802 11 Wireless LANs Wireless Technologies for LANs

802. 11 Wireless LANs • Wireless Technologies for LANs – Radio – Infrared light (as in TV remote control) – Ideal for mobile devices – Useful when wiring would be costly

802. 11 Wireless LAN Standards • Standards come from the 802. 11 Working Group – Initially, 1 Mbps and 2 Mbps standards for radio – Too slow for wide acceptance – Now, 11 Mbps – Prices falling but still high – On positive side, avoids the labor cost of installing wiring

802. 11 Wireless LANs • Normally use an Access Point – Bridges wireless device to server on main wired LAN • Box about the size of a hard cover book Access Point UTP Switch Or Hub Server RJ-45 Port

802. 11 Wireless LANs • Ad Hoc Mode – Clients and servers communicate directly – Good for wireless conference rooms – Not scalable Server

802. 11 Wireless LANs • Media Access Control (CSMA/CD+ACK) – CSMA/CA – CSMA with Collision Avoidance • Tries to avoid collisions – When line is clear, station may send (CSMA), – but before it sends, must wait a random amount of time – This prevents stations that have been waiting to transmit from all transmitting at once when the currently transmitting station is finished

802. 11 Wireless LANs • Media Access Control – When a frame is received correctly, the receiver immediately sends back an acknowledgement – This allows the sender to know if it needs to resend Frame ACK

802. 11 Versus Bluetooth • 802. 11 – Designed for site radio LANs • Bluetooth – Created by an industrial consortium – Designed to link nearby objects (within a few meters) – Personal area networking (cellphone, computer, printer, etc. )

802. 11 Versus Bluetooth • Bluetooth – 721 kbps transmission speed • 56 kbps back channel – Up to 10 piconets in an area • Each with a maximum of eight devices – Named for King Harald Bluetooth who unified Denmark and Norway in the 10 th Century

802. 11 Versus Bluetooth • Possible Interference – 802. 11 and Bluetooth use the same frequency band – May interfere if they are active in the same area – 801. 15 Working Group is working on coexistence methods

Radio LANs • Radio Propagation Problems – Shadow zones where signal does not reach – Multipath transmission, in which signals traveling different paths interfere with one another Multiple Paths Shadow Zone