• Table of Contents

Network Protocol Models

The OSI (Open Systems Interconnection) model and the TCP/IP model are both conceptual frameworks that describe how network protocols work and interact.

Figure 5.1 OSI vs TCP/IP Model.

Today, the Internet primarily relies on the TCP/IP model rather than OSI. Nonetheless, the OSI model remains valuable for visualizing and diagnosing network operations and problems.

Open Systems Interconnection (OSI) Model

he OSI model is made up of seven layers, each with its specific functions and responsibilities. We’ll describe OSI layers “bottom up” from the physical , up to the application layer that directly serves the end user.

1. Physical Layer: This layer deals with the physical connection between devices. It defines aspects like cables, connectors, and transmission rates.
   • Examples include Ethernet cables, USB connectors, and optical fibers.
2. Data Link Layer: Similar to the network layer, except it facilitates data transfer between two devices on the same — single — network.
   • Ethernet switches and network interface cards (NICs) operate at this layer.
3. Network Layer: This layer handles routing and logical addressing. Routers operate here to determine the best path for data to travel between networks.
   • IP addresses are used at this layer.
4. Transport Layer: Responsible for end-to-end communication, this layer ensures data integrity and reliability.
   • TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) operate here.
5. Session Layer: It establishes, maintains, and terminates connections between applications.
   • Examples include setting up and closing connections for web browsing sessions.
6. Presentation Layer: It ensures that data sent by the application layer is in a format that the recipient can understand.
   • Encryption and data compression are examples.
7. Application Layer: The top layer where user applications interact with the network.
   • It includes protocols like HTTP, FTP, and SMTP.
   • These facilitate various application-level services like web browsing, file transfer, and email.

Transmission Control Protocol/Internet Protocol (TCP/IP) Model

The TCP/IP model is more focused on the actual protocols used in the Internet:

1. Link Layer: Combines functions of both the OSI Physical and Data Link layers. It deals with hardware addressing, like MAC addresses.
2. Internet Layer: Similar to the OSI Network Layer, it handles logical addressing and routing using IP.
3. Transport Layer: Similar to the OSI Transport Layer, it provides end-to-end communication. TCP and UDP operate here.