MACAW Review

This paper explains the design of MACAW, a wireless media-access
protocol based on MACA, by presenting a linear progression through the
reasoning and motivations behind each of its features. The protocol is
designed, tested and evaluated. MACAW is derived from MACA by adding
new control signals and by changing some of the algorithms (back-off,
congestion accounting on both endpoints, etc). The authors state their
goal to be the development of a wireless media access protocol and the
exploration of some of the basic performance and design issues inherent
in these protocols. Additionally, the goal of MACAW is to achieve both
a high overall throughput and a fair allocation of throughput.

The authors set out 4 design guidelines to guide them in the design of
their new protocol:

-- the relevant congestion is at the receiver end, not the sender
(compare to CSMA)

-- congestion is location dependent

-- to allocate media access fairly, learning about congestion levels
must be a collective effort

-- the media access protocol should propagate synchronization
information about contention periods so that contention is "fair" for
all devices

The design of MACAW is documented in a story-line fashion, with each
change motivated by a compelling example and confirmed within a network
simulator:

-- the back-off algorithm is changed from exponential because of
unfairness, instead the current back-off counter is shared among devices
via the packet header and the back-off is done by multiplying to
increase and subtracting to decrease (MILD)

-- bandwidth should be divided evenly across streams not hosts by
implementing separate queues in a device for each stream

-- ACK is added to improver performance with link-layer recovery

-- DS is added to indicate that a RTS/CTS exchange succeeded and that
data transmission is to follow

-- RRTS is added for situations in which a device cannot respond to an
RTS because of deferral

-- a separate back-off value is necessary for each station and the
back-off values of both ends need to be inserted into the packet header

Several issues remain unresolved such as implementing multicast
communication and precisely defining fairness. Additionally several
enhancements are left un-investigated: piggy-packing ACK on CTS packets,
NACKs instead of ACKs, full carrier-sense instead of DS packets, power
variation, etc. MACAW outperforms MACA in fairness and throughput
comparisons in the presence of congestion and noise, and performs nearly
as well in the absence of both. Despite these performance
improvements, the authors conclude that their design is still preliminary.
Received on Mon Sep 25 2006 - 10:21:47 EDT