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MACAW: A Media Access Protocol for Wireless LANs

MACAW: A Media Access Protocol for Wireless LANs. Jennifer Ogunlowo Sarah El-Helw. Macaw topology . http://en.wikipedia.org/wiki/Macaw. Introduction. Media access protocols CSMA/CA MACA MACAW Current CSMA/CA inspired by MACAW MACAW is inspired by MACA

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MACAW: A Media Access Protocol for Wireless LANs

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  1. MACAW: A Media Access Protocol for Wireless LANs Jennifer Ogunlowo Sarah El-Helw

  2. Macaw topology  http://en.wikipedia.org/wiki/Macaw

  3. Introduction • Media access protocols • CSMA/CA • MACA • MACAW • Current CSMA/CA inspired by MACAW • MACAW is inspired by MACA • MACAW caused the development of the new present day CSMA/CA

  4. Background • Initial CSMA and MACA protocols are incompetent in performance • Initial CSMA • No collision avoidance • Senses carrier before transmission • Relevant contention at the sender only • MACA • Collision avoidance at the receiver only • Proper Collision avoidance should be at both sender and receiver.

  5. Problem and Motivation • To Develop a wireless infrastructure that supports hosts: • With fairness regardless to location • Avoids collisions • Media Access Fairness • At the time MACAW was being produced, MACA and CSMA weren’t sufficient to meet these goals.

  6. MACA- Multiple Access with Collision Avoidance • Signal with Control Packet • RTS- Request to send • Length of data transmission contained within • No transmissions by other nodes occur when RTS is heard • CTS- Clear to Send • Length of data transmission contained within • Signal to begin sending • Nodes that hear CTS defer until data transmission is complete. • Uses BEB (Binary Exponential Back-off) • Back-off doubled after every collision. • Reduced to minimal back-off (BOmin)after successful RTS-CTS frame

  7. Wireless LAN issues CTS Hidden Terminal A B C RTS RTS A B C Exposed Terminal CTS Node assumes collision when no CTS message is returned for its RTS and consequent timeout.

  8. Four Issues in Media Access Protocols that motivated MACAW: • In MACA relevant contention is at the receiver not at the sender • Congestion is location dependent in wireless • Media Access should be fair • Media access protocol should provide information about contention periods

  9. MACAW- Multiple Access with Collision Avoidance for Wireless • MACAW uses different back-off algorithm, BEB copy and MILD (Multiplicative Increase, Linear Decrease) • MACAW uses RTS-CTS-DS-DATA-ACK frame sequence. • Problem • With MACA, very likely that the least-backed-off station will ‘win’ the bandwidth during contention. • Problem is no sharing of collision avoidance with nodes

  10. Back-off Algorithm • MACAW Solution: • Stations copy back-off counter value into its own back-off counter (BEB Copy). • Uses MILD (Multiplicative Increase, Linear Decrease) to adjust BO values. • Finc(x) = MIN[1.5x, BOmax] • Fdec(x) = MAX[x-1, BOmin]

  11. MACAW Back-off Algorithm Results • MACAW achieves higher throughput than MACA. MACA vs. MACAW performance MACAW performance

  12. Multiple Stream Model • MACA + CSMA allocate bandwidth to stations. • MACAW maintains separate queues for each stream in each station and runs back-off algorithm for each queue • Allocates bandwidth to streams. • Provides per stream fairness Downstream Upstream

  13. ACK • If data lost, MACA relies on recovery at TCP layer. • Much longer compared to link layer • To solve this, MACAW protocol adds ACK messages • ACK would now occur at link layer • Sent by receiver immediately after data is received.

  14. ACK contd. • Data received and no ACK • Receiver returns ACK exchange to next RTS • ACK received • Sender decreases back-off • ACK not received • Sender increases back-off • Successful RTS-CTS exchange, no ACK • No change to back-off counter MACA vs. MACAW

  15. DS : Data-Sending Packet • Solves the exposed terminal problem • DS sent before DATA packet • Informs other nodes of RTS-CTS success. • Informs nodes about impending data transmissions and length of DATA packet. • Provides sync information about contention periods.

  16. DS Performance RTS B also sends DS to A A B C Exposed Terminal CTS No CTSCan’t tell if RTS or CTS successful B1 P1 P2 B2 MACA vs. MACAW

  17. RRTS – Request-for-Request-to- Send packet • Solves hidden terminal problem. • Station corresponds with first received RTS. • Stations that overhear RRTS defer long enough until after RTS-CTS success. CTS RTS Hidden Terminal A B C D RTS Defers transmission of CTS RRTS

  18. Back-off Algorithm Revisited • Three cases where BEB copy might have problems: • Presence of noise source to sender or receiver. • Leakages of high BO between two adjacent cells. • Unresponsive offline pad. • Per-destination back-off algorithm suggested as solution. • Multiple Stream Model • Stations communicating with same receiving station use the same BO value. • BO value of both ends inserted into a packet header.

  19. Performance Evaluation Model • During low loads, MACAW performs worse than MACA. • During high congestion levels, MACAW performs better with 37% overall throughput and more fair.

  20. Critique • Definition of fairness in wireless networks • No justification for noise levels presented. • Complex Implementation of MACAW • BEB definitions not included with MACA.

  21. Current Implementation Present Day CSMA/CA MACAW Old CSMA MACA

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