802.11 basics

Richard Dunn

CSE 802.11

July 2, 2003

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

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

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)– 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

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)

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

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

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

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

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

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