RSSI Explained: Wi-Fi Signal Strength in dBm for the Network+ Exam

RSSI stands for Received Signal Strength Indicator. It is the measurement your device reports for how strong a wireless signal is at the receiver, and it is the number behind the bars in the corner of your screen.

RSSI is expressed in dBm, it is always negative, and closer to zero is better. -45 dBm is excellent. -85 dBm is nearly unusable.

That negative-number convention trips people up constantly, so it is worth sitting with. The unit is decibel-milliwatts — a logarithmic measure of power relative to one milliwatt. Wi-Fi signals are a tiny fraction of a milliwatt by the time they reach your laptop, so the value is always below zero. -50 is stronger than -75, because -50 is the larger number.

Reading the Scale

RSSI	Quality	What works
-30 to -50 dBm	Excellent	Everything. You are close to the access point.
-50 to -60 dBm	Very good	HD video, voice, large transfers. The target for most designs.
-60 to -67 dBm	Good	Reliable for most use. -67 dBm is the usual design minimum for voice and video.
-67 to -70 dBm	Fair	Web and email hold up; streaming starts to suffer.
-70 to -80 dBm	Poor	Slow, unreliable, frequent retransmissions.
Below -80 dBm	Unusable	Association may succeed; useful throughput will not.

The number to memorize is -67 dBm. It is the conventional design threshold for voice over Wi-Fi and real-time applications, and it appears in wireless survey requirements constantly.

Signal Strength Is Not the Whole Story

This is where troubleshooting gets interesting, and where exam scenarios like to go. A strong signal does not guarantee a good connection.

SNR — signal-to-noise ratio — is the gap between your signal and the background noise, and it often matters more than RSSI alone. A signal at -60 dBm with a noise floor at -95 dBm gives an SNR of 35 dB, which is excellent. That same -60 dBm signal with a noise floor at -70 dBm gives an SNR of 10 dB, which will perform badly. Same RSSI, completely different experience.

Aim for SNR above 25 dB for solid performance; below 15 dB, throughput collapses.

Other things a good RSSI will not save you from:

• Co-channel interference. Neighboring access points on the same channel force everyone to take turns. Strong signal, poor throughput.
• Non-Wi-Fi interference. Microwaves, cordless phones, Bluetooth, and video senders raise the noise floor in the 2.4 GHz band.
• Client density. Sixty devices sharing one access point perform badly regardless of signal strength.
• Asymmetry. Your laptop hears the access point at -55 dBm, but the access point may hear your laptop far more faintly, because the AP transmits with more power and a better antenna. This is the classic cause of a device that shows full bars and still drops.

Sticky Clients and Roaming

Devices decide when to roam to a better access point using RSSI thresholds, and they are often stubborn about it. A laptop that associated at -45 dBm and walked to the far side of the building may cling to that original AP at -80 dBm rather than switch to one at -50 dBm nearby. The behavior is called a sticky client, and the roaming decision is made by the client, not the network — which is why it is so hard to fix from the infrastructure side.

Mitigations worth knowing: 802.11k for neighbor reports, 802.11v for network-assisted steering, 802.11r for fast transition between APs, and minimum-RSSI settings that let an access point refuse or disconnect clients below a threshold.

Measuring It

On Windows, netsh wlan show interfaces reports signal as a percentage rather than dBm. On macOS, holding Option and clicking the Wi-Fi menu shows RSSI directly. On Linux, iwconfig or iw dev wlan0 link gives the value. Dedicated site-survey tools and the wireless controller give the most reliable picture, including what the AP hears from the client.

One caveat that occasionally appears in questions: RSSI is not standardized across vendors. The 802.11 specification leaves the scale to the implementer, so a raw RSSI value from one chipset is not directly comparable to another's. Values reported in dBm are comparable; raw RSSI integers are not.

Network+ (N10-009) Exam Tips

• Know the expansion and the unit: Received Signal Strength Indicator, measured in dBm, always negative.
• Closer to zero is stronger. Expect a question asking you to pick the strongest of four values — the answer is the one nearest zero.
• -67 dBm is the design threshold for voice and video.
• SNR versus RSSI. If a scenario describes strong signal but poor performance, the answer is usually interference or a poor signal-to-noise ratio.
• Wireless troubleshooting causes: attenuation, reflection, refraction, absorption, co-channel and adjacent-channel interference, and simple distance.
• Sticky clients and roaming as a cause of intermittent wireless problems.

A+ and Security+ Angles

A+ keeps it practical: weak signal as a cause of slow or dropping wireless, and the fixes — move closer, reposition the access point, change channels, add coverage.

Security+ touches RSSI through wireless attacks. An evil twin or rogue access point often presents an unusually strong signal to lure clients, so an unexpected RSSI jump for a familiar SSID is itself an indicator. Site surveys and heat maps are the tools for finding APs that should not be there.

Key Takeaways

• RSSI is received signal strength in dBm, always negative, and closer to zero is stronger.
• -67 dBm is the number to remember as the design minimum for voice and video.
• SNR often matters more than RSSI. Strong signal plus high noise still performs badly.
• Signal strength is directional. What your device hears is not what the access point hears.
• Sticky clients cause problems that look like coverage problems, and the client makes the roaming decision.
• Raw RSSI is vendor-specific; dBm values are the comparable measure.