Computing stuff tied to the physical world

Relaying RF12 packets

In Software on Jan 12, 2011 at 00:01

Since the RF12 driver does not implement a full OSI network “stack”, there are no such things as routers and mesh networks in Jee-land. This means you’re basically limited to the range of a single 868/915 MHz point-to-point packet connection.

There are a number of ways to increase the range. One is to use a directional antenna (I’ve never tried this, but it has been mentioned on the discussion forum). Another option is to lower the transmission baud rate inside the radio and the bandwidth settings, so that the power is “beamed” into a narrower frequency range. Both of these are based on RF properties.

A third option if you can’t get from here to there in one go is to take multiple “hops”. That’s what a mesh network tries to do fully automatically, adapting on the fly to varying reception conditions and network topologies.

I’m more interested in much simpler approaches, which can easily be implemented in a little 8-bit ATmega with limited code storage. There’s a lot we can do, even within these pretty hard constraints – and it’s fun to push those little boundaries!

So here’s another option: a dedicated “packet relay” node, which listens to a specific RF12 net group most of the time, and when a packet comes in, it turns around and sends the packet out over another net group. Apart from picking two net groups for this, the mechanism should be fairly transparent.

Here’s what happens when you install such a relay:

Screen Shot 2011 01 11 at 23.44.59

That only covers packets going one way. A refinement is to also deal with ACK packets. In that case, the relay should wait a little while to see whether an ACK comes in, and if it does, send that ACK back out to the original packet source:

Screen Shot 2011 01 11 at 23.45.05

Here’s a groupRelay.pde sketch, which implements this:

Screen Shot 2011 01 11 at 23.58.18

It’s pretty straightforward. But it’s only made for broadcast scenarios, i.e. the originating node must be using a broadcast packet (i.e. send to special node “0″). The relay can then simply re-broadcast the information.

In the case of ACKs, the originating node id will be in the header. The relay saves this, sends out a packet with an ACK-type request of its own, waits briefly for such an ACK to come in, and when it does, sends an ACK back to the originating node.

Note how the loss of packets will have the same effect as without a relay. The only difference being that there are 2 hops (or 4 w/ ACK) where packets can be lost or get damaged.

Tomorrow, I’ll explain a little bit what’s going on and what sorts of trade-offs this leads to.