Open In App

What is OSI Model? – Layers of OSI Model

Last Updated : 06 May, 2024
Improve
Improve
Like Article
Like
Save
Share
Report

OSI stands for Open Systems Interconnection, where open stands to say non-proprietary. It is a 7-layer architecture with each layer having specific functionality to perform. All these 7 layers work collaboratively to transmit the data from one person to another across the globe. The OSI reference model was developed by ISO – ‘International Organization for Standardization‘, in the year 1984.

The OSI model provides a theoretical foundation for understanding network communication. However, it is usually not directly implemented in its entirety in real-world networking hardware or software. Instead, specific protocols and technologies are often designed based on the principles outlined in the OSI model to facilitate efficient data transmission and networking operations.

Prerequisite: Basics of Computer Networking

What is OSI Model?

The OSI model, created in 1984 by ISO, is a reference framework that explains the process of transmitting data between computers. It is divided into seven layers that work together to carry out specialised network functions, allowing for a more systematic approach to networking.

Table diagram of the OSI model illustrating communication between sender and receiver and the addition of headers to the transmitted data

What are the 7 layers of the OSI Model?

The OSI model consists of seven abstraction layers arranged in a top-down order:

  1. Physical Layer
  2. Data Link Layer
  3. Network Layer
  4. Transport Layer
  5. Session Layer
  6. Presentation Layer
  7. Application Layer

Physical Layer – Layer 1

The lowest layer of the OSI reference model is the physical layer. It is responsible for the actual physical connection between the devices. The physical layer contains information in the form of bits. It is responsible for transmitting individual bits from one node to the next. When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together.  

Data Bits in the Physical Layer

Functions of the Physical Layer

  • Bit synchronization: The physical layer provides the synchronization of the bits by providing a clock. This clock controls both sender and receiver thus providing synchronization at the bit level.
  • Bit rate control: The Physical layer also defines the transmission rate i.e. the number of bits sent per second.
  • Physical topologies: Physical layer specifies how the different, devices/nodes are arranged in a network i.e. bus, star, or mesh topology.
  • Transmission mode: Physical layer also defines how the data flows between the two connected devices. The various transmission modes possible are Simplex, half-duplex and full-duplex.

Note:

  1. Hub, Repeater, Modem, and Cables are Physical Layer devices. 
  2. Network Layer, Data Link Layer, and Physical Layer are also known as Lower Layers or Hardware Layers.

The data link layer is responsible for the node-to-node delivery of the message. The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer. When a packet arrives in a network, it is the responsibility of the DLL to transmit it to the Host using its MAC address
The Data Link Layer is divided into two sublayers:  

  1. Logical Link Control (LLC)
  2. Media Access Control (MAC)

The packet received from the Network layer is further divided into frames depending on the frame size of the NIC(Network Interface Card). DLL also encapsulates Sender and Receiver’s MAC address in the header. 

The Receiver’s MAC address is obtained by placing an ARP(Address Resolution Protocol) request onto the wire asking “Who has that IP address?” and the destination host will reply with its MAC address.  

Functions of the Data Link Layer

  • Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.
  • Physical addressing: After creating frames, the Data link layer adds physical addresses (MAC addresses) of the sender and/or receiver in the header of each frame.
  • Error control: The data link layer provides the mechanism of error control in which it detects and retransmits damaged or lost frames.
  • Flow Control: The data rate must be constant on both sides else the data may get corrupted thus, flow control coordinates the amount of data that can be sent before receiving an acknowledgment.
  • Access control: When a single communication channel is shared by multiple devices, the MAC sub-layer of the data link layer helps to determine which device has control over the channel at a given time.
Function of DLL

Note:

  1. Packet in the Data Link layer is referred to as Frame. 
  2. Data Link layer is handled by the NIC (Network Interface Card) and device drivers of host machines. 
  3. Switch & Bridge are Data Link Layer devices.

Network Layer – Layer 3

The network layer works for the transmission of data from one host to the other located in different networks. It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available. The sender & receiver’s IP addresses are placed in the header by the network layer. 

Functions of the Network Layer 

  • Routing: The network layer protocols determine which route is suitable from source to destination. This function of the network layer is known as routing.
  • Logical Addressing: To identify each device inter-network uniquely, the network layer defines an addressing scheme. The sender & receiver’s IP addresses are placed in the header by the network layer. Such an address distinguishes each device uniquely and universally.

Note:

  1. Segment in the Network layer is referred to as Packet
  2. Network layer is implemented by networking devices such as routers and switches.  

Transport Layer – Layer 4

The transport layer provides services to the application layer and takes services from the network layer. The data in the transport layer is referred to as Segments. It is responsible for the end-to-end delivery of the complete message. The transport layer also provides the acknowledgment of the successful data transmission and re-transmits the data if an error is found.

At the sender’s side: The transport layer receives the formatted data from the upper layers, performs Segmentation, and also implements Flow and error control to ensure proper data transmission. It also adds Source and Destination port numbers in its header and forwards the segmented data to the Network Layer. 

Note: The sender needs to know the port number associated with the receiver’s application. 

Generally, this destination port number is configured, either by default or manually. For example, when a web application requests a web server, it typically uses port number 80, because this is the default port assigned to web applications. Many applications have default ports assigned. 

At the receiver’s side: Transport Layer reads the port number from its header and forwards the Data which it has received to the respective application. It also performs sequencing and reassembling of the segmented data. 

Functions of the Transport Layer 

  • Segmentation and Reassembly: This layer accepts the message from the (session) layer, and breaks the message into smaller units. Each of the segments produced has a header associated with it. The transport layer at the destination station reassembles the message.
  • Service Point Addressing: To deliver the message to the correct process, the transport layer header includes a type of address called service point address or port address. Thus by specifying this address, the transport layer makes sure that the message is delivered to the correct process.

Services Provided by Transport Layer 

  1. Connection-Oriented Service
  2. Connectionless Service

1. Connection-Oriented Service: It is a three-phase process that includes

  • Connection Establishment
  • Data Transfer
  • Termination/disconnection

In this type of transmission, the receiving device sends an acknowledgment, back to the source after a packet or group of packets is received. This type of transmission is reliable and secure.

2. Connectionless service: It is a one-phase process and includes Data Transfer. In this type of transmission, the receiver does not acknowledge receipt of a packet. This approach allows for much faster communication between devices. Connection-oriented service is more reliable than connectionless Service.

Note:  

  1. Data in the Transport Layer is called Segments
  2. Transport layer is operated by the Operating System. It is a part of the OS and communicates with the Application Layer by making system calls. 
  3. The transport layer is called as Heart of the OSI model. 
  4. Device or Protocol Use : TCP, UDP  NetBIOS, PPTP

Session Layer – Layer 5

This layer is responsible for the establishment of connection, maintenance of sessions, and authentication, and also ensures security.

Functions of the Session Layer

  • Session establishment, maintenance, and termination: The layer allows the two processes to establish, use, and terminate a connection.
  • Synchronization: This layer allows a process to add checkpoints that are considered synchronization points in the data. These synchronization points help to identify the error so that the data is re-synchronized properly, and ends of the messages are not cut prematurely and data loss is avoided.
  • Dialog Controller: The session layer allows two systems to start communication with each other in half-duplex or full-duplex.

Note:

  1. All the below 3 layers(including Session Layer) are integrated as a single layer in the TCP/IP model as the “Application Layer”. 
  2. Implementation of these 3 layers is done by the network application itself. These are also known as Upper Layers or Software Layers. 
  3. Device or Protocol Use :  NetBIOS, PPTP.

For example:-

Let us consider a scenario where a user wants to send a message through some Messenger application running in their browser. The “Messenger” here acts as the application layer which provides the user with an interface to create the data. This message or so-called Data is compressed, optionally encrypted (if the data is sensitive), and converted into bits (0’s and 1’s) so that it can be transmitted.  

Communication in Session Layer

Communication in Session Layer

Presentation Layer – Layer 6

The presentation layer is also called the Translation layer. The data from the application layer is extracted here and manipulated as per the required format to transmit over the network. 

Functions of the Presentation Layer

  • Translation: For example, ASCII to EBCDIC.
  • Encryption/ Decryption: Data encryption translates the data into another form or code. The encrypted data is known as the ciphertext and the decrypted data is known as plain text. A key value is used for encrypting as well as decrypting data.
  • Compression: Reduces the number of bits that need to be transmitted on the network.

Note: Device or Protocol Use:  JPEG, MPEG, GIF

Application Layer – Layer 7

At the very top of the OSI Reference Model stack of layers, we find the Application layer which is implemented by the network applications. These applications produce the data to be transferred over the network. This layer also serves as a window for the application services to access the network and for displaying the received information to the user. 

Example: Application – Browsers, Skype Messenger, etc. 

Note: 1. The application Layer is also called Desktop Layer.  

          2.  Device or Protocol Use :  SMTP

Functions of the Application Layer

The main functions of the application layer are given below.

  • Network Virtual Terminal(NVT): It allows a user to log on to a remote host.
  • File transfer access and management(FTAM): This application allows a user to
    access files in a remote host, retrieve files in a remote host, and manage or
    control files from a remote computer.
  • Mail Services: Provide email service.
  • Directory Services: This application provides distributed database sources
    and access for global information about various objects and services.

Note: The OSI model acts as a reference model and is not implemented on the Internet because of its late invention. The current model being used is the TCP/IP model. 

What is the Flow of Data in OSI Model?

When we transfer information from one device to another, it travels through 7 layers of OSI model. First data travels down through 7 layers from the sender’s end and then climbs back 7 layers on the receiver’s end.

Let’s look at it with an Example:

Luffy sends an e-mail to his friend Zoro.

Step 1: Luffy interacts with e-mail application like Gmail, outlook, etc. Writes his email to send. (This happens in Layer 7: Application layer)

Step 2: Mail application prepares for data transmission like encrypting data and formatting it for transmission. (This happens in Layer 6: Presentation Layer)

Step 3: There is a connection established between the sender and receiver on the internet. (This happens in Layer 5: Session Layer)

Step 4: Email data is broken into smaller segments. It adds sequence number and error-checking information to maintain the reliability of the information. (This happens in Layer 4: Transport Layer)

Step 5: Addressing of packets is done in order to find the best route for transfer. (This happens in Layer 3: Network Layer)

Step 6: Data packets are encapsulated into frames, then MAC address is added for local devices and then it checks for error using error detection. (This happens in Layer 2: Data Link Layer)

Step 7: Lastly Frames are transmitted in the form of electrical/ optical signals over a physical network medium like ethernet cable or WiFi.

After the email reaches the receiver i.e. Zoro, the process will reverse and decrypt the e-mail content. At last, the email will be shown on Zoro’s email client.

Advantages of OSI Model

The OSI Model defines the communication of a computing system into 7 different layers. Its advantages include:

  • It divides network communication into 7 layers which makes it easier to understand and troubleshoot.
  • It standardizes network communications, as each layer has fixed functions and protocols.
  • Diagnosing network problems is easier with the OSI model.
  • It is easier to improve with advancements as each layer can get updates separately.

OSI Model – Layer architecture

Layer No

Layer Name

Responsibility

Information Form (Data Unit)

Device or Protocol

7 Application Layer Helps in identifying the client and synchronizing communication. Message SMTP
6 Presentation Layer Data from the application layer is extracted and manipulated in the required format for transmission. Message JPEG, MPEG, GIF
5 Session Layer Establishes Connection, Maintenance, Ensures Authentication and Ensures security. Message (or encrypted message) Gateway
4 Transport Layer Take Service from Network Layer and provide it to the Application Layer. Segment Firewall
3 Network Layer Transmission of data from one host to another, located in different networks. Packet Router
2 Data Link Layer Node to Node Delivery of Message. Frame Switch, Bridge
1 Physical Layer Establishing Physical Connections between Devices. Bits Hub, Repeater, Modem, Cables

OSI vs TCP/IP Model

Some key differences between the OSI model and the TCP/IP Model are:

  1. TCP/IP model consists of 4 layers but OSI model has 7 layers. Layers 5,6,7 of the OSI model are combined into the Application Layer of TCP/IP model and OSI layers 1 and 2 are combined into Network Access Layers of TCP/IP protocol.
  2. The TCP/IP model is older than the OSI model, hence it is a foundational protocol that defines how should data be transferred online.
  3. Compared to the OSI model, the TCP/IP model has less strict layer boundaries.
  4. All layers of the TCP/IP model are needed for data transmission but in the OSI model, some applications can skip certain layers. Only layers 1,2 and 3 of the OSI model are necessary for data transmission.

Did you Know?

TCP/IP protocol ( Transfer Control Protocol/Internet Protocol ) was created by U.S. Department of Defense’s Advanced Research Projects Agency (ARPA) in 1970s.

We have discussed about What is OSI model?, What are layers of OSI model, How data flows in the 7 layers of OSI model, and the differences between TCP/IP protocol and OSI protocol.

What is OSI Model? – FAQs

Is OSI layer still used?

Yes, the OSI model is still used by networking professionals to understand data abstraction paths and processes better.

What is the highest layer of the OSI model?

Layer 7 or Application layer is highest layer of OSI model.

What is layer 8?

Layer 8 doesn’t actually exist in the OSI model but is often jokingly used to refer to the end user. For example: a layer 8 error would be a user error.



Previous Article
Next Article

Similar Reads

Difference Between OSI Model and TCP/IP Model
Data communication is a process or act in which we can send or receive data. For data communication two models are available. We will discuss these two models in this article and also see the differences between these two articles. OSI ModelTCP/IP ModelOSI ModelOSI stands for Open Systems Interconnection. It has 7 layers Physical layer, Data Link l
3 min read
Similarities between TCP/IP model and OSI model
Pre-Requisite: Layers of OSI Model, TCP/IP Model OSI Model or Open Systems Interconnection is an architecture of 7 layers in which each layer has its work to perform whereas TCP/IP is a concise version of the OSI Model. TCP/IP Model consists of 4 layers rather than the 7 layers of the OSI Model. There are so many similarities between the TCP/IP mod
5 min read
This is exactly why we still use the OSI model when we have TCP/IP Model
What is the OSI Model?OSI is an acronym for Open Systems Interconnection. The International Organization for Standardization (ISO) created the OSI model (ISO). It's a model for how applications communicate over the internet. In order to facilitate interoperability between diverse devices and applications, the OSI model describes computing functions
10 min read
Critique of OSI Model and Protocols
Open System Interconnection (OSI) model is reference model that is used to describe and explain how does information from software application in one of computers moves freely through physical medium to software application on another computer. This model consists of total of seven layers and each of layers performs specific task or particular netw
4 min read
How Communication happens using OSI model
The Open System Interconnection (OSI) model is a standard “reference model” created by an International Organization for Standardization (ISO) to describe how software and hardware components involved in network communication divide efforts and interact with each other. The OSI model defines a seven-layer set of functional elements, from physical i
4 min read
Physical Layer in OSI Model
The physical Layer is the bottom-most layer in the Open System Interconnection (OSI) Model which is a physical and electrical representation of the system. It consists of various network components such as power plugs, connectors, receivers, cable types, etc. The physical layer sends data bits from one device(s) (like a computer) to another device(
5 min read
Application Layer in OSI Model
Prerequisite : OSI Model Introduction :The Application Layer is topmost layer in the Open System Interconnection (OSI) model. This layer provides several ways for manipulating the data (information) which actually enables any type of user to access network with ease. This layer also makes a request to its bottom layer, which is presentation layer f
6 min read
Presentation Layer in OSI model
Prerequisite : OSI Model Introduction : Presentation Layer is the 6th layer in the Open System Interconnection (OSI) model. This layer is also known as Translation layer, as this layer serves as a data translator for the network. The data which this layer receives from the Application Layer is extracted and manipulated here as per the required form
6 min read
Session Layer in OSI model
Prerequisite : OSI Layer Introduction :The Session Layer is the 5th layer in the Open System Interconnection (OSI) model. This layer allows users on different machines to establish active communications sessions between them. It is responsible for establishing, maintaining, synchronizing, terminating sessions between end-user applications. In Sessi
5 min read
Why Does the OSI Reference Model Matter?
OSI (Open Systems Interconnection) model is a reference model that describes the movement of information between software applications in different computer systems. It is a seven-layered model used in computer networking. It is useful for developing new standards while considering the existing networking standards. With dynamically changing needs
3 min read
Article Tags :