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Enhancing HDX: Introducing FlexDFS – Not All DFS Solutions Are Created Equally

- May 15, 2015 - 1 Comment

Cisco Systems is announcing a new set of features that enhance its HDX (High Density Experience) suite. This blog is the second in a series that explains the new features that comprise the enhancements to HDX.

5 GHz is a great place to operate a WLAN. There is ample spectrum, and it’s far less crowded and noisy than 2.4 GHz.

However, the majority of 5 GHz spectrum is shared with radar (for both weather and military systems). Therefore, Wi-Fi Access Points not only need to detect radar in order to avoid interference but also need to avoid being an interferer to these systems.

This procedure is commonly referred to as DFS or Dynamic Frequency Selection.

For DFS operation, if radar is detected on a channel then the AP must abandon that channel from further operation for some minimum amount of time. Furthermore, the AP must ensure that any new channel it selects for operation is free from radar (and that detection also requires a minimum amount of time).

Finally, accurate detection of radar (i.e., avoiding false positives) also requires a lot of skill. Compounding the issue are many devices that emit “radar like” transmissions (including Wi-Fi clients and APs doing proprietary over the air detection and calibration).

As a result, many equipment vendors simply take the easy way out and avoid use of the channels requiring DFS.

Cisco believes it has the best DFS solution in the wireless industry and that it only gets better with  a new feature we’re calling Flexible Dynamic Frequency Selection (or for short, FlexDFS).

In a historic DFS solution, it was assumed that the WLAN was typically operating using a 20 MHz channel. In this case, only 20 MHz of spectrum would need to be abandoned if radar was detected.

But with the advent of 11n (40 MHz channel widths) and 11ac (80 MHz channel widths today and 160 MHz channel widths expected soon), Cisco realized that abandoning the entire operating channel was a highly inefficient solution. Why abandon the entire channel if radar is detected on only one 20 MHz segment? Furthermore, if the radar is detected only on a secondary channel, then why seek a new operating channel, if the primary operating channel can be automatically narrowed from 80 to 40 or 20?

The answers to these questions are in Cisco’s patent pending FlexDFS. In this case radar energy can be accurately and reliably identified down to 1 MHz using the CleanAir subsystem in the custom ASIC in Cisco APs. This narrowband identification not only helps prevent false detection in the primary channel of operation but also in the secondary channels of operation and in new channels to be considered (if necessary).

The following figure shows an example of how FlexDFS works.FlexDFS 1When coupled with the new DBS feature, the best channel width can be selected with confidence, and correspondingly, confidence can be established when operating at 80 MHz channel width. Keep in mind that a high-density deployment thrives on a large number of non-overlapping channels being available. Used in tandem, DBS and FlexDFS reinforce the usability of 5 GHz, not only by conserving spectrum, but also by reducing interruptions due to detecting and avoiding interference.

Finally, and significantly, FlexDFS does not require user enablement. It’s merely an enhancement to our existing, industry leading, always on DFS solution.

With FlexDFS, Cisco is continuously improving functionality. We believe we are taking the best and making it better.


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  1. Which WLC code version will these features (DBS, FlexDFS) be available in?