The OSI stands for Open Systems Interconnection, and it is made up of seven layers that computers use to communicate over a network. It is regarded as the first standard model for network communication, having been developed in the early 1980s.
Characteristics of the OSI Model
OSI models’ key characteristics are as follows:
- When specific levels of abstraction are required, a layer should be used.
- The function of each layer should be chosen in accordance with internationally accepted standards.
- There should be many layers so that different functions are not grouped together in the same layer. It should also be small enough to prevent the architecture from becoming overly complicated.
- Each layer in the OSI model performs primitive functions by relying on the layer below it. Every layer must be capable of providing services to the layer above it.
- Changes to one layer should not necessitate changes to other layers.
OSI Model Explained: The OSI 7 Layers
OSI model layers vs TCP/IP
- Application Layer
- Presentation Layer
- Session Layer
- Transport Layer
- Network Layer
- Data Link Layer
- Physical Layer
Application Layer
The application layer is mainly used by end-user software such as web browsers and email clients. This layer defines protocols that let the software to transfer the data while also presenting various data to users. Application layer protocols include the Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Post Office Protocol (POP), Simple Mail Transfer Protocol (SMTP), and Domain Name System (DNS).
Presentation Layer
The data preparation for the application layer is done by the presentation layer.It specifies how two devices should encode, encrypt, and compress data in order for it to be correctly received on the other end. The presentation layer prepares any data transmitted by the application layer for transmission over the session layer.
The presentation layer’s functions are as follows:
Translation: ASCII to EBCDIC conversion, for example.
Encryption/decryption: Data encryption converts data into a different form or code. The encrypted data is known as ciphertext, while the decrypted data is known as plain text. A key value is used to both encrypt and decrypt data.
Compression: This technique reduces the number of bits that must be transmitted over a network.
Session Layer
The session layer establishes communication channels, known as sessions, between devices. These layer functions include opening the session, keeping it open and functional while transferring data, and closing it down once the data has been successfully transferred. The session layer always defines checkpoints while transferring data, and devices can resume data transmission from the last checkpoint if any of the sessions is interrupted while transferring data.
The session layer’s functions are as follows:
Establishment, maintenance, and termination of sessions: The layer enables the two processes to create, use, and terminate a connection.
Synchronization: This layer enables a process to insert checkpoints into the data that serve as synchronisation points. These synchronisation points assist in identifying the error so that the data can be properly re-synchronized, message ends are not cut prematurely, and data loss is avoided.
Dialog Controller: The session layer enables two systems to begin communicating with each other in half-duplex or full-duplex mode.
Transport Layer
The Transport Layer is in charge of the two devices’ end-to-end communication. This includes transferring data from the session layer and segmenting it before sending it to layer 3. The receiving device’s transport layer is in charge of reassembling the segments into data that the session layer can consume.
The transport layer is also in charge of flow control and error handling. Flow control determines the best transmission speed to ensure that a sender with a fast connection does not overwhelm a receiver with a slow connection. On the receiving end, the transport layer handles error control by ensuring that the data received is complete and requesting retransmission if it isn’t.
The transport layer serves the following purposes:
- Segmentation and Reassembly:- This layer receives the message from the (session) layer and divides it into smaller units. Each segment produced has a header attached to it. The message is reassembled at the destination station by the transport layer.
- Service Point Addressing:- The transport layer header includes a type of address called service point address or port address in order to deliver the message to the correct process. The transport layer ensures that the message is delivered to the correct process by specifying this address.
Network Layer
The network layer has two primary functions. One approach is to divide segments into network packets, which are then reassembled on the receiving end. The other method is to route packets across a physical network by determining the best path.
The Network layer performs the following functions:
Routing: The network layer protocols determine which route from source to destination is best. This network layer function is also referred to as routing.
Logical Addressing: The network layer defines an addressing scheme in order to uniquely identify each device on the internetwork. The network layer inserts the IP addresses of the sender and receiver into the header. Such an address uniquely and universally distinguishes each device.
Data Link Layer
Directly connected nodes are used at the data link layer to perform node-to-node data transfer, where data is packaged into frames. Errors that may have occurred at the physical layer are also corrected by the data link layer.
Data Link Layer is again classified into two sublayers. They are:
- Logical Link Control (LLC)
- Media Access Control (MAC)
Logical Link Control Layer (LLCL)
- It is in charge of transferring packets to the Network layer of the receiving receiver.
- This layer extracts the network layer protocol address from the header.
- It also controls the flow.
Media Access Control (MAC)
- A Media access control layer connects the Logical Link Control layer to the physical layer of a network.
- It is used to send packets across the network.
Data Link Layer Functions
The Data Link layer executes the following functions:
Framing: Framing is a data link layer function. It allows a sender to send a set of bits that are meaningful to the receiver. The beginning and end of the frame can be marked with special bit patterns to achieve this.
Physical addressing: After creating frames, the Data link layer adds the sender’s and/or receiver’s physical addresses (MAC addresses) to the header of each frame.
Error control: The data link layer provides an error control mechanism that detects and retransmits damaged or lost frames.
Flow Control: Because the data rate on both sides must be constant or the data will be corrupted, flow control coordinates the amount of data that can be sent before receiving acknowledgement.
Access control: When multiple devices share a single communication channel, the MAC sub-layer of the data link layer assists in determining which device has control over the channel at any given time.
Physical Layer
It is the OSI’s lowest layer. It is in charge of the physical connection between the devices. The physical layer stores data in the form of bits. It is in charge of sending individual bits from one node to the next. When this layer receives data, it converts the received signal into 0s and 1s and sends them to the Data Link layer, which reassembles the frame.
Purposes of Physical Layer
The physical layer serves the following purposes:
- Bit synchronisation: The physical layer synchronises the bits by providing a clock. This clock drives both the sender and the receiver, allowing bit-level synchronisation.
- Bit rate control: The Physical layer also defines the transmission rate, or the number of bits sent per second.
- Physical topologies: The physical layer specifies how different devices/nodes are arranged in a network, such as a bus, star, or mesh topology.
- Transmission mode: The physical layer also defines how data flows between two connected devices. Transmission modes include simplex, half-duplex, and full-duplex modes are available.
Why is the OSI model important?
Although the modern Internet does not strictly adhere to the OSI Model (rather, it adheres to the simpler Internet protocol suite), the OSI Model is still extremely useful for troubleshooting network issues. The OSI Model can assist in breaking down problems and identifying their root causes, whether it involves a single user who is unable to connect their laptop to the Internet or a website that is down for thousands of users. A lot of unnecessary work can be avoided if the problem can be narrowed down to one specific layer of the model.
Advantages and Disadvantages of OSI model
Advantages of OSI model
- It’s a generic model that can be used to guide the creation of any network model.
- It’s a two-tiered model. Changes to one layer have little effect on the others if the interfaces between them do not change dramatically.
- It distinguishes services, interfaces, and protocols specifically. As a result, it is adaptable. Depending on the nature of the network, each layer’s protocols can be easily adjusted.
- Connection-oriented, as well as connectionless services, are supported.
Disadvantages of OSI model
- It is purely a theoretical model, which limits its practical application.
- The OSI model is extremely complicated.
- It is both slow and expensive.
- When it comes to deployment, the session layer and presentation layer serve very few purposes.