1.4. DSL Protocol Reference Model

Figure 1.6 shows the open systems interconnection (OSI) protocol reference model with additional detail shown for the physical layer. The structure within the physical layer is largely due to the contributions of Les Humphrey. The protocol reference model provides a structure to organize complex communications systems. The layered approach hides details of information from the subsystem, invoking the service of a particular layer. Thus, an application on a host requesting communication with its peer application does not need to know the details of its physical connection to a data network, communications used between itself and the network, or even the details of the protocols exchanged between the hosts supporting the application. The layered approach also simplifies the analysis of complex communication systems by segmenting the systems into several well-defined portions, facilitates reuse of portions of communications systems, and allows upgrades of portions of a communications system independently of each other.