14.8.2 What Is the OSI Model and Why Is It Important?
- Why Was the OSI Model Developed?
- What Is a Layered Architecture?
- What Are the Seven Layers?
- What Does Each Layer Do?
- Why Is Layering Such a Good Idea?
- How Does Information Move Through the Layers?
- Does Every Layer Communicate Directly with Its Equivalent Layer?
- Why Is the OSI Model Still Taught?
- Is the OSI Model Used Exactly in Practice?
- How Does the OSI Model Help with Troubleshooting?
- Will the OSI Model Remain Relevant?
- What Should You Remember?
Short Answer
The Open Systems Interconnection (OSI) Reference Model is a conceptual framework that describes how information moves through a communication network. Rather than treating networking as a single complex process, the OSI model divides communication into seven layers, each responsible for a specific set of functions. Although today's Internet is based primarily on the TCP/IP protocol suite rather than the complete OSI protocol family, the OSI model remains one of the most important tools for understanding, designing, and troubleshooting communication networks.
Why Was the OSI Model Developed?
As computer networking expanded during the 1970s and 1980s, many manufacturers developed their own proprietary networking systems.
Equipment produced by one company often could not communicate with equipment produced by another. This lack of interoperability made network expansion difficult and expensive. To overcome these problems, the International Organization for Standardization (ISO) developed the OSI Reference Model.
Its purpose was not to define one particular network, but to provide a common framework that all networking technologies could follow.
What Is a Layered Architecture?
Instead of treating communication as one large problem, the OSI model divides it into a series of smaller, more manageable tasks.
Each task is assigned to a separate layer. Every layer:
- performs a specific function;
- communicates with the layers immediately above and below it; and
- provides services to higher layers while using services provided by lower layers.
This modular approach greatly simplifies both network design and implementation.
What Are the Seven Layers?
The OSI model consists of seven layers.
From lowest to highest they are:
- Physical Layer
- Data Link Layer
- Network Layer
- Transport Layer
- Session Layer
- Presentation Layer
- Application Layer
Each layer contributes a specific function to the overall communication process.
Together they allow information to travel from one application on one computer to another application on a different computer.
What Does Each Layer Do?
Each layer has a distinct responsibility.
- The Physical Layer transmits bits across the communication medium.
- The Data Link Layer transfers frames reliably across a single communication link.
- The Network Layer determines how information is routed between different networks.
- The Transport Layer provides reliable end-to-end communication between applications.
- The Session Layer establishes, manages, and terminates communication sessions.
- The Presentation Layer translates, compresses, and encrypts information where required.
- The Application Layer provides network services directly to user applications.
Each layer therefore concentrates on a well-defined aspect of communication.
Why Is Layering Such a Good Idea?
Layering provides many engineering advantages.
It:
- reduces system complexity;
- simplifies equipment design;
- encourages standardization;
- allows independent development of protocols;
- makes troubleshooting easier; and
- improves interoperability between manufacturers.
Perhaps most importantly, changes made within one layer normally have little effect on the others.
For example, replacing a copper Ethernet cable with an optical fiber changes the Physical Layer but does not affect web browsers, email software, or application programs.
How Does Information Move Through the Layers?
When information is transmitted, it passes down through the layers at the sending device.
Each layer adds its own control information before passing the data to the next lower layer. At the receiving device, the reverse process occurs. Each layer removes the information intended for it before passing the remaining data upward. This process is often called encapsulation and decapsulation.
It allows every layer to perform its own function independently while contributing to the complete communication process.
Does Every Layer Communicate Directly with Its Equivalent Layer?
Conceptually, yes.
Each layer appears to communicate with the corresponding layer at the receiving device. For example:
- the Transport Layer communicates with the Transport Layer;
- the Network Layer communicates with the Network Layer; and
- the Data Link Layer communicates with the Data Link Layer.
In reality, however, information passes through every intermediate layer before being transmitted across the network.
The layered model therefore provides a convenient conceptual abstraction for understanding communication.
Why Is the OSI Model Still Taught?
Many people wonder why engineers continue studying the OSI model when the Internet uses TCP/IP.
The reason is simple. The OSI model provides a clear conceptual framework for understanding networking. It enables engineers to:
- classify networking functions;
- compare different protocols;
- analyse communication problems;
- explain new technologies; and
- design interoperable systems.
For these reasons, the OSI model remains the standard teaching framework used throughout networking education.
Is the OSI Model Used Exactly in Practice?
Not completely.
Modern communication systems rarely implement the OSI model exactly as originally specified. Many practical protocols combine functions from several OSI layers. For example:
- Ethernet operates primarily at the Physical and Data Link Layers;
- IP provides Network Layer services;
- TCP and UDP operate at the Transport Layer; and
- web browsers, email programs, and file transfer software operate at the Application Layer.
The OSI model therefore serves primarily as a reference architecture rather than a rigid implementation standard.
How Does the OSI Model Help with Troubleshooting?
One of the greatest strengths of the OSI model is its systematic approach to fault diagnosis.
Engineers can investigate problems layer by layer. For example:
- damaged cables are Physical Layer problems;
- incorrect MAC addressing involves the Data Link Layer;
- routing failures occur at the Network Layer;
- unreliable end-to-end delivery may involve the Transport Layer; and
- application errors occur at the Application Layer.
Breaking a complex problem into separate layers often makes troubleshooting much more efficient.
Will the OSI Model Remain Relevant?
Almost certainly.
Although networking technologies continue evolving, communication systems will always require functions such as:
- physical transmission;
- framing;
- routing;
- transport;
- session management; and
- application services.
Whether implemented exactly as defined by the OSI model or not, these functions remain essential components of every communication network.
The OSI Reference Model therefore continues to provide a valuable framework for understanding both current and future networking technologies.
What Should You Remember?
- The OSI Reference Model divides network communication into seven functional layers.
- Layering simplifies network design by separating complex communication tasks into manageable functions.
- Each layer provides services to the layer above while relying on services from the layer below.
- Encapsulation allows each layer to add the information needed to perform its specific function.
- The OSI model promotes standardization, interoperability, modularity, and easier troubleshooting.
- Although modern Internet protocols follow the TCP/IP architecture, the OSI model remains the principal conceptual framework for understanding communication networks.
- Understanding the OSI model makes it much easier to understand networking protocols, troubleshoot communication problems, and learn new networking technologies.