The Open Systems Interconnection reference model is a layered, abstract representation created as a guideline for network protocol design. The OSI model divides the networking process into seven logical layers, each of which has unique functionality and to which are assigned specific services and protocols.
In this model, information is passed from one layer to the next, starting at the Application layer on the transmitting host, proceeding down the hierarchy to the Physical layer, then passing over the communications channel to the destination host, where the information proceeds back up the hierarchy, ending at the Application layer.
The different layer of OSI model are:
- Application Layer
- Presentation Layer
- Session Layer
- Transport Layer
- Network Layer
- Data link Layer
- Physical Layer
Let's discuss these in brief -
Application Layer -
The Application layer, Layer seven, is the top layer of both the OSI and TCP/IP models. It is the layer that provides the interface between the applications we use to communicate and the underlying network over which our messages are transmitted.
Application layer protocols are used to exchange data between programs running on the source and destination hosts. There are many Application layer protocols and new protocols are always being developed.
Presentation Layer -
The Presentation layer has three primary functions:
- Coding and conversion of Application layer data to ensure that data from the source device can be interpreted by the appropriate application on the destination device.
- Compression of the data in a manner that can be decompressed by the destination device.
- Encryption of the data for transmission and the decryption of data upon receipt by the destination.
Presentation layer implementations are not typically associated with a particular protocol stack. The standards for video and graphics are examples. Some well-known standards for video include QuickTime and Motion Picture Experts Group (MPEG).
Session Layer -
As the name of the Session layer implies, functions at this layer create and maintain dialogs between source and destination applications. The Session layer handles the exchange of information to initiate dialogs, keep them active, and to restart sessions that are disrupted or idle for a long period of time.
Transport Layer -
The Transport layer provides for the segmentation of data and the control necessary to reassemble these pieces into the various communication streams. Its primary responsibilities to accomplish this are:
- Tracking the individual communication between applications on the source and destination hosts
- Segmenting data and managing each piece
- Reassembling the segments into streams of application data
- Identifying the different applications
Network Layer -
The Network layer, or OSI Layer 3, provides services to exchange the individual pieces of data over the network between identified end devices. To accomplish this end-to-end transport, Layer 3 uses four basic processes:
Data Link Layer -
The Data Link layer provides a means for exchanging data over a common local media.
The Data Link layer performs two basic services:
- Allows the upper layers to access the media using techniques such as framing
- Controls how data is placed onto the media and is received from the media using techniques such as media access control and error detection
As with each of the OSI layers, there are terms specific to this layer:
Frame - The Data Link layer PDU
Node - The Layer 2 notation for network devices connected to a common medium
Media/medium (physical) - The physical means for the transfer of information between two nodes
Network (physical) - Two or more nodes connected to a common medium
The Data Link layer is responsible for the exchange of frames between nodes over the media of a physical network.
Physical Layer -
The OSI Physical layer provides the means to transport across the network media the bits that make up a Data Link layer frame. This layer accepts a complete frame from the Data Link layer and encodes it as a series of signals that are transmitted onto the local media. The encoded bits that comprise a frame are received by either an end device or an intermediate device.
The delivery of frames across the local media requires the following Physical layer elements:
- The physical media and associated connectors.
- A representation of bits on the media
- Encoding of data and control information
- Transmitter and receiver circuitry on the network devices
At this stage of the communication process, the user data has been segmented by the Transport layer, placed into packets by the Network layer, and further encapsulated as frames by the Data Link layer. The purpose of the Physical layer is to create the electrical, optical, or microwave signal that represents the bits in each frame. These signals are then sent on the media one at a time.
It is also the job of the Physical layer to retrieve these individual signals from the media, restore them to their bit representations, and pass the bits up to the Data Link layer as a complete frame.