Why −67 dBm is the WiFi design target

Many enterprise WiFi networks are designed so that devices receive at least −67 dBm from an access point everywhere they are used. It is not a law or a formal standard, but a widely used rule of thumb, drawn from vendor guidance for voice over WiFi, that leaves enough margin for devices to keep clear, reliable connections and roam smoothly.

Where the number comes from

The −67 dBm figure became common through equipment vendors' design guidance for voice over WiFi. Voice is one of the most demanding uses of a wireless network: a call cannot pause and retry the way a web page or email can, and handsets are carried around while in use, so they must move between access points without dropping audio.

Designers found that keeping signal at about −67 dBm or stronger, together with a good signal-to-noise ratio, gave handsets enough headroom to use efficient data rates, recover from brief dips and find a new access point before the old one faded. Because a network designed for voice also handles most other applications well, the figure spread into general enterprise design.

What the target protects

Margin for real-world variation

Signal at any spot moves up and down as people walk past, doors open and close, and devices are held differently. A phone in a pocket or a hand can lose a noticeable amount of signal compared with a survey adapter on a laptop. Designing to −67 dBm leaves room for those losses without falling into the weak range where connections suffer.

Efficient data rates

Stronger signal, with good SNR, lets devices use faster rates. Faster rates mean each transmission takes less airtime, which frees the channel for other users. A network full of devices on slow rates runs out of capacity long before it runs out of coverage.

Smooth roaming

Devices decide when to move to another access point, often when signal drops below their own internal threshold. If coverage is designed so that each area is also covered by a second access point at a usable level, devices have somewhere to go before the call breaks up. See WiFi roaming and handoff for more on how that works.

The target is never just one number

Signal strength alone does not make a good network. −67 dBm is normally paired with other targets, such as:

• Signal-to-noise ratio, commonly around 25 dB for voice. A strong signal in a noisy place still performs badly. See RSSI, SNR and signal strength.
• Secondary coverage, meaning a second access point heard at a usable level, for roaming and redundancy.
• Limited co-channel overlap, so that access points on the same channel do not drown each other out.
• Capacity, so that each access point serves a reasonable number of devices for the applications in use.

When a different target makes sense

−67 dBm is a starting point, not a universal answer. The right target depends on the devices and applications in each area.

Why turning up the power is not the answer

It is tempting to meet the target by setting every access point to full power. That usually backfires. Access points become audible far beyond their intended area, increasing co-channel interference. Devices hear a loud access point but cannot be heard well in return, because their own transmitters are weaker. And devices tend to cling to a distant loud access point instead of roaming to a closer one. Good design meets the target with the right number and placement of access points at moderate power. Access point placement covers how.

How ehZee can help

We set signal, SNR and overlap targets for each area based on the devices and applications you actually use, then measure your network against them with AirMagnet Survey Pro. Our WiFi surveys service shows where those targets are met and where they are not, and what change will close the gap.

Common questions