| Question |
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What is the structure of 802.11ac preamble and how does it help other legacy clients in understanding that the channel is busy? |
| Environment |
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A RF environment in which 802.11ac and legacy clients co-exist |
802.11ac clients send a preamble starting with is understandable to the legacy clients.
Below is the format of a VHT preamble.
Following is the function of each of the component:
•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.
So SIG helps the legacy client understand how long the transmission would continue from 802.11ac or simply how long the channel will be busy.