Range and Wireless LANs - What's the Max?

My friend and colleague Keith Shaw, who writes the Cool Tools columns for Network World and stars in online videos under the same name, passed an e-mail on to me that posed an interesting question: what's the maximum range of a wireless LAN?

The answer should be simple - but it's not. In fact, it's like one of those trick questions on the SAT, where the answer cannot be determined without a lot more data.

Here's the problem. Range in any wireless link is determined by a large number of factors, including transmit power, the type and orientation of the antennas being used, receiver sensitivity, the particularly frequency being used, how much bandwidth (spectrum) is used, the source and nature of any interference, and on and on. .11n using MIMO? Outdoors? 2.4 or 5 GHz. (and which bands within 5 GHz.; allowed power levels vary). Etc. Etc. Etc. If we're speaking purely theoretically, though, consider the case of the Voyager 1 spacecraft.

Voyager 1, a deep-space probe launched in 1977, is now about ten billion miles from earth. It's still partially in working order, and we do receive some data from it at a data rate of up to about 1.4 Kbps. The radio is transmitting through a 3.7 meter directional antenna at up to 23 Watts. Receiving antennas on earth are around 30 meters or so. In short - we have a direct, line-of-sight path between transmitter and receiver with very-high-gain antennas, and, while there is always an inverse relationship between range and throughput, well, it works.

Consider, then, the typical Wi-Fi-based WLAN. While regulations vary from band to band and country to country, it is in some cases possible to transmit with up to 1 Watt plus an additional 3 Watts via antenna gain (a topic for another day) - meaning and effective radiated power of 4 Watts. How far could one go with this? Well assuming a decent receiver and directional antennas, miles and miles.

But - we never get the theoretical maximum because of interference, obstructions, and because the typical WLAN radio rarely puts out more than 20 mW, and in some cases a lot less. We get, then, the best data rates when we keep links very short, say, a few tens of meters - hey, these systems are designed for indoor use, after all. It's the use of omnidirectional antennas, though, that provides the biggest limitation to range. Upon transmitting, most of the power is going in the wrong direction. But we have no choice, because we don't know where the other end of the connection is, and implementing a directional antenna on a mobile device is either tough or expensive or both.

All is not lost, however. Anyone who needs a long-range data link might consider the use of a wireless bridge. These are often just WLANs with different firmware and directional antennas. Even in the unlicensed bands, five miles or so at megabit speeds is not out of the question. Your mileage (and thus data rate) will continue to vary with the broad range of conditions you might encounter, but a bridge is definitely preferable to a standard WLAN node if distance is a concern.

So - no simple answer, but there is a solution. Better than nothing...

• Wireless / Mobile
• wi-fi
• wireless LANs