Chapter 18 · General Network Concepts 173 Through OSI, a layer in one device can effectively communicate with its counterpart layer in another device on the network, even though the only actual physical connection between the two devices is the physical layer. OSI is incredibly powerful but can also be incredibly confus- ing--it is often difficult to discern separate functions for some of the layers, especially the ones near the middle. Fortunately, we are generally able to leave this task to networking experts, but it is important to understand the concepts, so a brief introduction to each of the OSI layers is offered here. 7--Application The application layer, the "highest" layer in OSI, is where the meaning of data resides, and this layer presents resources for applications programs run by users. The application layer includes resources for transferring files, messages, or even electronic mail. 6--Presentation The presentation layer is responsible for presenting the raw data to the application layer; file and data transformation and translation are the responsibility of this layer, allowing different applications to access the same network. Encryption might be included in this layer as well. 5--Session The session layer is responsible for managing and synchronizing the overall network conver-