What is OSI Model?
OSI, an acronym for Open Systems Interconnection, is a model designed by the International Standards Organization (ISO) to explain how applications converse over a network (Beal, 1999 & Rouse, 2014). The OSI model is, however, not popularly used by developers because the most common internet protocols, TCP/IP, do not point clearly to the OSI model.
The Layers of the OSI Model
The OSI Model is made up of seven layers namely: The Application Layer, The Presentation Layer, The Session Layer, The Transport Layer, The Network Layer, The Data-Link Layer, and The Physical Layer, discussed from the seventh to the first below.
Layer 7: Application Layer
The Application Layer is the first layer in the sending device and the last layer on the receiving device. It allows the network handler, either a human or an application, to connect to the network. Additionally, this layer is responsible for the graphical user interface required by the user to perform tasks like sending and receiving emails and files (Forouzan, & Fegan 2007, p.41). All applications that are require to connect to the network are found in this layer.
Layer 6: The Presentation Layer
The Presentation Layer, often a portion of the Operating System conducts a series of functions needed to convert the data being transmitted from one form to another. On the sending end, this layer transforms printable characters, from the application layer, to computer understood bit streams to be sent. On the receiving side, it transforms bit streams to printable characters that the user (human) can understand. According to White (2013), this layer acts as a platform for encrypting and decrypting confidential files, and compressing large files into small units (p. 18).
Layer 5: The Session Layer
The Session Layer is responsible for starting, managing and ending communication sessions. This layer furthermore: allows token management which is a service that sends software tokens to and from users in order to determine who talks and who listens, conducts authentication, and synchronizes points or create backup point used for easy recovery from an error or interruption.
Layer 4: The Transport Layer
The Transport Layer, like in a TCP/IP model, manages bundling of data into packets and also delivering them. This layer also ascertains that the data bundles arriving at the receiving destination are identical to packets sent from the original destination.
Layer 3: The Network Layer
The Network Layer is responsible for addressing and routing or switching of data packets from the source to the final destination. Addressing involves adding a header with both the sender’s and the receiver’s IP address to the packets. Routing or switching is the use of switches or routers to respectively switch or route addressed packets through a series of network hops to their final destination. Switches and routers are the only commonly used hardware devices in this layer.
Layer 2: The Data-Link Layer
The Data-Link Layer obtains data from the network layer and converts it into a frame, a process called framing. It assumes the addressing and routing role of the Network Layer if the sender and the receiver are connected directly. This layer also: controls the flow of data when the receiver is slower than the sender in order to ensure that the receiver is not overloaded, identifies and resend damaged and lost frames, deletes repeated frames and controls which device has access to a link where devices are interconnected. Some of the hardware components in this layer include: switches, routers, and bridges
Layer 1: The Physical layer
The Physical layer manages sending of bit streams through a physical network medium. It is concerned with the physical and electrical means of transmitting data between two connected devices. The physical layer, additionally, defines the rules, procedures and functions that the communicating devices must meet for transmission of data to occur. This layer has hardware components like: hubs, repeaters, routers and switches.
The OSI model subdivides and simplifies network protocol design. It was designed to increase compatibility of network devices from different manufacturers. This article examined the seven layers of the OSI model namely: The Application Layer, The Presentation Layer, The Session Layer, The Transport Layer, The Network Layer, The Data-Link Layer, and The Physical Layer and presented the functions, purposes, and hardware components of each layer.
Beal, V. (1999). The 7 Layers of the OSI Model. Retrieved from http://www.webopedia.com/quick_ref/OSI_Layers.asp.
Forouzan, B. A., & Fegan, S. C. (2007). Data communications and networking (4th ed.). New York, NY: McGraw-Hill.
Rouse, M. (2014). OSI reference model (Open Systems Interconnection). Retrieved from http://searchnetworking.techtarget.com/definition/OSI.
White, C. M. (2016). Data communications & computer networks: A business user’s approach (8th ed.). Boston, MA: Cengage Learning.