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Introduction to OSI Reference Model

The OSI model defines the basic building blocks of computer networking, and is an essential part of a complete understanding of modern TCI/IP networks.

Network Model Overview:

 An understanding of the concepts of the OSI model is absolutely necessary for someone learning the role of the Network Administrator or the System Administrator.

There is a set of rules for the communication process for one computer to send information to another computer and for that computer to receive and understand this information. There is a standard that ensure that every devices and products can communicate with each other over any network. This set of standards is called a model.

The OSI Model was developed by the International Organization for Standardization (ISO) in 1978 and was published in 1984. Open System Interconnect is an open standard for all communication systems.

This model is divided into 7 layers, and each layer has a distinctive job in the communication process. Here is a look at each layer from a top-down –

Here is a look at each layer from a top-down –

1) Application Layer:

This layer supports application and end-user processes. This layer interacts with software applications that implement a communicating component. This layer ensures that effective communication with another application program in a network is possible. This layer provides application services for file transfers, e-mail, and other network software services.

This layer is responsible for providing interface to the application user. This layer encompasses protocols which directly interacts with the user.

Some of the better-known Application layer protocols are

2) Presentation Layer:

The presentation layer takes the data from the application format, and then transforms data into the form that the application accepts. This layer formats and encrypts data to be communicated in different formats via different sources.

3) Session Layer:

The session layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures.

4) Transport Layer:

The Transport layer is primarily concerned with end-to-end connection reliability. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Transport layer can keep track of the segments and retransmit those that fail. The transport layer also provides the acknowledgement of the successful data transmission and sends the next data if no errors occurred. The transport layer creates packets out of the message received from the application layer. Packetizing is a process of dividing the long message into smaller messages.

5) Network Layer:

Network Layer Handles routing of data across network segments. The network layer establishes the route between the sender and receiver across switching points, which are typically routers. Network Layer provides data routing paths for network communication and for that it selects and manages the best logical path for data transfer between nodes.

6) Data Link Layer:

The data link layer provides a reliable link between two directly connected nodes, by detecting and possibly correcting errors that may occur in the physical layer. It makes sure each packet created by the networking layer is transmitted successfully. If an error occurs during transmission, it is corrected and retransmitted.

The data link layer is divided into two sublayers:

Media Access Control (MAC) layer - responsible for controlling how devices in a network gain access to data and permission to transmit it.

This layer is responsible for reading and writing data from and onto the line. Link errors are detected at this layer.

7) Physical Layer:

The physical layer consists of the basic networking hardware transmission technologies of a network. It defines the electrical and physical specifications of the data connection. It defines the relationship between a device and a physical transmission medium.

The physical layer is responsible for various encoding and signaling functions that transform the data from bits that reside within a computer or other device into signals that can be sent over the network.