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.