OSI Reference Model

Let’s learn about the OSI Reference Model in this tutorial.  The OSI Reference Model does not define a network standard but provides guidelines for creating network standards. The OSI (Open Systems Interconnection) Reference Model is a conceptual framework to understand how different network protocols interact in a communication system. It breaks down the complex process of networking into seven manageable layers, each with its specific responsibilities.

The seven layers of the OSI model are as follows:

• Application Layer
• Presentation Layer
• Session Layer
• Transport Layer
• Network Layer
• Data Link Layer
• Physical Layer

 

Physical Layer (Layer 1)

• What it does: This layer is responsible for transmitting raw bits (1s and 0s) over a physical medium like cables or wireless signals. It deals with hardware components, electrical signals, and the physical connection between devices.
• Examples: Cables (Ethernet, fiber optic), network interface cards (NICs), and Wireless radios.

Data Link Layer (Layer 2)

• What it does: The Data Link Layer ensures reliable transmission of data over the physical link. It handles error detection, error correction, and flow control. It also formats data into frames.
• Examples: Ethernet, Wi-Fi, MAC (Media Access Control) addresses.

Network Layer (Layer 3)

• What it does: This layer is responsible for routing data from the source to the destination across multiple networks. It handles logical addressing (like IP addresses) and ensures that data reaches its correct destination.
• Examples: IP (Internet Protocol), routers, subnetting.

Transport Layer (Layer 4)

• What it does: The Transport Layer ensures that data is delivered reliably and in the correct order. It handles flow control, error correction, and data segmentation (breaking down large chunks of data into smaller pieces).
• Examples: TCP (Transmission Control Protocol), and UDP (User Datagram Protocol).

Session Layer (Layer 5)

• What it does: This layer manages and controls the dialog between two devices. It ensures that data is properly synchronized and manages session establishment, maintenance, and termination.
• Examples: NetBIOS, RPC (Remote Procedure Call), SMB (Server Message Block).

Presentation Layer (Layer 6)

• What it does: The Presentation Layer translates, encrypts, and compresses data. It ensures that data is in a format that the receiving system can understand. It also handles character encoding and data encryption and decryption.
• Examples: SSL/TLS (for encryption), JPEG, GIF, ASCII, and EBCDIC encoding.

Application Layer (Layer 7)

• What it does: This is the topmost layer and is closest to the user. It provides network services directly to applications (such as web browsers or email clients). This layer handles application-specific tasks like file transfers, email, and web browsing.
• Examples: HTTP (HyperText Transfer Protocol), FTP (File Transfer Protocol), SMTP (Simple Mail Transfer Protocol), DNS (Domain Name System).

 

Layer Number	Layer Name	Responsibilities	Examples
7	Application Layer	User interface & application protocols	HTTP, FTP, DNS
6	Presentation Layer	Data formatting, encryption, compression	SSL/TLS, JPEG, ASCII
5	Session Layer	Manages sessions between devices	NetBIOS, RPC
4	Transport Layer	Reliable data transfer, error correction	TCP, UDP
3	Network Layer	Routing, logical addressing	IP, Routers
2	Data Link Layer	Error detection and correction, framing	Ethernet, Wi-Fi, MAC addresses
1	Physical Layer	Raw transmission of bits over medium	Cables, NICs, Wireless Radios