3Com 09-1765-001 manual following conditions

Theory of Operation 25

Wireless Client APs recognize wireless clients as they associate with the AP. The AP keeps a list of Association Process the wireless clients it services. Wireless clients associate with an AP based on the

following conditions:

■Signal strength between the AP and wireless client

■Wireless clients currently associated with the AP

■Wireless client Supported Rate (see the table below)

■Positive match between the wireless client and encryption keys (optional)

■Positive wireless client authorization by the Access Control List (optional)

The following table gives the wireless client support rates:

Data Rate	Requirement
11 Mbps	Optional
5.5 Mbps	Optional
2 Mbps	Optional
1 Mbps	Required

Wireless clients perform preemptive roaming by intermittently scanning for APs and associating with the best available AP. Before roaming and associating with APs, wireless clients perform scans to collect AP statistics and determine the direct-sequence channel used by the AP.

Scanning is a periodic process where the wireless client sends out messages on all frequencies defined by the country code. The statistics enable a wireless client to reassociate by synchronizing its frequency to the AP. The wireless client continues communicating with that AP until it needs to switch cells or roam.

Wireless clients perform scans at start-up. In a scan, a wireless client uses a sequential set of channels as the scan range. For each channel in range, the wireless client tests for Clear Channel Assessment (CCA). When a transmission-free channel becomes available, the wireless client broadcasts a probe with the wireless LAN service area and the broadcast HSA_ID. An AP-directed probe response generates a wireless client Acknowledgment (ACK) and the addition of the AP to the AP table with a proximity classification. An unsuccessful AP packet transmission generates another wireless client probe on the same channel. If the wireless client fails to receive a probe response within the time limits, it repeats the probe process on the next channel in the sequence. This process continues through all channels in the range.

A wireless client can roam within the coverage area by switching APs. Roaming is transparent and virtually instantaneous in high-level applications. Roaming occurs when:

■An unassociated wireless client attempts to associate or reassociate with an available AP.

■The supported rate changes or the wireless client finds a better transmit rate with another AP.

■The signal quality of a potential AP exceeds that of the current AP.