.4 GHz Band Channel number Centre frequency (GHz) Geographical usage 2.412 US, Canada, Europe, Japan 2.417 US, Canada, Europe, Japan 2.422 US, Canada, Europe, Japan 2.427 US, Canada, Europe, Japan 2.432 US, Canada, Europe, Japan 2.437 US, Canada, Europe, Japan 2.442 US, Canada, Europe, Japan 2.447 US, Canada, Europe, Japan 2.452 US, Canada, Europe, Japan 2.457 US, Canada, Europe, Japan, France 2.462 US, Canada, Europe, Japan, France 2.467 Europe, Japan, France 2.472 Europe, Japan, France 2.484 Japan 802.11g PHY Layer The 802.11g PHY layer was the third 802.11 standard to be approved by the IEEE standards board and was ratified in June 2003. Like 802.11b, 11g operates in the 2.4 GHz band, but increases the PHY layer data rate to 54 Mbps, as for 802.11a. The 802.11g uses OFDM to add data rates from 12 Mbps to 54 Mbps, but is fully backward compatible with 802.11b, SO that hardware supporting both standards can operate in the same 2.4 GHz WLAN. The OFDM modulation and coding scheme is identical to that applied in the 802.11a standard, with each 20 MHz channel in the 2.4 GHz band (as shown in Table 6-8) divided into 52 subcarriers, with 4 pilot tones and 48 data tones. Data rates from 6 to 54 Mbps are achieved using the same modulation methods and coding rates shown for 802.11a in Table 6-6. Wireless LAN Standards Although 802.11b and 11g hardware can operate in the same WLAN, throughput is reduced when 802.11b stations are associated with an 11g network (so-called mixed-mode operation) because of a number of protection mechanisms to ensure interoperability, as described Table 6-9. Table 6-9: 802.11b/g Mixed-Mode Interoperability Mechanisms Mechanism Description RTS/CTS Before transmitting, 11b stations request access to the medium by sending a request to send (RTS) message to the access point. Transmission can commence on receipt of the clear to send (CTS) response. This avoids collisions between 11b and 11g transmissions, but the additional RTS/CTS signalling adds a significant overhead that decreases network throu