Results
This section presents performance results for each of the
durationbased matching metrics described in chapter
4. To compare the performance of these metrics, a
rating system was devised as follows. Each metric was exercised
across four print databases using three packet capture samples
, , and as input for each 802.11 implementation. We
define for each 802.11 implementation I, a success
probability for a matching metric M. It is the
probability that M correctly identifies a sample, that is,
identifies that sample as originating with I when it does
indeed originate with I.
For example, consider Table 5.1. This print
database has 13 fingerprints hence there are 13 entries. The table
was produced by using the MediumCompare metric on a particular
sample. The tables tells us that this metric believes the sample
originated with the BroadcomMiniPCI (ID 10 in the table) since it
has rank zero. But this is incorrect. The sample originated with
the AppleAirport Extreme (ID 5), which has rank "1". So we take as
SimpleCompare's probability of succeeding when the sample originates
with AppleAirport Extreme to be (13  rank)/13 or (13  1)/13 since
the correct implementation is given rank "1" by the metric.
Now since there are three samples, we extend RI for a metric
M to be (eq 5.1) where rank is the rank assigned by
M to the 802.11 implementation I that actually produced
sample . If the probability that I occurs is then
the success rate of M is the unconditional probability of
success given by
Each term in this sum is the product of the probability of seeing a
sample from one of the 13 implementations and the probability of
M succeeding to identify it in that case. So M could
have a good overall success rate even though it performs badly when
trying to identify a sample as belonging to some 802.11
implementation if that implementation doesn't arise often. However,
we shall assume that implementations are equally likely to occur. In
that case, the sum above becomes .
Table 5.1:
Ordered list generated from a matching metric
Subsections
