The fundamental difference between a legacy Data frame and a QoS-Data frame, as defined by the IEEE 802.11e amendment, is the inclusion of a 2-byte (16-bit) QoS Control field in the MAC header. This field is present only in frames with the QoS subtype. It contains several subfields, most notably the Traffic Identifier (TID), which maps the frame to one of eight User Priorities (UPs). These UPs are then mapped to specific Access Categories (ACs) for prioritized channel access using EDCA (Enhanced Distributed Channel Access). The presence of this field is the defining characteristic that enables MAC-layer Quality of Service.

A. Differentiated Services Code Point (DSCP) is a Layer 3 marking in the IP header, not a field within the 802.11 MAC frame header.

B. The QoS Information Element is used in Management frames (e.g., Beacons) to advertise QoS capabilities, not within individual Data frames.

C. An 802.1Q VLAN tag is a Layer 2 mechanism for wired Ethernet and is not a native component of the 802.11 MAC header.

1. IEEE Std 802.11™-2020 (Revision of IEEE Std 802.11-2016). Section 9.3.2.1, "Data frame format," states: "The QoS Control field is present when the QoS subfield of the Subtype field is 1." Figure 9-28, "MAC frame format for QoS Data frames," visually depicts the location of the QoS Control field within the MAC header.

2. Coleman, D. D., Westcott, D. A., & Harkins, B. A. (2021). Certified Wireless Analysis Professional (CWAP) Official Study Guide: Exam CWAP-404 (2nd ed.). Wiley. Chapter 4, "802.11 MAC Architecture," in the section "QoS Data Frames," explains that the key differentiator for these frames is the 16-bit QoS Control field, which carries the priority value for the frame.

3. Hiertz, G. R., Denteneer, D., Stibor, L., Zang, Y., Costa, X. P., & Walke, B. (2010). The IEEE 802.11s MAC layer management and the mesh deterministic access. IEEE Wireless Communications, 17(1), 54-62. https://doi.org/10.1109/MWC.2010.5416349. In Section II.A, "The 802.11e QoS Facility," the paper states, "The main enhancement of 802.11e is the introduction of a new data frame format, the QoS data frame, which contains a QoS control field in its MAC header."