VoIP and Roaming in a Warehouse
A 400,000 sq ft distribution center with 40-foot racking needs voice coverage for forklift operators using Wi-Fi handsets. Existing APs are mounted on the ceiling at 30 feet with omni antennas; operators report dropped calls when moving between aisles and at the dock doors. A site survey shows -67 dBm RSSI in open aisles but -78 dBm behind racking, with 12 dB of co-channel interference on 5 GHz in the busiest zone. The client requires sub-150 ms roaming and clear voice. There are 60 APs on 5 GHz and 20 on 2.4 GHz.
1. Diagnose why roaming fails and propose an RF and configuration remedy.
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Diagnosis: ceiling-mounted omnis at 30 ft create large, overlapping cells with no clear roaming boundary; behind racking, signal drops to -78 dBm, so handsets cling to a usable-but-degraded AP until the link fails, producing long roam times. 12 dB co-channel interference worsens retries and delays roam decisions.
Remedy:
- Move to directional or downtilt antennas mounted on racking at 15–20 ft, creating aisle-shaped cells with deliberate overlap at aisle intersections.
- Target -65 dBm primary coverage with 15–20% overlap and keep secondary coverage above -72 dBm so clients roam before the link degrades.
- Reduce co-channel interference by tightening channel reuse (UNII-1/2/3 planning) and lowering transmit power on dense APs.
- Enable 802.11k/v for neighbor reports and BSS transition, and 802.11r (FT) if handsets support it; verify handset firmware supports fast transition.
- Set a minimum RSSI / roaming threshold so clients disassociate below roughly -70 dBm.
2. The dock doors show intermittent drops only when trailers are present. What is the likely cause and how do you validate it?
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Likely cause: metal trailers act as moving RF reflectors/absorbers, changing multipath and shadowing at the dock. Trailers parked in front of APs can attenuate signal by 10–20 dB and shift coverage unpredictably.
Validation:
- Perform a survey with trailers present and absent, comparing RSSI, SNR, and retry rates at the same points.
- Check channel utilization and retry counters on dock APs during trailer arrivals.
- Confirm whether handsets are roaming to a distant AP through the trailer (look for sticky-client behavior and asymmetric signal).
Mitigation: add APs or directional antennas aimed down the dock lanes, lower cell size, and ensure dock APs are on non-overlapping channels. Consider a dedicated SSID/AP group for voice with stricter roaming thresholds.