All About OSI Model
9 December, 2022
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Open Systems Interconnection is what OSI stands for. It was created in 1984 by the ISO, or International Organization for Standardization. There are seven layers in all, and each layer is responsible for a certain set of tasks. To transport data from one person to another across the world, all 7 of these layers cooperate.
The simplified TCP/IP model, rather than OSI, is the foundation of the modern Internet. The OSI 7-layer model is still extensively used, nevertheless, as it aids in both isolating and troubleshooting networking issues as well as helping to depict and convey how networks function.
7 Layers Of OSI Model
The seven OSI layers are present. Each layer has a particular purpose. Below is a list of the seven layers:
1. Physical Layer- Provides a physical medium through which bits are transmitted
2. Data Link Layer- Used for error-free transfer of data frames
3. Network Layer- Responsible for moving the packets from source to destination
4. Transport Layer- Provides reliable massage delivery from process to process
5. Session Layer- Establish, manage & terminate the session
6. Presentation Layer- Responsible for translation, compression & encryption
7. Application Layer- Provide services to the user
1. Physical Layer
The physical layer is the bottom layer in the OSI reference model. The physical layer is in charge of the wired or wireless connections that physically connect network nodes. Information in the form of bits is present in the physical layer. It is in charge of sending certain bits from one node to the next. As soon as data is received, this layer takes the signal and converts it into 0s and 1s before sending them to the Data Link layer, which rejoins the frame.
Reference: https://static.javatpoint.com/tutorial/computer-network/images/osi-model3.png
Functions Of the Physical Layer
1. Bit synchronization: A clock is provided by the physical layer, which synchronizes the bits. This clock regulates both the sender and the receiver, enabling bit-level synchronization.
2. Topologies: It specifies the topology, such as bus, star, or mesh, in which network devices are placed.
3. Data Transmission: It chooses the signal type that will be utilized to send data between two connected devices. There are three different transmission modes: simplex, half-duplex, and full-duplex.
2. Data Link Layer (DLL)
The data link layer creates and breaks connections between two network nodes that are physically close to one another. Frames are created from packets, which are then sent from source to destination. This layer's primary responsibility is to provide error-free data flow from one node to another over the physical layer. It is the task of DLL to transmit a packet to the host using its MAC address when it enters a network.
Reference: https://static.javatpoint.com/tutorial/computer-network/images/osi-model4.png
Data Link Layer has two lower tiers:
1. Logical Link Control:
It is in charge of transmitting the packets to the receiving receiver's Network layer. It extracts the network layer protocol's address from the header. It also controls the flow.
2. Media Access Control
The logical link control layer and the physical layer of the network are connected by a media access control layer. It is employed to move packets across the network.
Functions Of Data Link Layer
1. Framing: The raw bit stream from the physical layer is converted into frames by the data link layer. By including unique bit patterns at the start and end of the frame, this can be achieved.
2. Physical Addressing: The Data link layer adds the physical addresses (MAC addresses) of the sender and/or recipient to each frame's header after constructing it. The destination address listed in the header is where the frame is sent.
3. Error Control: The error control mechanism provided by the data connection layer allows for the detection and retransmission of broken or missing frames. A calculated value CRC (Cyclic Redundancy Check) is added to the message frame's trailer in the data link layer before it is delivered to the physical layer to control errors.
4. Flow Control: The Data-link layer's primary role is flow control. It is a technique used to maintain a constant communication flow on both ends, preventing data corruption. The amount of data that can be supplied before getting acknowledgment is coordinated by flow control.
5. Access Control: The MAC sub-layer of the data link layer assists in determining which device has control over the channel at any one time when two or more devices are connected to the same communication channel.
3. Network Layer
The network layer facilitates the transfer of data between hosts that are part of various networks. It also handles packet routing, which is the choice of the shortest route from a variety of options to transmit the packet. In order to route packets to a destination node, the network layer needs network addresses, which are commonly Internet Protocol addresses. The network layer inserts the IP addresses of the sender and receiver in the header.
Reference: https://static.javatpoint.com/tutorial/computer-network/images/osi-model6.png
Functions Of Network Layer
1. Logical Addressing: The network layer inserts the IP addresses of the sender and receiver in the header. Each device can be identified individually and universally by such an address.
2. Packetizing: It decapsulates the payload from the network layer packet at the destination after encapsulating the data received from upper levels of the network (also referred to as payload) at the source. Internet Protocol is used to accomplish that (IP).
3. Routing: Routing, a key element of the network layer, selects the optimum path from among the many options for getting from one place to another.
4. Transport Layer
The transport layer transfers services from the network layer to the application layer. Segments are the name of the data in the transport layer. As it offers a point-to-point connection between source and destination to deliver the data stably and retransmits the data if a mistake is discovered, this layer can be referred to as an end-to-end layer.
Reference: https://static.javatpoint.com/tutorial/computer-network/images/osi-model7.png
In this tier, two protocols are employed:
1. Transmission Control Protocol:
It is a common protocol that enables internet-based communication between the systems. It creates and keeps a connection open between hosts. Each segment uses a different route to cross the internet, and they arrive at their destination in different sequences. At the receiving end, the transmission control protocol reorders the packets in the proper order.
2. User Datagram Protocol
A transport layer protocol is called User Datagram Protocol. It is an unreliable transport protocol because, in this instance, the sender does not wait for an acknowledgment from the recipient after the packet is received. As a result, a protocol is now unreliable.
Functions Of Transport Layer
1. Service Point Addressing: Due to the fact that computers execute multiple programs at once, data is transmitted from the source to the destination not only from one computer to another but also from one process to another. The transport layer header contains a form of address known as a service point address or port address in order to send the message to the appropriate process. The transport layer ensures that the message is delivered to the appropriate process by supplying this address.
2. Segmaetaion and Reassembly: This layer receives the message from the upper layer (session) and divides it into smaller segments, giving each segment a sequence number that uniquely identifies it. The transport layer then reassembles the message based on its sequence numbers after the message has reached its destination.
3. Connection control: Two services are offered by the transport layer. both connection-oriented and connectionless services are available.
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Connection-Oriented Services: Before sending the packets, a connection-oriented service establishes a connection with the target machine's transport layer. All packets in a connection-oriented service take the same route. This form of transmission is trustworthy and safe.
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Connectionless Services: Each segment is treated as a separate packet by a connectionless service, and they all take distinct paths to the same place. The receiver in this transmission style does not confirm receipt of a packet. This method enables considerably quicker device connectivity.
5. Session Layer
The session layer is used to set up and maintain sessions, provide authentication, guarantee security, and coordinate communication between devices.
The session layer establishes sessions, or channels of communication, between devices. It is in charge of starting sessions, making sure they are active and open while data is being exchanged, and shutting them down once communication is complete. The session layer can also establish checkpoints during a data transmission, allowing devices to pick up where they left off in the event that the session is terminated.
Reference: https://static.javatpoint.com/tutorial/computer-network/images/osi-model8.png
Functions of Session Layer
1. Dialog Controller: The session layer functions as a dialogue controller that initiates a conversation between two processes, or we could say that it enables two systems to initiate half-duplex or full-duplex communication.
2. Synchronization: When transmitting data in a sequence, the session layer adds a few checkpoints. These checkpoints aid in locating the issue, preventing premature message termination and data loss, and allowing for correct data resynchronization.
6. Presentation Layer
The Translation layer is another name for the Presentation layer. The syntax and semantics of the information communicated between the two systems are its primary concerns. In order for data to be correctly received on the other end, it specifies how two devices should encode, encrypt, and compress data. Any data transmitted by the application layer is processed by the presentation layer before being delivered via the session layer.
Reference: https://static.javatpoint.com/tutorial/computer-network/images/osi-model9.png
Functions Of Presentation Layer
1. Translation: The data is transformed from a sender-dependent format into a common format, which is then transformed at the receiving end into a receiver-dependent format.
2. Encryption/ Decryption: To guarantee privacy, encryption is required. Data is converted into another form or code through data encryption. The decrypted data is referred to as plain text, and the encrypted data is referred to as ciphertext.
3. Compression: It lowers the number of bits that must be transmitted. Data compression is crucial for multimedia content like text, music, and video.
7. Application Layer
Users and application processes can access network services through an application layer. End-user applications like web browsers and email clients operate at the application layer. The network apps implement the application layer. The data generated by these applications must be transmitted via the network. The end users are given network services by this tier.
The Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Post Office Protocol (POP), Simple Mail Transfer Protocol (SMTP), and Domain Name System are a few examples of application layer protocols (DNS).
Reference: https://static.javatpoint.com/tutorial/computer-network/images/osi-model10.png
Functions of the Application Layer
1. File transfer, access, and management (FTAM): A user can access files on a distant computer, recover files from a computer, and manage files on a remote computer using an application layer.
2. Directory Services: An application that offers access to global data about numerous objects and services houses a distributed database.
3. Mail Services: The ability to forward and store emails is made possible by an application layer.
4. Directory Services: An application that offers access to global data about multiple objects and services has a distributed database.
Conclusion
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