Radio and Radio waves were the PHY layer for the 802.11

802.11 splits the PHY into two generic components

• Physical Layer Convergence Procedure (PLCP), to map the MAC frames onto the medium

• Physical Medium Dependent (PMD), system to transmit those frames

In 802.11, the PLCP adds a number of fields to the frame as it is transmitted "in the air."

The PLCP prepare the frame for transmission by taking the frame from the MAC sublayer & creating PLCP Protocol Data Unit (PPDU).

PMD sublayer then modulates and transmits the data as bits.

When the MAC Protocol Data Unit (MPDU) is handed down to the physical layer it is then referred to as PLCP Service Data Unit (PSDU). Note that PSDU & MPDU is refer to the same as it is depend on from which sublayer perspective you looking ( PSDU from PHY sublayer & MPDU from MAC sublayer)

When PLCP receives PSDU, it then prepares the PSDU to be transmitted & creates PPDU. The PLCP adds a preamble and PHY header to the PSDU and forms PPDU.

MSDU - MAC Service Data Unit (Layer3-7 Data) called as Packet

MPDU - MAC Protocol Data Unit (MAC Header and trailer + MSDU) called as frame

PSDU - PLCP Service Data Unit (=MPDU) The MPDU in PHY sublayer called as PSDU.

PPDU - PLCP Protocol Data Unit (PLCP Preamble + PLCP Header + PSDU)

PLCP Protocol Data Unit (PPDU)

The PPDU consist of 3 parts.

1. PLCP Preamble
2. PLCP Header
3. PSDU

When the PLCP layer receives the PSDU from the MAC layer, the appropriate PLCP Preamble & PLCP Header are added to the PSDU to create PPDU.

When transmitting data, the Tx STA alerts the Rx STA of transmission by sending PLCP Preamble at the beginning of transmission. IEEE 8021.11-2007 define 3 different preambles

• Long PLCP Preamble
• Short PLCP Preamble
• OFDM PLCP Preamble

802.11n amendment further defines 3 additional preambles in 3 different PPDU.

• non-HT legacy PPDU
• HT-mixed PPDU
• HT-Greenfield PPDU

To know more mrncciew PHY PPDU