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lecture07-datalink1.md

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Lecture 7 Data Link I

Datalink Functions

  • Framing: encapsulating a network layer datagram into a bit stream
  • Error control: error detection and correction to deal with bit errors
  • Flow control: avoid sender overrunning receiver
  • Media Access Control (MAC): which frame should be sent over the link next?
    • Easy for point-to-point links

Framing

  • A link layer function, defining which bits have which function
  • Minimal functionality: mark the beginning and end of packets (or frames)
  • Delimiter based
    • SYN: sync character
    • SOH: start of header
    • STX/ETX: start of text / end of text

Character and Bit Stuffing

  • Mark frames with specical character
  • Mark frames with special bit sequence
    • must stuff a zero any time seven 1s appear

Ethernet Framing

  • Preamble is 7 bytes of 10101010 followed by one byte of 10101011
  • Allows receivers to recognize start of transmission after idle channel

Link Flow Control and Error Recovery

  • Dealing with receiver overflow: flow control
  • Dealing with packets loss and corruption: error control
  • These issues are relevant at many layers
    • Link Layer: sender and receiver attached to the same wire
    • End-to-end: transmission control protocol (TCP) - sender and receiver are the endpoints of a connection

Flow Control

  • Stop-and-wait flow control
    • Sender waits to send the next packet until the previous packet has been acknowledged by the receiver
  • Window flow control
    • Receiver provides sender with a window that it can fill with packets
      • The window is backed up by buffer space on receiver
      • Receiver acknowledges the a packet every time a packet is consumed and a buffer is freed
    • Max Throughput = Window Size / RTT

Error Recovery

  • Error Correcting Codes (ECC)
    • Send extra redundant data to help repair losses
  • Automatic Repeat Request (ARQ)
    • Receiver sends ACK when it receives packet
    • Sender uses ACKs to identify and resend data that was lost
  • Use sequence numbers to recognize retransmissions
  • In practice
    • No flow or error control (e.g., 100Mbps Ethernet, just CRC for error detection)
    • Flow control only (e.g., Gigabit Ethernet)
    • Flow and error control (e.g., 802.11 WiFi)
  • Flow and error control solutions also used in higher layer protocols
    • e.g., TCP for end-to-end flow and error control

Datalink Architectures

  • Switches connected by point-to-point links - store-and-forward
    • Used in WAN, LAN, and for home connections
    • MAC scheduling
  • Multiple access networks - contention based
    • Multiple hosts are sharing the same transmission medium

Scheduling for Medium Access Control

  • Schedule frame transmissions to avoid collision in shared medium
  • Reservation Systems
    • Centralized systems: A central controller accepts requests from stations and issues grants to transmit
    • Distributed systems: Stations implement a decentralized algorithm to determine transmission order
    • Transmission organized into cycles (reservation interval + frame transmission)
    • Single or multiple frames per frame
    • Channelized or Random Access Reservations

Contention-based Access

  • Aloha
    • When node has packet to send, transmit at full channel data rate R
    • Two or more transmitting nodes -> collisions
    • Random access MAC protocol specifies:
      • How to detect collisions
      • How to recover from collisions (e.g., via delayed retransmissions)
    • Random from collision by trying after random delay
    • Poisson Process: memory less
  • Pure Aloha
    • Total rate of packets attempting transmission = newly generated packets + retransmitted ones = $\lambda' > \lambda$
    • Total traffic intensity: $G = N\lambda'm$
  • Slotted Aloha
    • Time is divided into equal size slots - equal to packet retransmission time
    • If collision, retransmit packet in future slots with probability p, until successful
    • The vulnerable period of the packet of size m has been reduced from 2m to only m
    • Can achieve higher throughput
  • Ethernet MAC
  • Ethernet Frames

Carrier Sense Multiple Access / Collision Avoidance (CSMA/CA)

csma_ca

  • Exponential Backoff
    • Why not random delay with fixed mean?
      • Few senders -> needless waiting
      • Too many senders -> too many collisions
    • Goal: adapt retransmission attempts to estimated current load
    • Delay is set as K slots - control K
      • First collission: choose K from {0, 1}
      • Second collision: choose K from {0, 1, 2, 3}
      • After ten collisions: choose K from {0, 1, 2, 3, ..., 1023}