The RF part of wireless networking is often what keeps good IT folks from really getting proficient with WLAN, and many good WLAN types never look beyond the frequency ranges used in 802.11 technologies to see the bigger RF world that we live in. It’s understandable, especially for those without some sort of professional or hobbyist background with signals. The world of WLAN spectrum can be hard enough to wrap your head around, but every now and then there is value in seeing the bigger “comms” picture. The more you understand about the way different frequencies behave in the most basic sense (and what services use those frequencies) the more comfortable you’ll become with really understanding the more mysterious parts of both access-type WLAN and point-to-point bridging.
There are masters’-level classes on RF and radio technologies, tech training courses, and infinite online tutorials and calculators covering all the variety that falls under the broad heading of “learning about RF and RF systems”. This is one of those areas that you never, ever stop learning about. And once the bug bites, it’s not uncommon to become a radio-technology junkie who’s interested in far more than just the goings on in the 2.4 and 5 GHz slices of the electromagnetic spectrum.
Let’s look at just a bit of information on “commonly used” frequencies:
- How long are their wavelengths
- What are their natural “free space path loss” characteristics (how they “fade”)
- At a common power and antenna config, how do they behave compared to each other?
Sounds like heady stuff, yes? It’s really not that bad- so stay with me here.
The following frequencies have meaning to me, and certainly to many of you as well. I’ll give you the wavelength of each, and tell you how much the signal fades after 1 km based on these values applied to each frequency:
- 100 mW (or 20 dBm) of power
- Simple 3 dB antenna at the transmitter and receiver
Whether the signal would be usable (any signal left after path loss)
(Table created by me, there is some minor rounding done)
Again, we see that with same power and antenna gain/sensitivity, the frequency in play makes a dramatic difference to what’s available (or not) at the receiving end.
The frequency is a product of wavelength; the lower the frequency gets, the longer the wavelength is. Lower frequencies also tend to require bigger antennas.
But this little exercise is of limited practical value, beyond helping to understand basic aspects of RF behavior at each of the frequencies I chose to show. High gain antennas, increased power levels (some technologies like Wi-Fi are limited to miniscule power levels while other technologies measure their outputs in Kilowatts), and environmental factors all influence the basic RF goings on at each frequency. Modulation types, quality of engineering, CPU and other silicon behind each given technology all also define performance of whatever technology is in play for a given spectrum. As I mentioned before, it does get complicated.
One of my favorite communications-oriented RF tutorial sites is at National Instruments. Although the American Radio Relay League (ARRL) is often thought of as a ham radio organization, they have a wealth of resources on all sorts of RF-related technologies and industry happenings.
If you’ve never built an antenna of some sort or another, you should. Whether it be a simple project for Over the Air TV or something weird for wireless penetration testing, its worth doing at least once. Research it, build it, improve upon it, and see how altering it changes the performance of whatever your application is (could be using a scanner to hear the local police comms or doing your own point to point wireless bridging), it’s fascinating to design and build something RF-related, at least once. You’ll find that seemingly unrelated wireless disciplines really do enhance the understanding of the actual wireless part of wireless networking.