802.11 Protocol Architecture
FIGURE 2.5b
Active Scanning
An access point must reply to all probes that contain the broadcast
SSID or an SSID that matches its own. This standard is ignored when
the vendor provides a proprietary mechanism allowing the network
administrator to disable probe responses to probes with broadcast
SSIDs. This feature is very common in today’s access points and
wireless LAN switches. With Ad Hoc networks, the station that
generated the last beacon frame will respond to probes. The advantage
of active scanning is that it identifies potential access points faster,
which may be necessary if the client station is experiencing a rapid
decrease in received signal strength from frames.
Synchronization
Stations within a BSS must remain in time synchronization with the
access point (or each other in the case of an Ad Hoc network) to ensure
that they are able to effectively implement all functions. Specifically,
time synchronization in a BSS is needed for PCF and Power-Save
modes. To support this, the access point (or individual station in an
IBSS) periodically transmits beacon frames. In addition to announcing
the presence of the BSS, a beacon contains the access point's clock
value in the timestamp field of the beacon’s frame body. Each station
receiving the beacon from the access point to which they’re associated
uses the timestamp information to update its own clock accordingly.
The timestamp is used by stations to update their clocks regardless of
any other factors.
