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The Case for Heterogeneous Wireless MACs

The Case for Heterogeneous Wireless MACs. Chun-cheng Chen Haiyun Luo Dept. of Computer Science, UIUC. C. A. B. CS. CS. CS. RSS. RSS. CS Thrshld. CS Thrshld. Problem#1: intra-BSS interference. Clear channel assessment (CCA): transmit iif RSS < CS threshold

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The Case for Heterogeneous Wireless MACs

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

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

  3. RSS CS Thrshld C A B CS RSS CS Thrshld 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

  4. C A B 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) CTS RTS

  5. C A B 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) ACK DATA

  6. C A B D 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 CCA is inconsistent. When and with which client should RTS/CTS be enabled ? Preamble Header RTS 72~144 bits 48 bits 20 bytes @ basic_rate: 2Mbps – 802.11b 6Mbps – 802.11a/g DATA

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

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

  9. A Problem#2: inter-BSS interference • 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 Exposed Receiver Hidden Sender B C D

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

  11. A 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 B ACK C D

  12. 1 1 6 6 11 11 1 1 1 6 6 11 11 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

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

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

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

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

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

  19. C C B B D D A A 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

  20. A 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 Set one bit in DATA header to notify receiver of protocol switch! B C DATA RTR D If success ratio too low  switchMac().

  21. Normalized inst. thrput 2 1 3 0 Success ratio time (sec) time (sec) Evaluation • 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 Learning period

  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. Questions ?

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