Comparing spread spectrum technologies
 
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Because I receive a fair amount of e-mails questioning the difference between direct sequence spread spectrum and frequency hopping spread spectrum, I thought I'd take a moment to address the basics of these radio transmission methods.
First, let's take a high-level look at spread spectrum technology. Spread spectrum technology is a radio transmission technology that embeds mechanisms to avoid interference - both from other wireless devices and intruders.
The HomeRF wireless LAN specification uses frequency hopping spread spectrum (FHSS) for radio transmission, while the 802.11b standards use direct sequence spread spectrum (DSSS). Although the two LANs use the same frequency band (2.4 GHz, unlicensed), they are not interoperable. The Bluetooth personal area network, which is currently heating up for wireless communications among peripheral devices, also runs in the 2.4 GHz band and specifies the use of FHSS.
Spread spectrum in general was originally the brainchild of the military, invented to prevent radio signals from being monitored or blocked by nefarious souls. For wireless communications in an unlicensed radio frequency band, FCC regulations require the use of a spread spectrum technology. The choice is DSSS or FHSS.
FHSS was invented first. It changes the frequency of a transmission at regular intervals faster than an intruder could retune a jamming device designed to block a transmission running on a particular frequency. For FHSS to be used successfully, the receiver must know the frequency-hopping pattern so that it can interpret the transmission.
DSSS came along later. Unlike frequency hopping, DSSS spreads chunks of data over multiple frequencies at the same time. DSSS is said to provide somewhat faster data transmission and shorter delays because the transmitters and receivers don't have to spend time retuning to different frequencies. On the other hand, FHSS is said to consume less power and allow more access points than DSSS to coexist in the same area without blocking one another from transmitting.
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Joanie Wexler is an independent networking technology writer/editor in Campbell, Calif., who has spent most of her career analyzing trends and news in the computer networking industry. She welcomes your comments on the articles published in this newsletter, as well as your ideas for future article topics. Reach her at joanie@jwexler.com.
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