CSCI 465 D ata Communications and Networks Lecture 23 Martin van Bommel CSCI 465 Data Communications & Networks 1

Wireless LANs IEEE standard – nicknamed WiFi – Operates in unlicensed frequency bands Permitted if limited transmission power MHz, GHz, GHz Several variations – LAN extension Access points (base stations) connected via wired LAN Cross-building interconnect – point-to-point wireless Ad-hoc network – peer-to-peer without access point CSCI 465 Data Communications & Networks 2

Initial Standard Defined in 1997 Operated at 1 Mbps or 2 Mbps Operated by hopping between frequencies or spreading signal across allowable spectrum Instantly declared by users as too slow CSCI 465 Data Communications & Networks 3

Transmission Techniques Spread-spectrum LANs – Signal spread across multiple frequencies OFDM LANs – Orthogonal Frequency Division Multiplexing Uses multiple carrier signals at different frequencies Bits spread over signals CSCI 465 Data Communications & Networks 4

Original Direct-sequence spread spectrum (DS-SS) 2.4 GHz band at 1 Mbps and 2 Mbps Up to seven channels – 5 MHz each Depends on bandwidth available – Unlicensed in North America – 13 channels available in most European countries – Only 1 available in Japan CSCI 465 Data Communications & Networks 5

802.11b & a b – Extension of DS-SS scheme – Data rates of 5.5 and 11 Mbps a – Uses 5GHz band and more bandwidth Less interference in 5GHz band – Up to 54 Mbps – Has variety of modulation and coding options CSCI 465 Data Communications & Networks 6

802.11g and n g – Extension to b in 2.4 GHz band – Combines techniques for up to 54 Mbps n – Multiple-input-multiple-output (MIMO) antenna Increases signal-to-noise ratio – Uses 40MHZ channel with OFDM – twice the rate – MAC enhancements – More throughput on shared system CSCI 465 Data Communications & Networks 7

MAC Sublayer Protocol Very different from Ethernet – Radios are half-duplex cannot listen while transmitting – Cannot use collision detection CSMA/CA – collision avoidance – Channel sensing before sending – Backoff period used before sending once idle – Frames are acknowledged – lack indicates collision – Exponential backoff after collisions CSCI 465 Data Communications & Networks 8

9 IEEE MAC Logic IFS = Inter-frame Space

Reliable Data Delivery includes frame exchange protocol – Station receiving frame sends ACK frame – If no ACK, retransmit Wireless unreliable – Noise, interference, and other effects – Even with error-correction codes, frames may not be successfully received Protocols to adjust speed and frame length CSCI 465 Data Communications & Networks 10