Wi-Fi Signal Strength: dBm, RSSI, Path Loss, and Coverage

Wi-Fi signal strength is measured in dBm — decibels relative to one milliwatt — and because received Wi-Fi signals are tiny fractions of a milliwatt, the numbers are always negative: -45 dBm is a strong signal, -85 dBm is a weak one. The scale is logarithmic, so every 3 dB drop halves the power and every 10 dB drop cuts it to one tenth. Signal weakens for two compounding reasons: free space path loss, the natural spreading of a radio wave over distance, and attenuation from obstacles in the path. When the received level falls below what a client needs for reliable decoding, you get drops, retries, and dead zones — and the fix is always some combination of moving APs, adding APs, or changing antennas.

The dBm scale and RSSI

RSSI (Received Signal Strength Indicator) is the client’s measurement of how much power arrives at its antenna, expressed in dBm. Because the scale is logarithmic, small-looking differences are enormous: -60 dBm is ten times more power than -70 dBm and a hundred times more than -80 dBm.

Practical thresholds worth memorizing:

• -30 to -50 dBm — excellent; client is close to the AP.
• -60 to -67 dBm — good; -67 dBm is the common design floor for voice and video.
• -70 dBm — the edge of acceptable for basic data.
• -80 dBm and below — unreliable; expect low data rates, retransmissions, and disconnects.
• -90 dBm — at or near the noise floor; effectively unusable.

A laptop reporting -85 dBm at a desk 150 feet from the AP, through a couple of walls, isn’t “having driver problems” — it’s simply operating below the level where 802.11 can maintain a stable connection. Distance plus obstructions have attenuated the signal to the ragged edge, and frequent drops are the expected behavior. Signal strength alone isn’t the whole story — what ultimately matters is how far the signal sits above the noise — but a signal this weak fails on both counts (the signal-versus-noise side is covered in data rates and airtime).

Where signal goes: path loss and attenuation

Free space path loss (FSPL) is the reduction in signal strength that happens purely because a radio wave spreads out as it travels, even through empty air with nothing in the way. Energy radiating outward from an antenna covers an ever-larger sphere, so the power arriving at any fixed-size receiving antenna keeps shrinking with distance. FSPL also grows with frequency — a 5 GHz signal loses more over the same distance than a 2.4 GHz signal — which is one reason higher bands have shorter range. This is the term the exam wants when nothing is blocking the path and the signal is still weaker far away: distance alone did it.

Attenuation from obstacles stacks on top of FSPL. Every wall, floor, window, and cabinet in the path subtracts additional dB. Drywall costs a few dB; concrete, brick, and metal cost far more. The extreme case is an enclosure with metal walls — a walk-in freezer’s insulated metal panels block RF almost completely, which is why a client 20 feet from an AP but behind that wall can show little or no signal while open-floor users nearby are fine. Distance was never the issue; the material was. (A full rundown of what materials do to RF lives in Wi-Fi interference sources.)

Transmit power, antenna gain on both ends, FSPL, and obstacle attenuation together form the link budget: what the radio sends, minus everything lost on the way, equals RSSI at the receiver.

Fixing weak signal and coverage gaps

Insufficient coverage has a short menu of real fixes, and the exam loves making you pick the right ones:

1. Add access points where coverage is missing. One AP by the entrance cannot blanket a deep office suite; users at the far end get nothing. Adding an AP (or several, properly channel-planned) at the far side is the direct remediation. Where running new cable is impractical, mesh nodes or range extenders can bridge the gap — with throughput trade-offs worth understanding first.
2. Relocate the AP to a central position so its cell covers the space evenly, rather than wasting half its coverage circle outside the building.
3. Change the antenna to match the space. Omnidirectional antennas radiate a donut-shaped pattern — right for open square areas, wasteful for long narrow ones. A 200-foot loading-dock corridor served from a central omni AP will starve at the far end; a directional antenna (patch or Yagi) aimed down the corridor concentrates the same energy along its length. Placing an additional AP in the corridor works too. Antenna selection is its own topic — see Wi-Fi antenna types.
4. Adjust transmit power — cautiously. Raising AP power can extend reach slightly, but clients must still talk back; an AP shouting farther than clients can reply creates one-way coverage. Power boosts also enlarge co-channel overlap.

What validates any of this is a wireless site survey: walking the space with measurement tools to map actual RSSI against requirements before and after changes (see wireless site surveys).

Symptom	Likely cause	Best fix
Weak everywhere far from a single AP	FSPL — one AP, too much area	Add APs / relocate centrally
Dead zone behind specific structure	Material attenuation (metal, concrete)	AP or antenna on the far side of the obstacle
Long narrow area starved at far end	Omni pattern mismatched to geometry	Directional antenna or additional AP in the corridor
Drops at cell edge, ~-80s dBm	Operating below reliable RSSI	Add/move APs; don’t just crank power

How the N10-009 exam tests this

• A scenario giving a specific reading like -85 dBm with distance and walls, asking what’s wrong — the answer is insufficient received signal (RSSI below reliable thresholds) from distance and attenuation, not a protocol or security issue.
• A definition question asking which term names signal weakening purely from a wave spreading over distance through open air — the answer is free space path loss, not attenuation-by-obstacle, absorption, or interference.
• A dead-zone scenario behind a metal or heavily insulated structure despite short distance — the answer is that the material blocks/attenuates RF; distractors blame the AP hardware.
• A choose-two remediation for users with no connectivity at the far end of a space served by one AP — expect “install an additional access point” paired with “relocate the existing AP more centrally” (or add appropriate antennas).
• A long-corridor coverage scenario — the answer pairs a directional antenna aimed down the corridor with adding an AP in the corridor.

dBm readings only become intuitive with exposure — timed practice questions provide it.

Quick reference

• dBm is logarithmic and negative: closer to 0 = stronger; -3 dB halves power, -10 dB is one tenth.
• RSSI targets: -67 dBm for voice/video design, -70 dBm minimum for data, -80s unreliable, -90 noise floor.
• Free space path loss = weakening from distance alone; higher frequency = more loss.
• Attenuation from materials stacks on FSPL; metal and Low-E glass are near-total blockers.
• Fix coverage with AP placement, additional APs, and antenna choice — not just transmit power.
• Omni antennas suit open areas; directional (patch/Yagi) suit corridors and long spans.
• Verify with a site survey: measure real RSSI where users actually work.