Review: MACAW:A Media Access Protocol for Wireless LAN

The paper develops a new meda access protocol, MACAW, for wireless LAN based
upon the MACA protocol. The authors observed that contention is at the
receiver (not the transmitter), congestion is location dependent, learning
about congestion levels should be collective to allow fair access of the
shared media, and finally that synchronization information about contention
periods should be propogated so that all devices can contend effectively.
With these observations in mind, and with the goal of delivering high
network utilization as well as fair access to the media, the paper discusses
changing the original MACA RTS-CTS-DATA protocol into a MACAW
RTS-CTS-DS-DATA-ACK protocol.
The protocol changes summarized:
1) Replace the binary exponential backoff (BEB) algorithm with
multiplicative increase and linear decrease (MILD) backoff algorithm. They
argue MILD backoff is more efficient than the BEB since it prevents the
backoff counter from varying widely.
2) Modify the backoff algorithm so that the current backoff counter is
included in the packet header, thereby distributing congestion information
to provide more fair contention.
3) Make separate queues (with separate backoff counters) for each stream at
a station in order to distribute bandwidth more fairly amongst the streams
(rather than amongst stations).
4) Add an ACK after data transmission to improve efficiency of reliable data
transfer since the TCP timeouts are long and result in long waiting periods.
5) Add a DS (data-sending) packet sent by the transmitter after it receives
the CTS from the receiver and before it sends the data. This informs other
stations that RTS-CTS was successful and the DATA-ACK is about to follow.
This synchronization allows other stations to defer transmission till after
the ACK is received and hence reduces collisions.
5) Add an RRTS if needed. This also provides synchronization in the case
that a transmitter is sending RTS and the reciever is getting it but has to
defer transmission of a CTS to prevent collision with another transmitter.
When the receiver is free to receive and transmit, it sends an RRTS to
inform the transmitter that it may start the RTS-CTS-DS-DATA-ACK cycle.
The authors eventually show that despite the modifications to message
sending protocol there are still cases where one transmitter can gain
complete control over the channel and starve another transmitter. They also
show that the changes to the backoff algorithm can cause problems in
non-homogenous congestion through leakage of the single backoff counter from
one cell to another that have different congestion levels leading to an
inaccurate view of contention. Also, it aggravates the problem when there is
no contention but either the RTS or CTS is corrupted due to noise, because
the backoff increases even though there is no congestion.
The paper evaluates two scenarios and shows that despite the increase in
overhead, there is an increase in throughput due to better congestion
handling (assuming offcourse that there is congestion). The experimental
results also show fairer division of throughput among streams in the same
cell and better handling of non-homogenous congestion (although they
previously mentioned that non-homogenous congestion was a problem).
I find the paper explains the protocols well, however, they seem to go on
adding overhead to handle specific cases and eventually still end up with
unsolved problems. I am unconvinced that MACAW is much better than MACA.
Received on Thu Sep 21 2006 - 09:59:14 EDT