DFS / TPC - 802.11h Radar Avoidance
IEEE 802.11h-2003 (incorporated into 802.11-2020) defines two mechanisms for 5 GHz coexistence with radar systems: DFS (Dynamic Frequency Selection) for radar avoidance and TPC (Transmit Power Control) for interference reduction. Both are mandatory for operating on UNII-2A and UNII-2C channels worldwide.
5 GHz UNII Band Plan
Indoor only (low power limit). No radar co-existence required. Most common enterprise indoor band. Wi-Fi 6E APs often prioritise here for backhaul.
DFS + TPC required. TDWR (Terminal Doppler Weather Radar) frequencies. Typical 60s CAC before first use. High-value channels - 4 clean non-overlapping 20 MHz slots.
DFS + TPC required. Largest DFS sub-band. Includes long-range weather radar frequencies. 11 non-overlapping 20 MHz channels = most spectrum in 5 GHz.
No DFS required. Outdoor use allowed (US FCC 15.407). Highest power limit in 5 GHz. Often used for outdoor point-to-point links and mesh backhaul.
UNII-1: no DFS, indoor only. UNII-2A + UNII-2C: DFS + TPC mandatory. UNII-3: no DFS, outdoor allowed, highest power. The 6 GHz band (UNII-5 through UNII-8) has no DFS requirement - no radar systems operate there. Wi-Fi 6E and Wi-Fi 7 benefits: clean 6 GHz spectrum with no DFS delays.
DFS Lifecycle - From CAC to Recovery
AP monitors the DFS channel for radar pulses BEFORE transmitting any 802.11 frames. If no radar detected in 60s → channel declared available. Some radars require longer CAC: weather radar (TDWR) = 10 minutes in some regions.
Once channel is in use, AP continues passive radar detection while serving clients. Energy detection threshold: −64 dBm for short pulses (1 µs) and longer bursts. A compliant AP must detect >3840 pulses/second at correct pulse repetition frequency (PRF) before declaring radar.
AP must stop transmitting on the DFS channel within 10 seconds of radar detection (FCC 15.407(h)(3)). Non-occupancy period (NOP) = 30 minutes - channel cannot be reused for 30 min after detection.
AP broadcasts Channel Switch Announcement IE (IE 37) in Beacon and Probe Response frames. Fields: Channel Switch Mode (0=STA may transmit before switch, 1=STA must not transmit), New Channel Number, Channel Switch Count (Beacon intervals remaining). Clients cease transmission before switch.
AP must perform full CAC on the new target channel before resuming service. If the AP has a monitor radio, it can scan alternate channels continuously and cache their DFS status (background scan), reducing perceived outage.
TPC - Transmit Power Control
TPC allows the AP to reduce transmission power when operating near radar systems to minimise interference. 802.11h defines two TPC frames and a set of IEs for power negotiation.
Broadcast in Beacons. Single byte: Local Power Constraint in dB. STA calculates its maximum allowed TX power = Max Regulatory Power (from Country IE) − Power Constraint. e.g., If Country IE says 23 dBm max and Power Constraint = 6 dB → STA uses ≤17 dBm.
AP → STA: TPC Request (Action frame, Category 0, Action 2). STA → AP: TPC Report (transmit power used + link margin in dB). AP uses this to determine if STA needs power reduction. Part of spectrum management negotiation.
Sent by STAs in Assoc Req when operating on DFS channels. Min TX power (signed dBm) + Max TX power (signed dBm). Informs AP of STA transmit capability for regulatory compliance. Required field for 5 GHz DFS association.
CSA - Channel Switch Announcement
The Channel Switch Announcement IE (IE 37) is included in Beacon and Probe Response frames when the AP needs to move the BSS to a new channel - either because of radar detection or for load-balancing.
DFS channel switches cause a service interruption of approximately 60 seconds (new channel CAC). Enterprise APs with dual radios can maintain one radio on a non-DFS channel (UNII-1/UNII-3) as a fallback, serving clients during the DFS transition. APs that pre-scan DFS channels with a monitor radio (background scan) can reduce perceived outage significantly by pre-qualifying alternate channels before the switch is needed.
PCAP Identification
wlan.tag.number == 37 // CSA IE present (radar event)
wlan.tag.number == 32 // Power Constraint IE
wlan.tag.number == 33 // Power Capability IE
wlan_mgt.tag.channel_switch_count == 0 // Switch happening NOW