OSI MODEL
OSI stand for OPEN SYSTEM INTERCONNECTION. it has been devoloped by ISO - INTERNATIONAL ORGANIZATION OF STANDARDIZATION, in the year 1984. it is a 7 layer architecture with each layer having specific functionality to perform. all these 7 layers work collaboratively to another across the globle.
types of layer
there are 7 types of layers------
sender receiver
application layer application layer
presentation layer presentation layer
session layer session layer
transport layer transport layer
network layer network layer
data link layer data link layer
physical layer physical layer
- physical layer(layer 1):-
the lowest layer of the OSI reference modle is the physical layer. it is responsible for the devices. the physical layer contains information in the form of bits.
e.g. [1100][0111][0011]
the functions of the physical layer:-
- bit synchronization:-
the physical layer provides the sychronization of the bits by providing a clock. this clock controls both sender and receiver.
- data link layer(layer 2):-
the data link layer is responsible for the node to node delivery of the massage. the main function of this layer is error - free from one node to another over the physical layer.
data link layer is devided in to two sub layer:-
- logical link control (LLC)
- media access control (MAC)
the function of data link layer:-
- framing:- framing is the function of the data link layer. it provides a way for a sender to transmit a set os bits that are meaningful to the receiver. this can be accomplished by attaching special bit patterns to the beginging and end of the frame.
- physical addressing:- after creating frames, data link layer add physical addresses (MAC address) of sender and/or receiver in the header of each frame.
- error control:- data link layer provides the mechanism of error control in which detects and retransmits damaged or lost frames.
- flow control:- the data rate must be constant or both sides else the data may get corrupted thus, flow control coordinates that amount of data, that can be sent before receiving acknowledgement.
- access control:- when a single communication channel is shared by multiple devices, MAC sub-layer
3. Network Layer:-
The network Layer controls the operation of the subnet. The main aim of this layer is to deliver packets from source to destination across multiple links (networks). ... It also divides the outgoing messages into packets and to assemble incoming packets into messages for higher levels.
Functions of Network Layer
Let’s be clear on the above terminologies that network layer performs:
Addressing – Maintains both the source and destination addresses at the frame header. The network layer performs addressing to find out the specific devices on the network.
Packetizing – The network layer works on the conversion of packets those received from its upper layer. This feature is accomplished by Internet Protocol (IP).
Routing – Being considered as the major functionality, the network layer chooses the best path for data transmission from a source point to the destination.
Internetworking – Internetworking works to deliver a logical connection across multiple devices.
4. Transport layer
The transport layer is the fourth layer in the open system interconnection (OSI) model, and is responsible for end-to-end communication over a network. It provides logical communication between application processes running on different hosts within a layered architecture of protocols and other network components.
Functions & Features
Segmentation : Large data is divided into smaller segments at the senders end and then these smaller segments are recombined in exact format before it is received at receivers end.
Error Control : Distortion, Un-delivery of data packets and noise in data packets results in error and this is prevented using Transport Layer too and this feature is called Error Control.
Flow Control : Transport-Layer ensures that faster data transmission and slow data absorption or slow data transmission and faster data absorption must not occur.
Transport-Layer performs Multiplexing & De-multiplexing functions too.
Connection oriented service using TCP(Transmission Control Protocol) Protocol & Connection less service using UDP(User Data-gram Protocol) Protocol are performed by transport layer.
- Session layer
The session layer (layer 5) is responsible for establishing, managing, synchronizing and terminating sessions between end-user application processes.
The main functions of the session layer are as follows −
. It works as a dialog controller. It allows the systems to communicate in either half-duplex or full-duplex mode of communication.
. It is responsible for token management. Through this, it prevents the two users to simultaneously attempt the same critical operation.
. It synchronizes communication. It adds synchronization points or checkpoints in data streams for long communications. This ensures that data streams up to the checkpoints are successfully received and acknowledged. In case of any failures, only the streams after the checkpoints have to be re-transmitted.
6.Presentation layer(layer 6):-
presentation layer also called the translation layer. the data from the application layer is extracted here and manipulated us per the required formate to transmit over the network.
the function of the presentation layer are :-
- translation:- for example,ASCII to EBCDIC.
- encryption/decryption:- data encryption translates the data in to another form of code. the encrypted data is known as the cipher text and the decrypted data is known as plain text. a key value is used for encrypting as well as decrypting data.
- compression:- reduces the number of bits that need to be transmitted on the network.
7. application layer(7):-
At the very top of the OSI refrence model starck of layers, we find application layer which is implemented by the network applications. these applications produce the data which has to be transferred over the network. this layer also serves as a window for the applicatin services to access the network and for displaying the received information to the user.
EXP:- browsers, skype massenger etc.
application layer is also called as desktop layer.
