11 February 2010

The Lessons Of Wi-Fi #4: All Wi-Fi Vendors Live By The Same Rules of Physics

You've invited Wi-Fi vendors to your facility to discuss a new Wi-Fi project. You need wireless access across an entire floor of your building which includes open plan seating, conference rooms, and executive offices. This will be the primary form of network access and it needs to work. All the time.

It's late afternoon. A Wi-Fi vendor sits across from you in his white suit and black shirt, the very model of semi-neo-avant garde stylin. His shiny white tasseled loafers are firmly planted on the corner of your desk. He looks you straight in the eyes and says that his access point transmits radio signals farther than anyone else's. "It uses special technology. Yes, it's expensive, but by packing sixteen super duper radios in one unit you'll save a bundle because you only need one access point to cover the entire floor." Wow! How can you go wrong?

Those who forget the lessons of Wi-Fi are doomed to repeat them. Lesson #4: we all live by the same laws of physics, and no Wi-Fi vendor has yet bent them to their will.

The maximum output of a radio at any given frequency is dictated by local regulatory agencies. In most countries 100 milliWatts is the upper limit of what an indoor access point is permitted to output. Regardless of vendor and irrespective of Wi-Fi chip vendor - Atheros, Broadcom, Intel, etc. There is a level playing field when it comes to building radios.

What vendors can do is twiddle with antennas, using directional antennas to focus the allowed radio energy into more well defined beams. And, indeed, doing so can project radio signals longer distances.

The issue is that Wi-Fi networks are bidirectional - there's something on the receiving end of those directional antennas. Low power clients like iPhones and netbooks aren't equipped with directional antennas, much less ones that are easily focused on access points. They may be able to hear distant access points but the access points may be unable to hear them - even if directional antennas are used - because they don' use high power radios.

Additionally,as we learned in Lesson #3, bit rate is inversely proportional to range. In a shared medium like 802.11 where only one device transmits at any one time, lower data rates mean less available air-time for data on that entire 802.11 channel. So even if an access point and its clients can communicate, the throughput from the clients to the access point will be relatively low. Not good for voice. Not good for video. Not good for you.

You don't get something for nothing, but you can find yourself with nothing from something. The Wi-Fi standards anticipated the use of multiple access points, and that's how clients are designed to work. Pushing the limits of how far a Wi-Fi signal can be made to propagate has heuristic value, but when it comes to real-world deployments it can jeopardize the functionality and reliability of your network.

It's best just to tell the vendor to take his shoes off your desk and sell his wares elsewhere - you're having none of it.

If you'd like to get the whole picture on Wi-Fi architecture you've only to download our free white paper, WLAN RF Architecture Primer
. And leave it to someone else to relearn the lessons of Wi-Fi.