A wireless site survey is a structured way to find out how WiFi behaves in a real building, or how it will behave before you build it. People often ask whether they need a passive survey, an active survey or a predictive one. The three terms sound like grades of the same thing. They are not. Each answers a different question.
This guide explains what passive and active surveys measure, when to run each, and how they fit with predictive (design) and validation (after install) surveys. It also covers what you get back and the mistakes that make survey results misleading.
What is the difference between a passive and an active wireless survey?
A passive survey measures the radio environment without joining the network, and an active survey measures the service a connected device actually receives. Passive asks "what is in the air?" Active asks "what does a client get?"
A third type, the predictive survey, sits apart from both. It is a software model built from a floor plan and needs no measurements on site. The table below sets the three side by side.
| Survey | Connects to the network? | What it measures | Question it answers |
|---|---|---|---|
| Passive | No. It listens only. | Signal strength, noise, signal-to-noise ratio, channel use, overlapping access points | How good is the radio coverage and how crowded is the spectrum? |
| Active | Yes. The test device associates and passes traffic. | Throughput, latency, packet loss, retries, roaming, real connectivity | What do connected clients actually experience? |
| Predictive | No. Nothing is measured on site. | Modeled coverage from floor plans and wall data | Where should access points go, and how many are needed? |
What does a passive wireless survey measure?
A passive survey measures the signals that access points broadcast, plus the background noise around them. The survey device listens only. It does not join any network and it sends no test traffic.
From those broadcasts it can record which access points are heard at each spot, how strong each signal is, which channels they use and how much noise is present. Hamina describes the same set: signal strength, noise levels, channel usage and access point locations, all taken from broadcast beacons.
- Signal strength: how loud an access point sounds at a given point. It is often shown as RSSI, the received signal strength indicator.
- Noise: radio energy that is not WiFi but still disturbs it.
- Signal-to-noise ratio (SNR): the gap between the signal and the noise. A wide gap means a cleaner link.
- Channel overlap: two access points on the same or neighboring channels compete for airtime. This is called co-channel interference.
- Rogue and neighboring networks: other access points, yours or not, that share the space.
What does an active wireless survey measure?
An active survey measures how a connected client performs, because the survey device joins the network and sends and receives real traffic. Wikipedia lists the typical readings as round-trip time, throughput, packet loss and retransmissions.
Joining the network means the surveyor needs credentials for the SSID being tested. It also means the result describes one device, on one network, at one moment.
- Throughput: how much data moves per second, upload and download.
- Latency: the delay for a packet to make a round trip. Voice and video calls feel it first.
- Packet loss: packets that never arrive and must be sent again or are dropped.
- Retries: frames that had to be resent at the radio level. A high retry rate often points to interference or a weak link.
- Roaming: how the device moves from one access point to the next while walking, and whether a call or session survives.
- Real connectivity: whether the device gets an IP address, reaches the services it needs and stays connected.
When should you run a passive survey and when an active one?
Run a passive survey to check coverage and spectrum, and add an active survey when a specific application or user complaint needs proof of real performance. In most projects the passive pass comes first because it shows the foundation the network stands on.
Hamina notes that active results can vary a lot with timing and environment, which is one reason to run them with a clear purpose rather than everywhere by default.
- Choose passive to find coverage gaps, weak spots, noisy channels and overlapping access points.
- Choose passive for a broad look at a whole building, floor by floor.
- Choose active when users report slow speeds, dropped calls or failed roaming.
- Choose active for voice over WiFi, scanners, video or any application with a firm performance need.
- Choose both after installation, when you must show that the finished network works for real devices.
How do passive and active surveys fit with predictive and validation surveys?
Predictive and validation are stages of a project, while passive and active are measurement methods. A predictive survey designs the network, and a validation survey checks it after install, using passive measurements, active measurements or both.
The usual order is simple. Model the network first with a predictive survey. Install it. Then validate it with passive readings for coverage and active readings for performance. Where plans are poor or the risk is high, a temporary access point on a pole can measure the real building before you buy, as described on our on-site survey page.
Hamina makes the same point about priority: a design does not validate an installed network, and if a budget forces a choice, the post-deployment survey matters most. A model is a prediction. A validation survey is the check.
What do you get back from a passive or active survey?
You get a report built around heat maps, which are color-coded floor plans showing a measurement at each spot, plus the findings and fixes that follow from them. A passive survey returns coverage and spectrum maps. An active survey returns performance numbers for the tested device and location.
Ask for the underlying project file as well as the PDF, so another engineer can open it later and compare a new survey with the old one.
- Heat maps for signal strength, signal-to-noise ratio and channel overlap.
- For active work, throughput, latency, loss and retry results, with the device and test method named.
- A list of problem areas with a recommended fix for each.
- Access point positions, channels and settings as found or as recommended.
- The project file, so the work can be reopened and repeated.
What mistakes make a wireless survey misleading?
The most common mistake is running a passive survey only and assuming clients will perform well. Strong signal does not prove good throughput, clean roaming or low retries. A client can see a full signal and still stall.
Three other mistakes come up often.
- Wrong adapter or device: a survey adapter hears the air differently from a phone or scanner. Survey with equipment that reflects the devices your users carry, and say in the report which device was used.
- Surveying too early: if cabling is not run and access point mounts are not fixed, the results describe a layout that will change. Settle cable routes, mounting positions and heights before you validate.
- Treating one active run as the whole truth: timing, load and environment change active results, so repeat key tests and record when they were taken.
How does Deploris scope passive and active surveys?
Deploris scopes a survey against a written brief that names the survey type, the areas and floors, the devices and applications that matter, and the deliverables. The brief says whether passive, active or both are included, so there are no surprises.
We work with Ekahau tools. Vendors name their survey modes in slightly different ways, so ask any provider exactly what will be measured and with which device. We do not publish prices, because cost depends on that scope. We quote against it.
Key takeaways
- Passive surveys measure the radio environment: signal, noise and channel overlap. Active surveys measure what a connected client gets.
- Predictive and validation describe when a survey happens. Passive and active describe how it measures.
- A passive survey alone cannot prove that clients will perform well.
- Survey with devices that match real users, and only after cabling and access point mounts are settled.
- Ask for heat maps, the named test device and the project file.