802.11ac channel bandwidth sharing
Matthew Gast, Aerohive's Director of Product Management, explains what 802.11ac is, how it works, and why it's needed in this series.
To guide the discussion, let’s look at 160 MHz of spectrum from channels 36 through 64. (Readers of the early release version of my book might recognize this as a figure from Chapter 3 of my 802.11ac book, which has a full discussion of this feature.) The figure shows a network operating on channel 60.
For each channel bandwidth, 802.11ac designates one channel as “primary,” meaning that it is used for transmission at that bandwidth. In the figure, the primary channels are shown in blue. A 20 MHz transmission will be sent on channel 60. Frames using a bandwidth of 40 MHz will be sent across channels 60 (the primary channel) and 64 (the secondary channel). Frames sent at 80 MHz will be sent across channels 52, 56, 60, and 64.
One of 802.11ac’s new features is bandwidth signaling on a per-frame basis. To transmit a wide-channel frame, all the constituent channels must be clear. For example, to transmit an 80 MHz frame, the station must check that channels 52, 56, 60, and 64 must all be clear. However, if channel 52 is busy (say, due to a neighboring AP using 52 as its primary channel), 802.11ac enables the receiver to refuse an 80 MHz transmission and fall back to a narrow 40 MHz channel.
With 802.11n, administrators were forced to choose between the raw peak speed of 40 MHz channels and the total overall network throughput of 20 MHz channels. With 802.11ac, frames are sent using the optimum channel bandwidth at the time the frame is transmitted, freeing network administrators from needing to hand-optimize channel plans.
It also means that network administrators see benefits for 802.11ac clients from the moment they add them. 802.11ac clients can use wider channels immediately on deployment, but when they are not required an 802.11ac AP can revert to older narrow channels.
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