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Beacon frame

A beacon frame is a type of management frame in the IEEE 802.11 standard for wireless local area networks (WLANs) that is periodically transmitted by access points (APs) in infrastructure basic service sets (BSSs) or by stations in independent BSSs (IBSSs) to announce the presence of the network and provide essential synchronization and configuration information to client stations.^[1]^[2] Beacon frames serve critical functions in WLAN operation, including advertising the service set identifier (SSID), supported data rates, security parameters, and operational capabilities to enable station discovery, association, and synchronization with the network.^[3] They are broadcast at fixed intervals called the beacon period, with a default value of 100 time units (TUs)—where 1 TU equals 1024 microseconds—corresponding to the target beacon transmission time (TBTT) to maintain network timing across devices.^[2] Additionally, these frames may include a delivery traffic indication map (DTIM) every few beacons to notify power-saving stations of buffered multicast or broadcast traffic.^[3] The format of a beacon frame comprises a media access control (MAC) header for addressing and control, a variable-length frame body with mandatory information elements such as a timestamp for synchronization, the beacon interval, capability information, SSID, and supported rates, and a frame check sequence (FCS) for integrity verification.^[1] Optional elements in the frame body can encompass country information, extended supported rates, robust security network (RSN) details, and high-throughput (HT) or very high-throughput (VHT) capabilities, allowing adaptation to specific network requirements and standards amendments.^[2]

Overview

Definition and Purpose

A beacon frame is a type of management frame in the IEEE 802.11 wireless local area network (WLAN) standard, transmitted periodically by access points (APs) in infrastructure basic service sets (BSSs) or by stations in independent basic service sets (IBSSs) to announce their presence and operational parameters within a basic service set (BSS).^[3]^[2] The primary purposes of beacon frames include advertising essential network details to enable stations (STAs) to discover and associate with the network, such as the service set identifier (SSID), supported data rates, channel information, and security settings like Wired Equivalent Privacy (WEP) or Wi-Fi Protected Access (WPA).^[4] These elements allow STAs to assess compatibility and join the BSS without prior interaction.^[3] Unlike probe request and probe response frames, which facilitate active scanning through on-demand exchanges between STAs and APs, beacon frames operate as unsolicited broadcasts that passively inform all receiving devices of the AP's or station's capabilities.^[3] Additionally, beacon frames support timing synchronization among network participants by including timestamp information.

Role in IEEE 802.11 Networks

Beacon frames serve as a critical component of the IEEE 802.11 medium access control (MAC) layer, functioning as a subtype of management frames that facilitate ongoing network maintenance and coordination within wireless local area networks (WLANs). These frames enable access points (APs) and stations to manage essential operations such as synchronization, association, and resource allocation without relying on higher-layer protocols. By periodically broadcasting management information, beacon frames ensure the stability and efficiency of the MAC sublayer processes, including contention-based access via the distributed coordination function (DCF) and point coordination function (PCF).^[5]^[6] In network formation, access points (APs) transmit beacon frames to establish and advertise infrastructure basic service sets (BSSs), while stations transmit beacons in independent basic service sets (IBSSs); BSSs form the fundamental building blocks of 802.11 networks.^[2] Each beacon announces key identifiers like the service set identifier (SSID) and basic service set identifier (BSSID), allowing stations to discover and join the BSS by aligning with the advertised parameters. When multiple BSSs are interconnected via a distribution system to create an Extended Service Set (ESS), beacons play a pivotal role in maintaining network cohesion, as they broadcast consistent ESS identifiers that support mobility and seamless handoffs between APs. This mechanism ensures that stations can roam across the ESS while perceiving a unified network topology.^[1]^[7] Stations interact with beacon frames primarily during the discovery phase through scanning procedures, where they evaluate potential networks for association. In passive scanning, stations listen on each channel for incoming beacons, assessing factors such as received signal strength indicator (RSSI) and supported capabilities to select an optimal BSS. Active scanning complements this by prompting APs with probe requests, eliciting probe responses that mirror beacon content, thereby accelerating discovery in environments with sparse beacon transmission. These interactions allow stations to make informed decisions on network selection, prioritizing parameters like channel utilization and security settings for reliable connectivity.^[8]^[9] Beacon frames also significantly influence power management in 802.11 networks by incorporating the Traffic Indication Map (TIM), a bitmap that signals the presence of buffered unicast traffic destined for power-saving stations. Stations in doze mode periodically awaken to receive beacons and consult the TIM to determine if data awaits retrieval, minimizing energy consumption while maintaining responsiveness. This integration supports legacy power save modes and extends to modern enhancements like target wake time (TWT) in later amendments, where beacons help schedule low-power intervals without excessive polling. By embedding such indicators, beacons balance network efficiency with battery life constraints in mobile devices.^[10]^[7]

Frame Structure

General Format

The beacon frame follows the generic IEEE 802.11 MAC frame format, consisting of a MAC header, a variable-length frame body, and a 4-octet frame check sequence (FCS) for error detection.^[11]^[12] The MAC header includes a 2-octet frame control field, a 2-octet duration field, three 6-octet address fields, and a 2-octet sequence control field, resulting in a fixed header length of 24 octets for non-high-throughput (non-HT) operations.^[12]^[2] In the frame control field, beacon frames are identified by a management frame type value of 00 and a subtype value of 8 (binary 1000).^[11]^[13] The From DS bit is set to 1 to denote origin from the distribution system (access point), with the To DS bit set to 0; this configures Address 1 as the broadcast destination address, Address 2 as the source address (BSSID), and Address 3 as the BSSID.^[14]^[15] The frame body length varies up to 2304 octets and includes mandatory fields along with optional information elements, allowing flexibility while adhering to the overall maximum MAC service data unit size.^[12] Beacon frames are encapsulated and transmitted within a physical layer protocol data unit (PPDU) using the basic PHY rate, the lowest mandatory data rate supported by the physical layer for reliable broadcast reception.^[1]^[12]

Key Information Elements

The body of a beacon frame in IEEE 802.11 consists of a sequence of fixed fields followed by one or more variable-length information elements (IEs) that convey essential network configuration and management data to stations. These elements are transmitted by access points (APs) or stations in infrastructure or independent basic service set (IBSS) modes, respectively, to enable network discovery and association.^[16]^[17] Mandatory elements appear first and are required in every beacon frame. The Timestamp field is 8 octets long and contains the current value of the timing synchronization function (TSF) timer at the point of transmission, allowing stations to synchronize their local clocks with the AP or IBSS coordinator.^[16]^[17] The Beacon Interval field follows, spanning 2 octets and specifying the periodicity of beacon transmissions in time units (TU), where 1 TU equals 1024 microseconds; a typical value is 100 TU, corresponding to approximately 102.4 milliseconds.^[16]^[17] Next is the Capability Information field, 2 octets in length, which uses bit flags to indicate supported network features, such as the infrastructure basic service set (ESS) mode (bit 0), privacy (WEP support, bit 4), short preamble option (bit 5 for DSSS PHY), spectrum management (bit 8 for DFS and TPC), and quality of service (QoS, bit 9).^[16]^[17]^[2] Following these fixed fields, the frame body includes a series of IEs in a defined order, with some being mandatory and others optional depending on the network type and PHY layer. Mandatory IEs include the Service Set Identifier (SSID), which has variable length (up to 32 octets) and uniquely identifies the network, broadcast as Element ID 0.^[16]^[17] The Supported Rates IE, also mandatory, is variable in length (typically 1 to 8 octets) and lists up to eight supported data rates in the basic service set (BSS), encoded as subfields with rate values and usage flags (e.g., 6 Mbps as 0x0C for basic support).^[16]^[17]^[2] Common optional IEs provide PHY- and mode-specific details. The FH Parameter Set IE (7 octets, Element ID 2) is included for frequency-hopping spread spectrum (FHSS) PHYs and specifies parameters like dwell time and hop set.^[16]^[17] The DS Parameter Set IE (1 octet data plus overhead, Element ID 3) is used for direct-sequence spread spectrum (DSSS) or orthogonal frequency-division multiplexing (OFDM) PHYs to indicate the current operating channel.^[16]^[17] The CF Parameter Set IE (6 octets data, Element ID 4) details contention-free (CF) period parameters, such as the count and period, if point coordination function (PCF) is supported.^[16]^[17] For ad-hoc IBSS networks, the IBSS Parameter Set IE (2 octets data, Element ID 6) specifies the association identifier (ATIM) window duration.^[16]^[17] All IEs in the beacon frame body adhere to a standardized format for extensibility: an Element ID field (1 octet) identifies the IE type (e.g., 0 for SSID, 1 for Supported Rates, 221 for vendor-specific), followed by a Length field (1 octet) indicating the size of the subsequent Information field (variable length, 0 to 255 octets), which holds the specific data.^[16]^[17] This structure allows for the addition of new standard or vendor-specific IEs without altering the core frame format, supporting ongoing amendments to the IEEE 802.11 standard.^[16]^[17]

Operational Function

Transmission and Broadcasting

In infrastructure basic service sets (BSSs), the access point (AP) generates beacon frames periodically to announce network parameters and maintain synchronization. The AP constructs these frames at regular intervals defined by the beacon interval parameter, which specifies the time between target beacon transmission times (TBTTs) in time units (TUs), where 1 TU equals 1024 microseconds. The default beacon interval is 100 TUs, corresponding to approximately 102.4 milliseconds, though this value can be configured based on network requirements. Each beacon frame includes a timestamp field capturing the current value of the AP's Timing Synchronization Function (TSF) timer, an 8-byte counter that increments in microsecond units to provide a local time reference. Additionally, every nth beacon—determined by the delivery traffic indication message (DTIM) period—is designated as a DTIM beacon, which incorporates a traffic indication map (TIM) element to inform power-saving stations about buffered multicast or broadcast traffic.^[18]^[2]^[19] In independent basic service sets (IBSSs or ad-hoc mode), beacon transmission is distributed among all stations. At each TBTT, awake stations initiate a contention process by setting a random backoff timer between 0 and 15 slot times (e.g., 20 μs per slot in DSSS PHY, yielding up to 300 μs delay). The station that first gains access to the medium transmits the beacon frame containing its current TSF timer value; other stations, upon receiving it, synchronize their TSF to this value and cancel their backoff timers. This distributed mechanism ensures periodic beacons without a central coordinator, though it may introduce slight jitter compared to infrastructure mode.^[20] Beacon frames are broadcast by the AP to all stations (STAs) within the basic service set (BSS) on the operating channel, ensuring reliable delivery without targeting specific recipients. The receiver address (RA) field in the MAC header is set to the broadcast address (all 1s), while the transmitter address (TA) uses the AP's BSSID, a unique identifier for the BSS typically derived from the AP's MAC address. To maximize reach and compatibility, beacons are transmitted at the lowest supported basic rate in the BSS, such as 1 Mbps using differential binary phase-shift keying (DBPSK) in legacy 802.11b networks, allowing even distant or low-capability STAs to decode them. This broadcasting approach eliminates the need for acknowledgments, as the frames serve an announcement role rather than reliable data delivery.^[16]^[21] The transmission timing of beacon frames is tightly coupled to the TSF timer, which the AP maintains as the central clock in infrastructure mode BSSs. The AP initiates beacon transmission at each TBTT aligned with its local TSF value, contending for the medium if it is busy; if the medium remains occupied across multiple TBTTs, the AP gains priority access to ensure timely delivery. In infrastructure mode, the AP acts as the TSF master, starting the synchronization process and providing the reference time via the beacon's timestamp, which downstream STAs use to adjust their local timers for network-wide alignment. This mechanism prevents clock drift and coordinates activities like power-save mode wake-ups across the BSS.^[22]^[18] For support of multiple BSSs on a single physical AP—often implemented as virtual APs with distinct SSIDs—beacon frames are transmitted in sequence within the same beacon interval to minimize timing overlaps and maintain efficient airtime usage. Each virtual AP's beacon follows immediately after the previous one at the shared TBTT, using the same basic rate and broadcast addressing, but with SSID and capability elements tailored to the specific BSS. This sequential approach avoids contention delays between beacons while allowing the AP to advertise multiple networks without fragmenting the interval. In modern implementations, optimizations like the Multiple BSSID element can consolidate information into fewer frames, but traditional multi-BSS deployments rely on this ordered transmission to ensure all beacons fit within the 102.4 ms default period.^[23]^[24]

Synchronization and Discovery

Stations (STAs) in IEEE 802.11 networks discover available basic service sets (BSSs) primarily through passive scanning, where they tune their radio to each supported channel and listen for beacon frames broadcast by access points (APs). This process allows STAs to detect networks without actively transmitting, minimizing interference and power consumption during initial network search. Beacon frames contain essential network parameters, enabling STAs to evaluate potential associations passively. In contrast, active scanning involves STAs transmitting probe request frames to solicit responses from APs; however, received beacon frames during this phase confirm network presence and provide supplementary details for validation. Upon reception of a valid beacon frame, STAs synchronize their local Timing Synchronization Function (TSF) timers by adopting the 64-bit timestamp value embedded in the frame, which represents the AP's TSF counter at the moment the frame's first bit reaches the PHY layer. This update occurs only if the received timestamp is greater than the local TSF value, ensuring the entire BSS maintains a common time reference for coordinated operations such as power management and contention-based access. The TSF mechanism achieves network-wide clock alignment with a drift tolerance of 4 μs, accounting for propagation delays and timer granularity, which supports precise timing for distributed coordination function (DCF) and hybrid coordination function (HCF) operations. Beacon frames are transmitted periodically by APs, typically at intervals of 100 TU (102.4 ms), providing regular opportunities for this synchronization.^[25] During the discovery phase, STAs parse key elements within the beacon frame, including the service set identifier (SSID), supported data rates, and capability information, to assess network compatibility and security requirements before initiating association. This parsing enables STAs to select appropriate BSSs based on factors like channel usage, encryption support, and quality of service (QoS) features. In roaming scenarios, beacon frames facilitate fast BSS transition (FBT) as defined in IEEE 802.11r, by conveying mobility domain information and pre-authentication data that reduce handover latency to under 50 ms, allowing seamless transitions between APs without full re-authentication. To optimize energy efficiency, STAs in power-save mode (PSM) leverage beacon frames for buffered traffic indication. These STAs enter a low-power doze state between beacon intervals and awaken periodically to receive the beacon, specifically examining the Traffic Indication Map (TIM) information element for indications of pending unicast or multicast frames at the AP. The TIM uses a bitmap to signal delivery traffic, prompting the STA to either poll for unicast data via PS-Poll frames or remain awake for multicast delivery following a Delivery TIM (DTIM) beacon. This mechanism enables STAs to achieve significant power savings, with doze periods extending up to the full beacon interval minus processing time, while ensuring timely access to network resources.^[26]

Evolution and Standards

Historical Development

The beacon frame was first specified in the IEEE 802.11-1997 standard as a subtype of management frames intended for periodic transmission by access points in infrastructure basic service sets (BSS) or by any station in independent basic service sets (IBSS) to announce essential network parameters, such as the service set identifier (SSID), supported rates, and synchronization timing. This design enabled stations to discover networks, maintain clock synchronization via the timestamp and beacon interval fields, and coordinate medium access without requiring constant probing.^[16] The frame's body included fixed elements like capability information alongside optional parameter sets tailored to the physical layer in use, emphasizing basic operational announcements over advanced features.^[16] The evolution of beacon frames advanced significantly with the 1999 amendments IEEE 802.11a and IEEE 802.11b, which introduced new physical layers while retaining the core MAC structure. IEEE 802.11a defined an orthogonal frequency-division multiplexing (OFDM) PHY operating in the 5 GHz band, prompting beacons to incorporate OFDM-specific parameters, including supported data rates from 6 to 54 Mbps and regulatory details like country codes to ensure compliance across regions. Concurrently, IEEE 802.11b enhanced the 2.4 GHz direct-sequence spread spectrum (DSSS) to high-rate DSSS (HR-DSSS), with beacons updated to include a DS Parameter Set element specifying the current operating channel and supported rates extended to 5.5 and 11 Mbps for backward compatibility with the original DSSS rates of 1 and 2 Mbps.^[27] These adaptations allowed beacons to convey PHY-layer specifics dynamically, facilitating station association in diverse frequency environments.^[28] A pivotal milestone occurred in the 2003 IEEE 802.11g amendment, which brought OFDM to the 2.4 GHz band for higher throughput up to 54 Mbps while supporting legacy 802.11b devices; beacons were augmented with the ERP Information element to indicate extended rate PHY (ERP) operation, non-ERP station presence, and protection modes like RTS/CTS to mitigate interference in mixed-mode networks.^[29] This element, mandatory in beacons from ERP access points, helped coordinate coexistence by signaling when legacy protection was required, thus broadening deployment in shared spectrum without disrupting earlier DSSS systems.^[30] The 2004 IEEE 802.11i amendment introduced significant security enhancements, adding the Robust Security Network (RSN) information element to beacon frames. This optional element advertises supported security protocols, including authentication and key management suites (e.g., WPA2-Enterprise with EAP) and pairwise/group cipher suites (e.g., CCMP/AES), replacing the insecure WEP and enabling standardized robust security advertisement for stations during discovery and association.^[31] Prior to 802.11i and throughout these early standards up to 802.11g, beacon frames emphasized fundamental discovery and synchronization functions but exhibited key limitations, including reliance solely on Wired Equivalent Privacy (WEP) for confidentiality, which natively supported only 40-bit keys and prompted vendors to add non-standard 104- or 128-bit extensions via proprietary mechanisms.^[32] Absent native provisions for robust security protocols like Wi-Fi Protected Access (WPA) or mesh topologies, early beacons lacked standardized elements for advanced authentication advertisement, often leading manufacturers to insert vendor-specific information elements at the frame's end for custom features such as extended key management or proprietary discovery aids.^[33] These constraints highlighted the frames' initial orientation toward simplicity, spurring later amendments to address security and extensibility needs.^[32]

Updates in Modern Amendments

The IEEE 802.11n-2009 amendment introduced High Throughput (HT) enhancements to beacon frames, incorporating HT Capabilities and HT Operation information elements to support advanced features such as Multiple Input Multiple Output (MIMO) spatial streams, channel bonding up to 40 MHz, and beamforming capabilities.^[34] These elements advertise the access point's HT-specific parameters, including supported modulation and coding schemes (MCS), protection mechanisms for legacy devices, and secondary channel offset, enabling stations to assess compatibility and optimize association.^[35] This update facilitated data rates up to 600 Mbps while maintaining interoperability with prior standards.^[36] Subsequent advancements in the IEEE 802.11ac-2013 amendment, known as Very High Throughput (VHT), extended beacon frame capabilities with VHT Capabilities and VHT Operation elements tailored for the 5 GHz band. These elements detail support for wider channels up to 160 MHz, up to eight spatial streams, and Multi-User MIMO (MU-MIMO) for simultaneous downlink transmissions to multiple devices.^[37] By including these in beacons, access points signal enhanced throughput potential—reaching several Gbps—while ensuring beacons are transmitted on 20 MHz subchannels for legacy device detection and association.^[28] The IEEE 802.11ax-2021 standard, or Wi-Fi 6, further evolved beacon frames through High Efficiency (HE) Capabilities and HE Operation elements, integrating features like Orthogonal Frequency-Division Multiple Access (OFDMA) for resource unit allocation, BSS Coloring to mitigate inter-BSS interference via color-based spatial reuse, and Target Wake Time (TWT) signaling for scheduled low-power wakeups.^[10] These additions enable beacons to convey multi-user efficiency parameters, such as RU allocation hints and coloring values, improving dense environment performance and battery life for IoT devices without disrupting ongoing transmissions.^[38] In the IEEE 802.11be amendment (published July 2025), or Wi-Fi 7, beacon frames incorporate Multi-Link Operation (MLO) elements to support concurrent usage across multiple frequency bands (e.g., 2.4 GHz, 5 GHz, and 6 GHz) by multi-link devices, enhancing throughput, latency reduction, and reliability through link aggregation and fast switching.^[39] These elements advertise per-link parameters, including EHT (Extremely High Throughput) capabilities for wider channels up to 320 MHz and enhanced MU features like puncturing for interference avoidance, while prioritizing power efficiency via refined TWT and other scheduling.^[40] Throughout these modern amendments, backward compatibility is preserved by retaining legacy information elements—such as SSID, supported rates, and ERP—alongside new ones, allowing older 802.11 devices to parse and associate based on recognized fields.^[10]

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