The Open Systems Interconnection (OSI) Model

Without delving too deeply into the nitty-gritty of how computer networks operate, the OSI layer model will assist us in understanding the big picture.

Common Models

Computer networks may be arranged in a variety of ways. The Open Systems Interconnection (OSI) model and the Transmission Control Protocol/Internet Protocol (TCP/IP) model are the two most used types.

Starting with the OSI model, we will go into great depth about each model and how they vary from one another.

The OSI Model

In the 1970s, the Organization for Standardization created the OSI Model (ISO). The Internet was still in its adolescence at this point, and its protocols were still developing. The OSI model offers a standard for interoperability across various computer architectures.

The Layers of the OSI Model

A communication system is divided into seven abstract layers that are stacked on top of one another in the model.

The OSI Model’s seven levels are listed below.

The OSI Network Stack
The OSI Network Stack

Mnemonic to remember the Layers of The OSI Model Network Stack

All People Seem To Need Data Processing

Mnemonic to Remember OSI Model
Mnemonic to Remember OSI Model

These layers are used to organize the hardware and software components of network protocols, which may be implemented in hardware, software, or a mix of both. The major goal of this “network stack” is to comprehend how the various parts of these protocols fit together and interact.

Here are some of the main duties assigned to each tier. Please take note that we are just mentioning a portion of each layer’s duties. 

Application Layer

  • The majority of the time, software is used to implement these applications or protocols.
  • The application layer is accessed by end users.
  • The majority of end-user applications, including online surfing and email, are found at the application layer.
  • An outbound message’s travel begins at the application layer, which supplies information to the layer below.

Presentation Layer

  • Presents information so that the application layer may quickly understand it and show it.
  • An example of this presentation is Encoding. The character encoding used by the application layer may vary from that used by the underlying layers. The translation is handled by the presentation layer.
  • Additionally, encryption, which makes data only accessible by the intended recipients, is often carried out at this layer.
  • The presentation layer adapts content presentation to the application, assuming that a user session is being maintained by the lower levels.
  • To reduce network traffic, the presentation layer may also use end-to-end compression.

Session Layer

  • The task of the session layer is to develop a service that handles user sessions on top of the transport layer’s services.
  • Session layer communications must go across the network to their destination through the transport layer. The mapping of messages given by the transport layer to the sessions must be handled by the session layer.
  • A session is a communication channel between local applications and remote services running on different end systems.
  • For instance, A customer could engage with an e-commerce site for one session, during which they search, explore, and choose goods before making a purchase and logging out.
  • The session layer presumes that the layers below it handle connection setup and packet transfer.

Transport Layer

  • Protocols are also mostly implemented in software at the transport layer.
  • The transport layer divides the data into smaller pieces since the application, display, and session layers may be transmitting big amounts of data. Depending on the protocol being utilized, these pieces are referred to as datagrams or segments.
  • Additionally, sometimes extra data is needed to successfully send the segment or datagram. This data is added to the segment or datagram by the transport layer.
  • A good illustration of this is the checksum, which helps verify that the message is delivered accurately to its intended location and isn’t damaged or altered along the way.
  • A header is what’s appended at the beginning of a segment or datagram to provide further information.
  • A trailer is a piece of information that is added towards the end.

Network Layer

  • Packets are the name given to network layer messages.
  • They make it easier for packets to be sent between end systems and aid in choosing the most efficient paths for messages to go between systems.
  • Routing protocols are programs that operate at the network layer and communicate with one another to share knowledge that enables them to route messages at the transport layer.
  • In a particular network, there are several connections (copper wire, optical fiber, wireless), and one goal of the network layer is to maintain a nearly equal use of all of them. Otherwise, there would be doubts about the financial viability of installing and maintaining them if certain connections are underutilized.

Data Link Layer

  • Allows hosts that are directly linked to interact. On a physical medium, these hosts may sometimes be the sole two objects. In such a situation, flow control and error detection/correction are problems that this layer attempts to solve.
  • Packets are encapsulated for transmission over a single connection.
  • Resolves transmission conflicts, which occur when two end systems deliver a message via a single connection at the same time.
  • Addressing is another link layer issue if the data link is a broadcast medium.
  • To combine their bandwidths, numerous data lines may be multiplexed into a single entity.
  • Similar to this, sometimes we divide a single data connection into virtual data links that seem like different network interfaces.

Physical Layer

  • Mostly made up of hardware.
  • Offers a reliable mechanical and electrical channel for data transmission.
  • Bits are sent. Not logical datagrams, packets, or segments.
  • Needs to cope with mechanical requirements related to the construction of the cables and the connections.

This way of conceptualizing networks as layers had a big influence on networking education, which is why many courses still use it as their main paradigm.

A more protocol-oriented method is, however, a superior way to educate networks now that protocols have developed. This is where the TCP/IP model enters the picture, which is what we are going to see in the next article.

Gaurav Karwayun is the founder and editor in chief of CodeIntelligent. He has over 10+ Years of Experience in the software industry. He has experience working in both service and product-based companies. He has vast experience in all popular programming languages, DevOps, Cloud Computing, etc. Follow him on Twitter.

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