DSSS tidbits.

Before I get started with this post I want to apologize to the readers here for taking so long to get a post up. Over the last few weeks I have been ramping up for the CCIE Security Lab exam which I took and passed on Friday, March 13, 2009. At last I can spend some more of my time in the Cisco Learning Network and here on this blog helping CCNA Wireless candidates prepare for the IUWNE exam. With that said I’ve decided to discuss a little bit about Direct Sequence Spread Spectrum (DSSS) encoding. If you think about the different ways that data could become corrupted in a wireless transmission it will make sense when you see how DSSS was designed to be resilient. The idea behind DSSS is that a transmission could be corrupted to a certain extent and still be readable. The way that DSSS does this is by taking a single bit of data that it wants to send and encoding and transmitting it as multiple bits. This does mean that we add data to what we transmit, but again it is designed to be resilient. Expanding on this thought we can introduce the term “chips.” No, we are not talking about Pringles here, we are talking about the result of taking a single bit and making it multiple bits, called chips. In DSSS we use something called the Barker Code in which a binary value of 1 is equal to the chip sequence of 10110111000 and the binary value of 0 is equal to the chip sequence of 01001000111. This means that a binary value of 101 is actually a chip sequence of 10110111000 01001000111 10110111000. Interesting right? Now here is how we send it, where 1 bit of data would use 2 MHz of frequency space the 11 found in the chip sequence use 22-MHz and are thus spread out across that spectrum. The receiving side could lose 9 of the 11 bits in the chip sequence and still know wether the data bit was a 1 or a 0. Can you see the resiliency? If you dig a little deepr you will find that in 802.11b which used the 2.4-GHz frequency range, each channel is 22-MHz wide. There are only three channels int eh 2.4-GHz range that do not overlap, channel 1, channel 6, and channel 11. Your Access-Point (AP) will operate on one of these channels offering a 22 MHz wide channel to send data and using the Barker Code and DSSS should get your data from point A to point B.....most of the time.

Copyright © 2009 IDG Communications, Inc.

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