Bluetooth Mesh - The Scalability

Scalability was the initial reason for us at Silvair to turn to Bluetooth Low Energy when looking for a low power mesh networking technology. I posted this story in The Launch episode. We knew it would be good. Better than anything else available. Primarily due to the extremely compact packet structure which flies over the fastest low energy radio. Mesh packets are like bullets flying through the air. Imagine what would be the density of the fire required for bullets to collide in air?

OK I admit every solution has limits. So what are the limits of Bluetooth Mesh? The answer is - as always - it depends. It depends what the network is doing (how many and what messages it keeps sending around) and how it is set up. Mesh has many parameters that may be fine tuned to make it perform even better.

As a general rule of thumb you may say at 200 devices (or less) you should not worry about any tuning at all. It is like 200 guns shooting in the same area. The likelihood that any two bullets collide is pretty low. So let them fire at will.

Above 200 devices, depending on how talkative they are, you may start experiencing some collisions. And Bluetooth Mesh has a number of tools in the toolbox to optimize the network and let it grow significantly while maintaining an excellent service ratio. The most important among them are:

1. TTL or Time To Live. It defines how many relay hops a message is allowed to travel. It is rarely the case for large networks that every sender has to be heard across the entire network. For example for an occupancy sensor it is probably enough to report to light fixtures in the same room. And maybe to a gateway. But not across the building, where nobody is interested in its status. Setting the Default TTL to a low value (even to 0 in some cases) is a good way to increase scalability.
2. Relays. They retransmit received messages, obviously multiplying traffic in a given space. Usually the default setting for the relay function is "On", in order to make setting up small networks seamless. In large networks it pays to carefully select how many devices are designated as relays and disable relaying where it is not needed. Our friends at Ericsson Research published a thorough case study modeling an office floor with close to 900 talkative mesh devices. It is a very good paper, based on real data captured from a live network. It shows that for a case like an office floor, it is enough to assign about 1.5% of nodes as relays.
3. Subnets. Mesh networks can be significant in size, spanning entire multi - storey buildings. It is extremely rare that devices on separate floors need to communicate withing each other. Except for administrative tasks like re-keying  the entire network or shutting down the whole building, typically each floor is self - contained. There is no need for messages on one floor to travel to other floors. Subnets are a great mechanism to confine network traffic. A mesh node can be a member of multiple subnets, so a good practice is to have the base network spanning all floors and then have a subnet define for each floor. And configure the nodes to transmit only on subnets they belong to, not on the main network (except the administrative tasks). A single mesh network can have over 4000 subnets. We are yet to build a structure that high. Until then subnets should help scaling any network you imagine.

With design that scalable we can't wait to see real networks being provisioned. Bluetooth mesh has all it takes to spread in numbers.