has two sublayers: the Logical Link Control [LLC] and the MAC). A description of PHY sublayers follows from [11]:PLCP Sublayer: the MAC layer communicates with the PLCP sublayer via primitives (a set of “instructive commands” or “fundamental instructions”) through a Service Access Point (SAP). When the MAC layer instructs it to do so, the PLCP prepares MAC Protocol Data Units (MPDUs) for transmission. The PLCP minimizes the dependence of the MAC layer on the PMD sublayer by mapping MPDUs into a frame format suitable for transmission by the PMD. The PLCP also delivers incoming frames from the wireless medium to the MAC layer. The PLCP appends a PHY‐specific preamble and header fields to the MPDU that contain information needed by the PHY layer transmitters and receivers. The 802.11 standard refers to this composite frame (the MPDU with an additional PLCP preamble and header) as a PLCP Protocol Data Unit (PPDU). The MPDU is also called the PLCP Service Data Unit (PSDU) and is typically referred to as such when referencing PHY layer operations. The frame structure of a PPDU provides for asynchronous transfer of PSDUs between stations. As a result, the receiving station's PHY must synchronize its circuitry to each individual incoming frame.PMD Sublayer: under the direction of the PLCP, the PMD sublayer provides transmission and reception of PHY layer data units between two stations via the wireless medium. To provide this service, the PMD interfaces directly with the wireless medium (that is, RF in the air) and provides modulation and demodulation of the frame transmissions. The PLCP and PMD sublayers communicate via primitives, through an SAP, to govern the transmission and reception functions.5 5 “Stream” refers to spatial concepts: streams are used for per client Wi‐Fi communication (multiple antenna communication); “channel” refers to the RF facility used to support simultaneous MU Wi‐Fi client communications; there are two or three bands in recent AP hardware that uses 2.4 GHz and one or two 5 GHz – the so‐called Dual‐Band APs or the so‐called Tri‐Band APs.
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