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 RJ-45Port

SwitchOr Hub Server

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 10th 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

ShadowZone

MultiplePaths