Physical Layer (PHY)
IEEE 802.11b (High–Rate) DSSS PHY specification

This section focuses on the 1999 High Rate extension of the PHY for the Direct Sequence Spread Spectrum (HR/DSSS) system. The 802.11b standard is known as the High Rate PHY for the 2.4 -GHz band designated for ISM applications. It is also known as WiFi.

Number of operating channels
The channel center frequencies and CHNL_ID numbers are shown in Figure . As shown in the figure, not all countries’ national regulatory bodies have allocated the same number of channels. The three nonoverlapping operating channels for North America are shown in Figure .

Modulation and channel data rates
Four modulation formats and data rates are specified for the High Rate PHY . The basic access rate is based on 1 Mbps differential binary phase shift keying (DBPSK) modulation. The enhanced access rate is based on 2- Mbps differential quadrature phase shift keying (DQPSK). The extended direct sequence specification defines two additional data rates. The High Rate access rates are based on the Complementary Code Keying (CCK) modulation scheme for 5.5 Mbps and 11 Mbps. The optional packet binary convolutional coding (PBCC) mode is also provided for enhanced performance up to 22 Mbps.

Complementary Code Keying (CCK) is used to increase the peak data rate of 802.11b from 2 to 11 Mbps, while still using DQPSK modulation. It does this by first increasing the data clock rate from 1 Mbps to 1.375 Mbps, and then taking data in 8-bit blocks (8 * 1.375 = 11). Six of the eight bits are used to choose 1 of 64 complementary codes, which are each eight chips long and clocked out at 11 MHz. The other 2 bits are combined with the code in the DQPSK modulator.