802.11 PPDU Formats

802.11 PPDU Formats

By CWNP On 08/24/2007 - 9 Comments

There are three overall PPDU structures possible in an 802.11n network, one of which was previously defined by Clause 17.


— Non-HT format: frames of this format are structured according to the Clause 17 (OFDM) or 19 (ERP) specification.  Support for Non-HT Format is mandatory.

— HT mixed format: frames of this format contain a preamble compatible with the non-HT receivers.  The non-HT Short Training Field (L-STF), the non-HT Long Training Field (L-LTF) and the non-HT SIGNAL field are defined so they can be decoded by non-HT Clause 17 and 19 stations.  The rest of the frame cannot be decoded by Clause 17 or Clause 19 stations.  Support for HT mixed format is mandatory.

— HT greenfield format: HT frames of this format do not contain a non-HT compatible part.  Support for HT greenfield format is optional. An HT station that does not support the reception of an HT greenfield format frame must be able to detect that an HT greenfield format frame is an HT transmission (as opposed to a non-HT transmission).  In this case the receiver must decode the HT-SIG and determine if the HT-SIG cyclic redundancy check (CRC) passes.

Two NEW preamble formats are defined by 802.11n:

For HT mixed format operation (HT Mixed Mode preamble), the preamble has a non-HT portion and an HT portion.  The non-HT portion of the HT mixed format preamble enables detection of the PPDU and acquisition of carrier frequency and timing by both HT stations and stations that are compliant with Clause 17 and/or Clause 19.  The non-HT portion of the HT mixed format preamble also consists of the SIGNAL field defined in Clause 17 and is thus decodable by STAs compliant with Clause 17 and Clause 19, as well as HT stations.  The HT portion of the HT mixed format preamble enables estimation of the MIMO channel to support demodulation of the HT data by HT stations.  The HT portion of the HT mixed format preamble also consists of the HT-SIG field that supports HT operation, and the SERVICE field.  

For greenfield operation (HT Greenfield Mode preamble), compatibility with Clause 17 and Clause 19 STAs is not required.  Therefore, the non-HT portions of the preamble are not included in the greenfield format preamble.

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08/24/2007 at 14:08pm
Although it seems unlikely that Greenfield (GF) packets would be used in a WLAN today because of the proliferation of legacy (abg) devices, some implementations will 'protect' the GF transmission with a CTS-2-Self or RTS-CTS.

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