| Legacy PS (PS-Poll) | STA sets PM bit=1 (via Null frame) → AP buffers its DL traffic → AP sets STA's AID bit in beacon TIM → STA wakes each beacon, reads TIM, sends PS-Poll per buffered frame → AP delivers (More Data bit signals additional frames). | 802.11-1997 original. One PS-Poll per frame = chatty and slow. Broken implementations here cause "IoT device misses traffic" tickets. |
| TIM / DTIM | TIM element (IE 5) in every beacon: bitmap of AIDs with buffered unicast. DTIM (Delivery TIM) every N beacons: AFTER a DTIM beacon the AP transmits buffered broadcast/multicast. | DTIM period tradeoff: 1 = fastest multicast/low latency, worst battery; 3 = common default; high values break mDNS/ARP timing for sleepy devices. Voice SSIDs: keep DTIM small. |
| U-APSD / WMM Power Save | Trigger-enabled ACs: any uplink QoS frame from the STA = trigger → AP releases buffered frames of delivery-enabled ACs in a Service Period, ends with EOSP=1. | Perfect for VoIP: each voice packet (every 20 ms) doubles as trigger. Negotiated via QoS Info bits in WMM IE at assoc. Legacy PS and U-APSD can coexist per-AC. |
| SM Power Save (11n) | MIMO client powers down extra RX chains; static (always 1 chain) or dynamic (RTS wakes chains). | Explains clients associating 2×2 but receiving like 1×1. |
| TWT - Individual (11ax) | STA and AP negotiate exact wake schedule (TWT Setup Action): wake interval = mantissa × 2^exponent µs, min wake duration, first TWT time. STA sleeps entire interval - no beacon wakes needed. | Battery king for IoT: sensor can wake every 10 min. Announced TWT: STA signals wake with PS-Poll/QoS Null; Unannounced: AP just transmits at scheduled time. Trigger-enabled TWT: AP sends Trigger in SP for UL OFDMA. |
| TWT - Broadcast (11ax) | AP advertises shared TWT schedules in beacon (TWT IE 216); STAs join a schedule rather than negotiate individually. | Scales to hundreds of IoT clients; also used to time-slice airtime (scheduling flavor of 11ax). |
| TWT - Restricted (R-TWT, 11be) | Protected service periods for latency-sensitive traffic: other 11be STAs must end TXOPs before R-TWT SP starts; member streams get prioritized access. | Wi-Fi 7 determinism tool for XR/industrial - the closest Wi-Fi gets to TSN-style scheduling. |
| WNM-Sleep (11v) | Extended sleep for minutes+ while staying associated; GTK/IGTK updates delivered at wake. | Rarely enabled; check both sides support before relying on it. |
| WUR (802.11ba) | Separate ~µW wake-up receiver listens for OOK wake-up packets on 4 MHz; main radio fully off until woken. | ~250 kbps control channel; wake latency ms-class with years of battery. Niche silicon so far. |
| Power save debugging | Classic failure: AP buffers frame → client sleeping longer than buffer timeout → drops. Or client PM-bit flapping. Capture filters: wlan.fc.pwrmgt==1, PS-Poll (type_subtype==26), Null (==36), EOSP bit. | Symptoms: ping spikes matching DTIM multiples, IoT devices dropping mDNS, VoWiFi one-way audio after silence. |