The 802.11n standard supports the legacy 802.11a/b/g PPDU formats. In addition it also supports two new PPDU formats. They are
- Mixed Mode PPDU format (allows co-existence with older clients)
- Green-field mode PPDU format (pure 11n client network) – optional
The above two formats are termed as HT PPDU formats. The HT-Mixed mode format and the HT Green field mode is given be
FIG COURTESY: 802.11-2012 Standard
- As can be seen – the legacy preamble is added to the HT-Mixed frame format so that Legacy stations are able to decode the preamble and back off from transmission.
- The HT-Greenfield mode is an optional mode and it does not have the legacy Preamble attached. The HT-Greenfield mode is to be enabled in a pure 11n network which does not have any legacy 802.11 client.
- The variable number of Data HT-LTFs depends on the number of space-time streams and can be 1, 2 or 4. The number of Extension HT-LTFs can be 0, 1, 2 or 4. The Extension HT-LTFs are added so that the receiver is able to make a estimate of additional dimensions of the channel in addition to those that are used by the data portion of the frame.
- In addition to the frame format shown above – signal extension can be present in the frame if the NO_SIG_EXTN Txvector parameter is set to false and any one of the below two conditions is true
- The FORMAT parameter is equal to HT_MF or HT_GF.
- The FORMAT parameter is equal to NON_HT, and the NON_HT_MODULATION parameter is equal to ERP-OFDM, DSSS-OFDM, or NON_HT_DUPOFDM.
- The HT Signal (HT-SIG) field contains frame parameters that the receiver can interpret and set itself for decoding the packet. The HT-SIG field is shown below and the description of the different fields is shown below
Fig Courtesy – 802.11 Standards