HT modes and protection mechanisms
Last Post: March 3, 2007:
As I understand it (big qualifier here), the three HT modes operate as follows:
HT STA non-HT mode transmissions conform to ERP (2.4 GHz) and OFDM (5 GHz) rules, sharing the same channel, band, and space. Except that they can receive HT frames and act on their PHY frame length and MAC duration values, they operate and look like either an ERP STA or an OFDM STA. In the 2.4 GHz space this includes the use of classic ERP protection mechanisms for nearby HR/DSSS and DSSS STAs.
HT STA HT_GF mode (GreenField) transmissions are mostly noise to ERP and OFDM receiving STAs, and are entirely noise to non-ERP receiving STAs, sharing the same channel, band, and space. Protection mechanisms are irrelevant. OFDM modulations are used exclusively but the preamble and headers include values unknown to ERP and OFDM receivers. In my opinion this mode will be used seldom and offers only a little more channel efficiency than an almost all HT environment using mixed mode with one associated quiet ERP STA.
HT STA HT_MM mode (Mixed Mode) transmissions accommodate ERP and OFDM receiving STAs, and are entirely noise to non-ERP receiving STAs, sharing the same channel, band, and space. OFDM modulations are used exclusively. The first part of the preamble and header looks like ERP (ERP-OFDM) or OFDM operating at 6 Mbps data rate with a fake frame bit length that is chosen to cause the receiving non-HT station to defer for the balance of the frame, even though a moment later the frame shifts into a specific HT modulation and becomes unintelligible. The extra surprising part is that the fake frame bit length is calculated to protect not only the immediate frame, but the entire frame exchange sequence! It does this by considering both the duration value from the MAC header and the true length value from the second part of the PHY header to produce the fake length value for the first part of the PHY header. While slick, this introduces some problems of its own such as an inability to cut short a normal frame exchange sequence deferral by sending a CF-END frame to reset NAVs.
In order to get the MM fake frame length protection goodness to all interested ERP or OFDM STAs, an HT STA may use RTS/CTS. Essentially each mixed mode RTS and CTS frame looks huge to receiving ERP or OFDM STAs and the normally essential duration value in the MAC header is unreadable and no NAV gets set. By the time they stop holding their breath for a huge and apparently corrupted control frame to pass, the HT frame exchange sequence has finished.
I hope this helps. Thanks. /criss