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The OSI Model: An Essential Foundation to Networking

Updated: Mar 26

No technology that is connected to internet is un-hackable. It's only a matter of time.


What is the OSI Model? The OSI, short for (the Open Systems Interconnection model) is a conceptual framework for understanding how network communication works. It was the first standard model adopted by all major computer and telecommunication companies in the early 1980s.

The OSI Model (aka ISO-OSI, i.e., International Organization of Standardization – Open System Interconnection) divides the communication process between two devices into seven layers. It provides a standard reference model that allows different networking technologies and protocols to interoperate and communicate.


Imagine you have two servers that need to share information. The message doesn't just magically teleport from an application on the first machine to the application on the other. Instead, it transits down the layers and eventually reaches the transmission line. Once it jumps across the gap to the other device, it has to repeat the process in reverse by ascending layers until it reaches the receiving application.

Core Definition

For any starting number N representing a layer that transmits a message, the OSI model can be used to explain the transmission few key concepts:

  • Protocol Data Units (PDUs) are abstracted messages that include payloads, headers, and footers.

  • Service Data Units (SDUs) are equivalent to the payloads.

At each subsequent transition from some layer N to some layer N-1, a layer-N PDU becomes a new N-1 SDU. This payload gets wrapped up in a layer N-1 PDU with the relevant headers and footers. On the opposite end, the data passes up the chain, unwrapping at each relevant stage until it's just a payload that the corresponding layer-N device can consume.

The 7 Layers of OSI

7 Layers of the OSI Model
7 Layers of the OSI Model

We'll describe OSI layers "top-down" from the application layer that directly serves the end user to the physical layer.

7. The Application Layer

  • The application layer is the highest layer of the OSI Model and is responsible for providing the interface between the network and the end user's application.

  • Standard network services such as file transfer, email, and web browsing are provided at the application layer. Protocols such as HTTPS (Hypertext Transfer Protocol Secure) and, FTP (File Transfer Protocol), SMTP (Simple Mail Transfer Protocol) operate at this layer, allowing users to access and transfer files and other resources over the network.

Functions of the Application Layer
Functions of the Application Layer
  • The application layer also provides the interface for user authentication and authorization. Protocols such as LDAP (Lightweight Directory Access Protocol) and Kerberos are used to verify the identity of users and grant them access to specific resources or services on the network.

6. The Presentation Layer

  • The presentation layer is responsible for formatting and encoding data in a standardized way independent of the application or system being used. It includes protocols like SSL (Secure Sockets Layer) that provide secure communication.

  • It deals with issues such as data compression and encryption.

  • An example of a presentation service would be converting an extended binary-coded decimal interchange code text computer file to an ASCII-coded file. The presentation layer could translate between multiple data formats using a standard format if necessary.

Functions of the Presentation Layer
Functions of the Presentation Layer

5. The Session Layer

  • The session layer establishes, maintains, and terminates connections between devices. Some standard protocols that operate at the session layer include Remote Procedure Call (RPC), NetBIOS (Network Basic Input Output System), and Windows Internet Name Service (WINS).

Functions of the Session Layer
Functions of the Session Layer

Some standard functions of the session layer include :

  • Setting up and tearing down communication sessions between devices.

  • Synchronizing the flow of data between devices.

  • Resuming communication after a temporary interruption or fault.

  • Negotiating the options and parameters for a communication session.

  • Managing access to shared resources during a communication session.

4. The Transport Layer

  • The transport layer provides end-to-end communication services and error recovery for the application layer. It includes protocols like TCP (Transmission Control Protocol) and (UDP) User Datagram Protocol that provides error correction, flow control, and data segmentation and reassembly.

  • Every protocol uses a unique decimal number to ensure that the data is sent and received on the intended application as it passes through the network or Internet.

Functions of the Transport Layer
Functions of the Transport Layer
  • TCP is a connection-oriented protocol that guarantees the delivery of the message, while UDP is a connectionless protocol that sends the data without error correction. Under the TCP and UDP are port numbers used to distinguish the specific type of application.

3. The Network Layer

  • The network layer is responsible for routing data between different networks. It includes protocols like (IP) Internet Protocol, (IPX) Internetwork Packet Exchange, and AppleTalk. These protocols provide the necessary functions for routing data across a network and ensuring it reaches its destination.

  • It is responsible for determining the best path for data as it travels from its source to its destination. The network layer also assigns logical addresses to devices on the network, which are used to identify the devices and route data to them.

Functions of the Network Layer
Functions of the Network Layer
  • The network layer is often considered the "heart" of the OSI model because it plays a central role in the operation of a network. It is a critical component of modern computer networks and is essential for allowing devices to communicate with each other and exchange information.

2. The Data Link Layer

  • The data link layer links two devices on the same physical network, such as a local area network (LAN). It ensures that data is transmitted correctly and without errors.

  • It includes protocols like (SDLC) Synchronous Data Link Protocol, (HDLC) High-Level Data Link Protocol, (SLIP)Serial Line Interface Protocol, (PPP)Point - to - Point Protocol, (LCP) Link Control Protocol, and (NCP) Network Control Protocol.

  • This layer comprises two parts—Logical Link Control (LLC), which identifies network protocols, performs error checking, and synchronizes frames. Media Access Control (MAC) uses MAC addresses to connect devices and define permissions to transmit and receive data.

Functions of the Data Link Layer
Functions of the Data Link Layer
  • Overall, the data link layer is crucial in ensuring data's reliable and efficient transmission over a network.

1. The Physical Layer

  • The physical layer is responsible for transmitting raw data over a communication channel, including the hardware, cables, and other components that make up the network.

  • It defines the physical characteristics of the communication channel, including the signaling used, the frequency range, and the data rate.

Functions of the Physical Layer
Functions of the Physical Layer
  • The physical layer ensures that data is transmitted accurately and reliably from one device to another.


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