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Wireless Local Area Networks

Properties

  • Communication range
    • Lower frequency → less power needed to gain longer distance
  • Latency
  • Architecuture
  • Cost of creating the network

Wireless LANs

  • IEEE 802.11 family, HiperLAN, …
  • IEEE standards dominant
    • Wi-Fi (Wireless Fidelity)
    • Certified 802.11 standard family compliant product
    • Trademark of Wi-Fi alliance
  • Supported by most types of devices
    • Built into most laptops, PDAs & (newer) mobile phones
    • Add capability via PCMCIA, CF cards, USB dongles, etc
  • Global standards → same client works everywhere
  • Core elements of ethernet present
    • 48-bit MAC
    • Frames delivery based on MAC
    • Additional reliability mechanisms
  • Supports both ad hoc and infrastucture networks

2.4 GHz ISM

  • Industrial Scientific & Medical
  • Used by most WLAN standards
  • Free for use, no regulations
    • Maximum transmission power
  • Create own network is free → no licenses needed
  • No regulation → interference problem
  • No strong authentication
  • Nominal speed 1Mb/s - 100Mb/s
  • Relatively high power consumption

802.11 family

Specs Speed Frequency Outdoor range
802.11a 54Mbps 5GHz 120 metersnon-free frequency
802.11b 11Mbps 2.4GHz 140 meters Wi-Fi
802.11g 54Mbps 2.4GHz 140 meters speed update to b
802.11n 100Mbps 2.4/5GHz 250 meters speed update to g

Network types

  • Three different ways to connect devices to each other
    • Independent Basic Service Set (IBSS)
      • Ad Hoc -network
      • No access to external networks
      • Devices connected to each other directly (like in eg. Bluetooth)
        • No need for base stations
    • Basic Service Set (BSS)
      • Ad Hoc network with base station
      • Connection to network via base station
    • Extended Service Set (ESS)
      • Infrastructure network
      • one or more acces points linked together to form access network to Internet
  • Access points
    • Directional / omnidirectional RF
    • Range is from 10's - 100's meters
    • APs usually have an ethernet connection
    • act as bridge between wireless and wired Internet
    • Many brands to chose from
      • Lower cost

Connection creation

  • Connection formed between AP ↔ mobile / mobile ↔ mobile
    • Desired network found by scanning
      • Service Set Identifier (SSID) → network identifier
      • Active scan
        • Send probe frame to inform network SSID device wants to join
        • Using broadcast SSID in probe results response from all networks within range
      • Passive scan
        • Listening communication on each WLAN channel to find out which is used by desired SSID
  • Responses
    • AP responds to the probes
    • On Ad-Hoc networks station that generated last beacon frame responds
  • After desired network is found - authentication & association are conducted
    • Open system authentication
      • Authentication based on MAC address
    • Shared key authentication
      • WEP / WPA must be enabled
  • 1 AP serves up to 100 devices
  • Latency
    • Few milliseconds to hundreds of milliseconds
    • Processing connections increases latency
      • The more the users the higher the latency
    • Roaming is possible
      • Not always supported
      • Compatibilties issues between different vendor products

Wi-Fi direct

Considerations

  • Who creates the network?
  • Whats the desired network size?
  • Cost of maintaining own network?
  • Billing arangements
  • Rely on TCP/IP
  • How to conduct service discovery?
    • Dedicated SSID for a device providing certain type of service?
  • Security?
Last modified: 2014/02/19 09:57