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802.11 basics

802.11 basics. Richard Dunn CSE 802.11 July 2, 2003. What is 802.11?. Series of standards from IEEE MAC layer standard plus physical layer standards using different technologies (a, b, g) Plus enhancements for security, QoS, interoperability. The physical layer.

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802.11 basics

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  1. 802.11 basics Richard Dunn CSE 802.11 July 2, 2003

  2. What is 802.11? • Series of standards from IEEE • MAC layer standard plus physical layer standards using different technologies (a, b, g) • Plus enhancements for security, QoS, interoperability

  3. The physical layer • 802.11 networks operate in unlicensed spectra • 83.5 MHz band @ 2.4GHz, 300MHz band @ 5GHz • Must prevent interference from other devices, as well as other wireless networks • Interference from other networks • Divide available bandwidth into channels • Adjacent networks use different channels

  4. The physical layer • Interference from other devices • Frequency hopping: within a channel, keep changing the carrier frequency to avoid interference on any particular frequency • Direct sequence: Make each channel wide, but resistant to noise • OFDM: Send a single signal over numerous dense subchannels

  5. ABC’s of 802.11 • 802.11 (no bloody a, b, c, or d) • 3 channels @ 1-2 Mbps in 2.4 GHz using FH or DS • 802.11a • 8+ channels @ 54Mbps in 5GHz using OFDM • 802.11b • 3 channels @ 11Mbps in 2.4GHz using better DS • 802.11g • 3 channels @ 54Mbps in 2.4GHz using OFDM

  6. MAC layer • Two types of access • Ad hoc: stations talk directly to each other, coordinate routing of messages • Infrastructure mode: stations talk to Access Point (AP), AP talks to stations

  7. Access Points • Can act as: • Contention moderator • Gateway to wired LAN or internet • Authenticator • May also connect to others for extended network (think Ethernet bus)

  8. Medium Access • Distributed Coordination Function (DCF) • Nodes contend for access • Best-effort, no guarantees • Only method for ad-hoc • Point Coordination Function (PCF) • Access point moderates channel • All nodes get chance to talk through polling • Can co-exist with DCF

  9. DCF • CSMA/CA • CSMA • All nodes compete, nodes wait for idle channel before transmitting • No collision detection (CD) • Can’t transmit and listen simultaneously • Must use ACKs for all messages

  10. DCF – Collision Avoidance • Don’t want to waste time/energy on full packets • Can send short RTS frames instead, wait for CTS • All frames have duration field to reserve channel for time of reply • Normal backoff if no CTS

  11. 802.11 performance • Physical layers advertise bandwidth • In reality, packet drops + overhead of CA reduces throughput significantly • Distance from AP also reduces • Each AP uses one channel • Users must contend for this throughput

  12. 802.11 performance • May overlap APs to increase throughput • All must operate on separate channel • May knit APs to get wider service area • Neighboring APs must use different channels, or suffer even less throughput • Key is number of available channels

  13. Security • First try: Clearly no receiver will pick up transmission… except any wireless card • Second try: AP authenticates by MAC address • Can simply watch for valid frames and fake it

  14. Security • Third try: Wired Equivalent Privacy (WEP) • Encrypt all communications using single key • Notoriously insecure due to cryptographic weaknesses • Finally: 802.11i • TKIP: Clients can regenerate new keys • RSN: Clients authenticate with server, set up session keys

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