1. CS541 Advanced Networking 1 Basics of Wireless Networking Neil Tang 1/21/2009

2. CS541 Advanced Networking 2 Outline  Wireless Communication Basics  Wireless Networks  Interference Model  IEEE 802.11 MAC Protocols

3. CS541 Advanced Networking 3 Wireless Communications  There is no physical link in wireless networks. Signals are transmitted on a certain frequency, propagate in the space and are captured by the receiver tuned to the same frequency.  Wireless communication is normally broadcast communication, i.e., all nodes within the transmission range of a particular node can receive the transmitted packets.  Transmissions in a common neighborhood will interfere with each other. If the Signal-to-Interference-Noise-Ratio (SINR) in the receiver is large enough, a packet can be correctly decoded.

4. CS541 Advanced Networking 4 Wireless Networks  Single-hop wireless networks: cellular network, wireless LAN.  Multihop wireless networks: mobile ad hoc network, wireless mesh network, wireless sensor network.

5. CS541 Advanced Networking 5 Interference Model Primary Interference: ABC ABC ABC

6. CS541 Advanced Networking 6 Interference Model Protocol Model (for the fixed transmission power case): AB CD a b

7. CS541 Advanced Networking 7 Interference Model Physical Model (for the variable transmission power case):

8. CS541 Advanced Networking 8 MAC Protocol – 802.11  Basic architecture: Infrastructure mode and ad hoc mode  MAC protocol: CSMA/CA (MACA)

9. CS541 Advanced Networking 9 Basic Architecture AP Infrastructure mode Ad hoc mode

10. CS541 Advanced Networking 10 Hidden Terminal Problem Transmission Range A B C Node A and C are hidden terminals.

11. CS541 Advanced Networking 11 Exposed Terminal Problem Transmission Range A B CD Node A and B are exposed terminals.

12. CS541 Advanced Networking 12 802.11  Distributed Coordination Function (DCF) : Simple protocol with no centralized control.  Point Coordination Function (PCF): APs control medium access and provide collision-free communications. Bandwidth (timeslots) will be carefully allocated to nodes which has signed up for transmission.

13. CS541 Advanced Networking 13 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 are used to determine the time for transmission. Network Allocation Vector (NAV) is used for virtual carrier sensing.  For each transmitted data frame, an ACK frame will be sent back to from the sender. Stop-and-wait protocol are used to provide reliability.

14. CS541 Advanced Networking 14 DCF