OSI model

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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 .


Communication in the OSI model using the example of layers 3 to 5

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 ):

OSI layer classification TCP / IP reference model classification Protocol examples units Coupling elements
7th Applications
application End to
( multihop )
Data Gateway , content switch , proxy , layer 4-7 switch
6th Presentation
5 Session
4th Transportation
transport TCP
TCP = segments
UDP = datagrams
3 Mediation / Package
Internet ICMP
Packages Router , layer 3 switch
2 Backup
(Data Link)
Network access
Point to
IEEE 802.3 Ethernet
IEEE 802.11 WLAN
Token Ring
Frame ( frames ) Bridge , Layer-2-Switch , Wireless Access Point
1 Bit transmission
Token Ring
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.

Hardware on this layer: repeaters , hubs , cables , connectors , etc. a.

Protocols and standards: V.24 , V.28 , X.21 , RS 232 , RS 422 , RS 423 , RS 499

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.

Hardware on this layer: bridge , switch (multiport bridge)

The Ethernet protocol describes both layer 1 and layer 2, where CSMA / CD is used as access control .

Protocols and standards that are based on other Layer 2 protocols and standards: HDLC , SDLC , DDCMP , IEEE 802.2 ( LLC ), RLC , PDCP , ARP , RARP , STP , Shortest Path Bridging

Protocols and standards that are based directly on Layer 1: IEEE 802.11 ( WLAN ), IEEE 802.4 ( Token Bus ), IEEE 802.5 ( Token Ring ), FDDI

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.

Hardware on this layer: router , layer 3 switch ( BRouter ).

Protocols and standards: X.25 , ISO 8208, ISO 8473 ( CLNP ), ISO 9542 (ESIS), IP , IPsec , ICMP .

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 .

Protocols and standards: ISO 8073 / X.224 , ISO 8602, TCP , UDP , SCTP , DCCP .

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.

Applications: web browser , e-mail program , instant messaging


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.

Structure of an Ethernet packet with maximum IPv4 / TCP data
Layer 4: TCP segment TCP headers Payload (1460 bytes)
Layer 3: IP packet IP header Payload (1480 bytes)
Layer 2: Ethernet frame MAC receiver MAC sender 802.1Q tag (opt.) EtherType (0x0800) Payload (1500 bytes) Frame check sequence
Layer 1: Ethernet packet + IPG preamble Start of frame Payload (1518/1522 bytes) Interpacket gap
Octets ( bytes ) 7th 1 6th 6th (4) 2 20th 20th 6-1460 4th 12

Brief summary

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.


ISO-OSI 7-layer model

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

Standardization documents:

  • 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.

Actor Corresponding OSI layer
Company employees / business partners application
assistant presentation
secretary meeting
In-house mail worker transport
Letter post Mediation
Distribution point Fuse
Mode of Transport Bit transfer

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.

See also


Web links

Wiktionary: OSI model  - explanations of meanings, word origins, synonyms, translations
Wikibooks: Network technology: OSI  - learning and teaching materials

Individual evidence

  1. Frequently Asked Questions about OSI with Answers. Retrieved June 22, 2020 (English).
  2. ITU-T X.200 (07/1994). International Telecommunication Union, accessed February 25, 2013 .
  3. ^ William Stallings: The Origins of OSI. Retrieved February 25, 2013 .
  4. Wetherall, David: Computer networks . 5th ed., Pearson new internat. Ed. Pearson Education, Harlow, Essex 2014, ISBN 1-292-02422-4 .
  5. Wetherall, David: Computer networks . 5th ed., Pearson new internat. Ed. Pearson Education, Harlow, Essex 2014, ISBN 1-292-02422-4 .
  6. 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.
  7. ISO / OSI 7-layer model. In: www.elektronik-kompendium.de. Retrieved November 2, 2016 .
  8. PH Heidelberg / Didactics of the ITG / OSI layer model - ZUM-Wiki. In: wiki.zum.de. Retrieved November 2, 2016 .
  9. ISO / OSI 7-layer model. Retrieved March 2, 2017 .
  10. ISO / OSI 7-layer model (version from December 2010) ( Memento from December 25, 2010 in the Internet Archive ) on elektronik-kompendium.de
  11. Layer 8 (Financial) and layer 9 (Political) of theOSI protocol stack (PDF; 334 kB) on flora. Approx
This version was added to the list of articles worth reading on January 9, 2007 .