| Question |
|
How does a 802.11ac station communicate effectively so that legacy client understand that an 802.11ac client transmission is on going? |
| Environment |
|
802.11ac + 802.11n + legacy clients in a mixed environment |
802.11ac stations have a VHT preamble, which is backward compatible with Legacy clients.
•L-STF and L-LTF - Assist the receiver to synchronize with the signal
•SIG- The length of the frame. Helps set NAV for legacy stations
•VHT-SIG-A - Includes the channel bandwidth, number of spatial streams, MCS information
• Transmitted as 20-MHz symbols,
• Replicated over all underlying 20-MHz channels.
•VHT-STF - to improve automatic gain control estimation in a MIMO transmission
• The field is repeated in each 80-MHz channel.
•VHT-LTF - one per spatial stream to be used for transmission. LTF fields allow the receiver to
•calculate the multipath characteristics of the channel and apply them to the MIMO algorithm.
•VHT-SIG-B – Length of Frame. Distribution of Spatial Stream in case of MU-MIMO.
SIG ensures that the NAV value is announced and all legacy clients understand for the amount of the time in future channel is being occupied by another client. This way its ensured that 802.11ac and legacy clients remain interoperable at the PHY level.