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 Thu Sep 21 2006 - 11:24:31 EDT