 is able to work in any combination of ERP and non-ERP modes. For example, the access point could operate only in an ERP-OFDM mode, in a mixed mode ERP-OFDM and ERP-HR/DSSS or in a mixed mode ERP-HR/DSSS/and not ERP. The DSSS/OFDM mode uses an 802.11b-compatible header and a payload using OFDM multiplexing. 3.2.1. PLCP sub-layer An ERP station must support three different formats for the PLCP header. The first format corresponds to the ERP-HR/DSSS mode. It includes the long preamble defined for the 802.11b interface, with a redefinition of the reserved bits. 802.11a/g Interfaces The second format corresponds to the ERP-HR/DSSS mode. It includes the optional short preamble for the 802.11b interface. The third format corresponds to the ERP-OFDM mode. It includes the preamble and header defined for the 802.11 interface. The DSSS-OFDM mode uses a format that includes the short or long preamble and the header of the 802.11b interface associated with the preamble and the header of the 802.11a interface. This mode is optional. 3.2.1.1. ERP-HR/DSSS mode The structure of the PLCP frame is described in Figure 3.6. Preamble header SIGNAL SERVICE LENGTH 16 bits 8 bits 8 bits 16 bits 16 bits Preamble PLCP header Long : 144 bits / 1 Mbps / BPSK Long preamble: 48 bits / 1 Mbit/s / BPSK Short 72 bits / 1 Mbps / BPSK Short preamble: 48 bits / 2 Mbps / QPSK Figure 3.6. PLCP frame for ERP-HR/DSSS mode The difference with the PLCP header of the 802.11b interface is in the SERVICE field bits set to support the optional packet binary convolutional code (PBCC): - bits b0, b1 and b4 are reserved and must be set to ZERO; - bit b2 is used to indicate that the transmission frequency and the symbol clock are derived from the same oscillator. For ERP mode, the locked clock bit must be set to ONE; - bit b3 is used to indicate whether the data uses the PBCC option; - bits b5, b6 and b7 are used to resolve DATA field length ambiguities for the optional PBCC mode; - bit b7 is also used to resolve DATA field length ambiguities for CCK (Complement