The Open Systems Interconnection Model

Introduction:

Welcome to the world of Networking! Buckle up and get ready to explore the intricate world of networking through the lens of the OSI model. Understanding the OSI model is crucial for anyone who wants to troubleshoot network issues, design a new network or simply understand how data flows through the internet. It provides a common language for network engineers to discuss networking concepts and troubleshoot issues. During the days of early computing, network devices manufacturers all faced similar issues. Which was a result of their various devices not having the capabilities of sending or receiving information from other device models. To combat this issue, the Defence Advanced Research Projects Agency (DARPA) developed the TCP/IP model and as network devices started to grow, the international organisation of standardization (ISO) developed the OSI model which enabled diverse communication systems to communicate using standard protocols.

What is OSI model?

The open system interconnection (OSI) Model is a framework that defines how data is transmitted between different devices in a network. It’s a conceptual model that helps network engineers, device vendors, as well as anyone interested in computer networking have a standardized way of thinking about how data moves through a network. It is a model that was derived from TCP/IP model which is a four layer model while the OSI model comprises of seven layers each with its own unique functions and set of protocols.

The TCP/IP model VS The OSI model

The Transmission Control Protocol/Internet Protocol (TCP/IP) though carries out the same function as the OSI model, it is still different from it. TCP/IP model is a four-layer network model. It comprises of:

1. Application layer: comprises of Application, Presentation and Session layer
2. Transport layer
3. Internet layer: comprises  of the Network layer
4. Network layer:  comprises of Datalink and Physical layer      

Layers of the OSI Model

• Application layer
• Presentation layer
• Session layer
• Transport layer
• Network layer 
• Datalink layer
• Physical layer

To remember this easily, you can use the mnemonic “A Priest Saw Two Nuns Doing Pushups”.

Application Layer: This layer  works at the user end to interact with rhe user application. File transfer, email and Quality of Service (QoS) are the major popular services of the application layer. Protocols  in this layer includes: HTTP, SMTP, SMB, Telnet, SSH, FTP, SNMP etc.

Presentation layer: This layer deals with data compression, encryption, and decryption. Protocols that operate in this layer are TLS, SSL, XDR, and MIME.

Session Layer: This layer helps to create, maintain, and terminate a session/connection between the source and destination device. NetBIOS, PPTP, PAP and L2TP are all protocols in this layer.

Transport Layer: This layer works on two-determined communication modes: connection oriented and connectionless. It enables reliable data transmission, some error correction and sending acknowledgements.  Port numbers are essential for this layer to function accordingly. TCP, UDP, SPX, DCCP and SCTP are all protocols in this layer.

Network Layer: This layer deals with data as Packets. Devices like the router and multilayer switches work in this layer. This layer enables addressing and ensures delivery of packets across a network using the IP address. IPv4, IPv6, IPX, ICMP, IGMP, AppleTalk, and IPSec are good examples of protocols in this layer.

Data-link Layer: This layer deals with data as Frames. Bridges and some switches operate in this layer. The principle purpose of this layer is to detect errors by adding headers to data packets. Protocols in this layer include ARP, IEEE 802.3, SLIP, PLIP, SDLS and CSUP.

Physical Layer: Data in this layer are referred to as Bits. This layer enables the movement of data bits on and off the communication media (coaxial cable, fibre optics, wireless).  Devices in this layer includes: hubs and network adapters. Protocols  in this layer are IEEE 802.3, IEEE 802.11, TIA 449, DSL, Bluetooth, PON and OTN.         

Example of how the OSI Model works

Every time a user is sending or receiving data in a network, data travels from different layers of the model. To enable us understand how the OSI model helps in the transmission of data, we are going to take an example; 

Sender:

1. There are 2 PCs in a network where PC 1 is sending data to PC. The data will first travel from the upper layer of the OSI model (Application, Presentation, Session layer) to the Transport layer.
2. Transport layer will convert this data into segments and then forward it to the Network layer.
3. Network layer will convert this segment into packets and then forward it to the Data link layer.
4. Data link layer will convert these packets into frames and then forward it to the Physical layer.
5. The physical layer will convert these frames into binary bits and then forward it to another physical layer that is on the receiver’s side.

Receiver:

1. Now the data will flow in reverse order. The physical layer on the receiver side will forward its binary bits to the data link layer.
2. Data link Layer is going to convert the binary bits into frames and then forward it to the network layer.
3. The network layer will convert this frames into packets and then forward them to the Transport layer.
4. The transport layer will convert these packets into segments and then forward it to the upper layer (Application, Presentation, Session layer) of the OSI model.
5. And the user will receive the data from the application layer in a readable format.

The above steps should now tell you how data are being transferred in a network.

Advantages of the OSI Model

1. Layered Approach: The OSI model divides network into different layers which simplifies the design and development of networking protocols and allows for easier troubleshooting and debugging of network issues.
2. Modular Design: Each layer of the OSI model is designed to be independent of other layers, which allows for easier integration of new technologies and devices into existing networks.
3. Standardization: The OSI model provides a standardized framework for network communication, which ensures that different devices and applications can communicate with each other regardless of their specific implementation.
4. Flexibility: The OSI model is flexible enough to accommodate different types of networks and devices, from wired and wireless networks to different types of server and client devices.
5. Interoperability: The OSI model promotes interoperability between different network devices and applications by defining a set of protocols and standards for network communication.

Limitations of OSI Model

1. Complexity: The OSI model is a complex framework that can be difficult to understand and implement for less experienced network engineers.
2. Overhead: The layered approach of the OSI model can introduce additional overhead and latency, which can negatively impact network performance.
3. Lack of Real-World Implementation: The OSI model is a theoretical model and has not been fully implemented in real-world networks, which may limit its practical usefulness.
4. Rapid Technological Change: The OSI model may not keep up with the rapid pace of technological change and may become outdated over time, requiring frequent updates and revisions.
5. Limited Scope: The OSI model is primarily focused on the lower layers of the network stack, which may not be sufficient for addressing higher-level networking issues such as security and application performance.

Conclusion:

The OSI model provides a useful reference for understanding how data is transmitted across different layers of the network stack, and can help diagnose issues that may arise during transmission. I encourage  you, my reader to share your experience or possibly ask questions about the OSI model in the comment section.