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Open Systems Interconnection (OSI) Model

The Open Systems Interconnection Model

History of the Open Systems Interconnection Model

During the late 1970’s the ISO (http://www.iso.org) developed a theoretical model for networking hardware and software called the Open Systems Interconnection model. The Open Systems Interconnection model was derived, in part, from standards defined within the Advanced Research Projects Agency Network (ARPANET). The OSI model was implemented to address a need to allow multi-vendor interoperability of networking systems; for, the ARPANET was confined to the interconnection of systems within the Defense Advanced Research Projects Agency (DARPA) of the United States Department of Defense.

Open Systems Interconnection Model Defined

The Open Systems Interconnection model is a standard created by the ISO that defines the procedures that must occur for an information system to communicate with another information system. The OSI model uses what is known as seven layer architecture. Each layer has a specific set of instructions that it must follow to meet the requirements for a specific function. Each layer uses the services of the layer directly below it, and is does not consider the services of other layers. The seven layers of the OSI model are:
7. Application Layer
6. Presentation Layer
5. Session Layer
4. Transport Layer
3. Network Layer
2. Data Link Layer
1. Physical Layer

* Note – Although Voice over IP (VoIP) is not directly related to the OSI model, without the inception of the OSI model and TCP/IP, VoIP would not be possible. VoIP is a family of technologies that allows consumers to place calls via a TCP/IP connection. It is argued that the greatest benefit to VoIP is the improvement of quality combined with lowered costs for voice/data.

Application Layer

The application layer contains programs that are used as network utilities such as web browsers, email clients, instant messaging. The application also includes networking services such as FTP, web server programs, etc.

The application layer of one system creates a service request and sends its request to the application layer of another system.

Presentation Layer

The presentation layer validates that data transmitted between two systems is without error. Some common tasks within the presentation layer are encryption and decryption, compression and decompression. When viewing a website the presentation layer of a web browser formats images and fonts.

Session Layer

The session layer manages communication protocols, such as timeout, half/full duplex, and synchronization.  When a communication session has been created the session layer monitors and resolves any communication error that may arise.

While purchasing from an ecommerce website, the session layer watches for a break within the encrypted connections. If the communication is disrupted, the session layer on the vendor’s system will completely abandon the order, cancelling any banking transaction before it completes.

Transport Layer

The transport layer creates data packets that will be transmitted over the network. The packet includes a header and trailer that contain networking addresses, error detection data, and sequencing data. Once the transport layer receives a packet, it searches for errors and requests for retransmission if necessary.

The transport layer is also responsible for parsing over-sized packets into several smaller packets by sequencing and validating each packet.

Network Layer

The network layer acts as a router, sending packets to the nearest central node. The central nodes store routing tables and routing information. End nodes communicate other end nodes and central nodes as they become activated.

Data Link Layer

The data link layer is the link between software and hardware. Device drivers are the method by which hardware and software communicate. For example, when printing a document (on a network or locally), the device driver transmits packets from storage to the network interface card or network interface unit

Physical Layer

The physical layer is the layer in which hardware devices actually communicate. The physical layer includes NIC’s, transmission lines, routers, etc.

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