Note that each layer at the sending end has a parallel in the receiving end.

Horizontal Layers in Post

In the post analogy, the letter writer and receiver appear to be in direct communication with each other. The writer writes, the reader reads, oblivious to the man-hours spent in the lower layers. Similarly, the post office at the sender’s side is in communication with some other post office. They cooperate in getting the letter delivered. At the lowest layer, there could be multiple hops. For instance, there is a bicycle pickup of letters from a box, delivered to the post office. Then, the letters are bundled and sent by pickup truck to an airport. The airport flies the postage to a different airport. The airport sends the postage to a post office by a pickup truck and the delivery ensues. Sometimes there are multiple entities horizontally, but we only see the sender and the receiver.