For years, I’ve used the metaphor of escalators and elevators to explain the difference between Asynchronous and Synchronous (TDM and VoIP). It’s not a perfect analogy, but people get it. It goes something like this:
Escalators are a consistent speed – people get on in a certain order and their arrival time and order sequence is predictable. Elevators are much more random, we don’t know when the door will open, or how long it will take to arrive at its destination, or even the route it will take. The order passengers/packets disembark is unrelated to the order they boarded. For all of these reasons, voice has traditionally worked best on escalators. But technology improvements allow voice to work on elevators, effectively the elevators are so fast now that the passengers can arrive, and re-sort themselves in proper order in the same amount of time the escalator took.
Got it? Yes, I know there are holes in this comparison- sometimes people on escalators manage to change their sequence, and so on, but as a teaching metaphor it works pretty well.