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In all the detail on DSSS and OFDM, I have missed which characterics of DSSS and OFDM make these distinct technologies. Nothing I've read appears to contrast the two.

As I understand it, both DSSS and OFDM are intended to send many bits in parallel. DSSS spreads data across a channel for redundancy, and the books say that bits are sent in parallel. OFDM divides a channel into "subcarriers" and likewise sends bits in parallel. This might suggest the main difference between the two, that DSSS somehow sends data in parallel on a single carrier, whereas OFDM accomplishes this with multiple carriers. Either way, both take up more bandwidth than 1 signal, so I don't get how they're much different.

What should a network engineer know, about how DSSS and OFDM are fundamentally different from each other?

Update: According to the current CWNA book, OFDM is not a spread spectrum technology.

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From what I've gathered, the primary differentiation between DSSS and OFDM comes in a congested or constrained RF environment. Because DSSS is transmitting as many bits as possible all at once, it runs the risk of having some of the transmission disrupted in transit should the RF become less than ideal. In contrast, OFDM sacrifices a bit of performance to transmit the data in those multiple carrier "packets" that allow for reassembly or retransmission in the event of a failure.

If I use an extra-large semi truck to haul freight, it works well so long as the highway doesn't have any obstructions (like narrow lanes or short overpasses). If I break my freight up into a greater number of smaller trucks, I can ensure that some of the freight arrives even if conditions (like a traffic jam) prevent the entire shipment from arriving.

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Taken from http://wiki.answers.com/Q/Difference_between_ofdm_dsss_fhss:

Orthogonal Frequency Division Multiplexing (OFDM) is a multiple-carrier (MC) modulation technique which creates frequency diversity. A high-speed data stream is converted into multiple low-speed data streams via Serial-to-Parallel (S/P) conversion. Each data stream is modulated by a subcarrier. That way, instead of having a frequency-selective fading wireless channel, where each frequency component of the signal is attenuated and phase-shifted in different amount, we have multiple flat-fading subchannels.

Spread Spectrum (SS) techniques convert a low-speed data stream into a high-speed data stream. That way, the bandwidth of the modulated carrier becomes much larger than the minimum required transmission bandwidth. This is like Frequency Modulation (FM): we trade transmission bandwidth with Signal-to-Noise (S/N) ratio, meaning that we can have error-free communication transmiting lower-power signals.

The fact is, OFDM is used for 802.11n and upwards, so it seems to be the better of the two. I've been a wireless consultant for a couple of years now, and I've never really been asked any questions regarding modulation of the different specifications. Its good to be aware of the differences, but not much more than that.

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