The 802.11 PHY provides multiple
transmission rates. For example, 802.11b supports rates of 1, 2, 5.5, and 11
megabits per second. A rate switching algorithm aims to dynamically adapt
the transmission rate per packet based on the channel conditions to optimize
performance. During the period where a node undergoes rate switching, the
behavior of the rate switching algorithm has a temporal impact on the transmission
of each packet. How the algorithm responds to the transient and longterm
changes to the condition of the wireless link will a?®ect the transmission
duration and inter-packet timing. The rate switching algorithm may also trigger
other services such as the RTS/CTS handshake or retransmission, causing
a greater impact on the behavior of the tra?±c. The actual rate switching algorithm
implemented in the radio interface of a sensor is unknown, as it is
considered sensitive proprietary property. However, [10] discusses algorithms
that are speculated to be used in current products.
6 Empirical Analysis of Rate Switching
We have pinpointed rate switching as an opportunity for distinguishing between
sensor nodes with RIs produced by di?®erent vendors. Before developing
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Cherita Corbett, John Copeland, and Raheem Beyah
our approach, we conducted an empirical analysis to characterize the rate
switching phenomenon. As a result of its accessibility and ease of node deployment,
we used 802.11WLAN nodes to emulate a sensor network.
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