When performing protocol analysis, you capture an 802.1 lac data frame on channel 52, transmitted at MCS 8. At what data rate was the PHY Preamble…

The 802.11ac (VHT) Physical Layer Convergence Procedure (PLCP) frame format is designed for backward compatibility with legacy 802.11a/n devices operating in the same 5 GHz band. To achieve this, every 802.11ac frame begins with a legacy preamble, which includes the L-STF, L-LTF, and L-SIG fields. The L-SIG (Legacy Signal) field, which contains rate and length information, is always transmitted at the most robust and basic OFDM rate of 6 Mbps (using BPSK modulation with a 1/2 coding rate). This ensures that all legacy devices can decode this initial part of the frame and correctly set their Network Allocation Vector (NAV). The specified MCS 8 data rate only applies to the VHT data portion of the frame, not the preamble.

A. 54 Mbps is a valid 802.11a data rate, but it is the highest, not the most robust base rate used for the mandatory L-SIG field in the preamble.

B. 86.7 Mbps is an 802.11ac data rate (MCS 9, 1SS, 20MHz, SGI), not the rate used for the legacy-compatible portion of the preamble.

D. 78 Mbps is the 802.11ac data rate for MCS 8 (1SS, 20MHz, SGI). This rate applies to the data payload, not the legacy preamble.

1. IEEE Std 802.11™-2020 (Revision of IEEE Std 802.11-2016), 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.

Section 22.3.2, "VHT PPDU format": This section explicitly states that the VHT PLCP Protocol Data Unit (PPDU) begins with a legacy preamble (L-STF, L-LTF, and L-SIG).

Section 22.3.8.2.2, "L-SIG (legacy signal) field": This clause specifies, "The L-SIG field is transmitted using BPSK modulation, rate 1/2."

Section 17.3.4, "OFDM PLCP sublayer," Table 17-4—"Rate-dependent parameters": This table defines the parameters for the legacy OFDM PHY. It shows that a BPSK modulation with a coding rate (R) of 1/2 corresponds to a data rate of 6 Mbps.