802.11 basics Richard Dunn CSE 802.11 July 2, 2003

Series of standards from IEEE 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

802.11 networks operate in unlicensed spectra 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

Interference from other devices 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

ABC’s of 802.11 802.11 (no bloody a, b, c, or d) 802.11a 802.11b 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

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

May also connect to others for extended network (think Ethernet bus) 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)

Distributed Coordination Function (DCF) 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

No collision detection (CD) 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

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

Physical layers advertise bandwidth 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

May overlap APs to increase throughput 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

Second try: AP authenticates by MAC address 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

Third try: Wired Equivalent Privacy (WEP) 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