1. basics
Richard Dunn
CSE
July 2, 2003

2. Series of standards from IEEE
What is ?
Series of standards from IEEE
MAC layer standard plus physical layer standards using different technologies (a, b, g)
Plus enhancements for security, QoS, interoperability

3. 802.11 networks operate in unlicensed spectra
The physical layer
networks operate in unlicensed spectra
83.5 MHz 2.4GHz, 300MHz 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

4. 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

5. ABC’s of 802.11 802.11 (no bloody a, b, c, or d) 802.11a 802.11b
3 1-2 Mbps in 2.4 GHz using FH or DS
802.11a
8+ 54Mbps in 5GHz using OFDM
802.11b
3 11Mbps in 2.4GHz using better DS
802.11g
3 54Mbps in 2.4GHz using OFDM

6. 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

7. 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)

8. 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

9. 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

10. 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

11. Physical layers advertise bandwidth
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

12. May overlap APs to increase throughput
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

13. 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

14. 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: i
TKIP: Clients can regenerate new keys
RSN: Clients authenticate with server, set up session keys