Brocade Mobility 5181 Access Point Product Reference Guide (Supporting software release 4.4.0.0) - Manual (Page 33)

Brocade Mobility 5181 Access Point Product Reference Guide 19
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Theory of operations
1
Direct-sequence spread spectrum
Spread spectrum (broadband) uses a narrowband signal to spread the transmission over a
segment of the radio frequency band or spectrum. Direct-sequence is a spread spectrum
technique where the transmitted signal is spread over a particular frequency range. The access
point uses Direct-Sequence Spread Spectrum (DSSS) for radio communication.
Direct-sequence systems communicate by continuously transmitting a redundant pattern of bits
called a chipping sequence. Each bit of transmitted data is mapped into chips by the Mobility 5181
Access Point and rearranged into a pseudorandom spreading code to form the chipping sequence.
The chipping sequence is combined with a transmitted data stream to produce the output signal.
Clients receiving a direct-sequence transmission use the spreading code to map the chips within
the chipping sequence back into bits to recreate the original data transmitted by the Mobility 5181
Access Point. Intercepting and decoding a direct-sequence transmission requires a predefined
algorithm to associate the spreading code used by the transmitting Mobility 5181 Access Point to
the receiving Client. This algorithm is established by IEEE 802.11 specifications. The bit
redundancy within the chipping sequence enables the receiving Client 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 Mobility 5181 Access Point uses different modulation schemes to encode more bits
per chip at higher data rates. The Mobility 5181 Access Point is capable of a maximum 54Mbps
data transmission rate (802.11a radio), but the coverage area is less than that of an Mobility 5181
Access Point operating at lower data rates since coverage area decreases as bandwidth increases.
Client association process
An Mobility 5181 Access Point recognizes Clients as they begin the association process. An Mobility
5181 Access Point keeps a list of the Clients it services. Clients associate with an Mobility 5181
Access Point based on the following conditions:
• Signal strength between the Mobility 5181 Access Point and Client
• Number of Clients currently associated with the Mobility 5181 Access Point
• Clients encryption and authentication capabilities
• Clients supported data rate
Clients perform pre-emptive roaming by intermittently scanning for Mobility 5181 Access Point’s
and associating with the best available Mobility 5181 Access Point. Before roaming and
associating, Clients perform full or partial scans to collect Mobility 5181 Access Point statistics and
determine the direct-sequence channel used by the Mobility 5181 Access Point.
Scanning is a periodic process where the Client sends out probe messages on all channels defined
by the country code. The statistics enable an Client to reassociate by synchronizing its channel to
the Mobility 5181 Access Point. The Client continues communicating with that Mobility 5181
Access Point until it needs to switch cells or roam.
Clients perform partial scans at programmed intervals, when missing expected beacons or after
excessive transmission retries. In a partial scan, the Client scans Mobility 5181 Access Point’s
classified as proximate on the Mobility 5181 Access Point table. For each channel, the Client tests
for Clear Channel Assessment (CCA). The Client 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 Mobility 5181 Access Point and updates the table.