In networking, layer models have been established to break down complex processes into individual steps. Each step or task is represented as a layer in a layer model, stacked on top of each other.
The OSI model is a reference for vendor-independent systems or a design basis for protocols and networks.
OSI stands for Open System Interconnection and was designed by the ISO (International Organization for Standardization), which is the International Organization for Standardization, as a basis for the formation of open communication standards.
The OSI model consists of 7 layers and is based on the DoD model. Compared to the DoD model, the OSI layer is more finely and detailed structured. Each layer defines specific tasks and functions for communication between two systems. For each layer, there are procedures and protocols that fulfill certain tasks and provide a specific service to the higher layer.
When communicating between two systems, the communication or data flow passes through all 7 layers of the OSI layer model at least twice: once at the sender and once at the receiver. Depending on how many intermediate stations the communication route has, the communication also passes through the layer model here multiple times in parts or as a whole.
Protocols in the OSI Layer Model
Protocols are a collection of rules for communication at a specific layer of the OSI model.
The protocols of one layer are largely transparent to the protocols of the layers above and below, so that the behavior of a protocol appears as if communicating directly with its counterpart on the opposite side. The transitions between the layers are interfaces that must be understood by the protocols. Because some protocols were developed for specific applications, it also happens that protocols span multiple layers and cover multiple tasks. In some connections, individual tasks are performed in multiple layers and thus executed multiple times.
Classification
The OSI layer model consists of 7 layers. Each layer is assigned a specific task. Individual layers can be adjusted, combined, or replaced.
Layers 1-4 are transport-oriented layers.
Layers 5-7 are application-oriented layers.
The transmission medium is not specified.
Layer 1: Physical Layer
Measures and procedures for the transmission of bit sequences
The physical layer defines the electrical, mechanical, and functional interface to the transmission medium. The protocols of this layer differ only according to the transmission medium and method used. However, the transmission medium is not part of Layer 1.
Layer 2: Data Link Layer
Logical connections with data packets and elementary error detection mechanisms
The data link layer ensures a reliable and functioning connection between end devices and the transmission medium. To avoid transmission errors and data loss, this layer includes functions for error detection, error correction, and data flow control. Physical addressing of data packets also occurs at this layer.
Layer 3: Network Layer
Routing and data flow control
This network layer controls the temporal and logical separate communication between end devices, independent of the transmission medium and topology. Logical addressing of end devices first occurs at this layer. Addressing is closely related to routing (finding the path from sender to receiver).
Layer 4: Transport Layer
Logical end-to-end connections
The transport layer is the link between the transport-oriented and application-oriented layers. Here, data packets are assigned to an application.
Layer 5: Session Layer
Process-to-process connections
This layer organizes connections between systems. Control and coordination mechanisms for connection and data exchange are implemented here.
Layer 6: Presentation Layer
Output of data in standard formats
The presentation layer converts data into various codecs and formats. Data is converted to or from the application layer into a suitable format.
Layer 7: Application Layer
Services, applications, and network management
The application layer provides functions for applications. This layer establishes the connection to the lower layers. Data input and output also occur at this level.
Brief Description of the OSI Layer Model
7th Layer: Application: Functions for applications, as well as data input and output.
6th Layer: Presentation: Conversion of system-dependent data into an independent format.
5th Layer: Session: Control of connections and data exchange.
4th Layer: Transport: Allocation of data packets to an application.
3rd Layer: Network: Routing data packets to the next node.
2nd Layer: Data Link: Segmentation of packets into frames and addition of checksums.
1st Layer: Physical: Conversion of bits into a signal suitable for the medium and physical transmission.
Note: The end devices of the end systems and the transmission medium are excluded from the OSI layer model. Nevertheless, it may be the case that the end devices are specified in the application layer and the transmission medium in the physical layer.
The OSI Layer Model in Practice
The OSI layer model is often used as a reference when it comes to representing the processes of communication or message transmission. However, the TCP/IP (DoD) model is much closer to reality. The problem with the OSI layer model is the standardization organization ISO, which was simply too cumbersome to quickly establish a framework for the tasks of protocols and transmission systems in networking. TCP/IP, on the other hand, was freely available, functional, and rapidly spread with other protocols. ISO had no choice but to consider TCP/IP in the OSI layer model. In addition to TCP/IP, other network protocols have been developed. However, these were eventually replaced by TCP/IP.
Almost all networks today operate based on TCP/IP.