I disagree. The author's point is that if you have a network connection, you can view it as: * A medium with a series of photons or EM waves propagating in one

I disagree. The author's point is that if you have a network connection, you can view it as: * A medium with a series of photons or EM waves propagating in one or both directions. * An exchange of packets between two boxes. * … * A conversation between two applications. It's the all the same thing, but it's an increasingly holistic or reductive view of that one thing as you take the thing in from the perspective from increasing or decreasing layers of the OSI stack.