Understanding the OSI Model: A Beginner’s Guide to Computer Networking

Introduction

The Open Systems Interconnection (OSI) model is a conceptual framework used to understand how computers communicate with each other over a network. It was developed by the International Organization for Standardization (ISO) in the 1980s as a way to standardize network communication and ensure interoperability between different computer systems. The OSI model is divided into seven layers, each with a specific function and set of protocols. These layers include the application layer, presentation layer, session layer, transport layer, network layer, data link layer, and physical layer.

By breaking down network communication into these seven layers, the OSI model provides a standardized way for different computer systems to communicate with each other. This has been critical in the development of modern computer networks, enabling the internet and other complex networks to function as they do today. Understanding the OSI model is essential for anyone interested in computer networking, as it provides a framework for understanding how different technologies work together to enable communication over a network.

Breaking OSI to Understand Easily

The OSI model is a way to think about how computers talk to each other over a network. It’s like a map that shows you the different steps that need to happen for two computers to communicate successfully.

Let’s imagine that you want to send a letter to your friend who lives far away. There are a few steps you need to follow to make sure the letter gets to your friend:

1. Write the letter – This is like the “Application layer” in the OSI model. You are creating a message that you want to send to your friend.
2. Put the letter in an envelope – This is like the “Presentation layer” in the OSI model. You are packaging your message in a way that your friend will be able to understand.
3. Write your friend’s address on the envelope – This is like the “Session layer” in the OSI model. You are establishing a connection with your friend by addressing the envelope to them.
4. Put the envelope in a mailbox – This is like the “Transport layer” in the OSI model. You are handing your message over to a service that will make sure it gets to your friend.
5. The postal service picks up the envelope and takes it to a sorting facility – This is like the “Network layer” in the OSI model. The postal service is making sure your letter is going in the right direction to get to your friend.
6. The sorting facility sorts the envelopes by destination and sends them to a regional hub – This is like the “Data link layer” in the OSI model. The sorting facility is making sure that your letter is going to the right place by looking at the address on the envelope.
7. The regional hub sends the envelope to your friend’s local post office – This is like the “Physical layer” in the OSI model. The regional hub is physically moving your letter closer to your friend.
8. The local post office delivers the letter to your friend’s mailbox – This is like the “Physical layer” in the OSI model. The local post office is physically delivering your message to your friend’s location.

As you can see, there are many steps involved in sending a letter to your friend. Similarly, there are many steps involved in computers communicating with each other over a network. The OSI model helps us understand those steps and how they all fit together to make communication possible.

Standard Documents

The OSI model was defined in ISO/IEC 7498 which consists of the following parts:

• ISO/IEC 7498-1 The Basic Model
• ISO/IEC 7498-2 Security Architecture
• ISO/IEC 7498-3 Naming and addressing
• ISO/IEC 7498-4 Management framework

ISO/IEC 7498-1 is also published as ITU-T Recommendation X.200.

Protocols Used

1. Physical Layer: This layer is responsible for transmitting raw bits over a physical medium, such as copper or fiber optic cables. Protocols used in this layer include Ethernet, Fast Ethernet, Gigabit Ethernet, and Asynchronous Transfer Mode (ATM).
2. Data Link Layer: This layer is responsible for organizing bits into frames and ensuring error-free transmission over a physical medium. Protocols used in this layer include Point-to-Point Protocol (PPP), High-Level Data Link Control (HDLC), and Asynchronous Transfer Mode (ATM).
3. Network Layer: This layer is responsible for routing data across multiple networks, including the internet. It provides logical addressing and determines the best path for data to travel between different devices on a network. Protocols used in this layer include Internet Protocol (IP), Internet Control Message Protocol (ICMP), and Address Resolution Protocol (ARP).
4. Transport Layer: This layer is responsible for ensuring that data is transmitted reliably and without errors between applications on different devices. It provides end-to-end communication between hosts and can detect and retransmit lost data. Protocols used in this layer include Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Stream Control Transmission Protocol (SCTP).
5. Session Layer: This layer is responsible for managing and coordinating communication sessions between different applications on different devices. It establishes, maintains, and terminates connections between applications and provides synchronization and recovery of data. Protocols used in this layer include Remote Procedure Call (RPC) and Session Initiation Protocol (SIP).
6. Presentation Layer: This layer is responsible for translating data into a format that can be understood by the application layer. It provides encryption, compression, and formatting services to ensure that data is presented correctly to the application layer. Protocols used in this layer include Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).
7. Application Layer: This layer is the layer closest to the end-user and is responsible for providing services and interfaces for applications to interact with the network. It includes protocols such as Telnet, Simple Network Management Protocol (SNMP), and Domain Name System (DNS).

Advantages

• Standardization: One of the primary advantages of the OSI model is its standardized approach to networking. The model provides a clear and consistent framework for developing and implementing network protocols, which ensures that different devices can communicate with each other seamlessly.

• Modularity: Another advantage of the OSI model is its modular structure. The model is divided into seven layers, each of which performs a specific set of functions. This modular structure makes it easier to design, develop, and test network protocols, as each layer can be developed independently of the others.

• Scalability: The OSI model is highly scalable, which means that it can be used to build networks of varying sizes and complexity. This makes it suitable for use in small office/home office (SOHO) environments, as well as large enterprise networks.

• Flexibility: The OSI model is flexible, which means that it can accommodate a wide range of network technologies and protocols. This flexibility allows network administrators to choose the best technology for a given application, without worrying about compatibility issues.

• Troubleshooting: The modular structure of the OSI model also makes it easier to troubleshoot network issues. By breaking down the network into separate layers, administrators can isolate problems to specific layers and troubleshoot them more efficiently.

1. Interoperability: The OSI model is designed to facilitate interoperability between different vendors and technologies. By using a standardized set of protocols, devices from different vendors can communicate with each other seamlessly.

Conclusion

Understanding the protocols used in each layer of the OSI model is essential for developing and maintaining computer networks. By using a standardized set of protocols, network administrators can ensure that different devices can communicate with each other reliably and efficiently. Its modular structure, standardization, scalability, and flexibility make it a powerful tool for network administrators, while its focus on interoperability and troubleshooting makes it a valuable asset for any organization.