OSI stands for Open System Interconnection. It is a reference model to acknowledge how the applications transfer data over the network. It is a conceptual framework for understanding the relationship between seven layers of this model. In the late 1970s, one project was administered by ISO while another undertaken by the International Telegraph and Telephone Consultative Committee. These two international standards bodies have developed a document which defines the similar networking model. In 1983 these two International standards were merged to form a standard called as Basic Reference Model for Open System Interconnection. It was published in 1984 by ISO as standard ISO 7498. This model has two major components, an abstract model of networking called the Basic Reference Model and a set of specific protocols (Zimmermann, 1980). This seven-layer model concept was provided by Charles Bachman at Honeywell Information Services. There are various aspects of the OSI model which is evolved from experience with NPLNET, ARPANET, EIN, CYCLADES network and the work in IFIP WG6.1. The new design of the OSI model is documented in ISO 7498. In this model, the protocol allows an entity in one host to interact with the corresponding entity at the same layer in a different host. The documents of OSI standard is available from ITU-T as X.200-series of recommendations, and some of the protocol specifications were also available as part of the ITU-T X series.
The main purpose of this model is to guide developers and vendors so that digital communication products and software programs they create can interoperate to facilitate a clear framework describing the various functions of networking. This model is developed by the ISO (International Standard Organization). It has divided the OSI model into seven different layers such as layer 1-4 are the lower layers, layer 5-7 are the upper layers which contain application-level data. Every layer of the OSI model has its own functionality or job, and after that, it transfers the data to the next layer.
OSI Model Layers
The main purpose of the OSI model is to process the communication between a source and a destination device in a network which is divided into seven distinct layers. In this architecture, each layer serves the above layer (Melendez & Petersen, 1986). The function of seven layers of OSI is provided with a combination of applications, network card device drivers, operating system and networking hardware which allows the network to send a signal over Ethernet or Fibre optic cable with the use of Wi-Fi or other wireless protocols. The seven-layer architecture of OSI model indicates the protocols which are used for exchanging data between two users in a network.
Layers of OSI Model
In the following image, we can see the layers of OSI model as shown below.
Comparison with TCP/IP Model
The designing of protocol in the TCP/IP model does not support hierarchical encapsulation and layering technique. The RFC 3439 includes a section entitled “Layering considered harmful”. TCP/IP model has a four-layered architecture (Meyer & Zobrist, 1990). The TCP/IP Model layers are often compared with the OSI layer scheme in various ways that are discussed below:
- The application layer contains the OSI application layer, the session layer and presentation layer.
- The transport layer provides end-to-end connectivity similar like in OSI model.
- Internet Layer is a subset of OSI network layer.
- Data link layer and Physical layer of OSI model are linked in Physical layer of the TCP/IP model.
The above comparison of TCP/IP model and OSI model is based on the real seven-layer protocol model which is defined in ISO 7498.
Advantages of OSI Model
- OSI model is a generic model which acts as a guidance tool for the development of any network.
- It supports both connection-oriented services as well as connectionless services.
- It flows layered architecture and changes in one layer will not affect the other layer ("TCP/IP and the OSI Model", 2001).
- It distinctly separates Interfaces, Services, protocols and it is flexible in nature. The protocols used in every layer are replaced conveniently depending upon the nature of the network.
Disadvantages of OSI Model
- It is very complex, and the initial implementation of OSI model is very complex and slow.
- It is a theoretical model which does not ponder the availability of efficient technology.
- In this model, there is a duplication of various service provided by layers such as flow control, addressing, error control.
- This model does not offer an adequate solution for practical network implementation.
Melendez, W., & Petersen, E. (1986). The upper layers of the ISO/OSI reference model (part I). Computer Standards & Interfaces, 5(1), 13-46. Retrieved from https://www.sciencedirect.com/science/article/abs/pii/092054898690067X
Meyer, D., & Zobrist, G. (1990). TCP/IP versus OSI. IEEE Potentials, 9(1), 16-19. Retrieved from https://ieeexplore.ieee.org/document/46812
TCP/IP and the OSI Model. (2001). Designing A Wireless Network, 73-114. Retrieved from https://www.sciencedirect.com/science/article/pii/B978192899445950008X
Zimmermann, H. (1980). OSI Reference Model--The ISO Model of Architecture for Open Systems Interconnection. IEEE Transactions On Communications, 28(4), 425-432. Retrieved from https://ieeexplore.ieee.org/document/1094702