WLAN Air Interface and Roaming
Domain 3 examines 802.11 frame handling, client roaming and band steering: how a client moves from one AP to another without dropping a voice or video session.
Frame types
Management frames build and maintain the link: beacon, probe, authentication and association. Control frames coordinate access: RTS/CTS and ACK. Data frames carry To DS/From DS flags; a WDS or four-address frame is used when traffic crosses a wireless distribution link.
Roaming toolkit
802.11k lets the AP provide a neighbor report so the client scans less. 802.11v adds BSS Transition Management, letting the network suggest a better BSS. 802.11r fast BSS transition pre-derives keys (PMK-R0/R1) and can use over-the-air or over-the-DS signaling, reducing reauthentication time. OKC is a simpler cached-key method and is not 802.11r.
Band steering
A dual-band AP can delay or suppress 2.4 GHz probe responses for dual-band clients, nudging them to 5 GHz. Steering must respect actual RSSI; pushing a client to a weak 5 GHz signal creates sticky behavior.
Worked example: a phone stays on 2.4 GHz at -72 dBm while its 5 GHz radio sees -64 dBm. The AP sends an 802.11k neighbor report and an 802.11v BSS transition request toward the 5 GHz BSS. If both sides support 802.11r, the handover uses FT; otherwise the client performs a full authentication.
Traps: 802.11k only supplies information, it does not force a handover; 802.11r needs support on both client and AP; 802.11v requests can be declined; RSSI is negative, so -60 dBm is stronger than -70 dBm.