Why one 802.11b device cuts your AP throughput in half
"One 802.11b barcode scanner. Ratified 1999. Still in service. Cutting effective throughput for every OFDM client in the BSS." ·
A warehouse had 40 802.11ax APs and a throughput problem. The engineers suspected RF. They had been running site surveys for two weeks. Nobody checked the client list for legacy devices. The warehouse used 802.11b-only barcode scanners from 2003. Every single AP was running protection mode, 24 hours a day.
What protection mode is
An 802.11b client cannot decode an OFDM PPDU. If the AP transmits OFDM without warning, the 802.11b client will transmit on top of it — collision. Protection mode adds a CTS-to-self (or RTS/CTS) before every OFDM frame, transmitted at a legacy CCK rate the 802.11b clients can decode. They read the Duration field, set their NAV, and stay quiet.
This is triggered automatically when the AP detects any non-ERP client, or when it hears a neighbouring Beacon with the NonERP_Present bit set in the ERP IE (Element ID 42).
The airtime cost
A CTS at 11 Mbps adds ~100 µs overhead before every OFDM frame. The typical throughput impact in a busy BSS is 30-50% reduction. You are prepending a slow legacy frame to every fast modern frame.
Finding it in PCAP
ERP IE. Expand it — check NonERP_Present: 1 and Use_Protection: 1. Both set means protection mode is active.
CTS frames at 11 Mbps or lower. If you see one before every data burst, protection mode is active in the capture.
The fix
Remove 1, 2, 5.5, and 11 Mbps from the basic rate set. 802.11b-only clients cannot associate — they get a rate mismatch. This is a hard cut. The correct long-term fix is replacing the legacy devices. A barcode scanner that cost $4,000 in 2003 can be replaced for $400 now.