A beacon frame is a type of management frame in Wi-Fi networks that announces the presence of a wireless network. Sent by access points (or by devices in peer-to-peer setups) roughly every 100 milliseconds, it carries the information your phone, laptop, or tablet needs to discover a network, connect to it, and stay synchronized once connected.
How Beacon Frames Work
Every Wi-Fi access point continuously broadcasts beacon frames on its operating channel. This is the mechanism behind what you see when you open your device’s Wi-Fi settings and a list of nearby networks appears. Your device is either passively listening for these beacons or actively sending out probe requests, but beacons are the default way access points make themselves discoverable.
In the 802.11 protocol, frames are classified by type and subtype. Beacon frames fall under management frames (type 0) with a subtype value of 8. Unlike data frames that carry your actual internet traffic, management frames handle the administrative overhead of maintaining a wireless network: things like authentication, association, and in this case, network advertisement.
Because every device in range needs to be able to read a beacon, access points transmit them at the lowest data rate the network supports. In a mixed network where some older devices connect at 1 Mbps, beacons go out at 1 Mbps. This ensures universal readability but comes at a cost: beacon traffic at that slow rate can reduce overall network throughput by about 4% when the beacon interval is set very short (25 ms).
What’s Inside a Beacon Frame
A beacon frame has three main parts: a MAC header, a frame body, and a frame check sequence that verifies the data wasn’t corrupted in transit. The frame body is where all the useful information lives, and it contains both fixed fields and variable-length information elements.
The fixed fields include:
- Timestamp: A counter that lets all devices on the network synchronize their clocks with the access point.
- Beacon interval: Tells devices how often to expect the next beacon. Expressed in time units (TUs), where one TU equals 1,024 microseconds. The standard default is 100 TU, which works out to 102.4 milliseconds.
- Capability information: A set of flags that describe what the network supports, such as whether encryption is required or whether the network is operating in infrastructure mode versus ad hoc mode.
After the fixed fields come information elements, which vary depending on the network’s configuration. These include the SSID (the network name you see on your device), supported data rates, which channel the network operates on, and parameters specific to the physical layer in use. One access point might include a parameter set for direct-sequence spread spectrum, while another uses OFDM parameters for higher-speed operation. These are mutually exclusive since only one physical layer is active at a time.
Power Saving With the Traffic Indication Map
One of the most important information elements in a beacon is the Traffic Indication Map, or TIM. This is a bitmap where each bit corresponds to a connected device. When your phone goes into sleep mode to save battery, the access point buffers any incoming data meant for it. The TIM bit for your device gets set to 1, signaling that data is waiting.
Your device doesn’t need to stay awake constantly to check for new data. Instead, it wakes up briefly at each beacon interval, reads the TIM, and checks its assigned bit. If the bit is 0, the device goes right back to sleep. If it’s 1, the device stays awake long enough to request and receive the buffered data. This mechanism, known as legacy power save mode, is fundamental to how wireless devices manage battery life while maintaining a network connection.
Hidden Networks and SSID Behavior
When you configure a network as “hidden” on your router, the access point doesn’t stop sending beacons. It simply sets the SSID field to null, meaning the network name is blank. Your device still receives the beacon and knows a network exists on that channel, but it can’t display a name. To connect, you need to manually enter the SSID, which prompts your device to send a directed probe request. The access point then responds and the connection proceeds normally.
This is why hiding your SSID offers minimal security. The beacons still go out, and the SSID gets revealed in other management frames during the connection process. Anyone with a packet sniffer can see it.
Beacon Frames in the 6 GHz Band
Wi-Fi 6E and Wi-Fi 7 introduced the 6 GHz band, which changes how beaconing works. The 6 GHz band has dozens of available channels, and having a device passively scan each one for beacons would take too long and waste battery. Instead, the 6 GHz band relies on out-of-band discovery.
Access points that operate across multiple bands (2.4 GHz, 5 GHz, and 6 GHz simultaneously) include a Reduced Neighbor Report information element in their 2.4 GHz and 5 GHz beacons. This element contains the SSID, the unique identifier, and the channel of the co-located 6 GHz radio. Your device discovers the 6 GHz network by reading a beacon on a lower band it’s already scanning, then jumps directly to the correct 6 GHz channel. This makes discovery faster and more energy-efficient.
Wi-Fi 7 also requires beacon protection for both access points and clients, adding cryptographic integrity checks to prevent attackers from forging or tampering with beacon frames.
Why the Beacon Interval Matters
The default 100 TU interval (about 10 beacons per second) is a balance between responsiveness and overhead. Shorter intervals mean devices discover the network faster and power-saving clients check for buffered data more frequently, but each beacon takes airtime away from actual data traffic. At a 25 ms interval, the throughput penalty becomes noticeable.
Longer intervals reduce overhead but slow down network discovery and increase latency for power-saving devices waiting to retrieve buffered frames. In dense environments like office buildings or stadiums with many access points, beacon overhead adds up. Each access point on the same channel takes turns sending its beacons, and the time spent on beacon transmissions at low data rates can meaningfully reduce the capacity available for user traffic. Network administrators in these environments sometimes increase the beacon interval to 200 or 300 TU to reclaim airtime.