In the DFS blog we have glanced through the basics of DFS and the WLC configuration related to it. In this blog I would like to deep dive into DFS concepts which are critical to make a wireless AP compliant to operate in regulatory domains across the world. Every regulatory domain will have its own criteria but the concepts remain the same.
DFS Recap
Dynamic Frequency Selection is the process of detecting radar signals that must be protected against interference from 5 GHz radios by dynamically switching the operating frequency of the radio to a non-DFS channel or other DFS channel without any radar signal.
DFS Concepts
Availability of additional 5 GHz channels
There are limited number of non-DFS channels. DFS allows you to use additional 5 GHz channels which are DFS channels when there is no DFS activity on those channels.
Channel Availability Check
A DFS function that monitors a Channel to determine if a Radar Waveform above the DFS Detection Threshold is present. Before an AP can start beaconing on a DFS channel, it must passively scan the channel for any existing radar activity on the channel. If there is any activity on the channel the AP cannot transmit on that channel and will have to switch to a new channel. The CAC timer varies from 60 sec to 600 sec based on channel, channel width and regulatory domain.
Radar Detection
While the AP is transmitting on a DFS channel, it must constantly monitor for any radar activity on the channel. If the AP detects any radar activity it must cease transmission immediately on that frequency.
Channel Hop
While the AP is transmitting on DFS channel, if the AP detects any radar signal it performs the following actions-
- Blocks new transmissions on the channel
- Broadcasts an 802.11h channel switch announcement
- Disassociates the client devices associated to it
- Selects a different channel which is permitted by the regulatory domain
No-Occupancy Period
The time during which a channel will not be utilized after a radar waveform is
detected on that channel. If radar activity is detected on a particular DFS channel, the AP cannot transmit any signal on that channel for a duration of 30 minutes to prioritize radar activity over Wi-Fi traffic.
Channel Move Time
The time to cease all transmissions on the current channel upon detection of a radar waveform above the DFS Detection Threshold. Once radar activity is detected, the AP must stop transmission on that channel within 10 seconds . During this time the AP should completely vacate the flagged frequency and re-establish the connection on a clear channel.
Channel Closing Transmission Time
The total duration of transmissions, consisting of data signals and the aggregate of control signals, by the AP during the channel move time is termed as Channel Closing Transmission Time.
Zero Wait DFS
Enterprise APs leverage their dedicated scanning radio to continuously scan the DFS channels for any active radar activity. As of result of this continuous scanning the AP pre-emptively knows if there is any active radar activity on the DFS channels and hence the AP can start transmitting on the DFS channel without going through the CAC timer.
Related Blogs
Cisco Wireless Features
Wi-Fi 7 802.11 be
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