Steven C. Horii, MD

Figure 2. Diagram shows two simiplifed communication protocol stacks of three layers each. Vertical communication (within a stack) uses services, whereas horizontal communication (stack to stack) uses protocols. Stack-to-stack communication is physical only at the lowest layer. Peer-to-peer communication (ie, communication between corresponding layers in different stacks) occurs when information is moved down one stack, over the physical connection, and up to the corresponding layer in the other stack.