Communication using the relay system

Together, these seven layers work in tandem to facilitate communication across end systems. The top four layers (the Application, Presentation, Session, and Transport Layers) are generally considered to be the upper layers, while the lower three layers (Network, Data Link, and Physical Layers) are considered to be the lower layers. It is important to note that not all seven layers are required to be implemented on all of the devices that are present in the network. Some devices simply act as relay agents, supporting the lower layer protocols, while not decapsulating and processing the upper layers:

To illustrate the preceding diagram, consider that the two end devices being used are computers. Applications (such as web browsers) run on these computers and communicate at layer 7, but data for those applications may pass through relay devices that cannot run these applications. These relay devices may be equipment such as routers (devices that read addresses at the Network Layer and move packets between networks) or switches (devices that read addresses at the Data Link Layer and move frames between their ports). Routers, switches, broadcast domains, and collision domains will be discussed in depth in later chapters.

Now that we've explored the seven layers of the OSI reference model and described how systems exchange data using the processes of encapsulation and decapsulation, let's take a step back and establish some context for what we've learned. The OSI model, while being an important tool for explaining concepts and helping professionals develop methodical approaches to troubleshooting, is not widely implemented in the industry today. The reasons for this are numerous, but the most important factor is the existence of a second suite of protocols called the TCP/IP protocol suite or, alternatively, the IP suite. However, the distinctions between both models is often blurred, and many professionals combine the concepts of the OSI reference model with the devices and protocols that are in use today.

In the following section, we will examine this ubiquitous IP suite and consider how it compares to the OSI reference model.