The international standard organization (ISO) is a multinational body dedicated to a worldwide agreement on international standards.
An ISO standard that covers all aspects of network communications is the Open Systems Interconnection OSI model layers.
An open system may be a model that permits any two different systems to speak no matter their underlying architecture.
The purpose of the OSI model layers is to open communication between different systems without requiring changes to the logic of the underlying computer hardware and software.
The OSI model is not a protocol it is a model for understanding and designing a data network architecture that is flexible, robust, and interoperable.
The Open System Interconnection (OSI model layers) is a framework for the design of network systems that allow for communication across all types of computer systems.
It consists of 7 separate but related layers, each of which defines a segment of the process of moving information across a network.
Function Of The OSI model layers
In this section, we briefly describe the functions of each seven-layer in the OSI model:-
- Application layer
- Presentation layer
- Session layer
- Transport layer
- Network layer
- Data Link layer
- Physical Layer
The physical layer coordinates the functionsneededto transmit a bitstream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium.
Itconjointlydefines the procedures and functions that physical devices and interfacesgot toperform for transmission to occur. The position of the physical layer is relevantto the transmission mediumand also theelectrical circuitlayer.
The responsibilities of the physical layer include the following:-
- Physical characteristics of interface and media– The physical layer defines the characteristics of the interface between the devicesand also thetransmission medium.
- Representation of bits:- The physical layerinformationincorporatesa stream of bits (sequence of 0s and 1s)with nointerpretation. To be transmitted, bitsshouldbe encoded into signals electrical and optical. The physical layer definesthe kindofcoding.
- Data rate:- The transmission rate of the number of bits sent every second is additionally outlined by the physical layer. The physical layer defines the period of touch, that is however long it lasts.
- Synchronization of bits:- The sender and receivershouldbesynchronousat the bit level. The senderand also thereceiver clocksshouldbesynchronous.
- Line configuration:- The physical layer cares about the affiliation of devices to the medium. in an exceeding purpose-to-purpose configuration, 2 devices square measure connected along through an obsessive link. in an exceedingly multipoint configuration, a link is shared between many devices.
- Physical topology:- Thetopologydefineshoweverdevicessquare measureconnectedto formto network. Devicesare oftenconnectedemploying amesh(every device connectedto eachalternativedevice), atopology(devicessquare measureconnected through central devices),a hooptopology (every device is connected tosuccessive, forming a ring), or atopology(every device ona standardlink).
- Transmission mode:- The physical layer conjointly defines the transmission mechanism between 2 devices simplex, half-duplex, or full-duplex. within the simplex mode, just one device will send the opposite will solely receive. The simplex mode could be unidirectional communication. within the half-duplex mode, 2 devices will send and receive, however not at the same time. in an exceedingly full-duplex mode, 2 devices send and receive at identical times.
The data link layer transforms the physical layer, a raw transmission facility, into a reliable link and is responsible for node to node delivery.
It makes the physical layer appear error-free to the upper layer (Network layer) and the relationship of the data link layer to the network and physical layers.
The responsibilities of the data link layer include the following:-
- Framing:-the info link layer divides the stream of bits received from the network layer into manageable data units called frames.
- Physical addressing:-If frames are to be distributed to a different system on the network, the data link layer adds a header to the frame to define the physical address of the sender (source address) and receiver (destination address) of the frame. If the frame is intended for a system outside the sender network, the receiver address is the address of the device that connects one network to the next.
- Flow control:-Data absorbed by the receiver is less than the rate produced in the sender, the data link layer imposes a flow control mechanism to prevent massive the receiver.
- Error control:-The data link layer adds reliability to the physical layer by adding a mechanism to direct and transmit damaged or lost frames. It also uses a mechanism to stop the duplication of frames. Error control is generally achieved through a trailer added to the top of the frame.
- Access control:-When 2 or more devices are connected to an equivalent link, link-layer protocols are necessary to work out which device has control over the link at any given time.
The network layer is responsible for the source to destination delivery of a packet possibly across multiple network links.
The data link layer oversees the delivery of the packet between 2 systems on the same network links, and the network later ensures that each packet gets from its point of origin to its final destination.
If 2 systems are connected to the same link, there is usually no need for a network layer. If the 2 systems are connected to different network links with attached devices between the network links, there is often a need for the network layer to accomplish source-to-destination delivery.
The relationship of the network layer o the data link and transport layer.
The responsibilities of the network layer include the following:-
- Logical addressing:-The physical addressing implemented by the info link layer handles the addressing problem locally. If a packet passes the network boundary, we’d like another addressing system to assist distinguish the source and destination systems. The network layer adds a header to the packet coming from the upper layer that, among other things, involves the logical addresses of the sender and receiver.
- Routing:-When independent networks or links are connected together to create an internetwork or a large network, the connecting devices called routers and gateways route the packets to their final destination. One of the functions of the network layer is to give this mechanism.
The transport layer is accountable for the supply to destination (end-to-end) delivery of the entire message.
The network layer oversees the finish to complete delivery of individual packets, it does not acknowledge any relationship between those packets.
It treats all severally as if every bit belonged to a separate message. The transport layer, on the other hand, ensures that the whole message arrives intact and then as, overseeing each error management and flow management at the supply to destination level.
The connection of the transport layer to the network and session layers. The transport layer mat creates anaffiliationbetweenthe twofinishports. Anaffiliationcould be asingle logical path between thesupplyand destinationthat gowithall packetsin a verymessage.
Transmission of all packetsto 1pathway, the transport layer hasa lot ofmanagementover sequencing, flow, and error detection and correction.
The responsibilities of the transport layerincludethe following:-
- Servicepurposeaddressing:-Computers usually run many programs at identical times. supply to destination delivery suggests that delivery not solely from one pc to the next but conjointly from a specific method on one pc to a specific method on the other. The transport layer header so should embody a sort of address referred to as a service purpose address. The network layer gets every packet to the correct pc the transport layer gets the entire message to the correct method on its pc.
- Segmentation andreconstruction:-A message divided intocommunicablesegments,everyphasecontaining a sequencevariety. These numbersmodifythe transport layer toput togetherthe messageproperlyuponinwardat the destination andto identifyand replace packets that were lostinsidethe transmission.
- Connectionmanagement:- A connectionless transport layer treatseveryphaseasANfreelancepacket and delivers it to the transport layer at the destination machine. A connection-oriented transport layer makes respect tothe transport layer at the destination machineinitialbefore delivering the packets.in the end,the informationistransferred, and theaffiliationis terminated.
- Flowmanagement:-The transport layer isaccountable forflowmanagement. However, flowmanagementat this layer is performedfinishto completerather thanacross one link.
- Errormanagement:-The transport layer is accountable for flow management. However, flow management at this layer is performed finish to complete rather than across one link. The causing transport layer makes positive that the entire message arrives at the receiving transport layer while not an error.
The service provided by the primary three-layer (physical, data link, and network) is not adequate for a few processes. The session layer is the network dialog controller. it establishes, maintains, and synchronizes the interaction between the communication system.
Specific responsibilities of the session layer include the following:-
- Dialog management:- The session layer permits 2 systems to enter into a dialog. It permits the communication between 2 processes to require place either in half-duplex (one approach at a time) or full-duplex (two-approach at a time).
The presentation layer is bothered with the syntax and linguistics of the knowledge changed between the two systems. the connection between the presentation layer and thus the applying and session layer.
Specific responsibilities of the presentation layer include the following:-
- Translation:- The method (running program) in 2 systems is typically exchanging data within the kind of character string, numbers, and so on. the knowledge ought to be modified to bitstreams before being transmitted. as a result of totally different|completely different} computers using different cryptography systems, the presentation layer is chargeable for ability between these totally different cryptography ways.
The presentation layer at the sender changes the information from its sender-dependent format into a regular format. the presentation layer at the receiving machine changes the same format into its receiver-dependent format.
- Encryption:- To carry sensitive data, a system should be able to assure privacy. cryptography implies that the sender transforms the first data to a different kind and sends that ensuing message out over the network. cryptography reverses the first method to rework the messages back to their original kind.
- Compression:- Information compression reduces the amount of bits t be transmitted. information compression becomes significantly necessary in the transmission of transmissions like text, audio, and video.
The appliance layer allows the user, whether or not human or computer code, to access the network. It provides user interfaces and support for services like email correspondence, remote file access and transfer, shared management, and alternative forms of distributed info services.
Specific services provided by the appliance layer include the following:-
- Network virtual terminals:- A network virtual terminal may be a computer code version of a physically terminal and permits a user to go surfing in a foreign host. the appliance creates a computer code emulation of a terminal at the remote host. That user’s laptop talks to the computer code terminal. The remote host believes it’s a human activity with one amongst its own terminals and permits you to brand on. File transfer, access, and management (FTAM) – This application permits a user to access files in an exceedingly remote laptop, and to manage or manage files in an exceedingly remote laptop.
- Mails services:- This application provides the premise for email forwarding and storage.
- Directory services:- This application provides distributed info sources and access to world info regarding varied objects and services.
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