The Physical Layer generates and receives the signals that represent bits moving over a single physical transmission medium. That medium may be a copper cable, optical fiber, or radio channel. This layer is not aware of any other nodes that can be reached by that medium.

The Data Link Layer exchanges data frames with a single peer process across a single physical link. This layer can detect and recover from simple data errors. Each Data Link Layer frame contains a data packet passed down from the Network Layer above.

The Network Layer is aware of several peer processes, one for each data link, and takes responsibility for moving packets between links to send them on their way to their final destination. The Network Layer address is concerned with getting a single packet from one end of a network to the final destination.

The Transport Layer is aware of the entire network, and is the first end-to-end layer. A Transport Layer process is concerned with transmitting a complete message from a process on one node to the peer process at the destination node.

The job of the uppermost three OSI layers (Session, Presentation, and Application) is to provide services to user applications. By providing a suite of protocols and standardized services to accomplish tasks that would have to be recoded over and over again, these upper layers allow user applications to more easily share data and communicate with one another. Together, Layers 5 through 7 provide capabilities for:

In addition to these application services, a number of common network applications are part of the Application Layer (Layer 7):

The OSI Summary Diagram summarizes the process of encapsulation and decapsulation, which is the most important concept of this unit. As data is passed down through the protocol stack, each layer encapsulates it by adding its protocol header. Note that the Data Link Layer is the only layer that truly encapsulates the data by adding both a header and trailer when constructing a frame.