The ISO / OSI reference model ( English Open Systems Interconnection model ) is a reference model for network protocols as a layer architecture . It has been published as a standard by the International Telecommunication Union (ITU) since 1983 and also by the International Organization for Standardization (ISO) since 1984 . Its development began in 1977.
The purpose of the OSI model is to enable communication across a wide variety of technical systems and to promote further development. For this purpose, this model defines seven successive layers (Engl. Layers ) each narrowly defined tasks. Network protocols defined in the same layer with clear interfaces are easily interchangeable, even if they have a central function like the Internet Protocol .
In a computer network , different types of services are provided to the various clients by other hosts. The communication required for this turns out to be more complicated than it might appear at the beginning, since a large number of tasks have to be mastered and requirements with regard to reliability, security, efficiency, etc. The problems to be solved range from questions of the electronic transmission of signals to a regulated sequence in communication to more abstract tasks that arise within the communicating applications.
Due to this large number of tasks, the OSI model was introduced, in which the communication processes are divided into seven levels (also called layers). The requirements are implemented separately on each individual layer .
The instances used must work according to defined rules on both the sender and the recipient side in order to enable the processing of data. The definition of these rules is described in a protocol and forms a logical, horizontal connection between two instances of the same layer.
Each instance provides services that can be used by an instance directly above it. To provide the service, an instance itself uses the services of the instance immediately below. The real data flow is therefore vertical . The instances of a layer are exchangeable if they can be exchanged at both the sender and the receiver.
The seven layers
The degree of abstraction of the functionality increases from layer 1 to layer 7.
The OSI model at a glance (see in comparison the TCP / IP reference model ):
|TCP / IP reference model
( multihop )
|Gateway , content switch , proxy , layer 4-7 switch
TCP = segments
UDP = datagrams
|Mediation / Package
|Router , layer 3 switch
|IEEE 802.3 Ethernet
IEEE 802.11 WLAN
|Frame ( frames )
|Bridge , Layer-2-Switch , Wireless Access Point
|Bits , symbols
|Network cable , repeater , hub
Layer 1 - bit transmission layer (physical layer)
The physical layer is the lowest layer. This layer provides mechanical, electrical and other functional tools to activate or deactivate physical connections, to maintain them and to transfer bits over them. These can be, for example, electrical signals, optical signals (light guides, lasers), electromagnetic waves (wireless networks) or sound. The methods used for this are referred to as transmission methods . Devices and network components that are assigned to the physical layer are, for example, the antenna and the amplifier , plug and socket for the network cable , the repeater , the hub , the transceiver , the T-piece and the terminating resistor (terminator).
On the bit transfer layer, digital bit transfer is carried out on a wired or wireless transmission link. A transmission medium can be shared on this layer by means of static multiplexing or dynamic multiplexing . In addition to the specifications of certain transmission media ( e.g. copper cables , fiber optic cables , power grid ) and the definition of plug connections , this requires further elements.
In addition, the way in which a single bit should be transmitted must be resolved at this level : In computer networks, information is transmitted in the form of bit or symbol sequences . In copper cables and in radio transmission , modulated, high-frequency, electromagnetic waves are the information carriers, in fiber optics light waves of one or more specific wavelengths. Depending on the modulation, the information carriers can not only assume two states for zero and one , but possibly many more. A coding must therefore be defined for each type of transmission . This is done with the help of the specification of the physical layer of a network.
Layer 2 - data link layer
The task of the data link layer is to ensure reliable, i.e. largely error-free, transmission and to regulate access to the transmission medium , including section security layer , data security layer , connection security layer , connection level , procedure level . These dividing the Bitdatenstromes used in blocks - as frames or frame called - and adding checksums as part of the channel coding . In this way, incorrect blocks can be recognized by the recipient and either discarded or even corrected; however, this layer does not provide for a renewed request for discarded blocks.
A " data flow control " enables a receiver to dynamically control the speed at which the other side can send blocks. The international engineering organization IEEE saw the need to regulate competitive access to a transmission medium for local networks , which is not provided for in the OSI model.
IEEE layer 2 into two sub-layers (sub layers) divided: LLC ( Logical Link Control , layer 2b) and MAC ( Media Access Control , layer 2). In an older definition of the OSI layers, layer 2 does not contain many media access control components; these functions must be taken over by higher OSI layers there.
Layer 3 - Network Layer
The network layer (engl. Network Layer ; also packet level or network layer ) provides for circuit-oriented services for the switching of connections and packet-oriented services for the forwarding of data packets and the congestion avoidance (engl. Congestion Avoidance ). In both cases, the data transmission goes over the entire communication network and includes the route search ( routing ) between the network nodes . Since direct communication between sender and destination is not always possible, packets must be forwarded by nodes on the way. Forwarded packets do not reach the higher layers, but are given a new intermediate destination and sent to the next node.
The most important tasks of the network layer include the provision of cross-network addresses, the routing or the construction and updating of routing tables and the fragmentation of data packets. But negotiating and ensuring a certain quality of service also falls within the remit of the network layer.
In addition to the Internet Protocol , the NSAP addresses also belong to this layer. Since a communication network can consist of several sub-networks with different transmission media and protocols, this layer also contains the implementation functions that are necessary for forwarding between the sub-networks.
Layer 4 - Transport Layer
The tasks of the transport layer (also known as end-to-end control , transport control ) include segmentation of the data stream, congestion avoidance , and ensuring error-free transmission.
A data segment is a service data unit that is used for data encapsulation on the fourth layer (transport layer). It consists of protocol elements that contain layer 4 control information. The data segment is assigned a layer 4 address as addressing, i.e. a port . The data segment is encapsulated in layer 3 in a data packet .
The transport layer offers the application-oriented layers 5 to 7 uniform access so that they do not have to take the properties of the communication network into account.
Five different service classes of different quality are defined in layer 4 and can be used by the upper layers, from the simplest to the most convenient service with multiplex mechanisms , error protection and error correction procedures .
Layer 5 - Session Layer
Layer 5 (control of logical connections; English session layer ; also session layer , communication layer , communication control layer ) ensures process communication between two systems. Among other things, the RPC ( Remote Procedure Call ) protocol is found here . The session layer provides services for an organized and synchronized exchange of data in order to deal with breakdowns in the session and similar problems . For this purpose, restart points, so-called check points, are introduced at which the session can be synchronized again after a failure of a transport connection without the transmission having to start again from the beginning.
Protocols and standards: ISO 8326 / X.215 (Session Service), ISO 8327 / X.225 (Connection-Oriented Session Protocol), ISO 9548 (Connectionless Session Protocol)
Layer 6 - Presentation Layer
The presentation layer (engl. Presentation Layer ; also data presentation layer , data delivery level ) sets the system-dependent representation of the data (for example, ASCII , EBCDIC ) in an independent form, and thus enables the syntactically correct data exchange between different systems. Tasks such as data compression and encryption also belong to layer 6. The presentation layer ensures that data sent from the application layer of one system can be read by the application layer of another system. If necessary, the presentation layer acts as a translator between different data formats by using a data format understandable for both systems, the ASN.1 (Abstract Syntax Notation One).
Protocols and standards: ISO 8822 / X.216 (Presentation Service), ISO 8823 / X.226 (Connection-Oriented Presentation Protocol), ISO 9576 (Connectionless Presentation Protocol)
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. The applications themselves do not belong to the layer.
The levels of the widespread network system “TCP / IP over Ethernet” do not exactly correspond to the OSI model and are therefore partly across OSI layers.
|Layer 4: TCP segment
|Payload (1460 bytes)
|Layer 3: IP packet
|Payload (1480 bytes)
|Layer 2: Ethernet frame
|802.1Q tag (opt.)
|Payload (1500 bytes)
|Frame check sequence
|Layer 1: Ethernet packet + IPG
|Start of frame
|Payload (1518/1522 bytes)
|Octets ( bytes )
7th layer / application : functions for applications as well as data input and output.
6th layer / representation : Conversion of the system-dependent data into an independent format.
5th layer / session : Control of the connections and the data exchange.
4th layer / transport : assignment of the data packets to an application.
3rd layer / switching : Routing of the data packets to the next node.
2nd layer / security : segmentation of the packets into frames and addition of checksums.
1st layer / bit transmission : Conversion of the bits into a signal suitable for the medium and physical transmission.
The OSI reference model is often used when designing network protocols and understanding their functions. Network protocols have also been developed on the basis of this model, but these are used almost exclusively in public communication technology, i.e. by large network operators such as Deutsche Telekom . The TCP / IP protocol family is mainly used in the private and commercial sector . The TCP / IP reference model is specially tailored to the connection of networks (internetworking) .
The network protocols developed according to the OSI reference model have in common with the TCP / IP protocol family that they are hierarchical models. There are, however, essential conceptual differences: OSI precisely defines the services that each layer has to provide for the next higher. TCP / IP does not have such a strict layer concept as OSI. The functions of the layers are not precisely defined, nor are the services. It is permitted for a lower layer to be used directly by a higher layer, bypassing intermediate layers. TCP / IP is therefore considerably more efficient than the OSI protocols. The disadvantage of TCP / IP is that there is a separate network protocol for many small and very small services. OSI, on the other hand, has defined a large scope of performance for each of its protocols, which has many options. Not every commercially available OSI software has implemented the full scope of performance. For this reason, OSI profiles were defined, each containing only a certain set of options. OSI software from different manufacturers works together when the same profiles are implemented.
For the classification of communication protocols in the OSI model, see also:
The reference model for telecommunications
The concept of the OSI model comes from the data world, which always transports user data (in the form of data packets). In order to map the telecommunications world onto this model, additions were necessary. These additions take into account the fact that in telecommunication there is signaling for connection setup and disconnection that is separate from the data streams , and that in telecommunication the devices and facilities are configured, monitored and interference suppression remotely using a management protocol . For these additions, ITU-T has expanded the OSI model by two additional protocol stacks and standardized a generic reference model (ITU-T I.322). The three protocol stacks are referred to as
- User data (user plane)
- Signaling (Control Plane)
- Management (Management Plane)
Each of these “planes” is structured in seven layers according to OSI.
The standardized reference model is being further developed in the ISO . The current status can be found in the ISO / IEC 7498-1: 1994 standard. The technical committee "Information Processing Systems" set itself the goal of enabling information processing systems from different manufacturers to work together. This is where the name "Open Systems Interconnection" comes from.
The Open Communication Systems Committee of DIN also took part in the work within the framework of ISO , which then adopted the ISO standard as a German industrial standard in the original English version of the text. Also, ITU-T took it: In a series of standards X.200, X.207, ... specified the individual layers not only the reference model, but also the services and protocols.
Further names for the model are ISO / OSI model , OSI reference model , OSI layer model or 7-layer model
- ISO 7498-1, the same text as DIN ISO 7498, has the title Information technology - Open Systems Interconnection - Basic Reference Model: The basic model .
- ITU-T X.200, X.207, ...
The OSI model can be described using the following analogy from business life:
A company employee wants to send a message to his business partner . The employee is to be equated with the application process that initiates the communication . He speaks the message on a voice recorder . His assistant puts the message on paper . The wizard thus acts as a presentation layer. Then he gives the message to the secretary, who handles the dispatch of the message administratively and thus represents the session layer. The in-house mail employee (same transport shift) gets the letter on its way. To do this, he clarifies with the network layer (same as letter post) which transmission paths exist and selects the appropriate one. The postal worker applies the necessary notes to the envelope and passes it on to the distribution point that corresponds to the security layer. From there, the letter arrives with others in a means of transport such as a truck or plane and, after possibly several intermediate steps, to the distribution point that is responsible for the recipient.
|Corresponding OSI layer
|Company employees / business partners
|In-house mail worker
|Mode of Transport
On the recipient's side, this process is carried out in reverse order until the business partner finds the message spoken on a dictation machine.
This analogy does not show which options for checking and correcting errors are provided by the OSI model, since these are not available when sending letters.
There are some donkey bridges / computer science mottos about the names of the individual OSI layers, which are often used to make it easier to remember. Well sometimes one of the most popular sayings is " P lease D o N ot T HROW S alami P izza A way" ( physical layer , data link layer , etc.). A German version is " A lle d eutsche S tudent t Drink v arious S locate B ier" (application layer, presentation layer, ...). A very catchy German mnemonic for the English name of the layers is " A ll P Riester s aufen T equila n oh d he P redigt" and the English version " A ll P eople S eem t o N eed D ata P rocessing" .
Further layers not located in the OSI model
Among IT specialists, the OSI model is occasionally expanded to include additional layers that do not exist in the OSI model. Since the top, seventh layer is closest to the user in the model, z. For example, in addition to the terminals themselves, the user can also be assigned to an 8th layer if this is considered useful for a communication case description. Among other things, financial and political circumstances are also viewed as higher strata and therefore understood as strata 8 and 9.
- Data encapsulation (network technology)
- Interface control information
- IP address
- IP packet
- Layer 8
- Mobile IP
- Protocol stack
- Service access point
- Protocol data unit
- Gerd Siegmund: Basics of switching technology . R. v. Decker; Heidelberg; 1992, ISBN 3-7685-4892-9
- P. Stahlknecht, U. Hasenkamp: Introduction to business informatics . Jumper; Berlin; 2002, 10th edition, ISBN 3-540-41986-1
- Andrew S. Tanenbaum : Computer Networks. 5th, updated edition, Pearson Studium, Munich 2012, ISBN 978-3-86894-137-1
- Günter Müller , Torsten Eymann , Michael Kreutzer: Telematics and communication systems in the networked economy . Oldenbourg; Munich, Vienna; 2003, ISBN 3-486-25888-5
- Roland Bauch: Networks - Basics. Herdt-Verlag ; 6th edition, 1st update, December 2009
- The OSI reference model
- ISO / IEC standard 7498-1: 1994 ( ZIP format ) with PDF, 7.3 MB (English, associated license agreement )
- ITU-T X.200 (the same contents as from ISO) (English)
- Basics of computer networks: ISO reference model for data communication - script on Netzmafia.de
- OSI Reference Model — The ISO Model of Architecture for Open Systems Interconnection , Hubert Zimmermann, IEEE Transactions on Communications, vol. 28, no. 4, April 1980, pp. 425-432. (PDF; 776 kB)
- Frequently Asked Questions about OSI with Answers. Retrieved June 22, 2020 (English).
- ITU-T X.200 (07/1994). International Telecommunication Union, accessed February 25, 2013 .
- William Stallings: The Origins of OSI. Retrieved February 25, 2013 .
- Wetherall, David: Computer networks . 5th ed., Pearson new internat. Ed. Pearson Education, Harlow, Essex 2014, ISBN 1-292-02422-4 .
- Wetherall, David: Computer networks . 5th ed., Pearson new internat. Ed. Pearson Education, Harlow, Essex 2014, ISBN 1-292-02422-4 .
- According to Tanenbaum , the layer is called the communication control layer; The session layer is just a literal translation and can be misleading depending on the specific implementation.
- ISO / OSI 7-layer model. In: www.elektronik-kompendium.de. Retrieved November 2, 2016 .
- PH Heidelberg / Didactics of the ITG / OSI layer model - ZUM-Wiki. In: wiki.zum.de. Retrieved November 2, 2016 .
- ISO / OSI 7-layer model. Retrieved March 2, 2017 .
- ISO / OSI 7-layer model (version from December 2010) ( Memento from December 25, 2010 in the Internet Archive ) on elektronik-kompendium.de
- Layer 8 (Financial) and layer 9 (Political) of theOSI protocol stack (PDF; 334 kB) on flora. Approx