o OSI stands for Open System Interconnection may be a reference model that describes how information from a software application in one computer moves through a physical medium to the software application in another computer.
o OSI consists of seven layers, and every layer performs a specific network function.
o OSI model was developed by the world organization for Standardization (ISO) in 1984, and it’s now considered as an architectural model for the inter-computer communications.
o OSI model divides the entire task into seven smaller and manageable tasks. Each layer is assigned a specific task.
o Each layer is self-contained, in order that task assigned to every layer are often performed independently.
Characteristics of OSI Model:
o The OSI model is split into two layers: upper layers and lower layers.
o The upper layer of the OSI model mainly deals with the appliance related issues, and that they are implemented only within the software. the appliance layer is closest to the top user. Both the top user and therefore the application layer interact with the software applications. An upper layer refers to the layer just above another layer.
o The lower layer of the OSI model deals with the info transport issues. the info link layer and therefore the physical layer are implemented in hardware and software. The physical layer is that the lowest layer of the OSI model and is closest to the physical medium. The physical layer is especially liable for placing the knowledge on the physical medium.
Functions of the OSI Layers
There are the seven OSI layers. Each layer has different functions. an inventory of seven layers are given below:
1. Physical Layer
2. Data-Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer
o The main functionality of the physical layer is to transmit the individual bits from one node to a different node.
o It is that the lowest layer of the OSI model.
o It establishes, maintains and deactivates the physical connection.
o It specifies the mechanical, electrical and procedural network interface specifications.
Functions of a Physical layer:
o Line Configuration: It defines the way how two or more devices are often connected physically.
o Data Transmission: It defines the transmission mode whether it’s simplex, half-duplex or full-duplex mode between the 2 devices on the network.
o Topology: It defines the way how network devices are arranged.
o Signals: It determines the sort of the signal used for transmitting the knowledge .
o This layer is liable for the error-free transfer of knowledge frames.
o It defines the format of the info on the network.
o It provides a reliable and efficient communication between two or more devices.
o It is especially liable for the unique identification of every device that resides on an area network.
o It contains two sub-layers:
o Logical Link Control Layer
o It is liable for transferring the packets to the Network layer of the receiver that’s receiving.
o It identifies the address of the network layer protocol from the header.
o It also provides flow control.
o Media Access Control Layer
o A Media access control layer may be a link between the Logical Link Control layer and therefore the network’s physical layer.
o It is employed for transferring the packets over the network.
Functions of the Data-link layer
o Framing: the info link layer translates the physical’s raw bit stream into packets referred to as Frames. the info 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.
o Physical Addressing: the info link layer adds a header to the frame that contains a destination address. The frame is transmitted to the destination address mentioned within the header.
o Flow Control: Flow control is that the main functionality of the Data-link layer. it’s the technique through which the constant rate is maintained on both the edges in order that no data get corrupted. It ensures that the transmitting station like a server with higher processing speed doesn’t exceed the receiving station, with lower processing speed.
o Error Control: Error control is achieved by adding a calculated value CRC (Cyclic Redundancy Check) that’s placed to the info link layer’s trailer which is added to the message frame before it’s sent to the physical layer. If any error seems to occurr, then the receiver sends the acknowledgment for the retransmission of the corrupted frames.
o Access Control: When two or more devices are connected to an equivalent channel , then the info link layer protocols are wont to determine which device has control over the link at a given time.
o It may be a layer 3 that manages device addressing, tracks the situation of devices on the network.
o It determines the simplest path to maneuver data from source to the destination supported the network conditions, the priority of service, and other factors.
o The link layer is liable for routing and forwarding the packets.
o Routers are the layer 3 devices, they’re laid out in this layer and wont to provide the routing services within an internetwork.
o The protocols wont to route the network traffic are referred to as Network layer protocols. samples of protocols are IP and Ipv6.
Functions of Network Layer:
o Internetworking: An internetworking is that the main responsibility of the network layer. It provides a logical connection between different devices.
o Addressing: A Network layer adds the source and destination address to the header of the frame. Addressing is employed to spot the device on the web .
o Routing: Routing is that the major component of the network layer, and it determines the simplest optimal path out of the multiple paths from source to the destination.
o Packetizing: A Network Layer receives the packets from the upper layer and converts them into packets. This process is understood as Packetizing. it’s achieved by internet protocol (IP).
o The Transport layer may be a Layer 4 ensures that messages are transmitted within the order during which they’re sent and there’s no duplication of knowledge .
o The main responsibility of the transport layer is to transfer the info completely.
o It receives the info from the upper layer and converts them into smaller units referred to as segments.
o This layer are often termed as an end-to-end layer because it provides a point-to-point connection between source and destination to deliver the info reliably.
The two protocols utilized in this layer are:
o Transmission Control Protocol
o It may be a standard protocol that permits the systems to speak over the web .
o It establishes and maintains a connection between hosts.
o When data is shipped over the TCP connection, then the TCP protocol divides the info into smaller units referred to as segments. Each segment travels over the web using multiple routes, and that they arrive in several orders at the destination. The transmission control protocol reorders the packets within the correct order at the receiving end.
o User Datagram Protocol
o User Datagram Protocol may be a transport layer protocol.
o It is an unreliable transport protocol as during this case receiver doesn’t send any acknowledgment when the packet is received, the sender doesn’t await any acknowledgment. Therefore, this makes a protocol unreliable.
Functions of Transport Layer:
o Service-point addressing: Computers run several programs simultaneously thanks to this reason, the transmission of knowledge from source to the destination not only from one computer to a different computer but also from one process to a different process. The transport layer adds the header that contains the address referred to as a service-point address or port address. The responsibility of the network layer is to transmit the info from one computer to a different computer and therefore the responsibility of the transport layer is to transmit the message to the right process.
o Segmentation and reassembly: When the transport layer receives the message from the upper layer, it divides the message into multiple segments, and every segment is assigned with a sequence number that uniquely identifies each segment. When the message has received the destination, then the transport layer reassembles the message supported their sequence numbers.
o Connection control: Transport layer provides two services Connection-oriented service and connectionless service. A connectionless service treats each segment as a private packet, and that they all travel in several routes to succeed in the destination. A connection-oriented service makes a reference to the transport layer at the destination machine before delivering the packets. In connection-oriented service, all the packets travel within the single route.
o Flow control: The transport layer also liable for flow control but it’s performed end-to-end instead of across one link.
o Error control: The transport layer is additionally liable for Error control. Error control is performed end-to-end instead of across the only link. The sender transport layer ensures that message reach at the destination with none error.
o It may be a layer 3 within the OSI model.
o The Session layer is employed to determine , maintain and synchronizes the interaction between communicating devices.
Functions of Session layer:
o Dialog control: Session layer acts as a dialog controller that makes a dialog between two processes or we will say that it allows the communication between two processes which may be either half-duplex or full-duplex.
o Synchronization: Session layer adds some checkpoints when transmitting the info during a sequence. If some error occurs within the middle of the transmission of knowledge , then the transmission will happen again from the checkpoint. This process is understood as Synchronization and recovery.
o A Presentation layer is especially concerned with the syntax and semantics of the knowledge exchanged between the 2 systems.
o It acts as a knowledge translator for a network.
o This layer may be a a part of the OS that converts the info from one presentation format to a different format.
o The Presentation layer is additionally referred to as the syntax layer.
Functions of Presentation layer:
o Translation: The processes in two systems exchange the knowledge within the sort of character strings, numbers then on. Different computers use different encoding methods, the presentation layer handles the interoperability between the various encoding methods. It converts the info from sender-dependent format into a standard format and changes the common format into receiver-dependent format at the receiving end.
o Encryption: Encryption is required to take care of privacy. Encryption may be a process of converting the sender-transmitted information into another form and sends the resulting message over the network.
o Compression: Data compression may be a process of compressing the info , i.e., it reduces the amount of bits to be transmitted. Data compression is extremely important in multimedia like text, audio, video.
o An application layer is a window for users and application processes to access network service.
o It handles issues like network transparency, resource allocation, etc.
o An application layer isn’t an application, but it performs the appliance layer functions.
o This layer provides the network services to the end-users.
Functions of Application layer:
o File transfer, access, and management (FTAM): An application layer allows a user to access the files during a remote computer, to retrieve the files from a computer and to manage the files during a remote computer.
o Mail services: An application layer provides the power for email forwarding and storage.
o Directory services: An application provides the distributed database sources and is employed to supply that global information about various objects.