Brocade Mobility 7131 Access Point Product Reference Guide (Supporting software release 4.4.0.0 and later) - Manual (Page 40)

28 Brocade Mobility 7131 Access Point Product Reference Guide
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MUs perform pre-emptive roaming by intermittently scanning for Mobility 7131 Access Point’s and
associating with the best available access pointMobility 7131 Access Point. Before roaming and
associating, MUs perform full or partial scans to collect Mobility 7131 Access Point statistics and
determine the direct-sequence channel used by the Mobility 7131 Access Point access point.
Scanning is a periodic process where the MU sends out probe messages on all channels defined by
the country code. The statistics enable an MU to reassociate by synchronizing its channel to the
access pointMobility 7131 Access Point. The MU continues communicating with that Mobility 7131
Access Point until it needs to switch cells or roam.
MUs perform partial scans at programmed intervals, when missing expected beacons or after
excessive transmission retries. In a partial scan, the MU scans access points classified as
proximate on the Mobility 7131 Access Point access point table. For each channel, the MU tests for
Clear Channel Assessment (CCA). The MU broadcasts a probe with the ESSID and broadcast
BSS_ID when the channel is transmission-free. It sends an ACK to a directed probe response from
the access point Mobility 7131 Access Point and updates the table.
An MU can roam within a coverage area by switching Mobility 7131 Access Point access points.
Roaming occurs when:
• Unassociated MU attempts to associate or reassociate with an available Mobility 7131 Access
Point access point
• Supported rate changes or the MU finds a better transmit rate with another Mobility 7131
Access Point access point
• RSSI (received signal strength indicator) of a potential access point Mobility 7131 Access Point
exceeds the current access pointMobility 7131 Access Point
• Ratio of good-transmitted packets to attempted-transmitted packets that fall below a
threshold.
An MU selects the best available access pointMobility 7131 Access Point and adjusts itself to the
access point Mobility 7131 Access Point direct-sequence channel to begin association. Once
associated, the Mobility 7131 Access Point access point begins forwarding frames addressed to
the target MU. Each frame contains fields for the current direct-sequence channel. The MU uses
these fields to resynchronize to the Mobility 7131 Access Point access point.
The scanning and association process continues for active MUs. This process allows MUs to find
new Mobility 7131 Access Pointaccess points and discard out-of-range or deactivated Mobility
7131 Access Point access points. By testing the airwaves, MUs can choose the best network
connection available.
Operating Modes
The Mobility 7131 Access Point access point can operate in a couple of configurations.
• Access Point - As an Access Point, the Mobility 7131 Access Point access point functions as a
layer 2 bridge. The wired uplink can operate as a trunk and support multiple VLANs. Up to 16
WLANs can be defined and mapped to access point Mobility 7131 Access Point WLANs. Each
WLAN can be configured to be broadcast by one or both Mobility 7131 Access Point access
point radios. An Mobility 7131 Access Point or Mobility 7131N Access Point can operate in both
an Access Point mode and Wireless Gateway/Router mode simultaneously. The network
architecture and access point configuration define how the Access Point and Wireless
Gateway/Router mode are negotiated.