What is OSI Model in Computer Networking?

The Open Systems Interconnect (OSI) model is a conceptual framework that describes networking or telecommunications systems as seven layers, each with its own function.

The layers help network pros visualize what is going on within their networks and can help network managers narrow down problems (is it a physical issue or something with the application?), as well as computer programmers (when developing an application, which other layers does it need to work with?). Tech vendors selling new products will often refer to the OSI model to help customers understand which layer their products work with or whether it works “across the stack”. 

Characteristics of OSI Model:

OSI Model

  • The OSI model is divided into two layers: upper layers and lower layers.
  • The upper layer of the OSI model mainly deals with the application related issues, and they are implemented only in the software. The application layer is closest to the end user. Both the end user and the application layer interact with the software applications. An upper layer refers to the layer just above another layer.
  • The lower layer of the OSI model deals with the data transport issues. The data link layer and the physical layer are implemented in hardware and software. The physical layer is the lowest layer of the OSI model and is closest to the physical medium. The physical layer is mainly responsible for placing the information on the physical medium.   

OSI Model Explained: The OSI 7 Layers

OSI 7 layers

We’ll describe OSI layers “top down” from the application layer that directly serves the end user, down to the physical layer. 

1. Physical Layer 

The Physical Layer, which represents the system’s electrical and physical representation, is at the bottom of our OSI model. This can range from the kind of connection used to the radio frequency connectivity (such as in a Wi-Fi network) to the pin arrangement, voltages, and other physical specifications. When a networking issue arises, a lot of networking experts jump directly to the physical layer to make sure that all the cables are connected correctly and, for example, that the router, switch, or computer’s power plug hasn’t been pulled.

Functions of a Physical layer:

  • Line Configuration: It defines the way how two or more devices can be connected physically.
  • Data Transmission: It defines the transmission mode whether it is simplex, half-duplex or full-duplex mode between the two devices on the network.
  • Topology: It defines the way how network devices are arranged.
  • Signals: It determines the type of the signal used for transmitting the information. 

2. Data Link Layer

The Data Link Layer provides node-to-node data transfer (between two directly connected nodes), and also handles error correction from the physical layer. Two sublayers exist here as well–the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. In the networking world, most switches operate at Layer 2. But it’s not that simple. Some switches also operate at Layer 3 in order to support virtual LANs that may span more than one switch subnet, which requires routing capabilities. 
  • It contains two sub-layers:
    • Logical Link Control Layer
      • It is responsible for transferring the packets to the Network layer of the receiver that is receiving.
      • It identifies the address of the network layer protocol from the header.
      • It also provides flow control.
    • Media Access Control Layer
      • A Media access control layer is a link between the Logical Link Control layer and the network’s physical layer.
      • It is used for transferring the packets over the network. 

Functions of the Data-link layer

  • Framing: The data link layer translates the physical’s raw bit stream into packets known as Frames. The Data link layer adds the header and trailer to the frame. The header which is added to the frame contains the hardware destination and source address.
OSI Model

  • Physical Addressing: The Data link layer adds a header to the frame that contains a destination address. The frame is transmitted to the destination address mentioned in the header.
  • Flow Control: Flow control is the main functionality of the Data-link layer. It is the technique through which the constant data rate is maintained on both the sides so that no data get corrupted. It ensures that the transmitting station such as a server with higher processing speed does not exceed the receiving station, with lower processing speed.
  • Error Control: Error control is achieved by adding a calculated value CRC (Cyclic Redundancy Check) that is placed to the Data link layer’s trailer which is added to the message frame before it is sent to the physical layer. If any error seems to occurr, then the receiver sends the acknowledgment for the retransmission of the corrupted frames.
  • Access Control: When two or more devices are connected to the same communication channel, then the data link layer protocols are used to determine which device has control over the link at a given time. 

3.  Network Layer

The network layer has two main functions. One is breaking up segments into network packets, and reassembling the packets on the receiving end. The other is routing packets by discovering the best path across a physical network. The network layer uses network addresses (typically Internet Protocol addresses) to route packets to a destination node. 

Functions of Network Layer:

  • Internetworking: An internetworking is the main responsibility of the network layer. It provides a logical connection between different devices.
  • Addressing: A Network layer adds the source and destination address to the header of the frame. Addressing is used to identify the device on the internet.
  • Routing: Routing is the major component of the network layer, and it determines the best optimal path out of the multiple paths from source to the destination.
  • Packetizing: A Network Layer receives the packets from the upper layer and converts them into packets. This process is known as Packetizing. It is achieved by internet protocol (IP). 

4. Transport Layer 

The Transport Layer deals with the coordination of the data transfer between end systems and hosts. How much data to send, at what rate, where it goes, etc. The best known example of the Transport Layer is the Transmission Control Protocol (TCP), which is built on top of the Internet Protocol (IP), commonly known as TCP/IP. TCP and UDP port numbers work at Layer 4, while IP addresses work at Layer 3, the Network Layer.

The two protocols used in this layer are:

  • Transmission Control Protocol
    • It is a standard protocol that allows the systems to communicate over the internet.
    • It establishes and maintains a connection between hosts.
    • When data is sent over the TCP connection, then the TCP protocol divides the data into smaller units known as segments. Each segment travels over the internet using multiple routes, and they arrive in different orders at the destination. The transmission control protocol reorders the packets in the correct order at the receiving end.
  • User Datagram Protocol
    • User Datagram Protocol is a transport layer protocol.
    • It is an unreliable transport protocol as in this case receiver does not send any acknowledgment when the packet is received, the sender does not wait for any acknowledgment. Therefore, this makes a protocol unreliable.

Functions of Transport Layer:

  • Service-point addressing: Computers run several programs simultaneously due to this reason, the transmission of data from source to the destination not only from one computer to another computer but also from one process to another process. The transport layer adds the header that contains the address known as a service-point address or port address. The responsibility of the network layer is to transmit the data from one computer to another computer and the responsibility of the transport layer is to transmit the message to the correct process.
  • Segmentation and reassembly: When the transport layer receives the message from the upper layer, it divides the message into multiple segments, and each segment is assigned with a sequence number that uniquely identifies each segment. When the message has arrived at the destination, then the transport layer reassembles the message based on their sequence numbers.
  • Connection control: Transport layer provides two services Connection-oriented service and connectionless service. A connectionless service treats each segment as an individual packet, and they all travel in different routes to reach the destination. A connection-oriented service makes a connection with the transport layer at the destination machine before delivering the packets. In connection-oriented service, all the packets travel in the single route.
  • Flow control: The transport layer also responsible for flow control but it is performed end-to-end rather than across a single link.
  • Error control: The transport layer is also responsible for Error control. Error control is performed end-to-end rather than across the single link. The sender transport layer ensures that message reach at the destination without any error. 

5. Session Layer 

The session layer creates communication channels, called sessions, between devices. It is responsible for opening sessions, ensuring they remain open and functional while data is being transferred, and closing them when communication ends. The session layer can also set checkpoints during a data transfer—if the session is interrupted, devices can resume data transfer from the last checkpoint.

Functions of Session layer:

  • Dialog control: Session layer acts as a dialog controller that creates a dialog between two processes or we can say that it allows the communication between two processes which can be either half-duplex or full-duplex.
  • Synchronization: Session layer adds some checkpoints when transmitting the data in a sequence. If some error occurs in the middle of the transmission of data, then the transmission will take place again from the checkpoint. This process is known as Synchronization and recovery. 

6. Presentation Layer   

The area that is unrelated to data representation at the application layer is represented by the Presentation Layer. In general, it denotes the preparation, translation, or conversion of application format to network format or vice versa. To put it another way, the layer “presents” data to the network or the application. Data encryption and decryption for safe transmission is an excellent illustration of this; this takes place at Layer 6.

Functions of Presentation layer:

  • Translation: The processes in two systems exchange the information in the form of character strings, numbers and so on. Different computers use different encoding methods, the presentation layer handles the interoperability between the different encoding methods. It converts the data from sender-dependent format into a common format and changes the common format into receiver-dependent format at the receiving end.
  • Encryption: Encryption is needed to maintain privacy. Encryption is a process of converting the sender-transmitted information into another form and sends the resulting message over the network.
  • Compression: Data compression is a process of compressing the data, i.e., it reduces the number of bits to be transmitted. Data compression is very important in multimedia such as text, audio, video. 

7. Application Layer  

The OSI model’s Application Layer is the one that is “closest to the end user.” It displays incoming data to the user after directly receiving information from users. Amazingly, apps do not exist at the application layer themselves. Instead, in order to communicate with apps on the other end, the layer supports communication through lower layers. Examples of communications that rely on Layer 7 are TelNet and FTP, web browsers (such as Google Chrome, Firefox, Safari, etc.), and TelNet.  

Functions of Application layer:

  • File transfer, access, and management (FTAM): An application layer allows a user to access the files in a remote computer, to retrieve the files from a computer and to manage the files in a remote computer.
  • Mail services: An application layer provides the facility for email forwarding and storage.
  • Directory services: An application provides the distributed database sources and is used to provide that global information about various objects.

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