This is what I'm trying to understand.
Functionally, a pump is very simple: It delivers a precise does of insulin with a single "pulse". All pumps work this way, regardless of the actual mechanism metering the dose (in an Omnipod, it's a ratchet mechanism actuated by the contraction of an SMA wire when energized). There is a maximum rate the pump can deliver these small doses at.
That's it. There really isn't any more to an insulin pump that distinguishes between "basal", "bolus", or anything else. The rest is all programming.
So, that's what I'm trying to fully understand in the Loop system: Which software is involved in control during bolus delivery? I think I've learned that Loop is running the show, from the iPhone, using the temp basal feature of the pump to turn it into essentially a "dumb pump", analogous to a simple pump with a knob on it to turn insulin delivery up or down, Loop turning the knob.
In that case, "bolus" and "basal" really have nothing to do with the pump. They're concepts that exist in Loop, managed by Loop, presented to the user for control by Loop. Loop is in control of ALL insulin delivery -- the pump does not initiate insulin delivery or adjust the rate of delivery.
In terms of timing, let's look at the Omniipod for an example (as I know the specs). The dosing increment is 0.05U. The maximum rate it can impulse doses is every 2 seconds. So, it can deliver insulin at a maximum 0.05U/2s, or 0.025U/s (although insulin is delivered in 0.5U amounts every 2 seconds when a bolus is being delivered). This is equivalent to 90U/hr.
The omnipod has a hard limit for basal rates of 30U/hr, so at best using the temp basal mechanism as the "knob to turn" and deliver boluses would mean at best boluses would take 3x as long to deliver. A 10U bolus delivered through the Omnipod PDM interface (and software) will take 6:40 to deliver while Loop would be limited to taking 20 minutes to deliver the same bolus.
This is quite significant in terms of how the insulin will act in blunting any spikes from carbs eaten.