DSSS
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DSSS
- Direct sequence spread spectrum (DSSS) is a spread spectrum technology that uses fixed channels. DSSS 802.11 radios are known as Clause 16 devices.
- Frequency space in which either FHSS or DSSS radios can transmit is the license-free 2.4 GHz industrial, scientific, and medical (ISM) band.
- FHSS and DSSS technologies cannot communicate with each other and often have a hard time coexisting.
- DSSS 802.11 radios can transmit in channels subdivided from the entire 2.4 GHz to 2.4835 GHz ISM band.
- DSSS is set to one channel.
- The IEEE is more restrictive for FHSS radios, which are permitted to transmit on 1 MHz subcarriers in the 2.402 GHz to 2.480 GHz range of the 2.4 GHz ISM band.
- Direct sequence spread spectrum (DSSS) was originally specified in the primary 802.11 standard and provides 1 and 2 Mbps RF communications using the 2.4 GHz ISM band.
- Legacy DSSS channels had to have at least 30 MHz of spacing between the center frequencies to be considered nonoverlapping.
- In a deployment of legacy DSSS equipment using a channel pattern of 1, 6, and 11, the channels were considered overlapping because the center frequencies were only 25 MHz apart.
Spread Sequence
- Because 802.11 uses an unbounded medium with a huge potential for RF interference, it had to be designed to be resilient enough that data corruption could be minimized.
- Each bit of data is encoded and transmitted as multiple bits of data, making communication more resistant to data corruption.
- The system converts the 1 bit of data into a series of bits that are referred to as chips.
- To create the chips, a Boolean XOR is performed on the data bit and a fixed-length bit sequence pseudorandom number (PN) code.
- Using a PN code known as the Barker code, the binary data 1 and 0 are represented by chip sequences which is then spread across a wider frequency space.
- Although 1 bit of data might need only 2 MHz of frequency space, the 11 chips will require 22 MHz of frequency carrier space.
- When the data is converted to multiple chips and some of the chips are not received properly, the radio will still be able to interpret the data by looking at the chips that were received properly.
- When the Barker code is used, as many as 9 of the 11 chips can be corrupted, yet the receiving radio will still be able to interpret the sequence and convert them back into a single data bit.
Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
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