Warehouse WLAN: Co-Channel Interference and a PoE Budget Question
A 120 m × 60 m distribution warehouse with 12 m steel racking is covered by 12 ceiling-mounted access points. Handheld barcode scanners are 2.4 GHz-only 802.11n devices; the APs are dual-band but no client uses 5 GHz. Radios are set to channels 1, 6 and 11 in a repeating pattern down each aisle run, with three APs on channel 1 within line of sight of each other. Operators report scanner sessions dropping at aisle transitions, and the WLAN controller shows a 45 % retry rate on 2.4 GHz during shift change. The switch feeding the APs is a 24-port PoE+ unit with a 370 W total PoE budget; six PoE dome cameras at 12.95 W already draw from it, and each AP is rated 12.9 W at 802.3af but 'up to 25.5 W with 802.3at'. Facilities have asked whether enabling 802.3at everywhere will fix the drops.
1. Critique the 2.4 GHz channel plan and describe the changes you would make to reduce the 45 % retry rate.
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2.4 GHz has only three non-overlapping channels (1, 6, 11), so a plan must guarantee physical separation between same-channel cells, not just repeat the pattern. Three APs on channel 1 within line of sight means each scanner hears all three, defers to their transmissions, and retries - RSSI looks excellent while throughput collapses, which matches the 45 % figure.
Changes:
- Reduce the number of 2.4 GHz radios in use and lower transmit power so cells shrink and same-channel APs are physically separated.
- Keep 20 MHz channels; 40 MHz in 2.4 GHz consumes the whole band.
- Disable 1, 2, 5.5 and 11 Mbps rates so clients roam rather than cling to a distant AP at low rate.
- Mount/aim APs along the aisles, not behind steel racking, and survey for nulls caused by multipath.
- Move any dual-band client (laptops, newer scanners) to 5 GHz to free airtime.
2. Facilities want 802.3at enabled on every AP. Evaluate that proposal with numbers.
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Budget check at 802.3at: 12 APs × 25.5 W = 306 W, plus 6 cameras × 12.95 W = 77.7 W, total 383.7 W against a 370 W budget. The switch would deny power to whichever devices it brings up last - probably APs, causing an outage rather than a fix. 802.3at also requires Cat5e or better and adds heat in bundled cable.
At 802.3af: 12 × 12.9 W = 154.8 W, plus 77.7 W = 232.5 W, comfortably inside 370 W with headroom for a few more devices.
Conclusion: enabling 802.3at is not a remedy for retries - power is not the failure mode - and would actively break the closet. Only raise an individual AP to 802.3at if its datasheet shows it needs more than 12.95 W (for example 3×3 MIMO with a second radio), and then verify per-port and cable capability first.
3. Operators drop sessions specifically when walking between aisles. What is happening and what would you tune?
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This is a roaming behaviour problem, not a coverage hole: the scanner stays associated to the old AP until the link is effectively unusable, because the client makes the decision and the APs are probably transmitting too strongly to create a clear crossover point.
Tuning:
- Enable 802.11k neighbour reports and 802.11v BSS transition if the controller and scanner firmware support them.
- Set a minimum-RSSI / roaming threshold near −70 dBm so a client on a weak AP is pushed off.
- Reduce AP power so neighbouring cells overlap by roughly 15–20 % at about −67 dBm - enough to roam, not enough to create co-channel interference.
- Validate with a passive survey along the actual walking paths, and confirm the scanners support the features before promising a result; firmware that ignores 802.11k/v will only respond to the RSSI threshold.