Understanding the OSI Model: Concepts and Applications in Networking

In the 1980s, an organization called ISO (International Organization for Standardization) developed a network architecture model known as the Open System Interconnection (OSI) Reference Model for Open Networking. The OSI Model was designed to support the development of networking methods during that time.
Today, although the OSI layer model has become less common than the more simplified TCP/IP model, it is still used by some IT services providers around the world to design and implement computer networks. But how is the OSI layer model applied in networking, and what are its key uses?
What Is the OSI Model?
The OSI (Open System Interconnection) model is a standard framework for computer network architecture. The OSI model divides network functions into several layers, each with specific rules and requirements that promote interoperability among different products and software. It was primarily developed as a reference model for IT companies to build and manage computer networks.
Layers in the OSI Model
The OSI reference model breaks down inter-computer communication into seven layers. Below, we explain each layer starting from the lowest to the highest layer.
Layer 1: Physical Layer
This is the foundational and lowest layer of the OSI model, responsible for defining the physical aspects of the network, including transmission media, bit synchronization, signaling methods, cabling, and network topology. This layer transmits data using electrical or optical signals to the receiving device.
Layer 2: Data Link Layer
The data link layer is responsible for establishing, maintaining, and terminating connections between two directly connected devices within a network. Its functions include error correction, flow control, and hardware addressing. This layer is divided into two sub-layers: Logical Link Control (LLC), which handles network protocol identification, error checking, and frame synchronization, and Media Access Control (MAC), which uses MAC addresses to connect devices and allow data transmission.
Layer 3: Network Layer
The network layer takes data from the data link layer and sends it to the appropriate destination. It determines the destination by using logical addresses such as IP (Internet Protocol) that are embedded within the data packets. Routers play a critical role in this layer.
Layer 4: Transport Layer
The transport layer ensures the reliable delivery of data by checking for errors and controlling the flow of data between systems and hosts. It manages data packet segmentation, ordering, and reassembly before passing it on to the session layer. Common protocols used in this layer include TCP (Transmission Control Protocol).
Layer 5: Session Layer
This layer defines and controls the sessions or connections between applications. It manages the setup, maintenance, and termination of communication sessions, and handles authentication and recovery in case of disruptions. Protocols like NFS, SMB, and RTP can be found at this layer.
Layer 6: Presentation Layer
The presentation layer is responsible for translating data formats for transmission to the application layer. It also handles encryption and decryption for secure data transmission. Protocols used in this layer include MIME, SSL, and TLS.
Layer 7: Application Layer
The application layer is where the end-to-end interaction between users and systems takes place in a network. It provides network services for end-user applications, such as web browsers. It also identifies communication partners, resource availability, and synchronizes communication.
While the OSI layer model is not as widely used today as the simpler TCP/IP model, it remains a recognized standard for network architecture, as endorsed by the International Organization for Standardization (ISO). The complexity of the OSI hierarchy is one reason for its decline in popularity compared to the more streamlined TCP/IP model. Nonetheless, IT service providers still offer the OSI model in certain cases, although it is less common.
No matter which network architecture model you choose to implement, it’s important to work with an IT service provider that offers robust support and service points in key global locations.