Figure 17: 5G Primary And Secondary Channels - Grandstream Networks GWN7610 Deployment Manual

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5G channel selection and channel bandwidth issues
When discussing 5G channel selection, two terms need to be introduced: primary channel and secondary
channel, as shown in Figure 17 below:

Figure 17: 5G primary and secondary channels

802.11ac adopts channelization design, and 20/40/80/160MHz is combined by channel channels with a
bandwidth of 20MHz.
As the figure above, the combination is not arbitrary. CH56 and CH60 have no way to form a 40MHz
channel, and CH56 can only be combined with CH52. Based on this channelization method, once the
20MHz primary channel and the channel width are specified, the working channel parameters are basically
determined automatically.
The 20MHz primary channel is the channel number seen on the GWN's WebUI page, while the 20MHz
secondary is another 20MHz channel in the 40MHz channel. For example:
When an AP is configured with a channel bandwidth of 80 MHz and a primary channel of CH60, then CH64
automatically becomes its 20MHz secondary channel. The combined channel of CH60 and CH64 (40MHz
bandwidth) automatically becomes 40MHz primary. Then CH52 and CH56 will automatically become
40MHz secondary.
So, what is the difference between 20MHz primary and 20MHz secondary?
In 5G, 802.11ac is designed with backward compatibility. Management or control frames, such as beacon
frames, are transmitted at a rate with 802.11a supported. That is, those frames are only sent in 20MHz
primary channel, while the secondary channel is an extension. When the channel is idle, it can be used to
increase the transmission data rate. However, idle channel detection is required to find out if they are
accessible.
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Grandstream Wireless Product Deployment Guide