Concept of Open Systems Interconnection (OSI) Model in Networking

Introduction
The OSI reference model is defined as Open Systems Interconnection. The Open Systems Interconnection (OSI) model is developed by the International Organization for Standardization. The Open Systems Interconnection (OSI) model is a protocol used to communicate between different networks. Protocols are nothing but a set of rules used to communicate between two networks.

The Open Systems Interconnection (OSI) model is divided in seven layers.

Reference from Google

OSI Model divided in two parts upper level and lower level

The OSI Model is also divided in two parts; upper level and lower level. The lower level contains a physical layer, data link layer and network layer. The upper level contains a transport layer, session layer, presentation layer and application layer. Here we give brief descriptions of each layer.

Reference from Google

Form of data on different-2 layer

Reference from Google

Layer 1: Physical layer
It makes the relationship between a device and a transmission medium, such as a copper cable or fiber optical cable. The physical layer is responsible for maintaining voltages and bits. The Physical Layer works with cable specifications, hubs, repeaters, network adapters, host bus adapters and more.

The major functions and services performed by the physical layer are as fallows:

• Establishes and terminates the connection among communicating devices.
• Participates in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.
• Responsible for Modulation, or conversion of data among the representation of digital data in user device and the corresponding signals transmitted over a communications channel.
• Responsible for sending and receiving data in the form of bits.

Layer 2: Data link layer

In the Data Link Layer, the Data Link Layer encodes and decodes Data Packets into bits. The Data Link Layer works between the Physical layer and network layer. The Data Link Layer is also responsible for handling errors in a physical layer, flow control and Synchronization. The Data Link Layer works on frames. The Data Link Layer divides data into frames. The Data Link Layer provides physical addresses to data.

The Data Link Layer is divided in two sun layers

• The Media Access Control (MAC) layer.
• Logical Link Control (LLC) layer.
  

The Media Access Control (MAC) layer
The Media Access Control (MAC) sub layer is responsible for how a computer on the network gains access to the data and permission to transit it on another network or device.

Logical Link Control (LLC) layer
The Logical Link Control (LLC) sub layer is responsible for error checking, flow control and synchronization. 

Layer 3: Network layer

The Network layer works on switches and routers. The Network layer creates logical paths, known as virtual circuits. The Network layer uses these logical paths or virtual circuits for transferring data from one node to another node. For this it sets the route of a packet from a source node to a destination node.

Main work of Network layer are as fallows  

• Routing
• Forwarding
• Addressing
• Internet working
  
• Error handling,
• Congestion control
• Packet sequencing.

The Network layer data is in the form of Packets or Datagrams. The Network layer provides logical addresses to data in a network.

Layer 4: Transport layer

The Transport layer is responsible for transferring data between end systems or hosts. The Transport layer works for end to end recovery and flow control. The Transport layer is also responsible for complete, accurate and reliable data transfer. To maintain the reliability the Transport layer uses flow control segmentation / desegmentation, and error control. It also uses the TCP protocol which is a connection oriented protocol.
The Transport layer divides data in the form of segments.

Layer 5: Session layer

The Session layer is responsible for managing, establishing and terminating connections between applications (local and remote applications). The Session layer makes a connection coordination among applications, setup connections, exchanges, and dialogues (connections) between applications. The Session layer provides the following functionality:

• Full-duplex
• Half-duplex,
• Simplex operation
• Establishes check pointing
• Adjournment
• Termination
• Restart procedures
   

Layer 6: Presentation layer

The responsibility of the presentation layer is to provide independence from data representation by translating between applications and network formats. For this the presentation layer uses encryption and decryption technology. The Responsibility of the presentation layer is to convert machine dependent data to machine independent data. The Presentation layer sends encrypted data on the network. The Presentation layer is also known as the syntax layer.

Layer 7: Application layer

The Application layer of the OSI model provides services. These services are used by the user of software applications. This layer interacts with software applications developed for use of services provided by the application layer of the OSI model.

Some services provided by the application layer of the OSI model is as fallows:

• File transfers
• E-mail
• Network software services
• Telnet
• FTP
  

Some Helpful Resources

Basics Of Data Communication Part: 1
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