Just like in human communication, the various network and computer protocols must be able to interact and work together for network communication to be successful. 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, we have two people, each with a voice that can say words out loud. At the second layer, the rules layer, we have 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.