The OSI reference model

In order to exchange data, systems must be able to intercommunicate. Communication is possible only if the computers involved use the same file formats for data exchange and observe an agreed behavior during transfer. The sum of the conventions and file formats for communication is referred to as a protocol. Protocols are defined by the manufacturer (for example openFT protocols) on the one hand, and on the other by committees which define manufacturer-independent protocols. ISO (International Organization for Standardization) provides the OSI Reference Model (Open Systems Interconnection), the bestknown model for communications architecture and the most comprehensive collection of protocols.

The OSI Reference Model structures the communications functions of computer systems and provides a foundation for standardization of protocols and services. It specifies which functions the components involved in communication must provide.

The OSI Reference Model consists of seven hierarchically structured layers. Each layer is assigned specific communication functions.

Layers	Designation	Functions
Layer 7	Application Layer	Coordinates and controls the performance of communication tasks for an application	A P P L I C A T I O N
Layer 6	Presentation Layer	Regulates the form of information presentation and thus permits user/device-independent communication
Layer 5	Session Layer	Regulates the sequence of communication
Layer 4	Transport Layer	Regulates the reliable exchange of data between two communications partners	T R A N S P O R T
Layer 3	Network Layer	Regulates the exchange of data between two terminal systems (computers)
Layer 2	Data Link Layer	Secures the transmission on individual subroutes of the entire transmission route (procedures)
Layer 1	Physical Layer	Provides the physical connection (via the medium used for transmission)

OSI Reference Model

The individual layers use the service of the layer immediately below and provide a precisely defined service to the layer above. Only the physical layer must provide its service together with the physical medium. The active elements within a layer, which provide the functions, are referred to a instances.

Each layer is specified by the service it provides, and the services it uses from the layer below it. During communication, the various computers interoperate on the same layer, using common protocols.

The functionality of each layer in the OSI Reference Model can be provided by various protocols as a rule. Decisive for the communication is that the direct partner instances use the same protocol for a particular task. For this purpose, profiles are defined.

A profile is understood as precise specification of which protocols or which protocol variants are to be used on which layer to perform a particular task. Profiles are stipulated by national or international organizations or communities.