How’s that for a title to get your attention, eh?
There’s an interesting mechanism in communication, which has kept me intrigued for quite some time now:
- with JeeNode and RF12-based sensors, wireless packets are often broadcast with only a sender node ID (most network protocols use both a source and a destination)
- CAN bus is a wired bus protocol from the car industry, its messages do not contain a destination address, there is just a 11- or 29-bit “message ID”
What both these systems do (most of the time, but not exclusively), is to tag transmitted packets with where they came from (or what their “meaning” is) and then just send this out to whoever happens to be interested. No acknowledgements: in the case of wireless, some messages might get lost – with CAN bus, the reliability is considerably higher.
It’s a bit like hormones and other chemicals in our blood stream, added for a specific purpose, but not really addressed to an area in the body. That’s up to various enzymes and other receptors to pick up (I know next to nothing about biology, pardon my ignorance).
Couple of points to note about this:
- Communicating 1-to-N (i.e. broadcasting) is just as easy as communicating 1-to-1, in fact there is no such thing as privacy in this context – anyone / anything can listen-in on any conversation. The senders won’t know.
- There is no guaranteed delivery, since the intended targets may not even be around or listening. The best you can do, is look for the effects of the communication, which could be an echo from the receiving end, or some observable side-effect.
- You can still set up focused interactions, by agreeing on a code / channel to use for a specific purpose: A can say “let’s discuss X”, and B can say “I’ll be listening to topic X on channel C”. Then both A and B could agree to tag all their messages with “C”, and they’ll be off on their own (public) discussion.
- This mode of communicating via “channels” or “topics” is quite common, once you start looking for it. The MQTT messaging system uses “channels” to support generic data exchange. Or take the human-centric IRC, for example. Or UDP’s multicast.
- Note that everything which has to do with discovery on a network also must rely on such a “sender-id-centric” approach, since by definition it will be about finding a path to some sender which doesn’t know about us.
Having no one-to-one communication might seem limiting, but it’s not. First of all, the nature of both wireless and busses is such that everything reaches everyone anyway. It’s more about filtering out what we’re not interested in. The transmissions are the same, it’s just the receivers which apply different filtering rules.
But perhaps far more importantly, is that this intrinsic broadcasting behaviour leads to a different way of designing systems. I can add a new wireless sensor node to my setup without having to decide what to do with the measurements yet. Also, I will often set up a second listen-only node for testing, and it just picks up all the packets without affecting my “production” setup. For tests which might interfere, I pick a different net group, since the RF12 driver (and the RFM12B hardware itself) has implicit “origin-id-filtering” built in. When initialised for a certain net group, all other packets automatically get ignored.
Even N-to-1 communication is possible by having multiple nodes send out messages with the same ID (and their distinguishing details elsewhere in the payload). This is not allowed on the CAN bus, btw – there, each sender has to stick to unique IDs.
The approach changes from “hey YOU, let me tell you THIS”, to “I am saying THIS”. If no one is listening, then so be it. If we need to make sure it was received, we could extend the conventions so that B nods by saying “got THIS” and then we just wait for that message (with timeouts and retries, it’s very similar to a traditional ACK mechanism).
It’s a flexible and natural model – normal speech works the same, if you think about it…
PS. The reason this is coming up, is that I’m looking for a robust way way to implement JeeBoot auto-discovery.