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 - 20:15:39 EDT