Open Systems Interconnection (OSI) is a benchmark model for how to communicate over a network. A reference model is a conceptual model for understanding relationships. Its goal is the interoperability of various communication systems with standard protocols.
Most providers involved in telecommunications attempt to describe their products and services in relation to the OSI model. OSI is rarely implemented as is. The TCP / IP protocol suite, which defines the Internet, does not exactly match the OSI model.
The model (OSI) for implementing protocols in seven layers. In fact, it’s not even real. The OSI model does not perform any function in the networking process. It is a conceptual model that allows us to better understand the complex interactions that occur.
Who developed the OSI model?
International Standards Organization (ISO) created the OSI model. It divides network communication into seven layers. In this model, layers 1 through 4 are considered the lower layers and are primarily concerned with moving data. Layers 5 through 7, called upper layers, contain data at the application level. Each layer does a very specific job and then passes the data to the next layer.
The 7 layers of the OSI model
Layer 7 – Application
The application layer is the OSI layer closest to the end user, which means that the application layer and the user interact directly with the software application. This layer interacts with the software applications which implement a communicating component. The functions of the application layer generally include communication identification, determining the availability of resources and synchronizing communications. The application layer provides the software with interfaces to use network services. Common protocols found at the application layer include HTTP, FTP, NFS, SNMP, Telnet.
Layer 6 – Presentation
The Presentation layer acts as a translator between the Application layer and the Session layer. At the Presentation layer, the data is formatted in a representation understandable for the network, for example: ASCII. The Presentation layer also supports encryption and decryption of data, such as password encryption. Common protocols found at the presentation layer include ASCII, EBCDIC, JPG, and MIDI.
Layer 5 – Session
The Session layer allows two applications on different computers to open, use and close a connection, that is, a session. A session is a very structured dialogue between two applications. The session layer is responsible for managing this dialogue. It performs name recognition and other functions, such as security, necessary to allow two applications to communicate on the network.
The Session layer synchronizes human tasks by placing checkpoints in the data flow. Checkpoints divide data into smaller groups for error detection. This layer also implements dialog control between communication processes, for example by controlling which side transmits, when and for how long. Common protocols used by the session layer include RPC, SQL, and NetBIOS.
Layer 4 – Transport
The transport layer is primarily responsible for ensuring that data is transferred from one point to another reliably and without errors. For example, the transport layer ensures that data is sent and received in the correct order.
The transport layer provides flow control and error handling, and helps solve problems associated with the transmission and reception of packets. Common examples of transport layer protocols are Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Sequenced Packet Exchange (SPX).
Layer 3 – Network
The network layer manages the addressing and routing of data based on logical addressing. Routers belong to the network layer because they use logical (IP) addresses to intelligently direct data from sender to recipient. A router determines which path data should take based on network conditions, service priority, and other factors. It also handles network traffic issues, such as switching, packet routing, fragmentation, and data congestion control.
Examples of network layer protocols are Internet (IP) and Internetwork Packet Exchange (IPX) protocols.
Layer 2 – Data link
The data link layer sends data frames from the network layer to the physical layer. It is responsible for controlling the signals entering and exiting the network cable. On reception, the data link layer encapsulates the raw bits of the physical layer in data frames. The electrical representation of data (bit patterns, encoding methods and tokens) is found only in this layer.
Layer 1 – Physical
The lower layer of the OSI model is the physical layer. This layer transmits the raw unstructured data, that is: bits, on the physical medium. The physical layer is entirely hardware-based and deals with all aspects of establishing and maintaining a physical link between communicating computers.
The physical layer defines how the cable connects to the network card. For example, it defines the number of pins in a connector and the function of each. It also defines the transmission technique that will be used to send data through the network cable.
This layer provides data encoding and bit synchronization. The physical layer is responsible for passing bits (0 and 1) from one computer to another and because different types of media physically transmit bits differently, the physical layer also defines how long each pulse is and how each pulse is bit is translated into appropriate electrical energy.
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