WiFi site survey: why measure before you install

Almost every troubled WiFi network has a similar origin: the access points were installed where it was easy to run a cable or where there was a power outlet, not where people needed them. The network is tested on a quiet day, it works, and the complaints come later: the meeting room where video calls drop, the storeroom with no signal, the afternoon when everyone connects at once.

A site survey is the study that prevents that scenario: the site is measured, the network is designed on data and the result is checked before handover. It is the difference between assuming the WiFi will work and knowing it.

Problems we see again and again

Coverage is not the same as capacity

A common mistake is designing only for the signal to reach. A modern access point can easily cover a large room, but a signal that reaches everyone does not mean there is enough capacity for everyone: the devices connected to the same radio share the channel and transmit in turns.

That is why a survey starts with questions that have nothing to do with signal: how many people will be in each space, how many devices each one uses, which applications (email, video calls, cloud systems) and when usage peaks. Those answers size the capacity; coverage is solved afterwards.

What a site survey measures

• Signal strength (RSSI). How much signal arrives from the access point, in dBm. Voice and video are usually designed for −67 dBm or better across the whole area; browsing needs less.
• Signal-to-noise ratio (SNR). Useful signal versus background noise. With about 25 dB or more, devices use high data rates; with a small margin, they slow down and retransmit.
• Channel interference. How many access points, yours and your neighbors', are heard on the same channel. 2.4 GHz has only three non-overlapping channels (1, 6 and 11), so space runs out fast.
• Overlap for roaming. How much coverage areas overlap, so a phone can move from one access point to another without dropping the call.
• Real performance. Throughput, latency and packet loss measured while connected to the network, just as users experience it.

Before and after: the same floor, a different design

The heat map is the core tool of a survey: it paints on the floor plan the signal a device receives at every spot. This example shows an office of about 700 m² with an elevator core in the middle and a training room behind a concrete wall.

Before, a single access point at full power covers half the floor well and leaves everything behind the core without service. After, five access points at lower power, each on its own channel, cover the entire floor.

Lowering the power is not a mistake. Smaller cells mean fewer users per access point and less interference between them. Besides, an access point at full power "shouts" louder than a phone can answer: the phone sees a good signal, but the access point cannot hear it well.

Materials matter

Every wall takes something away from the signal, and not all of them equally. That is why a serious design is not done on a blank floor plan: you need to know what the building is made of and, when in doubt, measure it on site with a test access point.

Three types of survey, three moments

The predictive survey estimates quantities and budget before buying. The passive survey confirms on site what the model assumed and uncovers what the floor plan does not show: neighboring networks, a metal ceiling, a storeroom with full shelves. The active survey is the final test, the one that proves the installed network meets the design. When there is non-WiFi interference, such as microwave ovens or industrial equipment, we add a spectrum analysis.

Our process

1. 1

   Assessment and survey

   We talk with the people who use the network: how many people, which devices, which applications and which spaces are critical. We gather floor plans and construction details.

2. 2

   Predictive design

   We model the network with the building materials and define the number, location, channels and power of the access points, along with the cabling and switches they need.

3. 3

   Walking the site

   We run the passive survey and measure the real attenuation of walls we are unsure about with a test access point. We adjust the design with what we find.

4. 4

   Phased implementation

   Each access point goes where the design says, with its certified network outlet, configured with the channel and power plan, coordinating the work with your operations.

5. 5

   Validation survey

   With the network running, we measure signal, interference and performance across the site, and fix whatever falls short.

6. 6

   Support

   We hand over the heat maps, the location and configuration of every device and the record of what was measured: the baseline for any future adjustment or expansion.

On site

A survey is done on foot

The technician walks every hallway, room and storeroom with a laptop or a dedicated meter, marking their position on the floor plan while the equipment records everything it hears. That is how the things no floor plan shows come up: the metal ceiling of a room, the neighbor's network on the same channel or the shelving that blocks the signal at the back of the storeroom.

What a survey changes

Further reading

AISCNET Learning, our technical training platform, has free material on these topics:

• WiFi That Performs: Channels, Rates & Airtime: why WiFi is slow even when it shows a good signal.
• WiFi Channel Planner: detects overlapping channels and co-channel interference.
• WiFi Airtime & Capacity: how much airtime each client uses and when a cell saturates.
• WiFi PHY Rates (MCS): which data rate you get depending on modulation, channel width and spatial streams.

And on this blog:

• High-density WiFi for events, auditoriums and halls: when the challenge is capacity.
• WiFi for large hotels: how we design coverage room by room.
• A well-segmented network: what happens behind the access point.

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