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Radio Resource Management That Never Sleeps

by Sriram Venkiteswaran on Jul 16, 2016

RRM (Radio Resource Management) is a collection of techniques in which access point radios continuously analyze the RF spectrum to do the following:

  1. Adjust the operating channel to mitigate co-channel interference.
  2. Adjust the operating power to optimize coverage.
  3. Provide coverage hole protection by automatically increasing the power when neighbor APs fail.
  4. Ensure clients are evenly load-balanced between the APs, between channels, and between bands.
  5. Ensure clients are connected to the best possible access points (address “sticky client” behavior) by steering them to the right access point.
  6. Ensure air time fairness by making sure less-capable clients do not consume excess bandwidth.

That’s a nearly complete list of the RRM features supported by various WLAN vendors. For these features to function, they need to have visibility into the entire WiFi RF spectrum: all channels across both the 2.4GHz and 5GHz bands.

The Traditional Approach: RRM That You Can’t Use When You Need It the Most

Background Scanning

Traditionally, most WLAN vendors have relied on background scanning for RRM. In background scanning, access points go off-channel (stop serving clients) for a short period of time to scan other channels one by one. Most access points do this every 10 seconds and scan for 100 ms. With 24 20-MHz channels in the 5 GHz band, it takes 24 x 10 seconds = 240 seconds (four minutes) to complete one full sweep of the 5 GHz band.

Mission-Critical WiFi Voice and Video

Increasingly, enterprises across different verticals are relying on voice and video applications to run their businesses. It’s well-known that performing background scanning degrades voice and video traffic, because the time taken away from serving clients hurts latency-sensitive applications. Most WLAN vendors automatically turn off background scanning when the AP detects any voice or video traffic. It is considered “best practice” to automatically turn off background scanning when the AP detects any voice or video traffic.

But in today’s enterprise WLANs, it is safe to assume that some voice or video applications are always running. This means the APs never get a chance to perform off-channel scans and build information about other channels, and this impacts the AP’s ability to take optimal RRM decisions. Let’s look at some examples.

1. Dynamic Channel Selection

Let’s assume the operating channel of an AP becomes bad due to external interference. If the AP cannot perform background scanning, it has no way to learn if there is a better channel it can switch to. This prevents the AP from being able to make the right RRM decisions, right when it’s needed the most: to deliver top-quality voice and video.

2. Sticky Clients

Let’s say there are two APs: AP1 (ch 36) and AP2 (ch 44), deployed in an enterprise with coverage overlap. A client initially connected to AP1 moves away from it, and at a certain point has better signal strength to AP2 (-60) as compared to AP1 (-75). Deciding when to roam is a client’s decision, and the roaming behavior/algorithm across various clients vary significantly. In this example, let’s assume the client to be very sticky and hence remains connected to AP1 even though it has better signal strength to AP2.

If AP2 was able to hear the client’s conversation with AP1, it would have known that it can serve the client better and could have requested AP1 to steer the client to AP2. But for this, the AP2 needs information about channel 44. If there were any voice or video traffic on the network, the AP would never perform background scanning and hence never be able to steer the client.

3. Auto Transmit Power

Let’s assume a bunch of APs deployed in an enterprise campus are on various channels. Now, for each AP to decide on optimal power, each one needs to know the signal strength of all neighboring APs — which again means each one needs to listen on all the channels. If the AP cannot perform background scanning because of voice or video traffic in the network, then auto power selection fails, and so will coverage hole recovery.

A Third Radio Means That RRM is Always On

This is where the third radio comes to the rescue, scanning all the channels (even in the presence of voice or video) and providing the necessary spectrum information for critical WLAN functions, one of them being RRM.

A dedicated third radio completes a full spectrum sweep across all the 2.4GHz and 5GHz channels much more quickly (4 seconds, assuming 100 ms scan time, vs. 240 seconds) compared to the traditional approach of background scanning. Thus, RRM decisions can be made faster, and the AP can react faster to changes in the RF environment.

Whether your network is heavy with voice or video or not, you want to ensure that your radio resources are always optimized, thereby improving the application performance and keeping users happy. That’s a key benefit of having a dedicated multi-function third radio, like the one in Mojo’s C-130.

Read About the C-130

Topics: 802.11ac, RRM, C-130, radio resource management