A reference model is a theoretical framework for understanding relationships. The Open System Interconnection Reference Model (The OSI Model) is a reference model of how applications communicate over a network. The OSI Reference model is used to guide developers and vendors so the digital software programs and communication products they create can interoperate, and to enable a clear model that defines the functions of a networking or telecommunication system. The OSI Reference model is composed of seven layers; the application, presentation, session, transport, network, data link, and physical layer. The key concept of the OSI model is that the communication process in a network between two endpoints can be divided into seven separate layers of related functions. This paper will delve at describing the function, purpose, and the hardware components residing in each of the seven layers of the OSI reference model.
Layer 7: Application Layer
This is the layer that is closest to the end user, meaning that there is a direct interaction between the OSI application layer and the user. The application layer interacts or provides an interface between software applications that implement a communicating component. Typically, the primary function of the application layer is synchronizing communication, determining resource availability, and identifying communication partners. The application layer determines the identity, as well as, the availability of communication partners for an application with data to transit when identifying communication partners. The hardware residing in the application layer includes the proxy servers, content filtering firewalls, application switches, and gateways.
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Layer 6: Presentation Layer
This is the layer that creates context between the application layer entities, whereby, the application-layer entities can utilize different syntax and semantics provided that the presentation services offers a mapping between them. This layer is responsible for the conversion of the data that is sent over the network from one type of representation to another. The presentation layer offers independence from data representation by translating between the network formats and application. The hardware components or devices residing in the presentation layer include gateways, firewalls, and PCs.
Layer 5: Session Layer
The session layer sets up, coordinates and terminates the connections between the local and remote application. Its functions include authentication and reconnection after an interruption. The hardware components or devices residing in the session layer include gateways, firewalls, and PCs.
Layer 4: Transport Layer
This layer provides the functional, as well as, the procedural means of transmitting variable length data sequences to the destination host while ensuring that the quality of service functions is maintained. The layer manages packetization of data and reassembles the message on the receiving end and that no data is lost by ensuring that all the packets contained in a single transmission are received. The hardware components or devices residing in the transport layer include gateways and firewalls.
Layer 3: Network Layer
The network layer handles the addressing and routing of the data. The layer ensures that it sends the data into the right direction and destination on outgoing transmission and also ensures it receives the incoming transmissions at the packet level. The hardware components or devices residing in the network layer include routers, brouters, and 3-layer switches.
Layer 2: Data Link Layer
This layer provides node-to-node transfer of data. The data link layer sets up links across the physical network, ensuring that the packets are put into a network of frames. It detects and may correct errors that might occur in the physical layer. The hardware components or devices residing in the data link layer includes bridge, 2-layer switches, network cards, and modems.
Layer1: Physical Layer
The physical layer is in charge of the transmission, as well as, reception of formless raw data between the physical transmission medium and a device. The physical layer conveys the bit stream, electrically, mechanically, or through radio waves, across the network. The hardware components residing or devices residing in the physical layer include hubs, repeaters, transceivers, cables, and fibers.
In the OSI model, control is passed from one layer to another, starting at the 7 th layer (the application layer) in one station, and then proceeding to the last layer, the 1 st layer (the application layer). The model takes the task of inter-networking, as well as, divides it into a vertical stack that is composed of the seven layers. The paper has outlined the purpose, functions, and the hardware residing in each of the seven layers of the OSI reference model.
