How is wireless access point (AP) coverage affected by frequency?

I've heard that Wi-Fi at 5 GHz goes only half as far as Wi-Fi at 2.4 GHz. Is that really true? If so, wouldn't I need twice as many APs to deploy an 802.11n WLAN using 5 GHz channels?

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It's true that RF is more easily attenuated at 5 GHz than at 2.4 GHz, but the amount of signal loss ultimately depends upon the environment. Areas with many dense obstacles cause more attenuation than open areas. Thus, you might find an access point (AP) operating at 2.4 GHz reaches almost twice as far in open space, but when you put that AP inside a building, the difference between 2.4 and 5 GHz reach can vary. With 802.11n, indoor signal propagation is also impacted by multi-path reflections that increase an AP's range.

AP coverage measuring tool
To measure your own AP's coverage area, try using a free tool like HeatMapper -- see Lisa Phifer's HeatMapper product review at Wi-Fi Planet.

Furthermore, effective AP coverage areas depend on performance targets like data rate. For example, if you're designing for minimum 130 Mbps data rate, each AP's coverage area will be much smaller than if you're designing for just 26 Mbps. Suppose you have an 802.11bgn AP that delivers 100+ Mbps up to 100 feet and drops connections at 240 feet. Using 802.11an, that same AP might deliver 100+ Mbps up to 70 feet and drop connections at 150 feet. These are just examples -- data rate depends on many variables -- but they show how the number of APs needed to cover an area depends on design specs.

Finally, 2.4 GHz signals and therefore distance can be degraded by interference -- not just from microwave ovens or cordless phones, but from neighboring WLANs that use up the small set of channels available in that band. When RF interference is present at 2.4 GHz, using 5 GHz instead can reduce or entirely avoid that interference, letting each AP come closer to achieving its maximum potential.

Bottom line: 5 GHz range is likely to be shorter -- this is why dual-band WLANs should always be designed to meet performance targets at 5 GHz. But the effective difference between 2.4 and 5 GHz coverage varies and really needs to be measured in-situ.

This was first published in June 2009

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