Meraki loves to chalk up the secret sauce in their products to “Meraki Magic” and boasts “anyone can do it”. Yet our inner engineering geek wants to open the curtain and see the real show. An example of that is Auto RF, which is a form of Radio Resource Management (RRM) that allows Meraki Wi-Fi access points to dynamically plan WLAN channels and radio transmit (TX) power. The following sections will break down what Auto RF is and how it works.
Auto RF is made up of two major components: Auto Channel and Auto TX Power. The goal is to provide an initial channel plan, and then adjust dynamically over time based on the environment. Both features are enabled by default, reducing the number of steps required to deploy Meraki access points effectively.
All currently shipping Meraki access points are built with a dedicated 2.4GHz/5GHz scanning radio, which constantly scans the entire usable spectrum. This radio, among other things such as location analytics and WIPS, is used to detect neighboring BSS’s and make off-channel scans without consuming airtime on client-serving radios. The scanning radio dwells on every channel to monitor duty cycle and detect levels of non-802.11 interference. It also sends probes on non-DFS channels to detect neighboring BSS’s and listens for beacons on all channels.
The current iteration of Auto Channel comes from an algorithm called TurboCA. Auto Channel is designed to react to degrading conditions while balancing client performance against the disruptiveness of changing channels. Fortunately, with the 802.11-2012 standard we have better adoption of 802.11h, which defines standardized Channel Switch Announcements (CSAs) that reduce the impact of moving to a new channel by notifying clients when they will change channels and what channel they are moving to, so that clients can follow. Auto Channel relies on this heavily where possible, but also takes into account that many clients do not support CSAs, so it tries not to change channels frequently unless necessary.
How does it work?
You can refer to the above linked TurboCA article for the full mathematical detail, but this section will summarize the process.
The goal of Auto Channel is to build a channel plan that minimizes channel overlap, optimizes cell sizes for better roaming, and maximizes channel efficiency by picking the best channel available for each AP. Then, it regularly rebuilds the plan in search of an optimization. The computation for the channel plan happens in the Meraki Cloud, where all Meraki access points report their logging data.
The key metrics for the algorithm are:
- Node (AP) Performance
- Network (AP Set) Performance
- Channel Quality (noise floor, non-802.11 interference, neighboring BSS’s, etc.)
- Channel Width
- AP Load (number of associated clients)
- Channel Switch Penalty
- Hop Limit
Node performance is a calculation of how well an access point should perform on a given channel and channel width. Network performance is the product of the performance of all nodes in a Meraki Network, which is important as an individual node score close to zero will bring down the Network performance score, ensuring that a channel plan will not create issues for one area while optimizing another.
Channel quality measures non-802.11 interference, duty cycle, and channel width. Channel switch penalty is a metric designed to reduce the number of channel changes for negligible benefit to reduce negative impact impact, and is weighted heavier on 2.4GHz where fewer clients support CSAs.
Hop Limit is used to determine how many neighboring APs we will consider when planning an AP’s channel. This basically determines the “aggressiveness” of the calculation. Meraki runs this calculation at 3 different intervals with different hop limits:
- Every 15 minutes with a hop limit of “0”.
- Every 3 hours with a hop limit of “1”, then “0”.
- Every 24 hours with a hop limit of “2”, then “1”, then “0”
With a hop limit of “0” an AP only considers itself and directly neighboring APs when planning its channel. By running this more frequently, an AP can react to significant events quickly (such as a jammed channel) but won’t change channels too frequently. The more aggressive plans are run less frequently to balance creating a more globally optimal plan vs. changing channels too frequently.
The Auto RF Process
The process starts by inputting the current channel plan (if one exists), and collecting the scanning results and load information from each AP. It then picks a pseudo-random AP and identifies the channel that will render the best “node performance” for that AP. This selection favors picking heavier loaded APs first, as more clients actively connected to an AP signals that it is more important. By doing this, more actively used APs will have a better chance at picking the best channel available rather than running last and taking whatever channels are left.
After all APs have been assigned a channel and the cloud has calculated the predicted node performance for each, the network performance of the plan is determined. If the network performance of the new plan is better than the current plan, the new plan becomes the proposed plan. The algorithm is run ten times to compare multiple possible configurations. Once all iterations are run, the proposed plan becomes the current plan, and updated APs will change their channels accordingly.
Note that previous iterations of Auto Channel (before 802.11h) would not switch channels if a client was currently associated. Because Meraki now changes channels while clients are associated, it can lead to disruptions with clients using real-time applications that don’t support CSAs. If this is causing a negative impact, Meraki Support can revert this behavior upon request.
As with all things RF-related, an automatic algorithm will not fit every environment. In challenging RF environments, or high density deployments, Auto Channel can fall short. In these scenarios, manual channel assignment may be a better option, but Auto Channel can still be used as a starting point to reduce the amount of manual configuration required.
APs with a static channel assignment will be used in the plan to identify a used channel, but will be used in the algorithm to generate a plan or calculate network performance.
DFS events will always override a static or auto channel plan and trigger an immediate channel change, as required by the FCC.
If a jammed channel is detected, meaning that levels of non-802.11 interference exceed 65% for longer than one minute, a channel change will occur without waiting for the next run of Auto Channel.
If an AP is being used for wireless mesh, it will not change channels as this will have a significant impact on all APs and clients using that mesh route.
AUTO TX POWER
As with Auto Channel, Auto TX Power calculations are done in the Cloud, and the process is run every twenty minutes. A neighbor report is collected from each AP in the network, which contains the Signal-to-Noise Ratio (SNR) for all neighboring APs in the same Meraki network. The AP also reports its currently connected clients along with their SNR.
Using these lists, the Cloud compiles a list of “direct neighbors” for each AP (defined as any AP in the Meraki network with an SNR of 8dB or greater), and calculates what the ideal TX power should be. For each AP, the Cloud attempts to keep the SNR for its strongest direct neighbor at 30dB and always higher than 17dB for every direct neighbor.
An AP will never reduce its transmit power if a client is connected with SNR <10dB. Generally, if a client is connected with SNR <10dB it is looking for a better AP to roam to. If it hasn’t roamed, it can be assumed that a better AP is not available, so reducing the transmit power will only worsen that client’s performance.
To prevent dramatic changes in TX power which could have unintended results, at each twenty minute run an AP can increase its transmit power by 1dB or lower by 1-3dB. When a new Meraki AP is deployed, it starts at the highest transmit power supported by AP within the regulatory domain of which it is a member, unless overridden by an RF Profile or otherwise statically configured. This means that it could take several iterations before an AP reaches its optimal transmit power level.
Auto TX Power will never set the transmit power lower than 5dBm on the 2.4GHz radios or 8dBm on the 5GHz radios to avoid setting a value which is unusably low where there are a high density of APs. There are valid use cases, such as when using directional antennas or in challenging RF environments, where such a low value is warranted. These environments usually require manual tuning anyway, in which case static values can be set in the Dashboard.
If an AP has an active mesh neighbor, it will not increase or decrease its transmit power. When using mesh, if an AP has no client serving SSIDs enabled it will always use its maximum available transmit power.
If an AP only has one direct neighbor, it’s considered risky to reduce transmit power so it’s not done as often.
The Meraki Dashboard allows for several tools to monitor the current channel plan and any changes that have been made by Auto RF.
The Wireless > Radio Settings page allows you to identify the current channel and transmit power being used by each AP, as well as the target power range that Auto TX Power is using:
Clicking on any AP takes you to the Status Page, where the RF tab displays a lot of information about client count, channel utilization, and any changes made to the access point by Auto RF. In the below screenshot, we can see that Auto TX power has adjusted the transmit power, and clicking Details will show exactly what was changed:
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