Bluetooth NLC vs DALI+: Capacity and Performance

Whenever I come across a discussion that involves wireless communications over the Thread protocol (such as this one), there is almost 100% chance people don't understand the underlying physics and traffic patterns. This is the result of Thread marketing since 2014, highlighting the IPv6 protocol as its key strength. And typically it goes like that: "Thread is based on IPv6 and there are more IPv6 addresses that grains of sand in the Universe". And people read this "because Thread is based on IPv6, it is a highly scalable wireless protocol". Treating the potential number of static addresses equally to the dynamic situation when these devices actually send data.

If we consider the networked lighting control domain, the most prevalent pattern today is to have a motion sensor in every luminaire. The motion sensor usually has multiple functions: it is a motion sensor of course. But it also is a light level sensor enabling daylight harvesting (or dimming lights down when there is sufficient natural light coming in). And it also outputs a light dimming signal (0-10V analog or DALI digital) to the LED driver.

In DALI+ systems typically there is a room or zone or area controller - a small programmable box that collects the signals from the sensors and tells the lights what to do. For example: there are people in the room (occupancy detected), the light level is 150 lux (too low), so increase the level by dimming up. 

From the underlying Thread network perspective this typically means four network messages:

1. Sensors sending the occupancy / light level data to their associated Thread routers;
2. The Routers relaying the messages to the controller;
3. The controller responding with a multicast (group) command to the lights;
4. The group command is too relayed by Thread routers.

Messages 1 and 2 are additionally acknowledged at the link layer. Messages 3 and 4 are usually sent 3 times each, for reliability, as there is no acknowledgment for multicast messages. 

Thread uses the IEEE 802.15.4 standard at 250kbps data rate. The 802.15.4 uses the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) scheme to to control how devices share a communication channel without interfering with each other (interference effectively kills the messages, so must be avoided).

Now let's assume each sensor reports an event (motion or light level change) once per second, on average. This is a good approximation, although in busy peak moments there may be more (many people moving or sunrise when light level changes significantly). And let's assume we want to have at most 10% collision / interference (it is still a lot). Then with these assumptions: how many sensors can the DALI+ over Thread network handle?

The short answer is: about 20. Yes, twenty. Not two hundred and definitely not thousands.

(I'd be happy to do a deeper dive into my calculations, so reach out if you want to understand the details)

Now let's compare this to Bluetooth® NLC. What are the key differences?

1. Bluetooth® NLC uses 1Mbps data rate (4x faster than Thread) and on top of that has ultra compact packet size.
2. The different security and network architecture allows any device to talk directly to any other device in a stateless fashion. Thread relies on network "links" that require child supervision / keep alive packets (that leads to this whole concept of "self-healing" which is another lie (at least a misconception, as Bluetooth® NLC never needs that and can be considered "always healthy").
3. Bluetooth® NLC uses distributed lighting control architecture (each sensor is also a controller), removing the need for the message routers and bypassing the congestion created by bi-directional traffic over multi-hop half-duplex network.

All in all, in Bluetooth® NLC we have much more efficient radio technology, and much more efficient network traffic pattern. Both combined, give us the practical limit of around 200 devices in the same area. That assumes each sending one message (sensor data) per second and leaves room for some much higher bursts while maintaining the expected high reliability and low latency).

So here we have it: DALI+: 20 per area, Bluetooth® NLC: 200. 10x (an order of magnitude) difference in the real world.

Also remember that 20 sensors per area is really a low number - very often entirely insufficient, as a typical area is an entire building floor. We have Bluetooth® NLC projects with much bigger areas -  as I said the 200 number is conservative.