1. CS440 Computer Networks 1 Wireless LAN (IEEE 802.11) Neil Tang 10/01/2008

2. CS440 Computer Networks 2 Outline  Basic Architecture  Physical Properties  MAC Protocols  WLAN with Multiple APs

3. CS440 Computer Networks 3 Wireless LAN (802.11)  Basic Architecture: Infrastructure mode and ad hoc mode  MAC Protocol: CSMA/CA (MACA)

4. CS440 Computer Networks 4 Basic Architecture AP Infrastructure mode Ad hoc mode

5. CS440 Computer Networks 5  Encoding & Modulation: FHSS (Frequency Hopping Spread Spectrum), DSSS (Direct Sequence Spread Spectrum), OFDM (Orthogonal Frequency Division Multiplexing)  Spectrum: 2.4GHz/5GHz  Error Detection: CRC  Maximum Data Rate: 11Mbps/54Mbps  Transmission Range: 100-1000m Physical Properties

6. CS440 Computer Networks 6 Hidden Terminal Problem Transmission Range A B C Nodes A and C are hidden terminals.

7. CS440 Computer Networks 7 Exposed Terminal Problem Transmission Range A B CD Nodes A and B are exposed terminals.

8. CS440 Computer Networks 8 MAC Protocols  Distributed Coordination Function (DCF) : Simple protocol with no centralized control.  Point Coordination Function (PCF): APs control medium access and provide collision-free communications.

9. CS440 Computer Networks 9 DCF Basic Idea: CSMA/CA (MACA)  Request-To-Send (RTS) and Clear-To-Send (CTS) are used to reserve space and time for transmission.  Both physical and virtual carrier sensing (Network Allocation Vector (NAV)) are used to determine the time for transmission.  ACK frame and stop-and-wait protocol  The exponential backoff algorithm

10. CS440 Computer Networks 10 DCF

11. CS440 Computer Networks 11 WLAN with Multiple APs AP2 AP1 Distribution System A B

12. CS440 Computer Networks 12 WLAN with Multiple APs AP Selection (Scanning)  The node sends a Probe frame.  All APs within reach reply with a Probe Response frame.  The nodes selects one of the access points based on signal strength or other metrics, and sends that AP an Association Request frame.  The AP replies with an Association Response frame.  Active scanning Vs. passive scanning

13. CS440 Computer Networks 13 Frame Format Address Fields  Normal Case (point-to-point): Addr1(destination), Addr2(source)  Complex Case (routing via distribution system): Addr1(ultimate destination), Addr2(intermediate sender), Addr3(intermediate destination), Addr4(original source) Addr4PayloadSeqCtrlAddr3Addr2Addr1CRC 0 -18,49648163248 Duration 16 Control 16