Power Saving - Black Box LW0050A Manual

The WEP is a simple algorithm based on RSA's RC4 which has the following
properties:
• Reasonably strong: Brute-force attack to this algorithm is difficult because
every frame is sent with an Initialization Vector which restarts the PRNG for
each frame.
• Self Synchronizing: The algorithm re-synchronizes for each message. This is
necessary in order to work in a connectionless environment, where packets
may get lost (as any LAN).
E.8 Power Saving
Wireless LANs are typically related to mobile applications. In this type of
application, battery power is a scare resource. This is the reason why the 802.11
standard directly addresses the issue of power saving and defines an entire
mechanism which enables stations to go into sleep mode for long periods of time
without losing information.
The main idea behind the power-saving mechanism is that the Access Point
maintains a continually updated record of the stations currently working in Power
Saving mode, and buffers the packets addressed to these stations until either the
stations specifically request the packets by sending a polling request, or until they
change their operation mode.
As part of its Beacon Frames, the Access Point also periodically transmits
information about which Power Saving Stations have frames buffered at the Access
Point, so these stations wake up in order to receive the Beacon Frame. If there is an
indication that there is a frame stored at the Access Point waiting for delivery, then
the station stays awake and sends a Polling message to the Access Point to get these
frames.
Multicasts and Broadcasts are stored by the Access Point, and transmitted at pre-
defined intervals (called DTIM), all stations—both stations working in Power
Saving mode and stations working in Normal mode, will be awake at that period
and will receive this kind of frames.
Unicasts are stored by the Access Point, and transmitted at station-defined intervals
(called Listen Intervals), when all stations who wish to receive this kind of frames
are awake. Unicast frames are transmitted upon request only, whereas, Multicast
frames are transmitted automatically at every DTIM interval.
APPENDIX E: IEEE 802.11 Technical Tutorial
135