FH PHY Convergence Procedure (PLCP)

Before any frames can be modulated onto the RF carrier, the frames from the MAC must be prepared by the Physical Layer Convergence Procedure (PLCP). Different underlying physical layers may have different requirements, so 802.11 allows each physical layer some latitude in preparing MAC frames for transmission over the air.

Framing and Whitening

The PLCP for the FH PHY adds a five-field header to the frame it receives from the MAC. The PLCP is a relay between the MAC and the physical medium dependent (PMD) radio interface. In keeping with ISO reference model terminology, frames passed from the MAC are PLCP service data units (PSDUs). The PLCP framing is shown in Figure 11-9.

Preamble

As in a wired Ethernet, the preamble synchronizes the transmitter and receiver and derives common timing relationships. In the 802.11 FH PHY, the Preamble is composed of the Sync field and the Start Frame Delimiter field.

Figure 11-9. PLCP framing in the FH PHY

Sync

The sync field is 80 bits in length and is composed of an alternating zero-one sequence (010101...01). Stations search for the sync pattern to prepare to receive data. In addition to synchronizing the sender and receiver, the Sync field serves three purposes. First of all, the presence of a sync signal indicates that a frame is imminent. Second, stations that have multiple antennas to combat multipath fading or other environmental reception problems can select the antenna with the strongest signal. Finally, the receiver can measure the frequency of the incoming signal relative to its nominal values and perform any corrections needed to the received signal.

Start Frame Delimiter (SFD)

As in Ethernet, the SFD signals the end of the preamble and marks the beginning of the frame. The FH PHY uses a 16-bit SFD: 0000 1100 1011 1101.

Header

The PLCP header follows the preamble. The header has PHY-specific parameters used by the PLCP. Three fields comprise the header: a length field, a speed field, and a frame check sequence.

PSDU Length Word (PLW)

The first field in the PLCP header is the PLW. The payload of the PLCP frame is a MAC frame that may be up to 4,095 bytes long. The 12-bit length field informs the receiver of the length of the MAC frame that follows the PLCP header.

PLCP Signaling (PSF)

Bit 0, the first bit transmitted, is reserved and set to 0. Bits 1-3 encode the speed at which the payload MAC frame is transmitted. Several speeds are available, so this field allows the receiver to adjust to the appropriate demodulation scheme. Although the standard allows for data rates in increments of 500 kbps from 1.0 Mbps to 4.5 Mbps, the modulation scheme has been defined only for 1.0 Mbps and 2.0 Mbps.[*] See Table 11-3.

[*] It is unlikely that significant further work will be done on high-rate, frequency-hopping systems. For high data rates, direct sequence is a more cost-effective choice.

Header Error Check (HEC)

To protect against errors in the PLCP header, a 16-bit CRC is calculated over the contents of the header and placed in this field. The header does not protect against errors in other parts of the frame.

No restrictions are placed on the content of the Data field. Arbitrary data may contain long strings of consecutive 0s or 1s, which makes the data much less random. To make the transmitted data more like random white noise, the FH PHYs apply a whitening algorithm to the MAC frame. This algorithm scrambles the data before radio transmission. Receivers invert the process to recover the data.