What information does a beacon frame contain?

A beacon frame contains mandatory and optional Information Elements (IEs): Mandatory: Timestamp (TSF, 8 bytes, microsecond counter), Beacon Interval (2 bytes, default 100 TU), Capability Information (ESS/IBSS, privacy, short preamble), SSID (0-32 bytes, empty for hidden networks), Supported Rates (1-8 rates), DSSS Parameter Set (channel number). Common optional: TIM (Traffic Indication Map, buffered frames for sleeping stations), Country (regulatory domain), HT Capabilities (802.11n: channel width, MCS, MIMO), VHT Capabilities (802.11ac: 80/160 MHz, MU-MIMO), HE Capabilities (802.11ax: OFDMA, BSS coloring, TWT), RSN (WPA2/WPA3: cipher suites, AKM). Wi-Fi 6E beacons can exceed 500 bytes with all IEs.

Why do beacon frames affect network performance?

Beacon overhead becomes significant in dense deployments: Each beacon is ~200-500 bytes transmitted at the lowest rate (typically 6 Mbps in 5 GHz). At 6 Mbps, a 300-byte beacon occupies 400 μs of airtime. With 100 TU interval: 400 μs / 102,400 μs = 0.4% airtime per AP. In a dense venue with 50 APs audible on one channel: 50 × 0.4% = 20% of airtime consumed by beacons alone. Solutions: increase beacon interval (200-500 TU for IoT/sensor networks), use FILS Discovery frames (802.11ax, much shorter), enable 6 GHz unsolicited probe response (Wi-Fi 6E), or use hidden SSID with reduced beacon IEs. Multiple SSIDs per AP multiply the problem: 8 SSIDs = 8× beacon load.

How do stations use beacon frames?

Stations use beacons for several purposes: 1) Network discovery: passive scanning listens for beacons on each channel (dwell time 100-200 ms per channel). 2) Timing synchronization: the TSF timestamp aligns the station's clock with the AP for TDMA-based operations (OFDMA scheduling in 802.11ax). 3) Power management: the TIM bitmap tells sleeping stations whether the AP has buffered frames. Station wakes at DTIM intervals (every 1-3 beacons), checks TIM, and sleeps again if no data pending. 4) Roaming decisions: stations compare RSSI of beacons from neighboring APs to trigger roaming when signal drops below threshold (-70 to -75 dBm typically). 5) Channel assessment: stations decode capability IEs to determine supported PHY features before association.

Wi-Fi / 802.11

Beacon Frame

/BEE-kun fraym/

A periodic IEEE 802.11 management frame broadcast by Wi-Fi access points to announce network presence, capabilities, and timing. Transmitted every 102.4 ms (100 TU default) at the lowest mandatory rate. Contains SSID, supported rates, channel, TIM (power save), RSN/WPA security, HT/VHT/HE capabilities, and TSF timestamp. In dense deployments (50+ APs), beacon overhead can consume 20%+ of airtime. Wi-Fi 6E introduces FILS Discovery frames to reduce overhead.

Interval: 100 TU (102.4 ms)

Size: 200–500+ bytes

Rate: Lowest mandatory

Understanding Beacon Frames

The beacon frame is the heartbeat of a Wi-Fi network. Every AP periodically broadcasts a beacon to let nearby stations know it exists, what it supports, and when buffered data is available. Stations performing passive scanning simply listen for beacons on each channel to discover available networks. This is how your phone populates the list of available Wi-Fi networks.

Beacons carry an ever-growing set of Information Elements (IEs) as each 802.11 amendment adds new capabilities. An 802.11ax (Wi-Fi 6) beacon with multiple security configurations, MU-MIMO parameters, OFDMA scheduling, and BSS coloring can exceed 500 bytes, a significant airtime cost when transmitted at the lowest rate to ensure maximum range.

Beacon Airtime Calculation

Single AP Beacon Overhead:
Beacon size: 300 bytes = 2400 bits
Rate: 6 Mbps (5 GHz mandatory)
Airtime: 2400/6M = 400 µs
Interval: 102,400 µs (100 TU)
Overhead: 400/102,400 = 0.39%

Dense Deployment (50 APs/channel):
50 × 0.39% = 19.5% airtime
With 4 SSIDs per AP: 78% (!)

FILS Discovery (Wi-Fi 6E):
Size: ~40 bytes vs. 300+ (beacon)
Airtime: ~53 µs vs. 400 µs
7.5x reduction per frame

Key Beacon Information Elements

IE	ID	Purpose	Size
SSID	0	Network name	0–32 B
Supported Rates	1	PHY rates	1–8 B
TIM	5	Power save bitmap	4–254 B
RSN	48	WPA2/WPA3 security	20–60 B
HT Capabilities	45	802.11n (MIMO, 40 MHz)	26 B
VHT Capabilities	191	802.11ac (80/160 MHz)	12 B
HE Capabilities	255/35	802.11ax (OFDMA, TWT)	~30 B

Common Questions

Frequently Asked Questions

What's in a beacon?

TSF timestamp, beacon interval, capabilities, SSID, rates, channel, TIM, RSN security, HT/VHT/HE capabilities. 200 to 500+ bytes. All IEs are TLV-encoded. Hidden SSID = zero-length SSID IE.

Why do beacons affect performance?

400 µs airtime at 6 Mbps. 0.4%/AP. Dense venue: 50 APs × 4 SSIDs = 78% airtime consumed. Solutions: longer interval, FILS Discovery (40 bytes vs. 300+), reduced SSID count, hidden SSID.

How do stations use beacons?

Passive scanning (discovery). TSF sync (OFDMA scheduling). TIM check (power save: wake at DTIM). RSSI comparison (roaming). Capability assessment (PHY features before association).

Related Terms

← BCU Beam Codebook →

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