Paper review : MACAW: A Media Access Protocol for Wireless LAN's (BDSZ94)

• Goal

  To provide a high overall thoughput and a fair allocation of throughput to streams for the wireless LAN's.

• Contribution

  This paper proposes a media access protocol, MACAW, which is derived from MACA and adopts an RTS-CTS-DS-DATA-ACK message exchange and a significantly different backoff algorithm.

• Main ideas

  The key observations and the improvements of the protocol followed include:

  • The relevant congestion is at the receiver, not the sender.
    Solution:
    Using the RTS-CTS-DATA message exchange (which leads to RTS-CTS-DS-DATA-ACK exchange for a variety of performance reasons).

  • Congestion is location dependent.
    Solution:
    Introducing separate backoff parameters for each stream, and then for each end of the stream.

  • Learning about congestion levels should be a collective enterprise (in order to allocate media access fairly).
    Solution:
    Introducing the notion of copying the backoff parameters from the overheard packets, which enables the protocol to propagate congestion information explicitly.

  • Synchronization information about contention periods should be propagated (so that all devices can contend effectively).
    Solution:
    Introducing DS packet and RRTS packet.

• Evaluation
  • Significance rating: 3
    MACAW provides improvements derived from a variety of considerations. (4)
    Nevertheless, the context is narrowed in the indoor wireless LAN, the author do not provide more realistic configurations in a larger scale; also, so many problems are left open. (-1)

  • Convincing rating
    The authors give a thorough analysis on the shortcoming and limitation of MACA protocol; hence provide some solutions to these problems. Several configurations are desinged as simulation models, and experiment data are provided to compare the performance differences. The simulation results of the last two (more realistic) configurations show that the combination of improvements results in a high overall thoughput and a fair allocation of throughput, which is exactly the goal of MACAW.

    From the data provided, we also notice that two assumptions are made by the authors:

    • The error rate is reasonable (not too low), say, 0.001, to balance the overhead of the RTS-CTS-DS-DATA-ACK message exchange.
    • The fairness is defined with respect to the streams.

  • Limitation
    The authors provide some open questions, e.g.,
    • How to make receiver generated control messages like CTS work in the case of multicast without involving several rounds of contention?
    • How to solve the problem which is unable to be handled by RRTS packet as shown in Fig. 7?
    Moreover, the more general questions, such as the investigation of the token-based approach fully, are also left.

• Conclusion

  Complex protocols and algorithms can be designed to improve some kind of performance, the result is the tradeoff on some other properties or cases. For example, reliability and fairness can be got with a high overall throughput in some models of wireless LAN, as characterized in the paper.

9/17/01