Last Post: September 19, 2006:
The IEEE 802.11 standard does not describe what must be done, nor what should be done, nor what could be done. It is up to vendors to decide.
All three of the alternatives I suggested above (these are not laid out in the standard) will work.
The incompatibility of DSSS stations inter-operating with HR/DSSS and
ERP stations which are using short preamble is that the short preamble frames in the channel are utterly unintelligible to the DSSS stations. The DSSS stations can sense the RF energy in the air and defer to that, but cannot read the frame length field in the physical preamble header nor the duration field in the data link MAC header. This degrades the CSMA/CA process and leads to more collisions and retransmissions.
Communication between the DSSS stations and their AP are not degraded since the AP will use long preambles for frames unicast to the DSSS stations and multicast to the BSS.
Theoretically an AP could wait until it counted retransmissions involving its asssociated DSSS stations and only then change the short preamble bit in the AP's capability information field to zero forcing all associated stations in the BSS to use the less bandwidth efficient but better for mixed PHY CSMA/CA long preamble. Meanwhile another BSS in the same channel and space may still be using short preambles.
Lest we forget, ERP stations when transmitting using OFDM data rates use neither long nor short preambles leading to the whole subject of ERP protection mechanisms. Again, these new ERP-OFDM frames are just noise to their older DSSS and HR/DSSS brethren. Yet another generation of backward compatibility mechanisms will come with the 802.11n amendment in another year or so.
I hope this helps. Thanks. /criss