TTM4110 - CH7 - The Link Layer (Wireless and Mobile Networks) Link layer is hop-by-hop The data link layer - wireless

  • How wireless links differ from wired
  • Sharing a radio broadcast channel: Multiple access
  • WLAN link layer addressing
  • IEEE 802.11 a/b/g/n/ac
  • Mobile 2G (GSM), 3G (UMTS), 4G (LTE)
  • Securing Wireless LANs

Access networks Wireless access through radio networks of local or wide-area reach

  • Shared wireless access network connects end system to router via base station or “access point” Wireless LANs (WLAN)
  • Within building < 30 m
  • 802.11b/g/a/n (WiFi): 11/54/>100 Mbps Mobile broadband - wider-area radio access
  • Access provided by telco/cellular operator
  • 10s of km (0,2 - 100 Mbps (Edge, UMTS/WDMA, HSPA, LTE)) Unguided physical media: Radio signals are carried within the electromagnetic spectrum
  • No physical “wire”, bidirectional
  • propagation environment effects
    • Reflection
    • Obstruction by objects
    • Interference
  • Lots of satellites in different orbits and missions Earth’s atmospheric transmittance to electromagnetic waves Some frequencies are more valuable than others

7.1 Introduction

Wireless hosts Wireless links Base station Network infrastructure Single-hop, infrastructure-based Single-hop, infrastructure-less Multi-hop, infrastructure-based Multi-hop, infrastructure-less Ulisensierte frekvensbånd: Lovdata om hvilket frekvensbånd som er tillatt Wireless and mobility are two different things

  • Wireless: Communication over a wireless link
  • Mobility: handling the mobile user who changes point of attachment to the (mobile/cellular) network

    7.2 Wireless links and Network characteristics

    Make communication across wireless links more challenging than wired communication Path loss

  • Radio signal attenuates as it propagates through matter
  • Lower frequency, larger coverage area
  • Signal strength = (distance)^(1/3) Interference between signals
  • Signal phase has an effect on amplitude when receive multiple signals
  • Signal sharing - standardized wireless network frequencies (e.g. 2.4GHz) shared by other devices (e.g. phone); devices (motors, microwave)
  • Frequency harmonic Multipath propagation
  • Radio signal reflects off objects ground, arriving destination at slightly different times - interference Wireless network characteristics: Physical layer adapts to channel conditions
  • The signal-to-noise ratio (SNR) is a measure of the strength of the received signal
  • A higher SNR, a lower bit error rate (BER) – easier to extract signal from noise
    • Given physical layer: increase power -> increase SNR->decrease BER
    • SNR given: a higher bit transmission rate will have a higher BER
  • SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) to channel conditions
    • SNR decreases, BER increase as node moves away from base station
    • When BER becomes too high, switch to lower transmission rate but with lower BER. On an analog channel a modem (modulator-demodulator) converts between digital bits and signals

7.3 IEEE 802.11 wireless LANs (“Wi-Fi”) Frame

802.11: passive/active scanning to associate with an access point

IEEE 802.11: CSMA/CA for wireless medium access carrier sense multiple access / collision avoidance

  • CSMA – carrier sense before transmitting
    • don’t collide with ongoing transmission by other node
  • CA - collision avoidance
    • may be difficult to receive/sense collisions when transmitting due to weak received signals: fading
    • can’t sense all collisions: hidden terminal

7.4 Cellular Internet Access