Physical Layer / The OSI Model

Physical Layer / The OSI Model

Physical Layer

As the job of the link layer is to move entire frames from one network element to an adjacent network element, the job of the physical layer is to move the individual bits within the frame from one node to the next. The protocols in this layer are again link dependent and further depend on the real transmission medium of the link (for instance, twisted-pair copper wire, single-mode fiber optics). For instance, Ethernet has many physical-layer protocols: one for twisted-pair copper wire, another for coaxial cable, another for fiber, and so on. In each case, a bit is moved across the link in a different way.

The OSI Model

After discussing the Internet protocol stack in detail, we should mention that it is not the only protocol stack around. Particularly, back in the late 1970s, the International Organization for Standardization (ISO) proposed that computer networks be organized around seven layers, called the Open Systems Interconnection (OSI) model [IS0 2009]. The OSI model took shape when the protocols that were to become the Internet protocols were in their infancy, and were but one of various different protocol suites under development; in reality, the inventors of the original OSI model perhaps did not have the Internet in mind when creating it. However, beginning in the late 1970s, many training and university courses picked up on the ISO mandate and organized courses around the seven-layer model. Because of its early impact on networking education, the seven-layer model continues to remain on in some networking textbooks and training courses.

The seven layers of the OSI reference model, shown in "Protocol Layering" figure (b), are application layer, presentation layer, session layer, transport layer, network layer, data link layer, and physical layer. The functionality of five of these layers is approximately the same as their similarly named Internet counterparts. Therefore, let's examine the two additional layers present in the OSI reference model - the presentation layer and the session layer. The role of the presentation layer is to provide services that allow communicating applications to interpret the meaning of data exchanged. These services contain data compression and data encryption (which are self-explanatory) as well as data description (which, as we will see in "Network Management", frees the applications from having to worry about the internal format in which data are represented/stored - formats that may differ from one computer to another). The session layer provides for delimiting and synchronization of data exchange, including the means to build a checkpointing and recovery scheme.

The reality that the Internet lacks two layers found in the OSI reference model poses a couple of interesting questions: Are the services given by these layers unimportant? What if an application requires one of these services? The Internet's answer to both of these questions is the same - It's up to the application developer to decide if a service is important, and if the service is important, it's up to the application developer to build that functionality into the application.


network layer, protocol stack, link layer

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