Importance and illustration of ISO Model
International Standards Organization/Open System Interconnection (ISO/OSI) model is a standard reference model for communication between two end users in a network. It can be helpful to have a basic understanding of how your network works in order to troubleshoot future problems.
It would be difficult to overstate the importance of the OSI model. Virtually all networking vendors and users understand how important it is that network computing products adhere to and fully support the networking standards this model has generated. When a vendor’s products adhere to the standards the ISO model has generated, connecting those products to other vendors’ products is relatively simple. Conversely, the further a vendor departs from those standards, the more difficult it becomes to connect that vendor’s products to those of other vendors.
In addition, if a vendor were to depart from the communication standards the model has engendered, software development efforts would be very difficult because the vendor would have to build every part of all necessary software, rather than being able to build on the existing work of other vendors.
The first two problems give rise to a third significant problem for vendors: a vendor’s products become less marketable as they become more difficult to connect with other vendors’ products.
Thus, the ISO model defines a networking framework for implementing protocols according to seven layers. Each layer is functionally independent of the others, but provides services to the layer above it and receives services from the layer below it.
The layers are in two groups. The upper four layers are used whenever a message passes from or to a user. The lower three layers are used when any message passes through the host computer. Messages intended for this computer pass to the upper layers. Messages destined for some other host are not passed up to the upper layers but are forwarded to another host.
The seven ISO layers are explained in more detail below:
Layer 7— The application layer: This is the layer at which communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. (This layer is not the application itself, although some applications may perform application layer functions). It represents the services that directly support applications such as software for file transfers, database access, email, and network games.
Layer 6—The presentation layer: This is a layer, usually part of an operating system, that converts incoming and outgoing data from one presentation format to another (for example, from a text stream into a popup window with the newly arrived text). This layer also manages security issues by providing services such as data encryption and compression. It’s sometimes called the syntax layer.
Layer 5—The session layer: This layer allows applications on different computers to establish, use, and end a session/connection. This layer establishes dialog control between the two computers in a session, regulating which side transmits, and when and how long it transmits.
Layer 4—The transport layer: This layer handles error recognition and recovery, manages the end-to-end control (for example, determining whether all packets have arrived) and error-checking. It ensures complete data transfer.
Layer 3—The network layer: This layer handles the routing of the data, addresses messages and translates logical addresses and names into physical addresses. It also determines the route from the source to the destination computer and manages traffic problems (flow control), such as switching, routing, and controlling the congestion of data packets.
Layer 2—The data-link layer: This layer package raw bit from the Physical layer into frames (logical, structures packets for data). It is responsible for transferring frames from one computer to another, without errors. After sending a frame, it waits for an acknowledgment from the receiving computer.
Layer 1—The physical layer: This layer transmits bits from one computer to another and regulates the transmission of a stream of bits over a physical medium. This layer defines how the cable is attached to the network adapter and what transmission technique is used to send data over the cable.
Besides, the principles that led to these 7 layers were the following:
Every time a new level of abstraction for a layer is necessary; every layer has well defined functions, the functions of each layer must be chosen in the objective of the international standardization of protocols. Boundaries between layers must be chosen so as to minimize the flows of data through interfaces.
The low layers (1, 2, 3 and 4) are necessary to the routing of information between the two concerned ends and depend on the physical medium. The higher layers (5, 6 and 7) are responsible for the data processing relative to the management of exchanges between information processing systems. In addition, layers 1 to 3 intervene between close machines, but not between ending machines that can be separated by several routers. On the contrary, layers 4 to 7 intervene only between distant hosts.
This material is referred from: