Bird_e

If you are only looking for a 1 way communication from the boxes to the receiver, your strategy is correct. If, however, you choose to do a 2 way communication, you could simply have an ACK/NAK type protocol which tells you if your packet has been received. However, as you would've already guessed, it needs more coding and possibly interrupts for receiver too. But, having 2 way comms means you can now eavesdrop on the channel before transmitting. This is like having collision avoidance.

Going with your strategy, you may not need to use interrupts at all. See, when you put the PIC to sleep, you could set the wakeup on change interrupt so that the pic wakes up from sleep. You do not need to enable or service the interrupt though. The PIC will simply continue after the sleep instruction. Timing for 30 seconds can be done with software loops and timers. So, no need to make it complicated unless you wish to.

You will need to work out a scheme where you can be sure that the packet is received by the receiver under conditions that could include 2 or more boxes talking at the same time for 30 Seconds.