OSI MODEL


OSI Model is also known as Open System Interconnection Model.

The OSI Model is the foundation for all the communication that takes place between computer and other network devices.

This model is based upon a proposal developed by International Standards Organisation.

This model is called OSI because this model allows any two different systems to communicate regardless of their architecture.

OSI Model is just a model, it is not a protocol that can be installed and run on the computer.

There are seven layers on the OSI. These are:

§  Physical Layer

§  Data Link Layer

§  Network Layer

§  Transport Layer

§  Session Layer

§  Presentation Layer

§  Application Layer


Physical Layer:

§  Physical Layer is the first layer of OSI Model.

§  It is responsible for the actual physical connection between the devices.

§  Such physical connection may be made by using twisted pair cable, fibber optics, co-axial cable or any wireless communication media.

§  Physical layer is connected with transmitting raw bits over a communication channel.

Functions of Physical Layer:

·         Transforming bits into signals:

§  The data at physical layer consist of sequence of bits.

§  The layer transform these bits into signals by using different encoding schemes so that data can be transmitted across the physical media.

·         Bit Synchronization:

§  The physical layer provides the synchronization of the bits by providing a clock.

§  This clock controls both transmitter and receiver.

·         Provides physical characteristics of interface and medium:

§  The physical layer manages the way of a device connect to network medium.

·         Bit rate control:

§  The physical layer defines the transmission rate i.e the number of bits sends in one second.

·         Transmission mode:

§  The physical layer also defines the way in which data flows between two connected devices.

§  The various transmission modes are – simplex,halfduplex,fullduplex.

·         Physical Topology:

§  The physical layer specifies the way in which different devices are arranged in a network.

·         Multiplexing:

§  The physical layer can use different techniques of multiplexing to improve the channel efficiency.

·         Circuit switching:

§  The physical layer also provides the circuit switching to interconnect different networks.

Data Link Layer:

§  Data Link Layer is the second layer of OSI Model.

§  It is responsible for reliable node to node delivery of data.

§  It receives the data from network layer and create frames, add physical address to these frames and pass them to physical layer.

Functions of Data Link Layer:

·         Framing:

§  The data link layer is responsible for dividing the stream of bits received from network layer into small data units called frames.

·         Physical Addressing:

§  After creating frames data link layer add physical addresses of sender and receiver in the header of each frame.

·         Flow Control:

§  It is the traffic regulatory mechanism implemented by data link layer that prevents the fast sender and slow receiver.

§  It controls the flow of data between sender and receiver.

·         Error Control:

§  Data Link Layer provides the mechanism of error control in which it detects and retransmits damaged or lost bits.

·         Feedback:

§  After transmitting the frames the system waits for the feedback.

§  The receiving device then sends the acknowledgement frame back to resource.

Network Layer:

§  Network Layer is the third layer of OSI Model.

§  It is concerned with controlling the operations of the subnet.

§  It is responsible for the source to destination delivery of a packet across multiple networks.

Functions of Network Layer:

·         Logical Addressing:

§  Network Layer provides another addressing system called logical addressing that deals with the transfer of data across the network.

§  Because Physical Addressing provided by the Data Link Layer handles the addressing problem if the data are to be send within the same network.

§  Therefore when packet is sent outside the network boundary network layer adds the logical address or network address of the sender and receiver to each packet.

·         Routing:

§  When independent networks are connected together to create internetwork then several possible routes are available to send the data from source to destination.

§  These networks are interconnected by special devices called routers orgateways that route the packet to their final destination.

§  The Network Layer is concerned with providing this routing service.

·         Congestion Control:

§  Network Layer is the responsible for the source to destination delivery packet across multiple networks.

§  If too many packets are present in the subnet at the same time, they will get in each other’s way forming battle necks.

§  The control of such congestion also belongs to network layer.

Transport Layer:

§  Transport Layer is the fourth layer of OSI Model.

§  Transport Layer is responsible for the source to destination delivery of entire message.

§  The Network Layer was concerned with source to destination delivery of individually packets.

§  It does not see any relationship between those packets.

§  But the Transport Layer divides the message receives from upper layer into packets at source and reassembles these packets again into message at the destination.

§  Transport layer may create a connection between the two end points.

§  The Transport Layer provides two types of services. These are Connection Oriented and Connection less Transmission

§  Connection Oriented: In this type of transmission, the receiving device sends an acknowledgement back to source after a packet or group of packet is received.

§  Connection less Transmission In this type of transmission, the receiving device does not send an acknowledgement back to source after a packet or group of packet is received.

Session Layer:

§  Session Layer is the fifth layer of OSI Model.

§  Session Layer has the responsibility of beginning, maintaining and ending of communication between devices by regulating the flow of data.

Functions of Session Layer:

·         Establishment, maintaining and ending a session:

§  This layer is responsible for beginning, maintaining and ending a session between sender and receiver.

·         Dialog Control:

§  Dialog Control is the function of session layer that determines which devices will communication first and the amount of data will be send.

§  These networks are interconnected by special devices called routers orgateways that route the packet to their final destination.

§  The Network Layer is concerned with providing this routing service.

·         Synchronization:

§  The Session Layer is responsible for synchronising the flow of data.

§  This Layer is responsible for adding check points or markers within message.

Presentation Layer:

§  Presentation Layer is the sixth layer of OSI Model.

§  It is concerned with syntax and semantics of the information exchange between two devices.

Functions of Presentation Layer:

·         Data Presentation or Transmission:

§  This functions ensures that the data being sent in the format that receiver can process.

§  Because Different Computers can use different encoding systems, the prentation layer is responsible for inter-operatability between these different encoding methods.

·         Data Encryption:

§  Data Encryption is a tool that allows us to hide the information from everyone except the person who originally sent the data and the receiver.

§  On the sender side, The Presentation Layer encrypts the data and sends the message.

§  When This Encrypted data arrives at destination, the presentation layer decrypts the message.

§  Decryption is just the reverse of encryption.

§  The message in its original form is known as plain text and encrypted message is called Cipher Text.

Application Layer:

§  Application Layer is the top most layer of OSI Model.

§  It provides services that directly support user applications such as database access, email and file transfer.

§  Application Layer has several protocols that provide different services to the users.

Functions of Application Layer:

·         Network Virtual Terminal:

§  A Network Virtual Terminal is a software version of a physical terminal and allows a user to log on to a remote log called remote logging.

·         File Transfer:

§  Application Layer allows a user to access a file in remote host to make changes in local computer.

·         Mail Services:

§  Application Layer provides various e-mail services such as e-mail forwarding and storage.

·         Directory Services:

§  Application Layer provides the distributed database sources access for global information about various objects and services.

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