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IEEE 802.11 Direct Sequence Spread Spectrum (DSSS)

IEEE 802.11 DSSS combines a data signal at the sending station with a higher data rate bit sequence, which many refer to as a chipping code. The IEEE 802.11 Working Group has set its minimum processing gain requirements at 11, which means that the chipping code multiplies the data signal by 11. Direct sequence spread spectrum sends a specific string of bits for each data bit sent. A chipping code is assigned to represent logic 1 and 0 data bits. As the data stream is transmitted, the corresponding code is actually sent. For example, the transmission of a data bit equal to 1 would result in the sequence 00010011100 being sent.

IEEE 802.11 DSSS provides the following advantages (in comparison to FHSS):

  • Greater range than FHSS because of need for lower SNRs (at least 12dB) at the receiver

  • Highest potential data rates from individual physical layers, which is why DSSS was chosen for a higher-rate 802.11 physical layer (802.11b)

IEEE 802.11 DSSS provides the following disadvantages (in comparison to FHSS):

  • Less tolerance of signal interference because of operation over narrower (20MHz) bandwidth

  • Lower aggregate throughput using collocated access points (1 or 2Mbps versions)

  • Potential obsolescence as more companies continue to favor the higher-speed (802.11b) direct sequence products

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