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. If you are not able to write the assignment or find it difficult then go for Assignment Help California. It will help you to get better grades and good understanding of the subject concepts.
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 two devices that works on the different platforms. It was developed to curtail this disadvantage of communication between different platforms operating from different networks. 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 Fiber 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. The first 4 layers (Top to Bottom Approach) are responsible for host and the last 3 layers are responsible for networking.
Layers of OSI Model
Different layers of OSI model are as follows (Top to Bottom Approach):
1. Application Layer – Application layer has protocols that help’s the application to run. This layer is not an application but helps the web applications to run. It looks after the resource allocation, file transfer, access and manage. It also provides the facility of email messaging.
2. Presentation Layer – Presentation layer looks after the way in which data will be presented. Presentation layer also perform other functions like Translation. Encryption and Compression. If two platforms are on different network or operating system, then presentation layer converts it from one format to another, this process is called translation. This layer operates or is a part of operating system. In encryption, data is converted into a non – readable format from the sender side, physical layer is responsible for encryption. In compression, data bits are compressed and then the data is transmitted. Data compression is important for fast delivery of multimedia like audio, video, text, etc.
3. Session Layer – Session layer is responsible for creating a session between two communicating devices and maintaining it. It creates checkpoints at the time of data transmission. So that, when there is a problem in transmission the data can be sent from the checkpoint and not the whole route again. It is called synchronization and recovery. It allows the communication that can be half duplex or full duplex.
4. Transport Layer – As its name suggests, transport layer is used to transfer the data. This layer receives the data from the upper layer and divides it into segments (smaller units). It provides point to point connection so that the data delivered is reliable. There are two protocols in transport layer – TCP (Transmission Control Protocol) & UDP (User Datagram Protocol). In TCP, the data is divided into segments and sent over the internet. These data packets can travel from different routes and reach the destination. The order of the data packets shuffles, TCP arranges these packets in sequence again.
UDP is a protocol of transport layer. It is not reliable as compared to TCP because it does not send any acknowledgement or feedback to the receiver. Whereas, TCP send feedback to the receiver. UDP is used at places where only delivery is required and feedback is not necessary. Other functions performed by transport layer are service point addressing, Segmentation and reassembly, connection control, error control and flow control.
5. Network Layer – Network layer looks after the device addressing and manages or controls the data packet on the network. It tracks its location and determines the path through which the data should travel from source to destination. It also looks after the network traffic. Functions of network layer are internetworking – provides logical connections; addressing – it appends source and destination address in the header of the frame; packetizing – converts packets received from upper layer into packets; and routing – Determining the path of the data packet over the network.
6. Data-Link Layer – Data Link Layer defines the format of data on the network. It identifies each device on the local network uniquely. Data link layers performs functions like – framing, physical addressing, Flow control, Access Control, Error Control. Data is converted into frames; then physical address of the destination device is appended into the header of the frame. A frame consists of the header, packet and trailer. In the trailer, if there is any error that it is mentioned in it.
7. Physical Layer – Physical layer transfer the data into signals depending on the transmission media. If it is optic fiber cable, then the layer transfers the data into light signals. Its functions are data transmission, line configuration, topology and determining the signal. It defines the way in which two devices should be connected. It also defines the network topology of the devices.
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 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.
These were the advantages and disadvantages of the of the OSI model. Also the & layers of OSI model are explained thoroughly. If you are not able to write an assignment on these topics you can take assignment help USA.
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