Jeff Duntemanns Drive-By Wi-Fi Guide

Calculating Link Budgets

If you're going to pull all this knowledge together to attempt to span a significant distance using Wi-Fi gear in bridge mode, you should see if the challenge is realistic by calculating a link budget. The idea is to gather all the various gains and losses represented by elements of a Wi-Fi signal path, add them up, and see if the final signal strength falls above or below a Wi-Fi receiver's signal threshold. If it's above that threshold, you're 'in the black' and can establish the link; if it's below, you're 'in the red' with the equipment stated and the link may not work. By using lower-loss cable or higher-gain antennas you might be able to push the quality of the link back up into workable territory.

Here are the variables you need to consider. Calculate or look up their values using the gear you have:

Once you have all of these figures gathered together, you simply add them. Gains are positive numbers, losses are negatives. The resulting figure is the signal strength at the receiver.

You may have trouble determining the receiver sensitivity (and hence the signal threshold) of your particular Wi-Fi access point. Neither Linksys nor D-Link publish receiver sensitivity in their data sheets, online or off. Cisco does, and I'll use the Cisco 340 access point (with which I have some experience) in this example. You can ask a manufacturer's tech support people, but it's hard to tell if tech support even knows. If you can't find out your receiver's sensitivity, assume something on the order of -80 dBm. Most receiver specifications I've seen are a little better than that (in other words, they can pick up a signal with a lower strength of -83 or even -90 dBm) so it's a 'safe' assumption.

A Link Budget Example

It usually helps to work through an example with real hardware and real numbers. For this example I'll use the following hardware components:

First we add together all the gain factors into a single gain factor: 2 x +15 dBi for the antennas, or +30 dBi.

Next we add all the loss factors together into a single loss factor:

Total cable losses: 2 x -.92 dB = -1.84 dB

Total connector losses: 2 x -1 dB = -2 dB

Total link loss =

Path Loss -104 dB + cable loss -1.84 dB + connector loss -2 dB = -107.84 dB

Signal strength at the receiver will be the transmitter power plus antenna gain minus all the various loss factors:

Signal Strength = 15 dBm + 30 dBi antenna gain - 107.84 dB losses = -62.84 dBm

Most Wi-Fi access points have receivers that can pick up a signal at -80 dBm. Since -62.84 dBm is a stronger signal than -80 dBm (these are negative numbers, so 'less' is 'more') there is sufficient signal for the link to work.

Many network designers recommend a margin of 20 dB to cover difficult-to-calculate losses like reflections and other things that can interfere with the link's operation. If your signal comes in at more than 20 dB over receiver sensitivity, you're safely in business. In our example, we have 80 - 62.84 = 17.16 dBm margin. That's on the edge, but it will probably work. For the Cisco Aironet 340, the 11 Mbps sensitivity is -83 dBm, bringing us up to the comfortable 20 dBm margin.

What if you come up 'under budget?' Just as in monetary budgeting, where you can either increase revenues or cut expenses, in link budgets you can either reduce losses or increase gain. Gain antennas with more than 15 dBi gain are fairly common. Replacing the 15 dBi antennas in the example with a pair of 19 dBi antennas will give you an extra 8 dB of margin. Extra gain margin is good because there will always be factors in a link that you can't quantify, and those factors are rarely in your favor.

Note that because of the logarithmic nature of path loss, the loss across 2 miles is only 6 dB more than the loss across one mile-so with 8 extra dB of gain you can bridge another mile, with change!

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