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The TCP/IP Guide  9  The Open System Interconnection (OSI) Reference Model       9  Key OSI Reference Model Concepts

Interfaces: Vertical (Adjacent Layer) Communication 1 2 Data Encapsulation, Protocol Data Units (PDUs) and Service Data Units (SDUs)

Horizontal Communication

Let's consider how these corresponding layers communicate using protocols. First, recall that every layer in the model, except the bottom (physical) layer, is really a program or algorithm running on a computer. There is no way for, say, a Web browser and a Web server to actually connect together directly—they are just software programs, after all. Instead, the software running at various layers communicates logically. That is to say, through the use of software and procedures, a process running at layer 5 on one machine can accomplish logical communication with a similar process running at layer 5 on another machine.

Since machines are only physically connected at layer 1, this means that in order for a protocol at layer 5 to function, the data on the sending machine must “pass down” the data through the layers between layer 5 and layer 1. The data is then transmitted over the physical connection to layer 1 of the other machine, and “passed up” the protocol stack of the receiving machine to layer 5. This is how the two machines are logically linked at layer 5, even though they have no physical connection at that layer.

Thus, with the exception of the actual physical connection at layer 1, all horizontal communication also requires vertical communication—down the stack on one machine, and then back up the stack on the other. This process is illustrated in Figure 14. (The communication doesn’t always go all the way back up the stack for each connection, however, as in the case of routing.)

Interfaces: Vertical (Adjacent Layer) Communication 1 2 Data Encapsulation, Protocol Data Units (PDUs) and Service Data Units (SDUs)