The OSI Reference Model
The OSI model is shown in the following figure. This model is based on a proposal developed by the International Standards Organization (ISO) in the first step of the international standardization of protocols, different levels are used. The model is called the ISO, OSI Reference Model (Open Systems Interconnection) because it deals with connecting open systems—that is, systems that are open for communication with other systems. We will simply call this the OSI model for short. The OSI model has seven levels of layers. The principles that have been applied to achieve the seven levels can be summarized as follows:
- A level must be created where another abstraction is needed.
- Each level must fulfill a well-defined function.
- The function of each level should be chosen to define internationally standardized protocols.
- Level limits should be chosen to minimize the flow of information through the interfaces.
- The number of levels must be large enough so that it is not necessary to group different functions at the same level if necessary and small enough so that the architecture does not become difficult to manage.
The Physical Layer
The physical layer is engaged in the transmission of raw bits over a communication channel. The design issues are related to the fact that when one side sends a bit 1, it is taken from the other side as bit 1 and not bit 0.
The Data Link Layer
The main task of the data link layer is to convert the raw transmission medium into a line that does not contain undetected transmission errors. It does this by hiding real errors so that the network layer does not see them. Perform this task by asking the sender to split the input data into data frames and transmit them sequentially. If the service is reliable, the recipient confirms the correct reception of each frame by sending an acknowledgment frame.
The Network Layer
The network layer controls the operation of the subnet. A key design problem is how packets are routed from the source to the destination. Routes can be based on static tables “connected” to the network that rarely change, or more often can be updated automatically to avoid faulty components. You can also define at the beginning of each conversation, for example, a terminal session, such as connecting to the system on a remote computer.
The Transport Layer
The main function of the transport layer is to receive the data from the upper layer, to divide them into smaller units, if necessary, to transfer them to the network layer and to ensure that all parts are correctly delivered to the other end network layer. The transport level is the real end-to-end level; transports the data from the source to the destination.
The Session Layer
The session layer allows users of different machines to establish sessions between them. Sessions offer a variety of services, such as dialog management, token management, and synchronization.
The Presentation Layer
Unlike lower layers, which are mainly related to moving bits, the layer of presentation refers to the syntax and semantics of the information being transmitted. For computers with different internal data representations to be able to exchange data, the data structures that will be exchanged can be abstractly defined, as well as the standard encoding that will be used “on the wire”. The presentation layer is controlled these abstract data structures allow you to define and share higher-level data structures.
The Application Layer
The application layer contains many protocols that users typically need. The widely used application protocol is the HyperText Transfer Protocol (HTTP), which is the backbone of the World Wide Web. When the browser wants to get a web page, it sends the name of the desired page to the server hosting the page via HTTP. The server then sends the page again. Other application protocols are used to transfer files, electronic mails, and news from the network.