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Explain Concepts Related to the Open System Interconnection (OSI) Reference Model

As of 2025, 5.56 billion people worldwide use the internet regularly, meaning that more than two-thirds of the global population is connected to a computer network in one way or another. In the information age, everything from phones to servers, lightbulbs to cameras, and laptops to cars is intertwined through cables and radio waves.

Although the growth of global communication has been largely organic, building a modern network that is stable, secure, and efficient requires careful planning, regular maintenance, and expert troubleshooting. Today's IT networks come in many forms and are constantly evolving. To be an effective network engineer, you need the skills and knowledge to analyze, manage, and operate an almost infinite variety of network setups.

The CompTIA Network+ exam is designed to validate those skills and that knowledge. Earning this certification gives prospective clients and employers confidence that you can navigate the complexity of modern network engineering. It will also give you the confidence that you have the tools to progress in a vital, rewarding career.

This book covers everything you need to know to become CompTIA Network+ certified. Over the following chapters, you will learn about the core concepts, implementation strategies, operational practices, security essentials, and troubleshooting methodologies necessary for success in the exam.

However, before diving into the details of network functions, components, risks, and solutions, it's important to first understand what a network actually is. The Open Systems Interconnection (OSI) reference model provides a conceptual framework for understanding and standardizing the functions of a network. Rather than focusing on cables and hardware, the OSI model takes an abstract, layered approach that applies to all networks, regardless of their internal structure or underlying technologies. Developed by the International Organization for Standardization (ISO), the OSI model establishes a common language for describing and designing networks.

This chapter covers the first objective in Domain 1: 1.1 Explain concepts related to the Open System Interconnection (OSI) reference model.

Note

A full breakdown of objective 1.1 will be given at the end of the chapter.

What is the OSI model?

Modern IT networks can range from a small setup with just a few computers sharing a printer to vast infrastructures spanning multiple offices, continents, and cloud resources. No matter the size, architecture, or geographical spread, the job of the network engineer is to keep information flowing through that network reliably, efficiently, and securely. To do this, there is a range of physical and virtual appliances, protocols, techniques, and standards. The following chapters will cover these, but before doing so, it's worth looking at what a network is conceptually.

When you think of a network, you can break it down into its individual components, such as a router or a virtual private network (VPN). You can also consider it in terms of different functions, such as data routing, user interaction, or a mobile device connecting wirelessly. Often, when troubleshooting a problem, you won't start by identifying which piece of equipment has failed but rather by understanding how something isn't working. A user is more likely to complain that they can't access certain resources than to say that a routing table needs to be updated.

So, what does thinking about a network in terms of functions mean? This is where the OSI model comes in. The OSI model divides the process of network communication into seven distinct layers, with layer 1 being the physical layer, and layer 7 being the application layer. Each layer is dependent on the layer beneath it and serves the layer above, and each has its own specific functions and protocols. These layers, from top to bottom, are shown in Table 1.1.

Layer

Title

Function

Protocol/Devices

7

Application

Provides user access to network services. Examples include web browsers and email clients.

HTTP, HTTPS, Domain Name System (DNS), Simple Mail Transfer Protocol (SMTP), File Transfer Protocol (FTP), Post Office Protocol version 3 (POP3), Internet Message Access Protocol (IMAP), Telnet, Secure Shell (SSH), Secure Sockets Layer (SSL)/Transport Layer Security (TLS), web application firewalls (WAFs), intrusion prevention systems (IPSs), and sophisticated load balancers.

6

Presentation

Ensures that data is in a usable format and is where data encryption occurs.

ASCII, Unicode, SSL/TLS.

5

Session

Manages sessions between applications. Establishes, manages, and terminates connections between local and remote applications.

Logging in and creating a session, and logging out to terminate the session. Establishes connections.

4

Transport

End-to-end error communication, recovery, and flow control. Protocols such as TCP and UDP operate at this layer.

Datagrams: basic load balancers, transport layer firewall (also known as a Layer 4 firewall).

3

Network

Addressing and routing of data. Manages packet forwarding, including routing through intermediate routers.

Packets and IP addresses - routers, firewalls, packet filtering.

2

Data Link

Manages the reliable transmission of data frames over a physical medium, ensuring error-free communication between directly connected devices.

Frames, media access control (MAC) addresses - switch and bridge. Defines the format of the data on the network.

1

Physical

Connects devices using cables, wireless communication, hubs, and repeaters.

Bits: hubs and repeaters

Cables: coax, Ethernet, fiber, Wi-Fi, and Bluetooth

Table 1.1: The OSI layers, functions, protocols, and devices.

Table 1.1 gives an overview of what each of the layers does. Understanding the order and function of the layers is important for the exam. One way of learning the model is with the mnemonic shown in Figure 1.1.

Figure 1.1: A mnemonic for the functions of the seven layers of the OSI model

Note

When you prepare for the exam, you must know the function and the order of the layers. The mnemonic in Figure 1.1 can help you.

The following section will go into each layer in more detail.

Layer 1: Physical layer

The devices that operate at the physical layer are hubs, repeaters, and network cables. These cables can be Ethernet cables made out of copper, which transmit electrical signals, or fiber-optic cables, which transmit light pulses. The quality and type of cable determine how far and fast data can travel. Wireless also operates at this layer. Electronic devices that can connect to a local area network (LAN) using an Ethernet cable are known as Ethernet devices.

A hub is a basic...