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OSI reference model

The OSI reference model of the International Standards Organization (ISO) was designed from 1977 onwards as the basis for the formation of communication standards. The aim of Open Systems Interconnection (OSI) is communication in heterogeneous networks

, in particular between different computer worlds on the basis of application-supporting basic services. These basic services are, for example, file transfer, the virtual terminal, remote access tofiles and the exchange of electronic mail. For these and all other communication applications, in addition to the actual application data, additional structural and procedural information is required, which is defined as OSI protocols

.In the OSI reference model, communication is implemented by service elements or work units that have a fixed location and a defined task. The service elements can be implemented in terms of hardware, firmware and software. Elements with similar functions can reside on different systems and are arranged in layers

. Each layer describes the functions of the elements. An element that implementsobjects and provides operations

isreferred to as anentity

  • Division of the architecture into layers,
  • Division of layers into working units,
  • Cooperation of working units within a layer,
  • cooperation of working units in adjacent layers, and the
  • cooperation of working units in the same layers of different systems

interface between two layers is, from top to bottom, a principal/agent interface. A work unit within a layer performs a certain service

, using tools that are available to it locally or that are provided by a work unit in the next layer down. In addition to this interface, it is important to maintain a set of rules with like work units on another system. Such a set of rules is called aprotocol

The OSI reference model is divided into seven layers and is therefore also referred to as the 7-layer

model OSI reference model with the transport-oriented and application-oriented

model OSI reference model with the transport-oriented and application-oriented

layersThe layers can be divided into two groups: the transport-oriented and the application-oriented layers. The transport-oriented layers include layers 1 to 4; the application-oriented layers include layers 5 to 7. These layers each represent the framework for standards

and describe objects, operations, work units and protocols in a simplified and general manner. Thedescriptions are then refined in individual communication standards for the individual layers.

The seven layers of the OSI reference model

. Thelowest layer 1 is thephysical

layer.Thedata link


combines sequences of binary information into data packets or, if necessary, resolves larger units coming from a higher layer into smaller data packets


The data link layer essentially considers two-point connections.

Functions of the individual layers in the OSI reference model

Functions of the individual layers in the OSI reference model

This changes in thenetworklayer, whose most important task is routing, i.e. determining an optimal path through a branched network. Layer three implements an end-to-end connection

between two communicating stations. It ensures that a data packet arrives at the right place, regardless of the path it has taken through the branched network.The protocols of the next layer, the transportlayer, have an end-to-end character because they implement a connection between individual processes in different end systems. The transport-oriented layers 1 to 4 provide layer five, the communication control layer (session layer), with a universal transport service (process-to-process connection). A session of the communication control layer denotes the logical connection

between two working units of the top layer that communicate with each other. Thepresentation


provides special services

fortransforming the

data to an agreed standard format and for uniform interpretation. Theuppermost layer 7 is theapplication


, which provides thedistributed applications with logical communication support in the form of specific services such as e-mail, file transfer, virtual terminal (VT) or remote job entry

(RJE). TheOSI reference model defines serviceusers and serviceproviders


Each layer n is a service provider for the service user of the higher layer n+1. The working entities of layer n realize services that can be used by working entities of layer n+1. A service is achieved by accessing service access points, service access point (SAP). Each service access point has its own address

within a layer.

Service access point of a layer

Service access point of a layer

Service access points (SAP) for each layer are identified with a prefix, for example, session service access point (SSAP) for a communication control layer service access point (SAP), transport service access point (TSAP) for a network layer service access point, and data link layer service access point (LSAP) for a link layer service access point.

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Englisch: OSI reference model
Updated at: 10.02.2017
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