Extreme Networks Altitude 3500 Series, Руководство по продукции

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Altitude 3500 Series Access Point Product Reference Guide
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Intercepting and decoding a direct-sequence transmission requires a predefined algorithm to associate
the spreading code used by the transmitting access point to the receiving MU. This algorithm is
established by IEEE 802.11 specifications. The bit redundancy within the chipping sequence enables the
receiving MU to recreate the original data pattern, even if bits in the chipping sequence are corrupted
by interference.
The ratio of chips per bit is called the spreading ratio . A high spreading ratio increases the resistance of
the signal to interference. A low spreading ratio increases the bandwidth available to the user. The
access point uses different modulation schemes to encode more bits per chip at higher data rates. The
access point is capable of a maximum 54Mbps data transmission rate (802.11a radio), but the coverage
area is less than that of an access point operating at lower data rates since coverage area decreases as
bandwidth increases.
MU Association Process
An access point recognizes MUs as they begin the association process. An access point keeps a list of
the MUs it services. MUs associate with an access point based on the following conditions:
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Signal strength between the access point and MU
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Number of MUs currently associated with the access point
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MUs encryption and authentication capabilities
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MUs supported data rate
MUs perform pre-emptive roaming by intermittently scanning for access points and associating with the
best available access point. Before roaming and associating, MUs perform full or partial scans to collect
Altitude 35xx statistics and determine the direct-sequence channel used by the 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
point. The MU continues communicating with that 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 Altitude 35xxs classified as proximate on the 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 Altitude 35xx and updates the table.
An MU can roam within a coverage area by switching access points. Roaming occurs when:
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Unassociated MU attempts to associate or reassociate with an available access point
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Supported rate changes or the MU finds a better transmit rate with another access point
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RSSI (received signal strength indicator) of a potential access point exceeds the current access point
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Ratio of good-transmitted packets to attempted-transmitted packets falls below a threshold.
An MU selects the best available access point and adjusts itself to the access point direct-sequence
channel to begin association. Once associated, the 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 access point.
The scanning and association process continues for active MUs. This process allows the MUs to find
new access points and discard out-of-range or deactivated access points. By testing the airwaves, the
MUs can choose the best network connection available.