Review - MACAW: A Media Access Protocol for Wireless LAN's

This paper develops a new media access protocol for wireless LAN based on
MACA (Multiple Access Collision Avoidance), and addresses basic performance
and design issues of the proposed the MACAW. As three background reviews,
the authors describe Xerox Corporation's 'Nano-Cellular Radio Network'
providing a nanocell radio environment consisting of pads and base stations,
Carrier Sense Multiple Access (CSMA) providing the simplest carrier sensing
model but having problems of 'hidden terminal' and 'exposed terminal', and
Multiple Access Collision Avoidance(MACA) introducing RTS-CTS control
packets to solve the problems in CSMA. When designing MACAW, the paper pays
attention to the backoff algorithm and the RTS-CTS message exchange in order
to achieve the high network throughput and the fairness in media access in
wireless LAN. In terms of the backoff algorithm, the paper proposes a
multiplicative increase and linear decrease (MILD) avoiding wild oscillation
of backoff counter by decreasing the size of backoff interval, which
provides an adequate level of fairness on resource usage. With respect to
the message exchange, the paper proposes three new control messages added
into the basic RTS-CTS-DATA message exchange used in MACA. At first, an
acknowledge packet, ACK, is sent from the receiver to the sender when the
receiver completes the data reception, which increases the reliability at
the link layer under and advances the throughput performance. The next two
control message are a Data-Sending packet (DS) sent by the sender just
before sending a DATA packet and a Request-for-RTS (RRTS) sent by the
receiver during the next contention period when receiving an RTS but being
unable to respond immediately due to deferral. These two control packets
provide synchronization information regarding the contention period, which
enhances the fairness of access to the media. When evaluating the MACAW, two
values of the network throughput and the resource balance are measured.
Additional measurements regarding the total channel capacity are done in
order to quantify the overhead burned by the MACAW protocol. The paper shows
that the proposed MACAW outperforms the MACA in terms of improving fairness
almost without sacrificing the network throughput.

One of the major contributions of the paper is its investigation and
analysis of problems in MACA. Based on the endeavour, it gives four key
insights as follows. (1) The relevant contention is at the receiver, not the
sender; (2) the congestion is location dependent; (3) The media access
protocol should provide congestion information explicitly; (4) the media
access protocol should provide synchronization information about the
contention periods. Through the observations, the paper raises such problems
why the MACA cannot be applied into the wireless environment and what
modifications should be addressed. Throughout the paper, the authors focused
on the backoff counter and the unfairness of resource access as improvement
issues, and developed the proposed solution against these problems with
simple scenarios covering various communication use cases.

However, the paper has some weak points in terms of unclear criteria used in
its solution and impractical scenario. The MILD scheme increases the counter
interval by 1.5 times upon a collision and decreases the counter by 1 upon
success. And the authors show that the scheme works well with the scenario
presented in figure 3. However, what will happen if it increases by
1.6times and decreases by
1.1? What proves the selected values are the optimal? The authors should
give more refinement and explanation or present experimental results
comparing these values. Even though simple scenarios help readers understand
the problems and the solutions, they make the experiments to be seemed a
laboratory simulation, not practical evaluation. Even the last two
configurations simulate a part of the laboratory at PARC, which can degrade
the fidelity of the results.

In summary, the paper gives a significant perspective regarding the media
access protocol in wireless LAN environment even though it has small
shortcomings. It identifies problems inherent in MACA when applying for
wireless LAN and proposes appropriate and simple solutions with minimum
performance degradation. However, the paper can be improved by including
clearance in ideas and doing experiment in the field.
Received on Thu Sep 21 2006 - 09:54:35 EDT