RFUnderstanding WLAN Beacon Frames: Functionality and Fields
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Like other networks, a beacon frame in WLAN announces the availability of the network (i.e., an Access Point Router). It’s also used for maintenance tasks.
These beacon frames are transmitted at regular intervals so that mobile stations (i.e., stations or any WLAN clients, such as laptops, tablets, or any Wi-Fi gadgets) can find and identify the network. This helps WLAN devices match their parameters with the network, allowing them to join. The Access Point (AP) transmits the beacon frame in infrastructure networks. Therefore, stations need to be near the AP to receive the beacon frame.
WLAN Beacon Frame Fields
Figure 1 shows the fields carried by a WLAN beacon frame. Note that not all fields are present in all beacon frames. Frame fields may vary depending on the application and physical layer in use. FH and DS parameter sets are used based on the FH or DS physical layers, respectively. It’s important to note that at any given time, only one physical layer will be present in a WLAN station or AP (either FH or DS).
Table 1 below outlines the fields carried within the WLAN beacon frame’s body. The frame body is divided into mandatory and optional fields.
WLAN Beacon Frame Body Fields
| Field | Description |
|---|---|
| Timestamp | - |
| Beacon Interval | - |
| Capability Info | - |
| SSID | - |
| FH parameter set | This WLAN information element is present when the FH (Frequency Hopping) physical layer is used. |
| DS parameter set | This WLAN information element is present when the DS (Direct Sequence) physical layer is used. |
| CF parameter set | APs that support PCF (Point Coordination Function) will embed this information element in the beacon frame they transmit. |
| IBSS parameter set | This information element is present in beacon frames transmitted by APs in an IBSS (Independent Basic Service Set) network. |
| TIM | Stands for Traffic Indication Map, present only in beacon frames produced by APs. |
