Using Reference Models

In the early days of networking, various manufacturers were using incompatible networking protocols and systems for identifying and locating computers on a network. The idea of locking customers into a system by making the system proprietary seemed like a good idea in the beginning. But it soon became apparent to the industry that if everyone got on the same page, everyone could sell more hardware and software.

What the industry needed was a vendor-neutral organization to bring order out of the chaos. The International Organization for Standardization (ISO) became that organization.

This group created a reference model called the Open Systems Interconnection (OSI) model to act as a common blueprint for all vendors to work from, with the goal of promoting interoperability among vendors.

The ISO went about this job by creating standards. Standards are entirely voluntary in nature. They are not laws. No vendors are required to abide by them. In some cases, some vendors chose not to follow the standard until it became apparent that the standard had been widely adopted. Most vendors saw the creation of standards as a benefit and came onboard. In fact, many of the people working on the committees that created these standards came from these companies.

It's only natural that some vendors, particularly large ones, had more influence on decisions that were made than others. The ideal approach would have been to examine all proposals and select the best one on an impartial technical basis, but in reality, some industry voices were louder than others. Cisco was and remains a large player in how networking is done.

Regardless of each player's size and influence, all parties appreciated the benefits of reference models. In this section, you'll look at some of those benefits and then explore another model before diving into the OSI model itself in the next section.

Understanding the Benefits of Reference

Reference models provide a common blueprint from which software and hardware developers can work. The benefits of using reference models such as the OSI model and the TCP/IP, or DoD, model (discussed in the next section) are as follows:

• It encourages standardization by defining what functions are performed at particular layers of the model.
• It breaks the communication process into parts, taking a modular approach that allows each layer to perform its particular function without regard to the other layers. This ultimately aids in component development at each layer by providing an assurance that the layer can be made to communicate with the layers above and below it.
• It prevents changes in one layer from causing a need for changes in other layers, speeding development.
• It allows various types of software and hardware to communicate through a standardized communication process from layer to layer.
• It encourages vendors to build on each other's developments through use of a common framework.

During the following discussion of reference models, keep in mind that the layers, as they are called, represent logical functions that are a part of a communication process between two entities, usually computers. The goal of breaking a complicated communication process into parts, or modules(which is why this process is described as modular), is to avoid the need to completely reinvent the entire communication process when a new development takes place. By standardizing the interface between two layers (interface just means the way they exchange information), a change can be made on one layer without requiring a change at any other layer. As long as the standard interface between the two layers remains unchanged, the process continues to work smoothly.

Exploring Reference Model Examples

Although the OSI model is where we will spend the bulk of our focus, it is not the only reference model. The Department of Defense (DoD) created a four-layer model in the early 1970s based on the protocol that eventually became the protocol of the Internet and later the de facto standard for LAN protocols, called the TCP/IP or Internet model.

The TCP/IP model uses four layers, and the OSI model uses seven, but they explain the same process of packet creation or encapsulation. All of the same functions take place in each model; in the TCP/IP model, they are just organized logically into fewer layers. As you will see, some of the TCP/IP layers correspond to, or map to, multiple layers of the OSI model.

Figure 2.1 illustrates how the OSI and the TCP/IP models are related to one another. Refer to this figure as the layers of the OSI model are explained and keep in mind that the process of encapsulation is the same in both models; only the number and names of the layers are different.

FIGURE 2.1 OSI and DoD models