The Open System Interconnect (OSI) reference model is a model, developed by the International Standards Organization (ISO), which describes how data from an application on one computer can be transferred to an application on another computer. The OSI reference model consists of seven conceptual layers which each specify different network functions. Each function of a network can be assigned to one, or perhaps a couple of adjacent layers, of these seven layers and is relatively independent of the other layers. This independence means that one layer does not need to be aware of what the implementation of an adjacent layer is, merely how to communicate with it. This is a major advantage of the OSI reference model and is one of the major reasons why it has become one of the most widely used architecture models for inter-computer communications.
The seven layers of the OSI reference model, as shown in Figure 1, are:
- Data link
Figure 1: Diagram of the OSI reference model layers, courtesy of catalyst.washington.edu
Over the next few articles I will be discussing each layer of the model and the networking hardware which relates to that layer. This article, as you have probably guessed from the title, will discuss layer 1; the physical layer.
While many people may simply state that all networking hardware belongs exclusively in the physical layer, they are wrong. Many networking hardware devices can perform functions belonging to the higher layers as well. For example, a network router performs routing functions which belong in the network layer.
What does the physical layer include? Well, the physical layer involves the actual transmission of signals over a medium from one computer to another. This layer includes specifications for the electrical and mechanical characteristics such as: voltage levels, signal timing, data rate, maximum transmission length, and physical connectors, of networking equipment. For a device to operate solely in the physical layer, it will not have any knowledge of the data which it transmits. A physical layer device simply transmits or receives data.
There are four general functions which the physical layer is responsible for. These functions are:
- Definitions of hardware specifications
- Encoding and signaling
- Data transmission and reception
- Topology and physical network design
……. To be continued….