Zebra ML-1499-LAK1-01R Datasheet download pdf (Page 37)

AP-5131 Introduction

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Intercepting and decoding a direct-sequence transmission requires a predefined algorithm to

associate the spreading code used by the transmitting AP-5131 to the receiving MU. This algorithm

is established by IEEE 802.11b 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

AP-5131 uses different modulation schemes to encode more bits per chip at higher data rates. The

AP-5131 is capable of a maximum 54Mbps data transmission rate (802.11a radio), but the coverage

area is less than AP-5131’s operating at lower data rates since coverage area decreases as

bandwidth increases.

1.2.6 MU Association Process

An AP-5131 recognizes MUs as they begin the association process with the AP-5131. An AP-5131

keeps a list of the MUs it services. MUs associate with an AP-5131 based on the following conditions:

• signal strength between the AP-5131and MU

• number of MUs currently associated with the AP-5131

• MUs encryption and authentication capabilities

• MUs supported data rate

MUs perform pre-emptive roaming by intermittently scanning for AP-5131’s and associating with the

best available AP-5131. Before roaming and associating, MUs perform full or partial scans to collect

AP-5131 statistics and determine the direct-sequence channel used by the AP-5131.

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

AP-5131. The MU continues communicating with that AP-5131 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 AP-5131’s classified as proximate on

the AP-5131 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 AP-5131 and updates the table.

An MU can roam within a coverage area by switching AP-5131s. Roaming occurs when:

• Unassociated MU attempts to associate or reassociate with an available AP-5131

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Zebra ML-1499-LAK1-01R Datasheet Page 37