  We develop analytical models for estimating the energy spent by stations
(STAs) in infrastructure WLANs when performing TCP controlled file downloads.
We focus on the energy spent in radio communication when the STAs are in the
Continuously Active Mode (CAM), or in the static Power Save Mode (PSM). Our
approach is to develop accurate models for obtaining the fraction of times the
STA radios spend in idling, receiving and transmitting. We discuss two traffic
models for each mode of operation: (i) each STA performs one large file
download, and (ii) the STAs perform short file transfers. We evaluate the rate
of STA energy expenditure with long file downloads, and show that static PSM is
worse than just using CAM. For short file downloads we compute the number of
file downloads that can be completed with given battery capacity, and show that
PSM performs better than CAM for this case. We provide a validation of our
analytical models using the NS-2 simulator. In contrast to earlier work on
analytical modeling of PSM, our models that capture the details of the
interactions between the 802.11 MAC in PSM and certain aspects of TCP.
