1. 1 The Case for Heterogeneous Wireless MACs Chun-cheng Chen Haiyun Luo Dept. of Computer Science, UIUC

2. 2 Problem#1: intra-BSS interference Clear channel assessment (CCA): transmit iif RSS < CS threshold Downlink tx: all clients hear from AP C AB Tx RSS CS Thrshld time CS RSS CS Thrshld CS RSS CS Thrshld CS

3. 3 Problem#1: intra-BSS interference Clear channel assessment (CCA): transmit iif RSS < CS threshold Downlink tx: all clients hear from AP Uplink tx: clients may NOT hear from each other C AB CS Thrshld RSS CS RSS CS Thrshld

4. 4 Problem#1: intra-BSS interference Uplink tx: clients may NOT hear from each other Observation: they all hear from the AP Solution: four-way handshake (RTS/CTS/DATA/ACK) C AB RTS CTS

5. 5 Problem#1: intra-BSS interference Uplink tx: clients may NOT hear from each other Observation: they all hear from the AP Solution: four-way handshake (RTS/CTS/DATA/ACK) C AB DATA ACK

6. 6 Problem#1: intra-BSS interference Uplink tx: clients may NOT hear from each other Observation: they all hear from the AP Solution: four-way handshake (RTS/CTS/DATA/ACK) Caveats RTS/CTS involves 29-37% overhead Not necessary for all clients C AB D DATA RTS 20 bytes PreambleHeader 72~144 bits 48 bits @ basic_rate: 2Mbps – 802.11b 6Mbps – 802.11a/g CCA is inconsistent. When and with which client should RTS/CTS be enabled ?

7. 7 Problem#2: inter-BSS interference Optimal channel assignment 11 1 6 66 1 1 1 1 6

8. 8 Optimal channel assignment Clients on different BSSs interfere with each other Problem#2: inter-BSS interference D A B C

9. 9 Hidden/exposed terminal problem Exposed receiver and hidden sender CCA @ sender C and A is incomplete If RTS/CTS is enabled with C -> D, CCA @ A is complete but CCA @ C is still incomplete Problem#2: inter-BSS interference D A B C Exposed Receiver Hidden Sender

10. 10 Hidden/exposed terminal problem Exposed receiver and hidden sender When CCA is inconsistent, the sender with more complete CCA dominates starvation Problem#2: inter-BSS interference D A B C Flow A->B always dominates Inconsistent CCA @ sender A, C

11. 11 Problem#2: inter-BSS interference Hidden/exposed terminal problem Exposed receiver and hidden sender Hidden receiver CCA at senders C and B is incomplete RTS/CTS helps only if D and A within communication range D A B C ACK

12. 12 Problem#2: inter-BSS interference Can we have more orthogonal channels? Dividing frequency band into more orthogonal channels does not serve bursty traffic well Channel assignment may not be optimal 11 1 6 66 1 1 1 1 6

13. 13 Problem#2: inter-BSS interference What about Receiver-initiated MAC ? CCA @ receivers are still incomplete and inconsistent D A B C

14. 14 Road Map Motivations Goals Heterogeneous wireless MACs Intra-BSS interference mitigation Inter-BSS interference mitigation Evaluation Conclusion and future work

15. 15 Our Goals: Collision avoidance Starvation avoidance Minimize MAC overhead

16. 16 Heterogeneous Wireless MACs Idea: make the control tailored to context Turn on RTS/CTS only when necessary Control medium access from the node with better CCA MAC protocols will be Context-Dependent Because contexts are heterogeneous, MAC protocols will be heterogeneous Approach Define a set of MAC protocols Learn from the context, apply the best MAC

17. 17 Intra-BSS interference mitigation Example: A node enables RTS/CTS only if another node does not hear its transmission C AB D DATA D doesnt need to turn on RTS/CTS A, B, C need to turn on RTS/CTS

18. 18 If signal propagation is symmetric: All stations initially turn off RTS/CTS If signal propagation is asymmetric: Need explicit feedback relayed by AP When to turn on/off RTS/CTS? ACK RSS CSThrshld One pkt trans. time C AB D DATA ACK RSS CSThrshld One pkt trans. time ACK RSS CSThrshld A, C learned to turn on RTS/CTS! One pkt trans. time

19. 19 Inter-BSS interference mitigation Two hidden/exposed terminal problems not solved by 802.11: Observation: node A and D have complete and consistent CCA! Let A and D always initiate the transmission DA C B DA C B

20. 20 When to use which MAC? Example: All stations initially sender-initiated MAC: Define a set of control messages for inter-operability RTS/CTS/ACK/RTR Diff. protocols may interpret them differently D A B C If success ratio too low switchMac(). DATA Set one bit in DATA header to notify receiver of protocol switch! RTR

21. 21 time (sec) Success ratio Topology: prev. example Using: ns-2.28, TwoRayGround, 2Mbps basic rate, 11Mbps data rate CBR/UDP flows (flow 2 3 0~45sec; flow 0 1 10~45 sec), sender-initiated MAC initially Metrics: throughput, success ratio Evaluation 30 2 1 time (sec) Normalized inst. thrput Learning period

22. 22 Conclusion and future work Incomplete CCA leads to high pkt loss Inconsistent CCA leads to starvation Context-dependent clear channel assessment can be compensated with context-dependent, heterogeneous MAC protocols Dynamic, context-aware, heterogeneous protocols provide us a new dimension for protocol design Currently investigate global stability, learning algorithms

23. 23 Questions ?