What is OSI Modal | 7 Layers Explained

Kushan Madhusanka
8 min readJan 16, 2023

OSI stands for Open Systems Interconnect. This is a model that describes networking or telecommunications systems as seven layers, each with its own function. This is not how it always was. The OSI model, which is well-known to most people today, was developed in the 1970s when computer networking was just starting to gain popularity. Two distinct models were combined in 1983 and published in 1984. The OSI model is often described from top to bottom, with the numbers ranging from Layer 7 to Layer 1. The following are the layers:

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data-Link Layer

Physical Layer

The OSI model is divided into two parts such as upper layers and lower layers. The OSI model’s upper layers primarily address concerns connected to applications, which are exclusively implemented in software. The layer closest to the end user is the application layer. The software applications are interacted with by both the end user and the application layer. The layer just above another layer is referred to as an upper layer. The OSI model’s lower layers deal with data transmission difficulties. Both hardware and software are used to implement the data connection layer and the physical layer. The lowest layer in the OSI model and the one nearest to the physical medium is the physical layer. Information is primarily placed on the physical medium via the physical layer.

Let’s take a look at one by one:

Physical Layer

The first and lowest layer of the OSI reference model is called the physical layer. It is in charge of managing the transmission of raw, unstructured bitstream data through a physical medium. It is primarily concerned with the setup of the physical connection to the network and does not deal with the data contained in bits. It functions as an interface between real hardware and signaling systems. It is in charge of creating, maintaining, and deactivating the physical connection between two separate computer devices in a network. This layer specifies the hardware components, topologies, wiring, transmission methods, wiring, frequencies, and pulse widths that are utilized to represent binary signals, among other things.

Functions of the Physical Layer

  • Topologies

It specifies how various computing devices in a network must be placed in relation to one another. There are various topologies including star, mesh, ring, and bus.

  • Configuration

The method used to connect two or more communication devices to a link is referred to as this. The physical communication line used to convey data from one device to another is known as a link. Two devices must be simultaneously linked to the same link in the same manner for communication to take place. There are two different configurations such as Point-to-Point Configuration and Multipoint Configuration

  • Transmission Modes

It specifies the transmission direction between two devices. Simplex, Half-Duplex, and Full-Duplex modes are different types of transmission.

  • Bits synchronization

It has to do with bit synchronization. This indicates that a clock is used to synchronize the transmitter and receiver at the bit level. This clock provides synchronization at the bit level by controlling both the sender and the receiver.

  • Bits rate control

The physical layer maintains the flow of data rate, i.e the number of bits sent per second from the sender to a receiver.

  • Baseband and Broadband transmissions

Broadband sends analog signals via optical fibers and twisted cables while baseband uses a physical medium, such as wires, to carry digital messages.

Data-Link Layer

The second tier of the seven-layer OSI model of computer networking is the data link layer. This is the protocol layer that transfers data between network nodes in a local area network(LAN) or a wide area network (WAN).

One of the levels with the most liabilities and functionalities is the data link layer. The data link layer poses as the communication medium to the top layer while concealing the underlying hardware features. Bit-by-bit signal conversion and transmission across the underlying hardware are the responsibilities of the data link layer. The data connection layer receives data from hardware at the receiving end in the form of electrical impulses, puts it together into a recognizable frame format, and sends it to the top layer.

Delivering frames between LAN-connected devices falls under the purview of the data connection layer. The Data Link Layer’s main duty is to send the datagram over a specific link. It accomplishes this by segmenting the input data into data frames, which are typically composed of a few hundred or a few thousand bytes, and broadcasting the frames one after the other. If the service is dependable, the receiver sends back an acknowledgment frame after each frame to confirm reception.

Some Data-link protocols :

  • Synchronous Data Link Protocol (SDLC)
  • High-Level Data Link Protocol (HDLC)
  • Serial Line Interface Protocol (SLIP)
  • Point-to-Point Protocol (PPP)
  • Link Control Protocol (LCP)
  • Link Access Procedure (LAP)
  • Network Control Protocol (NCP)

Functions of Data Link Layer

  • Framing and link access
  • Reliable delivery
  • Flow control
  • Error control
  • Physical Addressing
  • Multi-access

Network Layer

The network layer is Layer 3 in the OSI paradigm. Managing sub-networks, internetworking, and host and network addressing are all responsibilities of the network layer.
A component of online communications called the network layer enables the connection and transmission of data packets between various devices or networks. Data is delivered in the form of packets to the receiving device through logical network channels under the network layer’s control.
With its basic capabilities, Layer 3 can offer such functions as:

  • management of service quality
  • Link management and load balancing
  • Security
  • Relationships between various protocols and subnets with various schemas.
  • Logical network architecture over physical network architecture.
  • It is possible to create end-to-end dedicated connectivity using L3 VPN and tunnels.

Routing is the primary emphasis of network layer devices. Routing can involve a variety of tasks that all work toward the same objective. These include:

  • Addressing devices and networks.
  • Populating routing tables or static routes.
  • Queuing incoming and outgoing data and then forwarding them according to quality of service constraints set for those packets.
  • Internetworking between two different subnets.
  • Delivering packets to a destination with the best efforts.
  • Provides connection-oriented and connectionless mechanisms.

Transport Layer

The transport layer, which is located at Layer 4 of the OSI modal, guarantees the timely delivery of messages through a network and offers mechanisms for error-checking and data flow management. Application message segments are transmitted by the transport layer into Layer 3, the networking layer. Once the segments are in the receiving side’s possession, they are put back together into messages and sent to Layer 7, or the application layer.
Transmission Control Protocol (TCP) and User Data Protocol (UDP) are the tools that Layer 4 (the transport layer) employs to complete its functions.

What services can be offered by the transport layer?

  • Connection-Oriented Communication

Before data is transmitted, devices at the endpoints of a network communication create a handshake protocol, such as TCP, to confirm a connection is secure.

  • Same Order Delivery

Gives each packet a number, ensuring that they are always delivered in a precise order.

  • Data Integrity

Checksums can be used to guarantee data integrity throughout all delivery tiers. These checksums provide the assurance that the data transferred and received are identical and free from corruption.

  • Flow Control

By controlling data flow, flow control makes sure that the data is provided at a rate that is suitable for both parties.

  • Multiplexing

When numerous internet browsers are opened on the same computer, multiplexing enables the usage of many apps at once via a network.

  • Traffic Control

Almost every component of a network may be impacted by this network congestion. The transport layer is able to recognize the signs of overloaded nodes and decreased flow rates and take the necessary action to address these problems.

  • Byte orientation

The transport layer permits the transfer of byte-oriented data streams if necessary. Some applications prefer to accept byte streams rather than packets.

Session Layer

The fifth layer in the OSI modal, the session layer, manages connections between several machines. The sessions layer keeps track of the computer-to-computer conversations, commonly known as sessions. The sessions between local and remote applications are established, managed, and terminated by this layer.
By starting the opening and closure of sessions between end-user application processes, the session layer controls a session. This layer interacts directly with both the presentation and the transport layers and manages either a single connection or numerous connections for each end-user application. Remote procedure calls are typically used in application contexts to implement the services provided by the session layer (RPCs).

Zone Information Protocol (ZIP), the AppleTalk protocol that manages the name binding procedure, and Session Control Protocol (SCP), the DECnet Phase IV session-layer protocol, are two other examples of session-layer implementations.

Session layer functions:

  • Establish a connection between the session entities
  • Map the session address to the shipping address.
  • Select the required transport quality of service(QOS) parameters.
  • Negotiate the session parameters.
  • Identify each session connection.
  • Transmit limited transparent user data.
  • Data transfer phase.
  • Connection release

Presentation Layer

In the OSI model, the presentation layer is sometimes referred to as the translation layer. The data is delivered to the OSI presentation layer, which formats and interprets it to ensure that data representation in the application layer and throughout the system is appropriate and consistent. The presentation layer, for instance, resolves any inconsistencies in syntax within the data that would interfere with the end-user system. Due to its handling of encryption, decryption, and the transformation of complex data into flat-byte strings, an easily transmittable format, the presentation layer is a crucial layer. Protocols and architecture are the two main networking aspects that the presentation layer of the OSI model controls. This layer completes data compression, decompression, encryption, and decryption.

Presentation layer functions:

  • Network security and confidentiality management, text compression, and packaging
  • Syntax conversion
  • Grammar negotiation
  • Connection management

Presentation layer protocols

  • Apple Filing Protocol (AFP)
  • Independent Computing Architecture (ICA), the Citrix system core protocol
  • Lightweight Presentation Protocol (LPP)
  • NetWare Core Protocol (NCP)
  • Network Data Representation (NDR)
  • Telnet (a remote terminal access protocol)
  • Tox, The Tox protocol is sometimes regarded as part of both the presentation and application layer
  • eXternal Data Representation (XDR)
  • 25 Packet Assembler/Disassembler Protocol (PAD)

Application Layer

In the OSI model, the application layer is the topmost layer. The shared communications protocols and interface techniques used by hosts in a communications network are specified at the application layer, which is an abstraction layer. Its proximity to the end user suggests that the application layer and the user may have direct communication with the software program.

Functions of the Application layer:

  • This layer allows users to log on as remote hosts.
  • The Application Layer provides protocols that allow the software to communicate and receive data and finally present it to users in a meaningful way.
  • This layer acts as a window via which users and application processes can access network resources.
  • The Application Layer provides various facilities for users to forward multiple emails and a storage facility.
  • This layer provides services such as email, file transfer, results distribution, directory services, network resources, etc.
  • This application layer generally performs host initialization followed by remote login to hosts.
  • This layer visualizes data, allowing individuals to grasp it rather than memorize it or see it in binary format (1s or 0s).
  • The application layer communicates with the operating system and guarantees that data is properly saved.
  • This layer allows users to interact with other software applications.

Application layer protocols:

  • HTTP
  • SMTP
  • TELNET
  • FTP
  • TFTP
  • NFS
  • DNS
  • DHCP

Here we have come to the end of the article. I have mentioned what the OSI modal is and its layers. Then I discussed each layer one by one. Hope you have got a better understanding.

See you in the next blog post. Bye Bye🍻🍸❤️❤️

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Kushan Madhusanka

Undergraduate of University of Moratuwa | Faculty of Information Technology