This extension of the DSSS system builds on the data rate capabilities of
the original 802.11 standard, to provide 5.5- Mbps and 11- Mbps payload data
rates. The earlier 1-Mbps and 2 -Mbps rates are still supported. To provide the
higher rates, 8-chip complementary code keying (CCK) is employed as the
modulation scheme. The chipping rate is 11 MHz, which is the same as the DSSS
system, thus providing the same occupied channel bandwidth. The basic High Rate
PHY uses the same PLCP preamble and header as the DSSS PHY so both PHYs can
coexist in the same BSS.
In addition to providing higher speed
extensions to the DSSS system, a number of optional features allow the
performance of the radio frequency LAN system to be improved.
The
following optional functions have been defined:
- An optional mode can replace the CCK modulation with packet binary
convolutional coding (HR/DSSS/PBCC). This optional extension also applies to
802.11g, which can operate at speeds up to 54 Mbps.
- An optional mode can allow data throughput at the higher rates of 2, 5.5,
and 11 Mbps to be significantly increased by using a shorter PLCP preamble.
This mode is referred to as HR/DSSS/short, or HR/DSSS/PBCC/short. This short
preamble mode can coexist with DSSS, HR/DSSS, or HR/DSSS/PBCC under limited
circumstances, such as on different channels. The standard and longer format is
shown in Figure
and the optional
shorter format is shown in Figure
. The
IEEE 802.11a extension to 802.11 includes a similar function, referred to as a
short or long training sequence.
- An optional capability for channel agility allows an implementation to
overcome some inherent difficulties with static channel assignments. This
option can be used to implement IEEE 802.11-compliant systems that are
interoperable with both FH and DS modulations.