OSI model

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The Open Systems Interconnection model (OSI model) is a method of thinking of computer networking in terms of abstraction layers. Different communication technologies with similar functions are grouped into different logical layers on the OSI Model. Each layer of the OSI Model makes use of functions provided by the layers below it and provides functions that are used by the layers above it.

Description of OSI layers[change | edit source]

According to the standard OSI Model there are seven layers. Each layer is dependant upon the layers below it to function.

OSI Model
Data unit Layer
Host
layers
7. Application Network process to computer programs
6. Presentation Data representation, security encryption, convert computer code to network formated code
5. Session Interhost communication, managing sessions between programs
4. Transport End-to-end connections, reliability and flow control
Media
layers
3. Network Path determination and logical addressing
2. Data link Physical addressing
1. Physical The physical infrastructure used to send and receive Signal_(electrical_engineering)

Some aspects of computer networking, such as management and security, use or are used on every layer.

Layer 1: physical layer[change | edit source]

The physical layer refers to electrical and physical aspects of devices. In particular, it specifies how a device sends and receives information, such as using copper wires or fiber-optic cables. Examples of this include Ethernet or fiber optic cables, phone cords used for dial-up or DSL services, the coaxial cable used to provide broadband internet, the wires used to connect various components of a computer or even the radio signals used in wireless communication. Other functions of the physical layer include the conversion of signals into something that another layer can use (referred to as a bit), and adjusting the signal to allow for multiple users to use the same connection.

Layer 2: data link layer[change | edit source]

The Data Layer is mainly the method in which information from the network is broken down into frames and transmitted over the physical layer. This layer is also responsible for some Error detection and correction and some addressing so different devices can tell each other apart in larger systems.

Layer 3: network layer[change | edit source]

The Routing Layer works to coordinate related parts of a data conversation to ensure that large files are transferred. In other words, while the data link layer deals with the method in which the physical layer is used to transfer data, the network layer deals with organizing that data for transfer and reassembly. This layer also handles aspects of Routing Protocols, finding the available [best] path(s) from one network to another to ensure delivery of the data.

Layer 4: transport layer[change | edit source]

The Transport Layer is the level at which system reliability and quality are ensured. This layer manages traffic flow through the network layer to reduce congestion on a network, and performs error checking ensuring quality of service by resending data when data has been corrupted. Some of the most popular methods of encryption and firewall security take place on this layer.

Layer 5: session layer[change | edit source]

The Session layer uses the stable communication system created by the transport layer to create and control conversations (or sessions) between two computers. Computer sessions consist of a series of requests and responses that are used by higher layers to manage communication between different systems. This allows for such things as password validation, Dynamic Host Protocols, and interactive media streaming.

Layer 6: presentation layer[change | edit source]

The Presentation Layer is where the human readable programming languages are translated into machine code instructions used by the lower layers. At this level it is often hard to distinguish this level from the Application Layer. In general terms, this layer works by taking care of the directions given by the user at the application layer.

Layer 7: Application layer[change | edit source]

This is the level that the user often interacts with. This is where data turns into websites, chat programs and so on. Many protocols run at this layer, such as DNS, FTP, HTTP, HTTPS, NFS, POP3, SMTP, and SSH.

References[change | edit source]