Tsahi Tal

Try picturing the following: a busy city-center multi-lane intersection in mid rush hour. There are no traffic lights due to a blackout, a tree has fallen blocking part of a pedestrian crossing, and a  lone police officer stands in the middle of the junction trying to direct traffic and pedestrians safely… wouldn’t want to be stuck there, would you? Now, imagine that you would desperately need the officer’s help since in this chaos someone has stolen your bag. A second officer would be very helpful in our horror scene. In a home Wi-Fi network, a busy Access Point can similarly suffer from limited capabilities making the challenge to observe, detect and react, let alone prevent chaotic scenarios, an impossible situation.

In this blog we will discuss the value and cost benefits of including a dedicated listening radio in a Wi-Fi AP, which very much like a second policeman, can be extremely useful and beneficial. 

Characterizing Wi-Fi networks co-existence in dense environments. What each AP sees about the other and how they are interfering with each other.

In Wi-Fi, each Access Point (AP) is a single, self-sustained entity. But in some cases APs that are close to one another may overlap. For example, in a multiple dwelling unit (MDU), it is likely that two neighbor apartments’ APs will be within each other’s coverage range (i.e. the APs are located so their Wi-Fi signals overlap), and may potentially choose to operate on the same or adjacent channels resulting in cross interferences and poorer connectivity experience on both networks.

The same phenomenon can easily happen in a home with multi-AP (e.g. Mesh) network. Though the two (or more) APs may appear as a single network (having the same SSID), in the absence of a governing software they act independently, and individually select their operation channels. Their non-coordinated operation may result with mutual interference to each other, just like the neighboring networks described earlier that compete with each other over the shared unlicensed spectrum. 

For multiple managed APs (such as mesh nodes), the channel’s usage policy might be limited by the architecture of the mesh solution itself. And granted, interferences may exist between the mesh network and other co-existing networks like a neighbor, etc.

The 5GHz, is a shared spectrum with radar systems (e.g. weather radars), and regulations strictly mandate the Dynamic Frequency Selection (DFS) rules for sharing the spectrum between unlicensed devices such as a Wi-Fi AP and such radar systems. In fact, most of the 5GHz spectrum is DFS governed - APs are required to clear channels from presence of radar system before they can start transmission on it (channel clearance time is typically 1min or 10min per each channel, rules vary between geographies), and are required to monitor sudden radar presence (In-Service-Monitoring) and must vacate the channel if radar is detected.

Channel agility in a conventional single AP environment

A single AP is expected to scan the spectrum and select the best channel to operate upon boot, and also periodically scan the spectrum to learn if there is a better channel on which to operate. In case it experiences interference, or detects that there is a DFS while operating on a DFS channel, the AP is required to change to an alternative operating channel.

Conventionally, channel scanning, DFS clearance, and the decision for changing a channel, and to which channel to transition, are all independent and done by the AP with the same interface (same radio) that is used to service clients. This is the common practice of AP engineering today. Very much like our policeman in the middle of the junction, if it is preoccupied with traffic direction and management it has very little attention for observation, monitoring and prevention.

Enhancement by centralized management software (SW)

When you have a centralized management software, the benefits are that your channel assignment policy looks across a larger view of your network and optimizes it as a whole. In such a case, the centralized software builds a cross-AP view, showing which APs are overlapping and potentially interfering with each other, and then based on each AP scan you can build a broader view of channel assignment, interferences, etc. Governing software that assumes such radio resource management roles, is growing in popularity with the growth of standardization in the form of EasyMesh (managing multi-APs) and Data Elements. These two Wi-Fi Alliance programs aim at enhancing network management and monitoring capabilities (Data Elements standardizes how the Wi-Fi network elements report their capabilities and statistics, EasyMESH is for management of multiple APs in your network). Nevertheless, each Wi-Fi AP is bounded by its ability to serve traffic, and observe and report with a single radio.

Added value and benefits of monitoring other channels during service

So what would a second “policeman” enable us when it comes to Wi-Fi networks? What are the functional benefits that it enables? Well, this is a very good question, let’s dive into some of the enhanced functionalities:

  • Spectrum Agility - as a shared unlicensed medium, two spectrum scans conducted by the same AP at the same location, at two different times (say couple of hours apart, or in some dense scenarios even a couple of minutes apart) may yield very different results, and consequently with a different selection of the best channel for the said AP to operate on. The spectral congestion statistics may change very rapidly over time, or at a slower change pace if there are less interfering neighbors or contending systems over the spectrum. Having the ability to refresh the AP’s view of the entire spectrum and its current congestion statistics faster can improve the AP’s performance enormously, and yield proactive actions (e.g. transition to a higher power channel if available) or to a less occupied channel, and not only reactive transitions (in case interference/blockage of the current channel becomes unbearable).

  • Off-Channel-CAC, a.k.a Zero-Wait-DFS - in Europe, the regulator allows pre-clearance of DFS channels even while currently operating on another channel (thus it is an off-channel scan from the channel the AP is actually active at that time). Once pre-clearance is completed, the AP can store the channel scan status and use the channel at any given time, without the undesired 1 min or 10 min service disruption of the Channel Availability Check (CAC) scan that is mandated if the AP would aim to use such channel without pre-clearing it. DFS channels are typically characterized by lesser interferers and are conventionally of higher transmission power, thus are very favorable for use for an AP that can avoid the downside of service disruption of clearing DFS status on them.
  • Zero-Wait-CAC a.k.a. Dynamic-In-Service-Monitoring - In North America, the FCC rules do not allow channel pre-clearance, a device must perform Channel Availability Check (CAC) of 1 min/10 min just before using a DFS channel. The AP needs to be able to detect the presence (or better said lack of) radar signal with high detection probability before it may use the channel. A capable AP, may operate on a non-DFS channel for example,while monitoring another DFS channel in parallel to operation and clear it while meeting the regulator specified reliability requirements; thus respecting the required CAC duration before switching and using the said DFS channel, while sustaining some level of operation during the clearance duration (thus referred to as “Zero-Wait-CAC”) 
  • Interference Characterization - if an access point can monitor a channel for a longer duration period, it can characterize better the interferences on the said channel. For instance it can detect if there is another Wi-Fi device operating on that network, and it can even decode the transmissions by the Wi-Fi device/s (e.g. the beacons) to identify better which device it is. This can help for instance to understand if the interference is coming from a neighboring AP that is within the management scope of the same service provider (or same managing multi-AP software). It can also allow the access point to identify non-Wi-Fi interferer types.
  • Sniffing & Debugging Connectivity Issues - a capable access point, can monitor a channel and decode communication packets and flows that are transacted over the channel. This practically is equivalent to a computer equipped with a sniffer radio and software tools that assists engineers onsite in debugging networks and connectivity issues. An access point that is equipped with a dedicated sniffer can save technician physical presence physically in the network premise, it can actually drive inputs to a debugging software that can identify (and in some cases automatically resolve) connectivity issues.

Added value and benefits of having a listening radio

In short, a dedicate listening radio enables all the above benefits and use-cases at their utmost capabilities with no penalty or compromise on service:

  • No degradation of prime channel service or radio capabilities
  • Scan spectrum faster and more frequent for better spectrum characterization and spectral agility
  • Clear DFS channels (CAC and Off-Channel-CAC) much faster
  • Perform zero-wait-CAC with lesser impact of service and higher reliability
  • Ability to characterize interferences better due to the longer monitoring durations and the dedicated monitoring compute resources
  • Ability to decode Wi-Fi packets and characterize networks on the scanned channels in parallel to service
  • Ability to sniff and decode Wi-Fi packets on a targeted channel and debug connectivity issues in parallel to service
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