Figure 1.

Layering. Application A wishes to communicate with application B. It does so by requesting services from the layer immediately below it, which in turn requests services from the layer below it, and so on. The bottom layer represents the physical wiring between the computers that run the applications. The message is transferred to the layers below application B, and the message percolates up. The two applications feel like they are talking to each other (dashed line), but the complex details of communication are hidden from them. Application A is not allowed to skip layers (curved line) because this breaks the integrity of the system.