From: Shobhit Raj Mathur (shobhit@cs.washington.edu)
Date: Sun Nov 21 2004 - 20:55:56 PST
MACAW: A Media Access Proctocol for Wireless LAN's
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This paper was motivated by the emergence of mobile deivices such as
Palmtops, PDAs etc at that time. The MAC protocol proposed for wireless
LANs prior to this paper (MACA) had many shortcomings which the proposed
protocol MACAW tries to address.
The congestion in a wireless LAN is at the receiver's end, hence carrier
sense approaches like ethernet are not useful for wireless LANs. MACAW
uses a RTS-CTS-DS-DATA-ACK protocol as opposed to RTS-CTS-DATA protcol
used by MACA. MACAW gains a considerable performance improvement by using
such a protocol. The ACK provided by the link layer allows for a much
fater recovery from packet losses. Packets are lost more often in wireless
networks due to collisions than in wired networks. If the link layer would
depend on the application layer to detect the packet losses it would
result in significant delays which can be avoided by using link level
ACKs.
The MACAW protocol propogates synchronization information about contention
periods so that all devices contend effectively. Using the Data-Sending
packet(DS) allows hosts which 'lose' the contention to compete effectively
for access when the medium is available next time. Another synchronization
information which MACAW uses is the RRTS. RRTS packet allows a receiving
host to contend for bandwidth even when it is in the presence of
congestion.
Congestion is not homogenous, it varies with the location of the receiver.
So instead of characterizing congestion by a single backoff parameter
MACAW uses seperate backoff parameters for each stream and infact for each
end of the stream. Backoff parameters are also copied from 'overheard'
packets so that all the nodes have the same view of the congestion levels.
These changes significantly improve the performance compared to MACA, but
the paper still does not identify other concerns in wireless networks such
as security. One important achievement of MACAW is the fair allocation and
utilization of bandwidth. It should be noted that the experiments were run
in the specific topography of Xerox Corporation's Palo Alto Reserach
Center. The authors themselves agree that they are not sure how this
protocol would perform in more general wireless LANs.
It is interesting to note that, MACAW deviates from the end-to-end
argument by using link level ACKs. This was necessary to improve
performance, otherwise the applications would face severe delays due to
packet losses. I think that the MACAW protocol is very close to the 802.11
protocol used today. Hence MACAW scaled well to diverse topologies.
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