The OSI Model
Open Systems Interconnection
Before comparing it with the TCP/IP suite, let's discuss the OSI Model. Developed in 1977 by the ISO (International Organization for Standardization) and the ITU-T (International Telecommunication Union - Telecommunication Standardization Sector), the OSI model is a popular layered framework. It consists of seven layers and provides a logical standard for devices to communicate with each other. The layers are categorized into upper and lower layers.
The OSI MODEL
OSI 7 layers
Understanding network communication is logical and serves as a standard for vendors and equipment. Let's delve into each layer for clarity.
The Physical Layer: This layer encompasses all physical elements, from cables to SFP ports, and is often called Layer 1. A common network troubleshooting method is to examine each layer sequentially.
Real-life tip: In my experience, most networking issues occur at this layer.
Note: The primary unit for the Physical Layer is bits.
Data Link Layer: This layer is responsible for framing data, packaging it, and transmitting it through the physical medium. Here, you'll find the Ethernet Protocol, commonly referred to as Layer 2.
Note: The primary unit of the Data Link Layer is frames.
Network Layer: Also known as Layer 3, it contains the Internet Protocol (IP) necessary for communication. It deals with addressing, encapsulating frames in IP headers, and routing them to the destination.
Note: The primary unit for this layer is packets.
Transport Layer: This is where TCP and UDP come into play, managing how devices negotiate the reception and transmission of packets. When devices establish a connection, they coordinate the transfer of information. It's also known as Layer 4.
Note: The unit for the Transport Layer is segments.
Session Layer: Known as Layer 5, it manages sessions between end-user application processes, such as web page requests, including authentication, authorization, and session restoration.
Presentation Layer: Also known as Layer 6, it formats data and is where encryption and decryption occur. The data, often scrambled, is processed at this layer to ensure it's in the correct format for the application layer.
