Table of Contents
What is the OSI model?
An OSI is the Open Systems Interconnection (OSI) model is the standard that covers all aspects of network communications from ISO. It was first introduced in the late 1970s. It has 7 layer architecture with each layer performing a particular task.
The OSI model is a model that defines a process of both, transmitting data from the sender and receiving data from the receiver. It is a 7 layered model in which each layer performs a subset of the required communication functions. Each layer relies on the next lower layer to perform more primitive functions and then this layer provides service to the next higher layer.
7 layers of the OSI model
The physical layer is the lowest layer of the OSI model. It contains the information in the form of bits and this layer is responsible for movements of individual bits from one hop (node) to the next in the form of 0’s and 1’s.
The functions performed by the physical layer are :
- Representation of bits
- Data rate: The layer defines the transmission rate also known as the Data rate or Bit rate
- Synchronization of bits: The layer synchronizes the bits at each bit level using a clock.
- Line configuration: It provides the line configuration of the network. The various line configurations are point-to-point or multipoint.
- Physical topology: The layer provides information about the topology of networks such as mesh, star, ring, or bus
- Transmission mode: It specifies the mode of transmission of data such as simplex, half-duplex, or duplex.
Data Link Layer
This is the second layer of the OSI model after the physical layer. It is responsible for the moving of messages in the form of frames from one hop(node) to the next. Once the message is received in a network, the Data link layer transmits the message to the host using the MAC address
The functions of the data link layer are:
- Framing: It is the process in which a special bit pattern is attached to the beginning and the ending of the frame. By doing this, the sender can transmit the set of bits to the receiver.
- Physical addressing: After the framing process, this layer adds the physical address in the form of a MAC address in the header of the frame
- Flow control: This functionality in the layer synchronizes the amount of data sent and received because the rate at which data is transmitted should be constant at the sender’s side and receiver’s end.
- Error control: This function enables the layer to detect the error in a particular frame and then retransmit the frame.
- Access control: Whenever the communication system is connected to multiple devices. The data link layer helps to determine which device to communicate with which device at a particular time.
It is the layer functioning after the Data link layer which works for the delivery of individual packets from the source host to the destination host through the shortest path available. The IP address for the source and the destination is available in the header of the layer.
Functions performed by the Network layer are :
- Logical addressing: The network layer performs a logical addressing task by identifying every system uniquely as the IP address of the source and the destination is already placed in the header of the address.
- Routing: The layer identifies the shortest route to transmit the data from the source to the destination.
This layer takes service from the network layer. The transport layer is responsible for the delivery of a message from one process to another. It is referred to as process-to-process delivery or End-to-End delivery. The layer also verifies whether the data is successfully transmitted otherwise the data is retransmitted.
Functions performed by the transport layer are :
- Port addressing: The port address of the source and destination is included in the header of the transport layer. This will ensure the correct transportation process.
- Segmentation and reassembly: after receiving the message from the earlier layer, this layer will divide the message into smaller units known as segments and the same process is reversed at the receiver’s end.
- Flow control
- Error control
In this layer, various services are performed such as :
Connection control: Connection control is mainly divided into
- Connection-oriented: In this transmission, After receiving a packet or set of packets, the receiving device replies with an acknowledgment to the source.
- Connectionless: The recipient of a packet sent in this manner does not acknowledge receipt of it. Compared to connection-oriented communication, it offers faster communication but is less dependable and secure.
This layer is placed after the transport layer, which is responsible for the dialog control and synchronization between the communicating system.
Functions performed by the Session layer are :
Dialog control: Due to the session layer, two systems can establish communication with one another
Synchronization (checkpoints): This layer adds checkpoints to a process. These data synchronization points aid in locating the problem so that the data can be correctly unsynchronized.
The presentation layer is responsible for translation, compression, and encryption. This layer receives the data from the application layer and translates the data as the rules and requirements.
Functions of the Presentation Layer :
Translation: EBCDIC-coded text file → ASCII-coded file
Encryption and Decryption: This layer translates the data into another form of the code. which enables Data encryption and data decryption using a Key value.
Compression: The number of bits from the data is reduced and then transmitted.
The topmost layer of the OSI model is responsible for providing services to the user. In addition, this layer acts as a window through which application services can access the network and show the user the data they have received.
Functions of Application layer :
Network virtual terminal (Remote log-in): This layer allows the user to log in remotely
File transfer and access: It allows the user to access, manage, and transfer files from a remote device.
Mail services: It allows various mail services.
Directory services (Distributed Database): It allows access to global information from worldwide about various topics.
Accessing the World Wide Web: The user can access the World Wide Web anywhere.
Advantages of OSI model
Modularity: The networking process is divided into discrete layers by the OSI model, each of which performs a particular function. Because each layer can be developed and updated independently, networks are easier to comprehend and design thanks to their modularity.
Interoperability: By clearly defining interfaces and protocols at each layer, the OSI model facilitates interoperability. This guarantees effective communication between various network devices and systems, regardless of their vendor.
Standardization: Network communication can be standardized with the aid of OSI. For network hardware and software to speak the same language, standardization is essential.
Troubleshooting: Troubleshooting is made easier with the layered approach. It is quicker to detect and fix issues when they arise because it is simpler to identify the precise layer where the problem is located.
Disadvantages of the OSI model
Complexity: The OSI model is rather complicated, and people who are unfamiliar with networking in particular may find it difficult to understand all seven layers and their purposes.
Real-world implementation: Although the OSI model offers a theoretical framework, real-world networking technologies and protocols don’t always fit neatly into its seven-layer architecture. Since many network protocols are multilayered, it is difficult to map them exactly to the OSI model.
Overhead: In actuality, overhead can be introduced by implementing all seven layers of the OSI model since each layer increases the amount of data that is transferred during communication. Network performance may be impacted by this.