Understanding 802.11 Pseudo Headers

Pseudo headers is representation of PHY information that is displayed with the captured frames from a Protocol analyzer(Wireshark). It play a crucial role in providing detailed information about signal strength, SNR, Data rates, Channels etc. While troubleshooting an issue, this information provides a detailed information about frame transmission and reception characteristics. When looking at any Wireless Captures, you must have seen the Radiotap Header which is the most well-known example of a Pseudo Header.

Lets explore some of the important fields of Radiotap header:

 




Header length : Duration of wireless frame is derived from the PHY Header length field. It determines how long it will take to transmit the PPDU.

Timestamp: It is the current timestamp of the packet and it helps STA to synchronize its timing with the Access Point.

Flags: The flags shows different PPDU related flags such as, Preamble Type: Short/Long, Frame Check Sequence (FCS) handling, Fragmentation support, Data padding capabilities, Short Guard.

Data Rate: This shows the Data Rate at which the frame was received.

Channel Frequency: This is the channel at which the Capture was collected. In this case Channel-100 of 5 GHz spectrum

Channel Flags: Important fields under this section is the Modulation scheme used and the Spectrum (2.4 or 5GHz) for the captured frame.

Antenna Signal Strength and Noise: This helps us understand the Signal strength at which the frame was received and helps us calculate the SNR; SNR = Noise-Floor- RSSI in this example -96 - (-48) = 48 dBm


The 802.11 pseudo header provides a a great deal of information which is critical for both network analysis and performance optimization. a good understanding these fields allows network engineers to:
  • Troubleshoot connectivity issues
  • Optimize network performance
  • Monitor signal quality
  • Ensure proper protocol operation

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