802.11g WLANs

The IEEE 802.11g standard, approved in June 2003, introduces an ERP to provide support for data rates up to 54 Mbps in the 2.4 GHz ISM band by borrowing from the OFDM techniques introduced by 802.11a. In contrast to 802.11a, it provides backward compatibility to 802.11b because 802.11g devices can fall back in data rate to the slower 802.11b speeds. Three modulation schemes are defined: ERP-ORFM, ERP-PBCC, and DSSS-OFDM. The ERP-OFDM form specifically provides mechanisms for 6, 9, 12, 18, 24, 36, 48, and 54 Mbps, with the 6, 12, and 24 Mbps data rates being mandatory, in addition to the 1, 2, 5.5, and 11 Mbps data rates. The standard also allows for optional PBCC modes at 22 and 33 Mbps as well as optional DSSS-OFDM modes at 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. This section describes the changes necessary to form the ERP-OFDM, ERP-PBCC, and DSSS-OFDM.


802.11g PLCP

The 802.11g standard defines five PPDU formats: long preamble, short preamble, ERP-OFDM preamble, a long DSSS-OFDM preamble, and a short DSSS-OFDM preamble. Support for the first three is mandatory, but support for the latter two is optional. Table 3-16 summarizes the different preambles and the modulation schemes and data rates they support or are interoperable with.


The long preamble uses the same long preamble defined in the HR-DSSS but with the Service field modified as shown in Table 3-17.


The length extension bits determine the number of octets, when the 11 Mbps PBCC and 22 and 33 Mbps ERP-PBCC modes are in use.

The CCK-OFDM Long Preamble PPDU format appears in Figure 3-29. You set the rate subfield in the Signal to 3 Mbps. This setting ensures compatibility with non-ERP stations because they still read the length field and defer, despite not being able to demodulate the payload. The PLCP header matches that of the previously defined long preamble, but the preamble is the same as for the HR-DSSS. Both the preamble and the header are transmitted at 1 Mbps using DBPSK, and the PSDU is transmitted using the appropriate OFDM data rate. The header is scrambled using the HR-DSSS scrambler, and the data symbols are scrambled utilizing the 802.11a scrambler.


Much like the DSSS-OFDM long preamble, the short preamble DSSS-OFDM PPDU format uses the HR-DSSS short preamble and header at a 2 Mbps data rate. With the HR-DSSS scrambler and the data symbols, the short preamble and header are transmitted with OFDM and use the 802.11a scrambler.


ERP-OFDM

As previously stated, the ERP-OFDM provides a mechanism to use the 802.11a data rates in the ISM band in a manner that is backward compatible with DSSS and HR-DSSS. In addition to utilizing the 802.11a OFDM modulation under the 2.4 GHz frequency plan, ERP-OFDM also mandates that the transmit center frequency and symbol clock frequency are locked to the same oscillator, which was an option for DSSS. It utilizes a 20 microsecond slot time, but this time can be dropped to 9 microseconds if only ERP devices are found in the BSS.