Where is the application layer implemented

OSI layer model

In computer and network technology, layer models have become established to break down complex processes into individual steps. Each step or each task is represented as a layer in a layer model on top of one another.

The OSI layer model is a reference model for manufacturer-independent communication systems or a design basis for communication protocols and computer networks.

OSI means Open System Interconnection (Open System for Communication Connections) and was designed by the ISO (International Organization for Standardization), that is the international organization for standardization, as the basis for the formation of open communication standards.

The OSI layer model consists of 7 layers and is based on the DoD layer model. Compared to the DoD layer model, the OSI layer model is finer and more detailed. Each layer defines certain tasks and functions for communication between two systems. For each layer there are procedures and protocols that perform specific tasks and provide the higher-level layer with a specific service.

The dashed lines are intended to show that there is a virtual connection on the layers between the end systems.
When communicating between two systems, the communication or the data flow runs through all 7 layers of the OSI layer model at least twice. Once at the sender and once at the recipient. Depending on how many intermediate stations the communication path has, the communication runs through the layer model several times in parts or in whole.

Protocols in the OSI layer model

Protocols are a collection of rules for communication on a specific layer of the OSI layer model. The protocols of a layer are largely transparent to the protocols of the higher and lower layers, so that the behavior of a protocol is as in direct communication with the counterpart on the other side.
The transitions between the layers are interfaces that the protocols must understand. Because some protocols were developed for very specific applications, it is also possible that protocols extend over several layers and cover several tasks. It happens that in some connections individual tasks are carried out in several layers and thus several times.

Classification of the OSI layer model

  • The OSI layer model consists of 7 layers.
  • A specific task is assigned to each shift.
  • Individual layers can be adjusted, combined or exchanged.
  • Layers 1..4 are transport-oriented layers.
  • Layers 5..7 are application-oriented layers.
  • The transmission medium is not specified.

Layer 1: bit transmission layer / physical layer

Measures and procedures for the transmission of bit sequences

The bit transmission layer defines the electrical, mechanical and functional interface to the transmission medium. The protocols of this layer only differ according to the transmission medium and method used. However, the transmission medium is not part of Layer 1.

Layer 2: Security Layer / Data Link Layer

Logical connections with data packets and elementary error detection mechanisms

The security layer ensures a reliable and functioning connection between the end device and the transmission medium. To avoid transmission errors and data loss, this layer contains functions for error detection, error correction and data flow control.
The physical addressing of data packets also takes place on this layer.

Layer 3: Network Layer

Routing and data flow control

The network layer controls the temporal and logical separate communication between the end devices, regardless of the transmission medium and the topology. The logical addressing of the end devices takes place on this layer for the first time. Addressing is closely linked to routing (finding the way from the transmitter to the receiver).

Layer 4: transport layer / transport layer

Logical end-to-end connections

The transport layer is the link between the transport-oriented and application-oriented layers. The data packets are assigned to an application here.

Layer 5: communication layer / session layer

Process-to-process connections

The communication layer organizes the connections between the end systems. To this end, control and monitoring mechanisms are implemented for the connection and the data exchange.

Layer 6: Presentation Layer

Output of data in standard formats

The presentation layer converts the data into various codecs and formats. Here the data to or from the application layer is converted into a suitable format.

Layer 7: application layer

Services, applications and network management

The application layer provides functions for the applications. This layer establishes the connection to the lower layers. Data input and output also take place at this level.

Brief description of the OSI layer model

7th shift / application: Functions for applications as well as data input and output.
6th shift / presentation: Conversion of the system-dependent data into an independent format.
5th shift / communication: Control of connections and data exchange.
4th shift / transport: Assignment of the data packets to an application.
3 layer / Mediation: Routing of the data packets to the next node.
2 layer / Fuse: Segmentation of the packets into frames and adding of checksums.
1st shift / Bit transfer: Conversion of the 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 is possible 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 very often used as a reference when it comes to depicting communication or message transmission processes. But actually the DoD layer model (TCP / IP) is much closer to reality.
The problem with the OSI layer model is the standardization organization ISO, which was simply too cumbersome to set up a framework for the tasks of protocols and transmission systems in network technology in the shortest possible time. TCP / IP, on the other hand, was freely available, worked and spread rapidly with other protocols. The ISO had no choice but to consider TCP / IP in the OSI layer model.
In addition to TCP / IP, other network protocols have also developed. However, at some point they were replaced by TCP / IP. Almost all networks today work on the basis of TCP / IP.

The various protocols, transmission and switching technologies are assigned to the layers of the OSI model in the following table.
Many protocols and transmission methods use more than just one layer. Therefore, a complete and correct representation of the table cannot be guaranteed.

Layer 7applicationTelnet, FTP, HTTP, SMTP, NNTP
Layer 6presentationTelnet, FTP, HTTP, SMTP, NNTP, NetBIOS
Layer 5communicationTelnet, FTP, HTTP, SMTP, NNTP, NetBIOS, TFTP
Layer 4transportTCP, UDP, SPX, NetBEUI
Layer 3MediationIP, IPX, ICMP, T.70, T.90, X.25, NetBEUI
Layer 2FuseLLC / MAC, X.75, V.120, ARP, HDLC, PPP
Layer 1transmissionEthernet, Token Ring, FDDI, V.110, X.25, Frame Relay, V.90, V.34, V.24

Even if only layers from 1 to 7 are officially defined, layer 0 is generally used for cabling and hardware. Layer 8 represents the user with his requirements or politics. Layer 9 is religion or a dogma.

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