|
|
2.3 | |
Physical Layer (PHY) | |
| 2.3.2 | |
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 Modulation and channel data rates 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.
|
. 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
. 
. 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.