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IEEE® 802.11 WLAN - Beacon Frame

This model shows transmission and reception of beacon frames in an 802.11 based wireless local area network (WLAN) as described in [ 1 ]. The beacon frame is a type of management frame. It identifies a service set formed by a number of 802.11 devices. The access point (AP) of a service set periodically transmits the beacon frame to establish and maintain the network. In an ad hoc network, the stations transmit the beacon frame.

Structure of the Example

The model has three main parts:

  • Model Parameters, where you can select the information sent within the beacon frame and the channel parameters,

  • 802.11 System, which is formed of the transmitter, channel, and the receiver,

  • Results, where you can view the most important received information, such as CRC flags, packet type, and received SSID.

The following section describes the transmitter and receiver in more detail.

Transmitter

The transmitter implements a lightweight MAC sublayer and physical layer that constructs the PHY layer frame. It transmits each PHY frame using several consecutive channel frames.

The MAC sublayer constructs the beacon frame, which is a type of a management frame. The following figure [ 1 ] shows the management frame format. This frame is also called a MAC protocol data unit (MPDU).

This example implements a beacon frame with a frame body that contains the following fields:

  • Timestamp

  • Beacon interval

  • Capability

  • SSID elements

  • Supported rates element

  • Direct sequence (DS) parameter set element

  • Contention Free (CF) parameter set element (optional)

You can set some of the parameters contained in these fields using the model parameters GUI. For example, the channel number parameter, which is sent in the DS parameter set element.

The transmitter turns on periodically to transmit a beacon frame. The beacon interval parameter determines the transmission period.

The PHY Framer and Modulator creates the PLCP protocol data unit (PPDU) by adding the physical layer convergence procedure (PLCP) preamble and header to the MPDU. The PLCP preamble contains 128 bits of ones (SYNC), which are later scrambled. The receiver determines the presence of a PPDU frame using this SYNC signal. The LENGTH field of the PLCP header determines the MPDU frame length. PLCP header also contains a 16-bit CRC.

The transmitter scrambles the PPDU frame, modulates using differential binary phase shift keying (DBPSK) at a rate of 1 Mbps, and applies spreading using a length 11 Barker code. This subsystem also pads extra chips to the spread symbols to reach a 1024-bit maximum PPDU length. This way, the system forces the MAC layer to work at a period of 1024 microseconds, which is a time unit (TU).

The transmitter emits 128 modulated symbols (1408 chips), which is a channel frame, at a time. Modulated symbols pass through a square root raised cosine pulse shaping filter. Pulse shaped symbols are sent through an AWGN channel that also applies a frequency offset and delay.

Receiver

The receiver consists of down-conversion and matched filtering, a receiver controller, and a detector.

The receiver controller searches for the scrambled SYNC signal using a correlator. Once a peak is detected, the controller delays the received signal to align at a channel frame boundary, down samples, and turns on the detector. The controller also provides indices to the detector to aid parsing the received frame.

The detector despreads, demodulates, and descrambles the received signal. The packet parser creates a PLCP buffer and an MPDU buffer. Once the PLCP buffer fills, the PLCP decoder turns on and decodes the PLCP frame. If the PLCP CRC checks, i.e. the CRC check flag is 0, then the PLCP decoder supplies the length of the MPDU to the MPDU decoder. Otherwise, the detector is turned off by the receiver controller.

When the MPDU buffer receives the number of bits that the LENGTH field of the PLCP specifies, the MPDU decoder is activated. If the MPDU CRC checks, i.e. MPDU CRC flag is 0, then the MPDU contents are parsed.

Results and Displays

When you run the simulation, it displays these textual results:

  • PLCP CRC flag (0 for no error, 1 for error or no frame)

  • MPDU CRC flag (0 for no error, 1 for error or no frame)

  • Frame type (0 for management frame)

  • Frame subtype (8 for beacon frame)

  • Selected MPDU information, such as beacon interval and SSID

You can see all the PLCP and MPDU information if you open the PLCP and MPDU display subsystems.

The simulation also displays the scatter plots of transmitted and received chips, and the despread symbols. You can see the rotating scatter plot due to the frequency offset. The differential demodulator is able to track the phase change and correctly demodulate the symbols.

Exploring the Example

This example allows you to modify beacon frame parameters such as beacon interval, SSID, supported rates, etc. You can also include or exclude the CF parameter set element from the generated beacon frame. You can also set the noise level (Es/No), delay, and frequency offset introduced by the channel.

Selected Bibliography

  1. IEEE Std 802.11-2007: IEEE Standard for Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, IEEE, New York, NY, USA, 1999-2007.

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