Frequency-Hopping PMD Sublayer

Although the PLCP header has a field for the speed at which the MAC frame is transmitted, only two of these rates have corresponding standardized PMD layers. Several features are shared between both PMDs: antenna diversity support, allowances for the ramp up and ramp down of the power amplifiers in the antennas, and the use of a Gaussian pulse shaper to keep as much RF power as possible in the narrow frequency-hopping band. Figure 11-10 shows the general design of the transceiver used in 802.11 frequency-hopping networks. It is required that the transceiver have a sensitivity of -80 dBm for both 1 Mbps and 2 Mbps transmission.

Figure 11-10. Frequency-hopping transceiver

PMD for 1.0-Mbps FH PHY

The basic frequency-hopping PMD enables data transmission at 1.0 Mbps. Frames from the MAC have the PLCP header appended, and the resulting sequence of bits is transmitted out of the radio interface. In keeping with the common regulatory restriction of a 1-MHz bandwidth, 1 million symbols are transmitted per second. 2GFSK is used as the modulation scheme, so each symbol can be used to encode a single bit. 802.11 specifies a minimum power of 10 milliwatts (mW) and requires the use of a power control function to cap the radiated power at 100 mW, if necessary.

PMD for 2.0-Mbps FH PHY

A second, higher-speed PMD is available for the FH PHY. As with the 1.0-Mbps PMD, the PLCP header is appended and is transmitted at 1.0 Mbps using 2GFSK. In the PLCP header, the PSF field indicates the speed at which the frame body is transmitted. At the higher data rate, the frame body is transmitted using a different encoding method than the physical-layer header. Regulatory requirements restrict all PMDs to a symbol rate of 1 MHz, so 4GFSK must be used for the frame body. Two bits per symbol yields a rate of 2.0 Mbps at 1 million symbols per second. Firmware that supports the 2.0-Mbps PMD can fall back to the 1.0-Mbps PMD if signal quality is too poor to sustain the higher rate.

Carrier sense/clear channel assessment (CS/CCA)

To implement the CSMA/CA foundation of 802.11, the PCLP includes a function to determine whether the wireless medium is currently in use. The MAC uses both a virtual carrier-sense mechanism and a physical carrier-sense mechanism; the physical layer implements the physical carrier sense. 802.11 does not specify how to determine whether a signal is present; vendors are free to innovate within the required performance constraints of the standard. 802.11 requires that 802.11-compliant signals with certain power levels must be detected with a corresponding minimum probability.