wifi enviormentsToday, all machines need to communicate with each other. The fact that they are heavier than other technologies is the reason why the Internet has become so fundamental in our everyday lives. This would never have been possible, however, without wireless communication, that is, devices capable of connecting to each other using electromagnetic waves in the air.

In the field of Wi-Fi environments, it’s common to refer to wireless links in terms of “short distances” – for example, a home or a business. From them, the devices, also known as stations, connect to an access point, which oversees providing the network service. All devices that need to connect to the network wirelessly, must use it.

One feature of access points is their ability to scan the various Wi-Fi frequency bands they are operating in to discover, on the one hand, which frequencies are operational and, on the other, which other access points are providing a service in the same band. For businesses, this can be used to find authorized or non-authorized access points, and thereby avoid malicious or misconfigured Wi-Fis that could harm users. Continuous monitoring of access points in a Wi-Fi area is highly recommended for this reason.

One of the main problems with scanning Wi-Fi frequencies in an access point is that it must provide an uninterrupted service to the stations. This conflicts with the scanning process, since whenever a scan is performed, the stations must be interrupted in order to change the frequency within the Wi-Fi band and discover other access points on external channels. Consequently, the end user might end up noticing this stop in services. Making a VoIP or video call is all it would take. In fact, the user would notice such interruption even if they used an application that worked with the network in real time and there was significant packet loss.

To address this problem, the access point would have to periodically scan its entire band and switch channels without deteriorating the quality of service (QoS). Various solutions are feasible in this regard. One is to use an auxiliary radio that facilitates monitoring. Another option is, rather than performing a complete scan at a certain time, perform a continuous scan.

CBS (Continuous Background Scanning) for VoW scanning and switching channels

Choosing a continuous scan comes from wanting to avoid sweeping all the frequencies at a single moment in time – an action that would reduce the QoS of the stations. Why? Simply because the radio must change all the channels in the frequency band and continue for a time listening to information frames (beacons) from other access points.

In this context, the access point must always monitor the type of traffic carried. In most access points today, traffic types are considered according to four QoS priority levels, with VoIP and video streaming traffic marked with the highest priority.

Based on this “priority queue” concept, the access point will carry out the scan avoiding performing a channel switch whenever voice or video traffic is queued in the channel being used to serve the stations.
As soon as the access point is rid of this type of traffic, it momentarily switches to the external channel to listen for information messages (beacons) from other access points. That same channel will be momentarily scanned several times in order to avoid deteriorating the quality of service of the stations using said access point. The total listening time on a channel is the same as without using CBS but is divided into shorter intervals that prevent cutting the priority traffic.

Reliable Wi-Fi network topology monitoring

To obtain a reliable Wi-Fi network topology monitoring with this scanning mode, the system is based on two assumptions:

1. The access points periodically send information messages from the AP (beacons) at a mean interval of 100 ms.
2. Scanning is carried out continuously and the same external channel is scanned several times before the end of the scan.

The configurable parameters in this scan model are: the scan time, the wait time in each external channel, and the rest time in which a channel switch is not made within the scan interval itself.

These parameters are usually configurable and highly dependent on the number of stations and the QoS traffic class that will be used within the Wi-Fi network. The normal way to calibrate these parameters is to perform a scan while VoIP traffic is being sent, and check that there is no packet loss and network monitoring matches the true network topology.
Teldat have dedicated teams of R&D engineers, specialized in Wi-Fi, working continuously to keep our products up-to-date with the latest advances in the markets.


About the author

Francisco Javier GutierrezFrancisco Javier Gutierrez
Telematics Engineer. He is part of Teldat's R&D Department. Within this department he is part of the new generation access points team

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