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In this article:
- The OSI Reference Model philosophy
- The Layers in the OSI Model
The OSI Reference Model Philosophy
The OSI Reference Model was developed as a part of the Open Systems Interconnection initiative. The OSI initiative, as it is called, was undertaken by the International Organization for Standardization (ISO) along with the ITU-T as an effort towards standardizing networking in 1982. This was because networking was developed by vendors and hence proprietary.
The OSI Reference Model is a hierarchy of seven layers that defines the requirements for communication between two computers. It is aimed at bringing various platforms closer to each other in terms of operations with each other. A typical characteristic of the layers in this model is that a layer uses the functionality of the layer immediately below it only and its own functionality is used by the layer immediately above it only. The whole series of these layers or ‘protocols’ is generally termed as a ‘Protocol Stack’. The higher layers are software while the lower ones are hardware.
It is notable that the whole OSI Model is not used as is. Only a part of it is used, hence keeping it from achieving full functionality.
The OSI Reference Model is shown below as an image:
The Layers in the OSI Model
The OSI Model comprises of seven layers. These layers are explained below:
- Layer 7: The Application Layer
This is the layer that is closes to the end user of a network. This layer is a way by which the user interacts with the network to gain access to information within it. This is done using a software application and hence the name of the layer. Telnet, File Transfer Protocol (FTP) and Simple Mail Transfer Protocol (SMTP) are some common Application layer implementations.
- Layer 6: The Presentation Layer
The Presentation layer provides a standard interface for the Application layer. It is at this level that MIME encoding, data encryption and compression is done.
- Layer 5: The Session Layer
Dialogues or communication between computers are called sessions. The Session layer controls these sessions. It is its responsibility to establish a connection, maintain and terminate such sessions. The connection here refers to the connection between a local and a remote application.
- Layer 4: Transport Layer
The layers that precede the Transport layer already have heavy loads on them. The transport layer aims at transferring data between the users without putting any load on the preceding layers. Hence, the transport layer is responsible for the reliability of a link. An example of such a layer is the Transmission Control Protocol (TCP).
- Layer 3: Network Layer
In any network, data travels from one place (called a source) to another place (called a destination). The Network layer controls this. The layer is responsible for taking the data from the source to its destination through routing while maintaining quality. It also performs error control. The Internet Protocol (IP) is the best example of this layer.
- Layer 2: Data Link Layer
The Data Link layer provides the functional means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. Ethernet is the best example for this layer.
- Layer 1: Physical Layer
The Physical layer defines all the electrical and physical specifications for the devices used in a network. This includes the layout of pins, voltages, and cable specifications. Hubs, repeaters and network adapters are all physical layer devices.
The physical layer has the following major functions and responsibilities:
- establishment and termination of a connection to a communications medium.
- participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.
- modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling. A Modem, a device used in computer networks performs this very function.
Parallel SCSI buses and physical-layer Ethernet standards are also at this level. Ethernet incorporates both this layer and the data-link layer. The same is true of the other local-area networks, such as Token ring network, FDDI, and IEEE 802.11.
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