03-12-2016 05:14 PM
The supported rates field has 8 octets and can specify eight different rates. The was sufficient to specify the data rates for the legacy 802.11 and 802.11b.
The 802.11g can support more than eight data rates (DSSS and OFDM) and this field was not sufficient to specify all the data rates. So the Extended Supported Rates was added that is 255 octets in lengh and can specify more data rates. The Extended Supported Rates is required only if there are more than eight supported rates.