All communication, whether it is face-to-face or over a network, is governed by rules called protocols. These protocols are specific to a type of conversation. In our day-to-day personal communication, the rules we use to communicate over one medium, like a telephone call, are not necessarily the same as the protocols for using another medium, such as sending a letter.
Think of how many different rules or protocols govern all the different methods of communication that exist in the world today. For example, consider two people communicating face-to-face. First, they must agree on how communication occurs. If the communication is using voice, they must agree on the language, and then they must actually have a message to share, and format that message in a way that is understandable. Each of these tasks describes protocols put in place to accomplish communication. Similarly, successful communication between hosts on a network requires the interaction of many different protocols. A group of inter-related protocols necessary to perform a communication function is called a protocol suite. Protocol suites are implemented by hosts and networking devices in software, hardware, or both.
One of the best ways to visualize how the protocols within a suite interact is to view the interaction as a stack. A protocol stack shows how the individual protocols within a suite are implemented. The protocols are viewed in terms of layers, with each higher-level service depending on the functionality defined by the protocols shown in the lower levels. The lower layers of the stack are concerned with moving data over the network and providing services to the upper layers, which are focused on the content of the message being sent. As the figure shows, we can use layers to describe the activity occurring in our face-to-face communication example. At the bottom layer, the physical layer, there are two people, each with a voice that can say words out loud. At the second layer, the rules layer, there is an agreement to speak in a common language. At the top layer, the content layer, there are words that are actually spoken. This is the content of the communication.
Were we to witness this conversation, we would not actually see layers floating in space. The use of layers is a model that provides a way to conveniently break a complex task into parts and describe how they work.