Physical Layer (PHY)
802.11b modulation

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.