcast linl/channel
  • shared wire or medium
• interference between two or more simultaneous tranmissions
  • collision may occur if node recieves two or more signals at the same time

MAC protocols

• coordinates nodes sharing a channel
• communication about channel sharing must use channel itself
  • no out-of-band channel for coordination

classes

• channel partitinioning
  • divide channel into smaller pieces: (time slots, frequency)
  • allocate piece to node for exclusive use
  • e.g., TDMA, FDMA
• random access: contention-based
  • channel not divided, allows for collisions
  • "recover" from collisions
  • e.g., ALOHA, CSMA
• "taking turns"
  • node takes turns, but nodes with more to send can take longer turns
  • e.g., token-passing

TDMA time division mulitple access

• acces to channel in "rounds"
• each station gets a fixed length slot (length = packet transmission time) in each round
• unused slots go idle

FDMA freqency division multiple access

• channel spectrum divided into frequency bands
• each station assigned fixed freqency band
• unused bands go idle
• 
• can communicate in parrallel

slotted ALOHA

assumptions

• all frames same size
• tiem divided into equal-size slots (time to transmit one frame)
• start to transmit only slot beginning
• nodes are synchronized
• if two or more nodes transmit in a slot, all nodes detect collision

operation

• when node obtains fresh frame, transmits in next slot

  • if no collision: node can send new frame in next slot
  • if collision: node retransmits frame in each subsequent slot with probability p until success

• simple

• decentralized

• single node transmit at full rate

• wasting slots: collisions idle slots

• requires synchronisation

pure ALOHA

• unslotted aloha, no sync
  • when first frame arrives, transmit immediately
• collision probability increases with no sync