Bluetooth Mesh for Commercial Lighting

It is not often in real life that technology (governed by the underlying laws of physics) makes a perfect fit for an application. But it just happens to be this way when Bluetooth mesh is used for commercial / industrial sensor - based lighting control.

Light fixtures (be it office - type "troffers" or industrial "high-bay" fixtures) are organized in a grid layout. The spacing of the grid is in the range of 10x10 ft in offices and up to several times that in warehouses and over factory floors. These grid configurations result from lighting - related requirements.

Now each light fixture becomes a radio mesh node. And when we apply the radio propagation principles for a given type of space (use the Bluetooth Range Estimator), it turns out we arrive at the magic number of 200, which is the typical number of mesh nodes within a single-hop radio range. IOW on average there are 200 nodes able to receive a radio transmission from any other node.

So why is the 200 number important?

First of all it is not always 200. It is a typical bell curve. So it may be 50 or it may be 400. But 200 is clearly the mid point of this distribution and the key input to scalability considerations.

Now when we look into what actually this mesh node is doing, we can isolate the following activities:

• Sensing and reporting occupancy (to reduce / turn off the lights when the space is unoccupied)
• Sensing and reporting ambient light level (to balance the light output based on the level of natural light)
• Reporting fixture - level energy use (for management purposes and also often required by the code)
• Reporting maintenance data (operating conditions and alarms / faults)
• Synchronizing the state with neighboring lights

The last part (the neighbor synchronization) is unique to Bluetooth mesh - based lighting systems. They behave like collectively intelligent systems with local autonomy. This means no centralized controller and each node making its own decisions but coordinating with neighbors. Like a flock of birds or a school of fish. This is super reliable (no point of failure) and super scalable (all traffic is local or semi-local and mostly in one direction: publication of data, no need for control commands to go over the network).

So back to our 200 number.  Considering the traffic pattern described above, we end up with roughly 200 radio packets per second. This assumes each node fires an event once every two seconds and each event results in two radio packets (the original and 1 extra retransmitted for enhanced reliability). The network is uniformly balanced, so in any given point there will be 200 messages per second "in the air". Each message fired randomly. What becomes super interesting is the 200 messages per second is the practical limit of the number of independent Bluetooth mesh messages on one channel. We published the science behind this in the 2016 IES AC paper titled "Connected Lighting in Commercial Environment – Challenges and Barriers to Adoption. Can Bluetooth Mesh be the Answer?". More than that and the network saturates with too many packets colliding and the system falls apart. What is important - Bluetooth mesh has this performance number way above any other competing technology. That is the key reason why Bluetooth mesh - based systems can FINALLY deliver on the promises of wireless commercial lighting control.

The benefits of the fully decentralized control architecture combined with the best performing radio technology and the most compact design of a secure packet structure has met the density / performance requirements of lighting systems. And on top of that this is a fully open standard.

Now how does the "200 messages in the air" number translates into practical requirements / limitations? First of all this is not any network / system size limit. This is the level of traffic any mesh node may experience at any given time. Does it grow with the size of the network? Not really, due to the decentralized architecture (it would with a central controller). It mostly depends on the density of the network - how many devices are in direct radio range. But the bottom line is the devices must be able to cope with that number of messages. IOW they must continue functioning, participating in the network as normal while being able to receive / authenticate / process (and retransmit if configured to do so) without any extra delays or dropping the traffic altogether.

Of course by being fully open, the standard does not automatically guarantee the performance of each individual implementation. We have seen very good products, fully capable of flawless operation in a crowded radio environment. But we have also seen poorly performing products, choking up at much lower traffic. What is important here is the Bluetooth mesh standard / technology enables creation of products meeting the high performance requirements. We at Silvair are proud of enabling with our implementation the best preforming ones. Effectively setting the benchmark and proving what is possible. 

And finally - I think - the best news is this is far from over. Remember we are still today at Bluetooth mesh 1.0, the specification adopted back in 2017. We have huge room to continue growing the performance numbers over the coming years. So expect more to come :)