OSI model
The OSI means open system interconnection model is conceptual and logical layout that defines network communication used by systems open to interconnection and communicate with other system.
It defines logical network and effectively describes computer packet transfer by using different layer protocol . the OSI model was developed by the international for standardization (ISO) in 1978.
The ISO was a committee different vendors system. The ISO was a committee represented by many different organizations whose goal was not to favor a specific method of communication but rather to develop a set of guidelines that would allow vendors to ensure that their products would inter-operate
Moreover, the OSI model was created to simplify communications between system. There are many events that need to take place in order to ensure that data first reaches the correct system and is then passed along to the correct application in a usable format. A set of rules was required to break down the communication process into a simple set of building blocks.
7 layer of OSI model -
Physical layer
The physical layer is responsible for the transmission and reception of unstructured raw data between a device and physical transmission medium. It converts the digital bits into electrical , radio or optical signals.Layer specifications define characteristic such as voltage levels , the timing of voltage changes,physical data rates, maximum transmission distance and physical connectors.This includes the layout of pins, voltage line impedance, cable specifications, signal timing and frequency for wireless devices. But tare control is done at the physical layer and may define transmission mode as simplex, half-duplex and full duplex .The components of a physical layer can be described in terms of network topology,Bluetooth,Ethernet and USB all have specifications for physical layer .It controls feature of medium, frame synchronization , data encoding , signal pattern etc.
SO in short we can say ,Physical layer is concerned with the following terms :
Physical characteristics of interfaces and media : The physical layer define the characteristic of the interface between the devices and the transmission media. It also define the type of transmission media.
Representation of bits : The physical layer data consists of a stream of bits . with no interpretation. To be transmitted , bits must be encoded into signals- electrical or optical . The physical layer defines the types of encoding ( how 0 and 1 are changed to signal )
Data rate : The transmission rate the number of bits sent each second - is also defined by the physical layer. In other words, the physical layer defines the duration of a bit , which is how long it lasts.
Synchronization of bits : The sender and receiver must not only use the same bit rate but must also be synchronized at the bit level. In other words, the sender and the receiver clock must be synchronized.
Line configuration : the physical layer is concerned with the connection of devices to the media. In a point to point configuration , two devices are connected together through a dedicate link. In a multi-point configuration , a link is shared between several devices.
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Data link layer
The data link layer provides node to node transfer ( between two directly connected nodes ), and also handles error correction from the physical layer. Here data packet are encoded and decoded into bits. It also flow control and frame synchronization.
The data link layer is divided into two sub layer . the media access control layer and the logical link layer (MAC and LLC )
MAC responsible for controlling how device how devices in a network gain access to a medium and permission to transmit data.
Note : LLC - It responsible for identifying and encapsulation network layer protocol , and controls error checking and frame synchronization.
The point to point protocol ( PPP ) is a data link layer protocol that can operate over several different physical layer, such as synchronous and asynchronous serial lines.
Network
Network layer provides switching and routing technologies , creating logical path,know as virtual circuits, for transmitting data from node to node.
The network layer provides the functional and procedural means of transferring variable length data sequence ( called packet ) from one node to another.Connected in different networks.A network is a medium to which many nodes can be connected. On which every nodes can be connected, On which every node has an address and which permits nodes connected to it to transfer massage to other nodes connected to it by merely providing the content of a message and the address of the destination node and letting the network find the way to deliver the message to the destination node.possibly , routing It through intermediate nodes. If the message is too large to be transmitted from one node to another on the data link layer between those nodes, The network may implement message delivery by spilting message into several fragments at one node . sending the fragments independently and reassembling the fragments at another node . It may , but dose not need to report delivery errors.
Transport layer
The transport layer deals with the coordination of the data transfer between end systems and hosts.how much data to send,at what rate where it goes etc.
It is responsible for braking a large data transmission into small packets ensuring that all the packets have been received eliminating duplicate packets and performing flow control to ensure that no computer overwhelmed by the number of message it receives.It provides data acknowledgement of the successful data transmission.
Although error control is performed by the data link layer.the transport layer can also perform error checking.
Transport layer provides two types of services:
Connection oriented transmission: In this type of transmission the receiving device sends an acknowledge back to the source after a packet or group of packet is received. It is slower transmission method.
Connection transmission : In this type of transmission the receiving devices dose not sends an transmission method.
Feature of transport layer
It is responsible for process to process delivery of the entire message .
TL looks after the delivery of entire message considering all its packets and make sure that all packets are in order. On the other hand network layer treated each packet independently.
Session layer
The session layer controls the connected between computers.In establishes,manages and terminates the connections between the local and remote application. It provides for full duplex,half duplex or simplex operation.
Establishing,Maintaining and Ending a session : When sending device first contact with receiving device,It sends synchronization packet to establish a connection and determines the order in which information will be send receiver sends acknowledgement.
So the session can be sent and end
Dialog control : This function determines that which device will communicate first and the amount of data that will be sent.
Dialog separation : Process of adding checkpoints and markers to the stream of data is called dialog separation.
Presentation layer :
It is sixth layer of OSI model. It is concerned with the syntax and semantics of the information exchanged between the two device.It was designed for data encryption, decryption and compression.
Data presentation or translation : Because different computers use different encoding system It ensures that the data being sent is in the format that the recipient can process.
Data encryption: Physical layer provides this facility by which hides the information from everyone except the person who originally send the information and the intended recipient when encrypted data arrives at destination . Physical layer decrypt the message.
Data compression : Physical layer shrinks large amount of data into smaller pieces such that It reduce the size of data.
Application layer
The application layer is the one at the top, It is closest to the end user,which means both OSI application layer and the user interact directly with the software application.The real traffic data will be often generated from the application layer.This may be a web request generated from HTTP protocol, a command from telnet protocol, a fill download request from FTP protocol etc.
Factions of application layer
Mail services: this application provides various email service.
File transfer and access:It allows users to access files in remote host, to retrieve files from remote computer for user etc.
Remote log in : A user can log into a remote computer and access the resources of that computer.
Accessing the world wide web.
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