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Introduction
24 AP-4131 Access Point Product Reference Guide
1.3.8 Supporting CAM and PSP Stations
CAM (Continuously Aware Mode) stations leave their radios on continuously
to hear every beacon and message transmitted. These systems operate
without any adjustments by the AP. A beacon is a uniframe system packet
broadcast by the AP to keep the network synchronized. A beacon includes
the Net_ID (ESS), the AP address, the Broadcast destination addresses,
a time stamp, a DTIM (Delivery Traffic Indication Message) and the TIM
(Traffic Indication Map).
PSP (Power Save Polling) stations power off their radios for short periods.
When a Spectrum24 MU in PSP mode associates with an AP, it notifies the AP
of its activity status. The access point responds by buffering packets received
for the MU. The Spectrum24 adapters use a PSP performance index from 1
to 5, where 1 provides the quickest response time and 5 provides the most
efficient power consumption.
The performance index determines how long the adapter stays in CAM after
transmit or receive activity. Regardless of the performance index used,
adapters switch to CAM for data reception/transmission. The awake interval
in PSP performance index 1 is long enough to allow for round-trip packet
response times. The packet response time in PSP performance index 5 is only
25 msec, the adapter goes back to sleep and requires another wake up
period to receive data.
When the MU wakes up and sees its bit set in the TIM, it issues a short frame
to the AP for the packets stored. The AP sends them to the MU and the MU
issues another short frame when the data has been received and is ready to
go back to PSP. A DTIM field, also called a countdown field, informs MUs of
the next window for listening to broadcast and multicast messages. When the
AP has buffered broadcast or multicast messages for associated MUs, it
sends the next DTIM with a DTIM Interval value. To prevent a PSP-mode MU
from sleeping through a DTIM notification, select a PSP mode value less than
or equal to the DTIM value. PSP-mode MUs hear the beacons and awaken to
receive the broadcast and multicast messages.