Sure, the local node can wake up at any time, but if it has to transmit over the mesh network, then it has to wait for the remote node to wake up.
How does the mesh network handle deep sleep, anyway? Do they synchronize clocks so they all wake up at the same time, or does a node just transmit continuously until someone reasons?
The mesh nodes have to run either in always-on mode (which makes perfect sense if they are powered by, say, a wall socket, which they often are) or in a sampled listening mode. Sampled listening shaves some 98% off from the radio duty cycle compared to always-on mode, but still is not enough for the mesh nodes to run on coin cell batteries. Give them a larger pack of batteries and they'll run for months though.
To send data to a node in sampled listening mode, the sender sends a string of smaller wake-up packets that indicate when the sender intends to send the real data packet. When the receiver picks up the wake-up packet, it knows when it should wake up again to receive the data packet, so it can safely go back to sleep again for a while. And if the sender knows that the receiver is in always-on mode, because it is powered by a wall-socket, the sender can skip sending those wake-up packets, saving a bit of power.
This requires clocks to be synchronized, but only loosely - millisecond synchronization is enough. The trick is to strike the right balance between communication responsiveness and power consumption.
The BOM indicates that the clock oscillator is just a vanilla quartz crystal. http://www.ti.com/lit/df/swrr135b/swrr135b.pdf Does the firmware do any clever temperature compensation using the onboard thermometer, or are you just sending lots of clock skew correction packets as the mesh warms up and cools down diurnally?
For this level of time synchronization it is enough to send a few extra bytes with synchronization information in each packet. If you'd be down at microsecond-level synchronization, you probably need to do temperature compensation - particularly in outdoor deployments where there can be a 40 C / 100 F difference between two nodes on a sunny winter day.
Yeah it's just a regular XO. There's some calibration data they stick in a special flash area at the factory that the radio firmware uses to handle temperature compensation for the chip's internal digital oscillator. I also believe you can run it in a XO-less mode with some caveats.
How does the mesh network handle deep sleep, anyway? Do they synchronize clocks so they all wake up at the same time, or does a node just transmit continuously until someone reasons?