802.11 Physical Layer (PHY):

• At the physical layer, IEEE 802.11 defines three physical characteristics for WLANs:
• Diffused infrared (baseband), DSSS, and FHSS. All three support a 1 to 2 Mbps data rate. Both DSSS and FHSS use the 2.4 GHz ISM band (2.4–2.4835 GHz). The physical layer provides three levels of functionality.
• These include: (1) frame exchange between the MAC and PHY under the control of the physical layer convergence procedure (PLCP) sublayer; (2) use of signal carrier and spread spectrum (SS) modulation to transmit data frames over the media under the control of the physical medium dependent (PMD) sublayer; and (3) providing a carrier sense indication back to the MAC to verify activity on the media (see Figure 1).

DSSS PHY:

• In the DSSS PHY, data transmission over the media is controlled by the PMD sublayer as directed by the PLCP sublayer. The PMD sublayer takes the binary information bits from the PLCP protocol data unit (PPDU) and converts them into RF signals by using modulation and DSSS techniques (see Figure 1).
• In the PPDU frame (figure 2), which consists of a PLCP preamble, PLCP header, and MAC protocol data unit (MPDU). The PLCP preamble and PLCP header are always transmitted at 1 Mbps, and the MPDU can be sent at 1 or 2 Mbps.
• The start of frame delimiter (SFD) contains information that marks the start of the PPDU frame. The SFD specified is common for all IEEE 802.11 DSSS radios.
• The signal field indicates which modulation scheme should be used to receive the incoming MPDU. The binary value in this field is equal to the data rate multiplied by 100 kbps.
• The service field is reserved for future use. The length field indicates the number of microseconds necessary to transmit the MPDU. The MAC layer uses this field to determine the end of a PPDU frame.
• The CRC field contains the results of a calculated frame check sequence from the sending station.

FHSS PHY:

• In FHSS PHY, data transmission over media is controlled by the FHSS PMD sublayer as directed by the FHSS PLCP sublayer. The FHSS PMD takes the binary information bits from the whitened PSDU and converts them into RF signals by using carrier modulation and FHSS techniques.
• The format of the PPDU is shown in Figure 3. It is consists of the PLCP preamble, PLCP header, and PLCP service data unit (PSDU). The PLCP preamble is used to acquire the incoming signal and synchronize the receiver’s demodulator.
• The PLCP header contains information about PSDU from the sending physical layer. The PLCP preamble and header are transmitted at 1 Mbps.

• The sync field contains a string of alternating 0s and 1s pattern and is used by the receiver to synchronize the receiver’s packet timing and correct for frequency offsets.
• The SFD field contains information marking the start of a PSDU frame.
• FHSS radios use a 0 0 0 0 1 1 0 0 1 0 1 1 1 1 0 1 bit pattern. The leftmost bit is transmitted first.
• The PLCP length word (PLW) field specifies the length of the PSDU in octets and is used by the MAC layer to detect the end of a PPDU frame.
• The PLCP signaling field (PSF) identifies the data rate of the whitened PSDU ranging from 1 to 4.5 Mbps in increments of 0.5 Mbps.
• The header error check field contains the results of a calculated frame check sequence from the sending station.
• Data whitening is used for the PSDU before transmission to minimize DC bias on the data if long strings of 1s or 0s are contained in the PSDU. The PHY stuffs a special symbol every 4 octets of the PDSU in a PPDU frame.