Statistical Analysis of Duration Field
As mentioned in chapter 3, the duration field is a 16
bit value which describes how long the station that currently has
access to the medium intends to keep it, after the current
transmission. Even though the duration field is 16 bits wide, it
only takes on a few discrete values. Common values are 0 (for
packets that are not acknowledged such as management frames
broadcast during a Contention Period), and the time it takes for a
SIFS (Short Interframe Spacing) interval plus an acknowledgment,
used in transmitting unicast data frames.
Variables that can affect the duration field include some parameters
of the local Basic Service Set specified in a beacon's fixed flags
field. These include short slot time, short pre-amble, and of
course, the data rates supported. The net result of this is that
ideally a unique fingerprint for a given implementation would be
taken across all possible variations of these parameters. For this
work, four databases were created. The databases currently have
human-friendly names (the name of the AP used to create them). In
the future, the number of databases will grow large enough that an
algorithmic naming scheme (rates-flags for example) will be
employed.
Since the performance of this technique varies with the parameters
of the Basic Service Set with which it is associated, a brief
introduction to the four networks it was developed and tested
against is given below.
Table 4.1:
Summary of databases created
Table 4.1 represents data about the four WLANs on which all
experiments in this chapter were performed. They were chosen to give
a good estimate of real world network deployments. Lexie is a b-only
Cisco aironet 350 . Mixed-wrt54g is a rev5 Linksys wrt54g running
in mixed mode. Mixed-Airplus is a D-link DI-524, and G-wrt54g is a
rev5 Linksys wrt54g in g-only mode. The models of the Access Points
used are mentioned to give the reader some sense of market
representation. The databases generated from each AP are not tied to
that specific AP. Clients should respond identically in any BSS with
the same set of parameters listed above.
Subsections
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