1.2 Key Characteristics of Various Switching Technologies
1.2.1 Switching and the OSI model

Most communication environments use the OSI model to separate the communications functions and applications processing into layers. Each layer in the OSI model serves a set of specific functions. This semester's focuses on Layers 2, 3, and 4 of the OSI model.

Each layer within one system uses its own protocols to communicate with peer layers in another system. Each layer protocol exchanges information, called protocol data units (PDUs), between peer layers. A given layer has specific names for each of its PDUs. The Table gives examples of specific PDUs for Layers 2, 3, and 4 and the device types that process those PDUs.

Each peer-layer protocol is serviced by the underlying layer. For example, Transmission Control Protocol (TCP) segments are encapsulated in Layer 3 packets, and Layer 3 packets are encapsulated in Layer 2 frames. A layer-specific device processes only those PDUs for which it is responsible - this is done by inspection of the PDU header.