OSI Reference Model

 OSI Model  

OSI stands for Open System Interconnections. This is a conceptual network model that helps to illustrate 'How the network communication works between the various devices in the network ?'.  Devices can communicate to each other without caring about the underlying architecture as this model provides interoperable architecture. 

Characteristics of OSI Model

• OSI Model consists of 7 imaginary layers.
• OSI Model is platform independent model
• Each Layer in the OSI Model has its function divided and responsibilities

Layer 1 Physical Layer

    Physical Layer is the bottom-most layer of the OSI Model 

Functions of Physical Layer 

Physical Characteristics of Interface and Medium 

    Physical characteristics of medium involves the transmission medium.

Representation of Bits 

    Physical Layer  data consists of the stream of bits ( sequence of 0s and 1s). The transmission of this stream of bits by converting these bits to electrical or optical signals and the coding schemes used for transmission is governed by the physical layer.

Transmission Rate/Data Rate

    The rate of transmission of bits (data) from the sender to the receiver is called the data rate. These number of bits transmitter per second is called as data rate.

Synchronization of Bits

 The clocks of the sender and the receiver must be synchronized with each other. Physical Layer oversees this thing.

Line Configuration

    Physical layer also oversees that 'How the devices are connected with each other ?'. In point-to-point interconnection the devices are connected with a dedicated link. In multipoint configuration the one device is connected with the other devices with a shared link.

Transmission Mode

        Transmission mode defines the direction of transmission between the devices. It may be simplex, duplex, or half-duplex. 

                In simplex mode the the transmission is done in only one direction. Only device will send and the other will receive

                In half-duplex the transmission can take place in both directions but not at same time.

                In duplex mode the transmission can take place in both direction simultaneously. 

Physical Topology

    Network topologies determines structural layout and data path of a communication network. Network topologies can be mesh, bus, star, ring, hybrid etc.

Layer 2 Data Link Layer

    Data link layer is the second layer in the OSI Reference Model. Data link layer helps to keep the physical layer data look clean to the upper layers. The data link layer is responsible for moving frame from one node to the another.

Framing

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

Physical Addressing

    Physical address of the sender and the receiver is added to the header of the frame. 

Flow Control 

    The speed of the data sent to the receiver is controlled at the data link layer so that the receiver does not get overwhelmed.

Error Control

    The detection and retransmission of damaged frame is done in the data link layer.

Access Control

   If two or more devices are connected to the link then the data link layer helps to determines which device has control over the link when.

Layer 3 Network Layer

Network layer is reponsible for source-to-destination delivery of packet. It differs from data  link layer in the manner that DLL oversees the node-to-node delivery while the network layer oversees the source-to-destination delivery.

Logical Addressing

Logical addressing is applicable when the source and destination nodes are situated in different networks. If they are in the same network then logical addressing may not be useful.

Routing

When independent networks are connected and using devices like switches and bridges then routing mechanism is implemented using the network layer.

Layer 4 Transport Layer

    Transport Layer mainly focuses on process to process communication. Transport layer implement Flow and Error control on its level to check that the data from the network layer can be arranged in a meaningful manner.

Service-point addressing

Communication between computers not only is limited to delivery of packet from sender to receiver and vice versa but process running on one node can also communicate directly with the process on another node. Therefore transport layer adds a service-point address (port address) to the the header of the packet. Network layer is responsible for the delivery of packet to node, while transport layer takes the responsibility to deliver the packet to the process.

Segmentation and reassembly

The message is divided in small segment so that they can be added with a sequence number. Sequence number helps to arrange the packets in order on the receiver side and replace the out of order packets.

Connection Control

Connection in the transport layer can be connectionless or connection oriented. If the transport layer is connectionless then connection is not established between the two (receiver and sender transport layer). In connection oriented communication, first the connection is established  and then packets are sent and then the connection is terminated.

Flow Control

Flow control is done end to end not on the link in transport layer.

Error Control

Error control is also performed process-to-process rather then across a single link.

Layer 5 Session Layer

Dialog Control

Session layer allows the two systems to enter into a dialog whether full-duplex or half-duplex mode.

Synchronization

It allows the system to add checkpoints to the data. This allows the transmission to resume from the last saved point rather than starting from the beginning.

Layer 6 Presentation Layer

Concerned with the syntax and semantics of the the information exchanged.

Translation

The data in the form of strings, characters and symbols needs to be translated to bits so that the lower layers can transmit the data. Presentation layer performs the task of translating the data into bit streams. 

Interoperability between different different types of system is provided with the help of presentations layer.

Encryption

Encryption and decryption of the message is done on the presentation layer.

Compression

Compression of the message is also done at the presentation layer.

Layer 7 Application Layer

 User Interface

  It is closest to the user. It is a part of User support layers. Application layer helps the user to communicate with the model and provides an interface for the communication between the device and the user.