Brocade Mobility 7131N-FGR Access Point Product Reference Guide (Supporting software release 4.0.0.0-35GRN and later) - Manual (Page 32)

20 Brocade Mobility 7131N-FGR Product Reference Guide
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Theory of operations
1
The serial port provides a Command Line Interface (CLI) connection. The serial link supports a
direct serial connection. The access point is a Data Terminal Equipment (DTE) device with male pin
connectors for the RS-232 port. Connecting the access point to a PC requires a null modem serial
cable.
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 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.
MUs 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 access point.
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 802.11b 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 access point uses different modulation schemes to encode more bits per chip at
higher data rates.
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:
• Signal strength between the and MU
• Number of MUs currently associated with the access point
• MUs encryption and authentication capabilities
• MUs supported data rate
MUs perform pre-emptive roaming by intermittently scanning for ’s and associating with the best
available access point. Before roaming and associating, MUs perform full or partial scans to collect
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 until it needs to switch cells or roam.