Review: Macaw

Xerox Corporation’s Palo Alto Research center is building a single
channel wireless LAN. A wide variety of mobile computing devices are
available that could be used is such a wireless LAN but the main issue
is of providing adequate wireless network connectivity for these
devices. This paper focuses on identifying inherent performance and
design issues to some of the existing wireless media access protocols
and based on these observations develop an improved wireless media
access protocol called MACAW that has both a high throughput and a fair
allocation of throughput to streams.

In order to understand the performance and design issues in the wireless
media access protocols the author has chosen to study the MACA protocol.
Packet level simulations were used to evaluate design decisions.
Simulator used is a modified version of network simulator used in
studies of wired networks. Two areas of MACA were investigated, backoff
algorithm (BEB) and basic RTS-CTS message exchange.

The results from the simulations showed that the BEB used by MACA does
not provide an adequate level of fairness in simple one cell
configuration because of the fact that BEB calculations adjusts
extremely rapidly. To rectify this, basic BEB algorithm was changed to
explicitly disseminate the congestion information to all nodes and to
further increase the fair allocation of throughput, multiple stream
model was purposed.

There were also some modifications that are suggested in message
exchange format of MACA. These include incorporating the ACK message
after each data transmission, sending a DS packet to indicate that data
transmission is about to start and use of RRTS to resolve the congestion
issue at the receiver. Simulation results showed that incorporation of
these extra messages increase the throughput of the resulting algorithm,
MACAW.

Further changes in the BEB algorithm were made to cater for different
congestion level in different heterogeneous environments. Author has
shown that there was an improvement of about 37% in throughput by using
MACAW over MACA. The scenarios used for design decisions were really
simple and the approach used was based on multiple access. It would have
been really significant if the author had tested the protocols in some
real world complex scenario by using different media access schemes
instead of only multiple access.
Received on Fri Sep 22 2006 - 20:30:19 EDT