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IEEE 802.11b Wireless LANs

IEEE 802.11b Wireless LANs. The Basics. IEEE 802.11 is the protocol of wireless LAN (WLAN) Security Wired Equivalent Privacy (WEP) not secure at all!. Some Features. Infrastructure mode or “ad hoc” mode Access Point (AP) sends “beacon frames” Mobiles choose AP based on signal strength

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IEEE 802.11b Wireless LANs

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  1. IEEE 802.11b Wireless LANs

  2. The Basics • IEEE 802.11 is the protocol of wireless LAN (WLAN) • Security • Wired Equivalent Privacy (WEP) • not secure at all!

  3. Some Features • Infrastructure mode or “ad hoc” mode • Access Point (AP) sends “beacon frames” • Mobiles choose AP based on signal strength • Multiple channel access protocols supported • Carrier Sense multi access with collision-avoidance CSMA/CA • MAC-layer can provide error control, retransmission, rate adaptation, etc. • Direct Sequence Spread Spectrum (DSSS) • signal spread across 14 22-MHz channels

  4. Compared with Ethernet Similarities: LAN; limited geographic coverage; multiple stations; shared transmission medium; CSMA-based Medium Access Control protocol; 48-bit MAC addresses; comparable data rates

  5. Compared with Ethernet • Differences: • wireless (air interface) versus wired (TP/ coax) • wireless propagation environment (multi-path) • higher error rate due to interference, etc. • successful frames are ACKed by receiver • mobile stations; “hidden node” problem; potential asymmetries • CSMA/CA versus CSMA/CD • multiple data transmission rates

  6. Where does 802.11 live in the OSI? Application Telnet, FTP, Email, Web, etc. Presentation Session TCP, UDP Transport IP, ICMP, IPX Network Logical Link Control - 802.2 (Interface to the upper layer protocols) Data Link MAC Wireless lives at Layers 1 & 2 only! 802.3, 802.5, 802.11 Physical Layer Convergence Protocol Physical LAN: 10BaseT, 10Base2, 10BaseFL WLAN: FHSS, DSSS, IR

  7. Wireless Cells Access Point coverage area is called a “Cell” Access Point Channel 6 ESSID: NAI Range per Access Point is 100m • Each Access Point coverage area is called a “Cell” • Computers can roam between cells 11 Mbps bandwidth “shared” by all devices in the Cell!

  8. Wireless Cells 1 1 6 11 11 1 Computers can roam between cells

  9. CSMA-CA + Acknowledgement Carrier Sense Multiple Access with Collision Avoidance How CSMA-CA works: • Device wanting to transmit senses the medium (Air) • If medium is busy - defers • If medium is free for certain period (DIFS) - transmits frame Latency can increase if “air” is very busy! Device has hard time finding “open air” to send frame! • DIFS - Distributed Inter-Frame Space (approx 128 µs)

  10. CSMA-CA + Acknowledgement Carrier Sense Multiple Access with Collision Avoidance others source destination “Air” is free for DIFS time period DIFS data send frame All other devices must defer while “air” is busy NAV: defer access SIFS Receive ACK back that frame was received intact! ack • Every frame is ack’ed - except broadcast and multicast! • SIFS - Short Inter-Frame Space (approx 28 µs)

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