Open Systems Interconnection (OSI) reference model by R4R Team

About the OSI model:

•This  model was developed by the International Organisation for Standardisation (ISO) in 1984. It is now considered the primary Architectural model for inter-computer communications.
• The Open Systems Interconnection (OSI) reference model is a descriptive network scheme. It ensures greater compatibility and interoperability between various types of network technologies.
• The OSI model describes how information or data makes its way from application programmes (such as spreadsheets) through a network medium (such as wire) to another application programme located on another network.
• The OSI reference model divides the problem of moving information between computers over a network medium into
  SEVEN smaller and more manageable problems .
• This separation into smaller more manageable functions is known as layering or 7 layers.

The Open Systems Interconnect (OSI) model has seven layers. This article describes and explains them, beginning with the 'lowest' in the hierarchy (the physical) and proceeding to the 'highest' (the application).

the 7 layers are in order as .....

Application

Presentation

Session

Transport

Network

Data Link

Physical

Characteristics of the OSI 7 Layers:

This  seven layers of the OSI reference model can be divided into two categories:

a)Upper layers
b) lower layers.

The upper layers of the OSI model deal with application issues and generally are implemented only in software. The highest layer, the application layer, is closest to the end user. Both users and application layer processes interact with software applications that contain a communications component. The term upper layer is sometimes used to refer to any layer above another layer in the OSI model.

The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium (the network cabling, for example) and is responsible for actually placing information on the medium.

Note: A handy way to remember the seven layers is the sentence "All people seem to need data processing." The beginning letter of each word corresponds to a layer.


All           —Application layer
People      —Presentation layer
Seem       —Session layer
To            —Transport layer
Need        —Network layer
Data        —Data link layer
Processing—Physical layer


Basics of OSI layers:

Here the main concept of OSI is that the process of communication between two endpoints in a telecommunication network can be divided into seven distinct groups of related functions, or layers. Each communicating user or program is at a computer that can provide those seven layers of function. So in a given message between users, there will be a flow of data down through the layers in the source computer, across the network and then up through the layers in the receiving computer. The seven layers of function are provided by a combination of applications, operating systems, network card device drivers and networking hardware that enable a system to put a signal on a network cable or out over Wi-Fi or other wireless protocol).

About the 7 Layers:

LAYER 7: APPLICATION:

The application layer serves as the window for users and application processes to access network services. This layer contains a variety of commonly needed functions:
The application layer is the OSI layer that is closest to the user.
It provides network services to the user’s applications
Resource sharing and device redirection
Remote file access
Remote printer access
Inter-process communication
Network management
Directory services
The application layer establishes the availability of intended
communication partners, synchronizes and establishes agreement on
procedures for error recovery and control of data integrity.
Electronic messaging (such as mail)
It differs from the other layers in that it does not provide services to any
other OSI layer, but rather, only to applications outside the OSI model.
Examples :of such applications are spreadsheet programs, word processing
programs, and bank terminal programs.

LAYER 6: PRESENTATION:

The presentation layer, defines the format used to exchange data among networked computers. Think of it as the network's translator. When computers from dissimilar systems—such as IBM, Apple, and Sun—need to communicate, a certain amount of translation and byte reordering must be done. Within the sending computer, the presentation layer translates data from the format sent down from the application layer into a commonly recognized, intermediary format. At the receiving computer, this layer translates the intermediary format into a format that can be useful to that computer's application layer. The presentation layer is responsible for converting protocols, translating the data, encrypting the data, changing or converting the character set, and expanding graphics commands. The presentation layer also manages data compression to reduce the number of bits that need to be transmitted.

LAYER 5: SESSION:

The session layer. This layer sets up, coordinates and terminates conversations. Services include authentication and reconnection after an interruption. On the Internet, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) provide these services for most applications.

LAYER 4: TRANSPORT:

This layer Responsible for the reliable transmission of data and service specification between hosts. The major responsibility of this layer is data integrity--that data transmitted between hosts is reliable and timely. Upper layer datagrams are broken down into network-sized datagrams if needed and then implemented using the appropriate transmission control. The transport layer creates one or more than one network connection, depending on conditions. This layer also handles what type of connection will be created. Two major transport protocols are the TCP (Transmission Control Protocol) and the UDP (User Datagram Protocol

LAYER 3: NETWORK:

The network layer defines the network address, which differs from the MAC address. Some network layer implementations, such as the Internet Protocol (IP), define network addresses in a way that route selection can be determined systematically by comparing the source network address with the destination network address and applying the subnet mask. Because this layer defines the logical network layout, routers can use this layer to determine how to forward packets. Because of this, much of the design and configuration work for internetworks happens at Layer 3, the network layer.

LAYER 2: PRESENTATION:

The data link layer provides access to the networking media and physical transmission across the media and this enables the data to locate its intended destination on a network.
The data link layer provides reliable transit of data across a physical link by using the Media Access Control (MAC) addresses.
The data link layer uses the MAC address to define a hardware or data link address in order for multiple stations to share the same medium and still uniquely identify each other.
Concerned with network topology, network access, error notification,ordered delivery of frames, and flow control.
Examples :- Ethernet, Frame Relay and FDDI.

LAYER 1: PHYSICAL:

This layer Handles the bit-level electrical/light communication across the network channel. The major concern at this level is what physical access method to use. The physical layer deals with four very important characteristics of the network: mechanical, electrical, functional, and procedural. It also defines the hardware characteristics needed to transmit the data (voltage/current levels, signal strength, connector, and media). Basically, this layer ensures that a bit sent on one side of the network is received correctly on the other side.

Data travels from the application layer of the sender, down through the levels, across the nodes of the network service, and up through the levels of the receiver. Not all of the levels for all types of data are needed--certain transmissions might not be valid at a certain level of the model.

To keep track of the transmission, each layer "wraps" the preceding layer's data and header with its own header. A small chunk of data will be transmitted with multiple layers attached to it. On the receiving end, each layer strips off the header that corresponds to its respective level.

The OSI model should be used as a guide for how data is transmitted over the network. It is an abstract representation of the data pathway and should be treated as such.