Wireless Batman: Playing Nice through Channel Blacklisting

In wireless communication, everyone shares the same space. So we need to make sure we are not overwhelmed by others, and ourselves avoid to overwhelm others. Particularly in industrial IoT settings.


We use channel hopping to avoid crowding the wireless spectrum while improving communication performance.

But sometimes channel hopping is not enough – sometimes you may need to select what channels to use. Or which channels to avoid. That’s why we use channel blacklisting.

First a little background to the different channels we use in the Thingsquare system: the 2.4 GHz band and the sub-1 GHz bands.

The 2.4 GHz Band

The 2.4 GHz band is a world-wide license-free band, which means that it is used for a whole bunch of wireless things. Both WiFi and Bluetooth use this band. In a typical home or office, this band is therefore very crowded.

A typical, crowded office WiFi environment.

The Thingsquare system uses the IEEE 802.15.4 radio standard for communicating on the 2.4 GHz band. It splits the band into 16 different channels, many of which directly overlap with WiFi.

The Sub-1 GHz Bands: 868 MHz and 915 MHz

The sub-1 GHz bands are different in different regions of the world. In the US, the frequencies between 903 and 928 MHz are available. In the European region, the frequencies between 863 and 870 MHz are available.

The sub-1 GHz band has strict requirements on how they can be utilized, much stricter than the 2.4 GHz bands. Devices using these bands must switch channels on a regular basis, both under European (ETSI) and US (FCC) regulations.

The Thingsquare system uses a configuration from the IEEE 802.15.4g family of frequency modulation settings, with our channel hopping technology on top of it, to ensure that we follow both the ETSI and FCC regulations. The Thingsquare system uses 50 channels, with different frequency settings for the US and European configurations.

Channel Blacklisting

The Thingsquare system uses channel hopping to allow systems to use the entire wireless spectrum and to avoid interference from other wireless equipment. By default, the channel hopping mechanism uses all its available channels, but sometimes not all channels are good for use. To tell the system to use only a specific set of channels, channel blacklisting is used.

The channel blacklisting mechanism allow selecting what channels the channel hopping sequence should use. Channels that are blacklisted are not used.

The channel blacklist is controlled by the gateway, which distributes it to all nodes in the network.

The gateway also measures the channel quality of each channel by keeping track of how many collisions that have been experienced on each channel.

Setting up a Channel Blacklist

To set up a channel blacklist for a gateway, open the app or web frontend. Navigate to the gateway in question.

Go to the Details tab, scroll down a bit, and tap the Blacklist Channels button. Tapping this button will request the channel statistics from the gateway and bring up the blacklist selection window.

In the blacklist selection window, the quality of each channel is listed. Note that the number of channels will differ from different frequency bands: the 2.4 GHz band uses 16 channels, whereas the sub-1 GHz bands use 50 channels. One channel is used as the synchronization channel, which can not be blacklisted.

To select channels to be blacklisted, tap or click on the selected channels in the graph. The channels that are selected for blacklisting will turn black.

After selecting which channels that should be blacklisted, scroll down to the end of the graph and tap the Apply blacklist button. This will cause the gateway to apply the channel blacklist to its network. Within a few minutes, the blacklist will have been distributed to the entire network.

Blacklisting channels on the 2.4 GHz band (left) and the sub 1-GHz band (right). There are way more channels on the sub 1-GHz band.

Channel Blacklisting in Action

A Thingsquare sub-1 GHz Sensortag starter kit.

To see channel blacklisting in action, we set up a Thingsquare starter kit with sub-1 GHz Sensortags and configure two different channel blacklists. We measure the radio spectrum with a frequency analyzer.

In the first setup, we blacklist channels 0-24. This should cause all communication to occur on the upper part of the available frequency spectrum. We use a spectrum analyzer to see the results. As expected, we see only frequencies on the upper part of the spectrum being used.

The left part of the spectrum is blacklisted, to the left of the yellow line, so the spectrum analyzer sees no activity there.

In the second setup, we blacklist the remaining channels and remove the blacklist from the 0-24 channels. This causes the communication to go on the lower part of the spectrum, the part that was previously unused. Again, we use the spectrum analyzer to see the results.

Now the right part of the spectrum is now blacklisted, so all activity is on the left part of the spectrum.

Conclusion

Wireless communication must be able to avoid bad channels. Channel hopping helps, but sometimes it is necessary to avoid certain channels completely. Channel blacklisting is a technique used in the Thingsquare system that allows for making the channel hopping mechanism skip the bad parts of the spectrum.


May 05, 2017

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