This article provides a detailed explanation of the OSI model and its seven layers. It also explores the TCP/IP model, compares it to the OSI framework, and examines network topologies, their characteristics, advantages, and applications.
Alexander S. Ricciardi
December 12, 2024
In networking,the Open Systems Interconnection (OSI) model is a standardized reference framework that describes how data flows in networks or how networked devices communicate with each other. In 1977 the International Organization for Standardization (ISO) developed OSI to standardize the interoperability of multivendor communications systems into one cohesive model (uCertify, 2019 a). The OSI Model is a reference model, it is not a reverence model (Wallace, 2020). In other words, the model does not need to be revered as a framework where every network component or device must neatly fit. However, it can be used as a tool to explain and understand where different network components or devices reside. This makes the model very useful for diagnosing and fixing network issues as it helps isolate problems within its different layers.
The OSI model is composed of seven layers:
Layer 1: The physical layer
Layer 2: The data link layer
Layer 3: The network layer
Layer 4: The transport layer
Layer 5: The session layer
Layer 6: The presentation layer
Layer 7: The application layer
Note that the application layer is the last in the OSI queue, as it is the closest to the user. However, graphically the layers are usually represented as a stack, bottom-up, as illustrated in
Figure 1.
Figure 1
OSI Layers
Note: From “The OSI reference model. CompTIA Network+ Pearson N10-007”, Figure 2.2, by uCertify. (2019 a).
Each layer represents a different network functionality as shown in Figure 2.
Figure 2
OSI vs. TCP/IP
Note: From “Objective 1.01 Explain, compare, and contrast the OSI layers” by vWannabe (n.d.).
In Figure 2, the OSI stack is compared to the TCP/IP stack model, which is a reference model based on the TCP/IP protocol suite. The TCP/IP model is used to describe communications on the Internet and simplifies the OSI layers into four categories which are Network Interface (Network access layer), Internet (Internet layer), Transport (Host-to-Host layer), and Application (Process/Application layer) see Figure 3.
Figure 3
OSI and TCP/IP
Note: From “The OSI reference model. CompTIA Network+ Pearson N10-007”, Figure 2.15, by uCertify. (2019 a).
The TCP/IP layers map to the OSI layers as follows:
Network Interface: Combines the physical and data link layers of the OSI model.
Internet: Corresponds to the network layer of the OSI model.
Transport: Maps directly to the transport layer of the OSI model.
Application: Consolidates the session, presentation, and application layers of the OSI model.
As shown above, for me, the OSI model is a great tool for understanding network systems and diagnosing issues. When connected to the TCP/IP model, it provides practical insights into troubleshooting and understanding Internet systems, which is where most of today's networks operate.
Another important concept to understand is network topology. Topology classifies the arrangement of devices and connections within a network, either physically (physical topology) or logically (logical topology). Below is an illustration of the most common topologies:
Figure 4
Network Topologies
Note: From “Lesson 1: Computer Network Fundamentals. CompTIA Network+ Pearson N10-007,“ various Figures, by uCertify (2019 b). Modify.
The table below describes the characteristics, advantages, and limitations of various topologies.
Table 1
Network Topologies
Note: Data from “Lesson 1: Computer Network Fundamentals. CompTIA Network+ Pearson N10-007“ by uCertify (2019 b).
As shown in Table 1, each topology has its pros and cons depending on the needs, budget, and future goals of a business. One topology may be more suitable than another. Below, Table 2 is a comparison between Star and Generic Mesh Topologies, that showcases their advantages, disadvantages, and the types of business applications or use cases they are best-suited for.
Table 2
Comparison of Star and Generic Mesh Topologies
Note: Data from “Lesson 1: Computer Network Fundamentals. CompTIA Network+ Pearson N10-007“ by uCertify (2019 b).
As shown in Table 2 a star topology is better suited for Small-to-medium businesses due to its low cost as mesh topology is better suited for large enterprise networks, data centers, and IoT networks that require fault tolerance.
To summarize, the OSI model is a foundational framework for understanding network communication and diagnosing connection issues. It is particularly helpful when used in conjunction with the TCP/IP model to troubleshoot and understand modern Internet systems. Additionally, network topology helps to define the structure of networks by setting the arrangement of devices and connections, both physically and logically, enabling businesses to select the most suitable configuration based on their specific needs and goals.
References:
uCertify. (2019 a). Lesson 2: The OSI reference model. CompTIA Network+ Pearson N10-007 (Course & Labs) [Computer software]. uCertify LLC. ISBN: 9781616910327
uCertify. (2019 b). Lesson 1: Computer Network Fundamentals. CompTIA Network+ Pearson N10-007 (Course & Labs) [Computer software]. uCertify LLC. ISBN: 9781616910327
vWannabe (n.d.). Objective 1.01 Explain, compare, and contrast the OSI layers. vWannabe.com. https://vwannabe.com/2013/07/29/objective-1-01-explain-compare-and-contrast-the-osi-layers/
Wallace, K. (2020, December 11). Networking foundations: Networking basics [Video]. LinkedIn Learning. Retrieved from: https://www.linkedin.com/learning/networking-foundations-networking-basics/a-high-level-look-at-a-network?autoSkip=true&resume=false&u=2245842.