6.1 Experimental Setup
Analysis was conducted at a local hotspot on the campus of Georgia Institute
of Technology. Over the course of seven days we captured all tra?±c on
the wireless network. We used a Toshiba laptop with a Linksys WPC11 wireless
card to collect tra?±c. We put the wireless card into monitor mode using
the wlanctl ??’ ng utility and stored the captured tra?±c using Ethereal. With
the card in monitor mode we are able to detect the transmission rate associated
with each packet collected, while Ethereal appends a timestamp to each
packet. We used timing information and the transmission rate to generate
statistics. Tra?±c was collected for a total of 13.3 hours over the course of
seven days. During our observation period, there were a total of 61 wireless
nodes that visited the hotspot.
6.2 Results
The results of our analysis show that rate switching is common at the hotspot.
While this is definitely true for the hotspot we monitored, it is likely that RF
interference occurs in most wireless environments. Therefore, rate switching
is likely a widespread, common phenomenon. Figure 1 shows the transmission
rate of each data frame transmitted by one of the nodes at the local hotspot.
This particular node switched transmission rates 279 times. Overall, Figure
Fig. 1. Node at local hotspot invoking rate switching.
2(a) shows how often rate switching occurred for all wireless nodes over the
entire observation period.
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