802.11 Physical (PHY) layer is divided into two sublayers
1. PLCP (Physical Layer Convergence Procedure) sublayer
2. PMD (Physical Medium Dependent) sublayer

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. Below diagram shows the upper layer information moving between the Data-Link & Physical layer.(Page 39 – CWAP Official Study Guide)

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
1. Long PLCP Preamble
2. Short PLCP Preamble
3. OFDM PLCP Preamble

802.11n amendment further defines 3 additional preambles in 3 different PPDU.
1. non-HT legacy PPDU
2. HT-mixed PPDU
3. HT-Greenfield PPDU

Long PLCP Preamble.

Short Preamble PPDU.

Comparison between Long PPDU & Short PPDU is as shown below

OFDM PLCP Preamble.
– Also known as OFDM training structure
– consist of 10 short symbols (t1-t10) & 2 long symbols (T1-T2).
– GI2 is long guard interval.
– Following the PLCP preamble SIGNAL & DATA field each with GI preceding them.
– total training length is 16μS
– short OFDM training symbol consist of 12 subcarriers.
– long OFDM training symbol consists of 53 subcarriers.

PLCP Header
Long & Short PLCP Header contain following 4 files
1. Signal (8bits) – indicate which modulation method will be used for PSDU/MPDU.
2. Service (8bits) – 5 out of 8 bits are used
Bit 3 to indication modulation method (0 – CCK: Complementary Code Keying, 1- PBCC: Packet Binary Convolution Code)
Bit 2 to indicate Transmit Frequency & Symbol clock dreived from same clock.
Bit 5-7 to resolve data length field ambiguities for ERP-PBCC-11 to ERP-PBCC-33
Bit 7 also used to supplement Lenght field for CCK 11Mpbs.
3. Length (16 bits) – Indicate number of microseconds (μS) that are required to transmit the PSDU.
4. CRC (16bits) – Provide Protection for other 3 fields (signal, service & length)Lo

In OFDM transmission, SIGNAL field is 24 bits long.
– First 4 bits (0-3) indicate the data rate (6,9,12,18,24,36,48,54).
– Next bit (bit 4) is reserved for future use
– Next 12 bits (bit 5-16) make up the PLCP Length field which indicate number of bytes in the PSDU.
– Bit 17 will be parity bit for 0-16 bits.
– Last 6 bits (bit 18-23) make up the SIGNAL TAIL with all 6 bits set to 0.

802.11n PPDU
Non-HT PPDU
– Consist of preamble that uses short & long training symbols (10 STF & 2 LTF).
– Support for non-HT legacy format is mandatory for 802.11n radios.
– non-HT transmit only in 20MHz channels.(same format used by 802.11a & 802.11g

HT-Mixed PPDU
– Preamble contain the non-HT short & long training symbol that can be decoded by legacy 802.11a (clause 17) or 802.11g (clause 19)
-Rest of the HT-mixed preamble & header cannot be decoded by legacy clients.
– Tranmission can occur both 20MHz & 40MHz.
– When 40MHz channel is used all broadcast traffic must be sent on legacy 20MHz (for legacy clients)

HT-greenfield PPDU
– Preamble not compatible with legacy clients.

References
1. CWAP Official Study Guide – Chapter 2