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

2. Macaw topology 

3. Introduction Media access protocols 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
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 Hidden Terminal A B C A B C Exposed Terminal
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 ACK received ACK not received
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 A B C Exposed Terminal B1 P1 P2 B2 MACA vs. MACAW
RTS
B also sends DS to A A B C
Exposed Terminal
CTS
No CTS Can’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