OSI Model

What is OSI Model?

                OSI stands for Open Systems Interconnection. It was developed by International Organization of Standardization (ISO) to achieve International standards in communication. OSI Model divides the communication process into 7 layers and each layers has some specific functions. 

7 layers of OSI model :

       1. Physical layer

       2. Data link layer

       3. Network layer

       4. Transport layer

       5. Session layer

       6. Presentation layer

       7. Application layer

Functions of OSI Layers :

1. Physical layer :

• Physical layer is the lowermost layer of the OSI model
• It is responsible for movement of individual bits from one node to the other node

Functions of physical layer :

• Physical characteristics of interfaces and medium :

                         The physical layer defines the characteristics of interface between the devices and the transmission medium. It also defines the type of transmission medium.

• Representation of bits :

                         The physical layer data consists of stream of bits (0's and 1's). To be transmitted, bits must be encoded into signals. The physical layer defines the type of encoding.

• Data rate :

                         Data rate is number of bits transmitted per second.

• Synchronization of bits:

                         The sender and receiver clock must be synchronized at the bit level.

• Line configuration :

                         Line configuration is how two or more devices are connected physically, either point to point or multipoint connection.

Point to point connection - Two devices are connected through a dedicated link

Multipoint connection - Two  or more devices are connected to a single link

• Physical topology :

                       Topology is how devices are connected to form a network. Devices can be connected using mesh topology, star topology, ring topology, bus topology, tree topology and hybrid topology.

• Transmission mode :

                        Physical layer defines the way in which the data flows between two connected devices i.e., either Simplex mode, Half-duplex mode or Full duplex mode.

2. Data link layer :

• The data link layer is responsible for moving frames from one node to the other node
• It has two sub layers :
• Data link control (DLC) / Logical link control (LLC) sub layer
• Media access control (MAC) sub layer

Functions of data link layer :

• Framing :

                     The data link layer divides the stream of bits received from the network layer into manageable data units called frames.

• Physical addressing : 

                     The data link layer adds a header to the frames which contains physical address (MAC address) of the sender and/or the receiver. 

• Flow Control :

                     The data rate must be constant on both the sender and receiver side. The data link layer coordinates the amount of data that can be sent before receiving acknowledgement.

• Error control :

                     Error control adds reliability by adding mechanisms to detect and retransmit damaged or lost frames.

• Access control :

                     When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.   

3. Network layer :

• The network layer is responsible for delivery of individual packets from source host to destination host.

Functions of network layer :

• Logical addressing :

                       The network layer adds a header to each packet which contains logical address (IP) address of the sender and receiver.

• Routing:

                      The network layer protocols determine which route is suitable and routes the packets from source to destination.

4. Transport layer :

• The transport layer is responsible for delivery of a message from one process to the other.

Functions of transport layer :

• Service-point addressing :

                          In order to deliver a message to correct process, the transport layer header includes a type of address called service point address (or Port address).

• Segmentation and Re-assembly :

                           In sender side, the transport layer divides the message into segments and adds a sequence number to each segment. In receiver side, the segments are rearranged based on the sequence number.

• Connection control :

                            The transport layer provides two services connection-oriented service and connectionless service.

Connection oriented service - Connection is established between source and destination. After data transmission, the connection is terminated.

Connectionless service - A connectionless service treats each segment as individual packets, and the packets travel through different routed to reach the destination.

• Flow control :

                             Like data link layer, the transport layer is also responsible for Flow control. 

• Error control :

                             Like data link layer, the transport layer is also responsible for Error control. 

5. Session layer :

• The session layer establishes, maintains, and synchronizes the interaction among communicating systems.

Functions of session layer :

• Dialog control :

                            The session layer allows two systems to enter into a dialog. It allows communication between two processes to takes place in either half-duplex or full duplex mode.    

• Synchronization :

                             The session layer allows a process to add check points or synchronization points to the data in a sequence. If some error occurs in the middle of the transmission of data, then the transmission will takes place again from the check point.

6. Presentation layer :

• The presentation layer is concerned with the syntax and semantics of the information exchanged between the two systems

Functions of Presentation layer :

• Translation :

                            Different computers use different encoding methods. In sender side, the presentation layer converts the data from sender dependent format into a common format. In receiver side, the presentation layer converts the data from common format into receiver dependent format.

• Encryption and decryption :

                            To carry sensitive information, the system must be able to ensure privacy. 

In sender side encryption takes place, i.e., the original information is transformed into another form/code.

In receiver side decryption takes place, i.e., the coded form of the message is transformed into original message.

• Compression and decompression :

                             Compression is reducing the number of bits in the data. Compression is done in sender side and decompression in receiver side.

7. Application layer :

• The application layer is responsible for providing services to the user.

Functions of Application layer :

• Network virtual terminal :

                             A network virtual terminal is the software version of a physical terminal which allows a user to log on to a remote host.

• File transfer, access and management :

                            The application layer allows a user to access files in a remote host, to retrieve files from a remote computer for use in local computer, and to manage or control files in a remote computer locally.

• Mail services :

                           The application layer provides the basis of email forwarding and storage.

• Directory services :

                           The application layer provides distributed database sources and access for global information about various objects and services.

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