How does 802.11ac devices ensure legacy clients understands them?

Aruba Employee
Aruba Employee
Question   What is the structure of 802.11ac preamble and how does it help other legacy clients in understanding that the channel is busy?
Environment   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.
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Last update:
‎06-24-2014 09:00 AM
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