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Enhancing HDX: Introducing Dynamic Bandwidth Selection – Automatically Choosing the “Best” Channel Width

- May 8, 2015 - 0 Comments

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

Every advancement in Wi-Fi technology comes with corresponding complexities and tradeoffs.  You just don’t get something for nothing.

For example, much of the speed improvements in the evolution from 11b to 11g/a to 11n to 11ac are achieved by simply doubling the RF channel width. Increasing channel width from 20 MHz to 40 MHz effectively enables doubling “over the air” speed. Increasing channel width from 40 MHz to 80 MHz doubles that speed again.

Of course, wider channels are more susceptible to interference (since a wider channel can “hear” more). Furthermore, with wider channels, the number of available so called “non-overlapping” channels decreases making mutual interference an increasing problem. Being able to send data over the air faster is very important, but if the devices in your WLAN are waiting more often to send data because the wider channel is more likely to be busy, then disappointment and unrealized expectations will occur. Keep in mind that because “air is shared” for Wi-Fi that it uses a “listen before talk” protocol.

Also, in a real world WLAN, it is highly unlikely to have homogeneous device types. The client mix will include legacy devices that simply can’t operate at 80 MHz (or 40 MHz). This means that spectrum could be wasted if the network is configured for a greater channel width than most of its devices can handle. This has far more consequences at 5 GHz than at 2.4 GHz since 40 MHz channels are unlikely to be usable at 2.4 GHz and 80 MHz channels cannot be used at 2.4 GHz.

Interestingly, 802.11ac does include a feature called RTS/CTS with bandwidth indication that is intended to address dynamic channel width (read more about this in 802.11ac: The Fifth Generation of Wi-Fi” section 2.3.4). The challenge is that this feature is not often used and cannot be used by either 11a or 11n clients.

Last, but far from least, no two wireless networks are the same – every wireless network is different. Even parts of the same wireless network will be different. Thus, there really is no “one size fits all” static configuration that helps offer optimization. The Wi-Fi network needs to adapt as conditions change.

So given all the variables and complexity how does one achieve confidence in using the wider channels widths? How does one arrive at choosing the “best” channel width?

The answer is Cisco’s Dynamic Bandwidth Selection (DBS).

DBS is a patent pending enhancement and extension to DCA (Dynamic Channel Assignment). DCA selects the best channel number but has to be manually configured for 20, 40, or 80 MHz channel width.

With DBS, the best channel number is still automatically assigned, but the DCA metrics are also used to select that best channel by accounting for all possible channels widths. The DCA algorithm compares 20, 40, and 80MHz channel width options. The best channel width is automatically calculated using:

      1. RF Neighbor Channel Widths
      2. BSS Channel Overlap ratio
      3. Channel Utilization
      4. non-Wi-Fi Noise
      5. Wi-Fi interference
      6. Number of client types/capabilities

The result is a highly powerful mechanism for establishing a WLAN for optimal spectrum efficiency. In other words, a spectrum conservation plan is realized in which the “right” amount of spectrum is used based on the needs of the client devices and the characteristics of the surrounding air.

The following figure helps explain DBS in action.

Dynamic bandwidth 1

The tradeoff is basically operating at “fastest over the air speeds” (wider channels) vs. “lowest latency” (the least amount of “wait time” to get onto the air). Nevertheless, in both cases, performance is automatically optimized. Wider channels are used if the RF neighborhood is relatively free from congestion due to competing wireless networks, narrower channels are used if the RF neighborhood is relatively dense.

Not only these, but network planning is greatly simplified since heterogeneity of client devices is assumed from the beginning.

Finally, all of this isn’t done only at network startup. DBS is about intelligently adapting channel width on a continuous basis.

Why is DBS important? Very simply, DBS can help provide confidence in deploying 80-MHz channels.

To summarize, since DBS is constantly monitoring the channel and BSS statistics it is effectively evaluating transient parameters (11n/11ac client mix, load and traffic flow types) and reacts to these fast changing statistics by varying the BSS’s channel width and/or adapting channel orientations that are unique and introduced via 11ac, e.g., select to whether transmit in 20, 40, or 80 MHz.

If all of this sounds really simple, that’s good, because it’s certainly easier to explain than it is to build!


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