g/networking • Posted by u/wireless_wendy • 1 months ago

Wireless site survey in a warehouse with 30 foot racking — predictive was useless Wireless

Distribution center, 180,000 square feet, racking to 30 feet, full when occupied and empty when I surveyed.

Ran a predictive survey off the floor plan, designed 34 APs, installed. Coverage was fine on install day. Three weeks later when the racks filled up, coverage collapsed in the aisles and the handheld scanners were unusable in about a third of the building.

What I learned the hard way and what I would do differently:

Predictive surveys assume free space. The model does not know that 30 foot racks full of dense product are effectively walls. An empty warehouse and a full one are different buildings, RF-wise.

Survey the loaded condition or do not bother. If you cannot survey loaded, at minimum do an AP-on-a-stick validation in a loaded aisle before you commit the design.

Aisle-directional antennas, not omnis. In racked aisles the aisle is a waveguide. Patch antennas aimed down the aisle massively outperform ceiling omnis, and it took a redesign to learn it.

Mount below the rack top where you can. An AP above the racks covers the tops of the racks beautifully and the floor not at all.

Ended up at 41 APs with directional antennas mounted at 24 feet, aimed down aisles. Works. Cost the client a change order I partly ate.

💬 6 replies

6 replies

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u/cat6_carlos • 1 months ago

The waveguide insight is the important one and it is counterintuitive. Everybody's instinct in a big open space is more coverage from higher up, and in a racked warehouse that is exactly backwards.

Filing this. I have a cold storage facility coming up with similar geometry and I was about to design it the way you did the first time.

u/wireless_wendy OP • 1 months ago

Cold storage adds another one — the product itself is mostly water and it is a much better absorber than dry goods. If anything, expect it to be worse than what I described. Survey loaded, seriously.

u/rack_and_stack_rob • 1 months ago

"An empty warehouse and a full one are different buildings" should be on a poster. The same is true for occupancy in offices, just less dramatically — a survey done on a Sunday in an empty open-plan floor is optimistic about a Tuesday with 200 people and 200 laptops in it.

u/estimator_erin • 24 days ago

The change order you partly ate is the part I would push back on for next time. A predictive survey is explicitly a design assumption, and if the client would not fund a loaded validation survey, that is a documented risk they accepted.

Write the assumption into the proposal. "Design based on predictive modeling of unloaded condition; validation survey under loaded condition recommended and not included." One sentence and the next change order is theirs.

u/wireless_wendy OP • 21 days ago

That sentence is going into my template today. I did not have language for the thing I knew was a risk, so I carried it.

u/av_wizard • 27 days ago

AP-on-a-stick validation is one of those steps that feels like it slows the project down and saves the project. Same category as walking a conference room with an SPL meter before quoting. An hour of measurement against a change order you eat.