OSI model

The OSI model (English Open Systems Interconnection Model) is a reference model for networking protocols as a layered architecture. It is published as a standard since 1983 by the International Telecommunication Union and since 1984 also by the International Organization for Standardization. Its development began in 1977.

The purpose of the OSI model is to allow communication via different technical systems of time and to promote the development. For this purpose, this model defines seven consecutive layers (Layers ), each with very limited tasks. In the same layer with clear interfaces defined network protocols are easily interchangeable, even if they have such as the Internet Protocol a central function.

Motivation

In a computer network the different hosts services of various types are provided, from the other participants in the network. The required communication is not as trivial as it seems at first glance, because it must overcome a variety of tasks and requirements of reliability, safety, efficiency, etc., are met with respect. The problems to be solved questions ranging from the electronic transmission of signals via a regulated order in which communications to more abstract tasks that arise within the communicating applications.

Because of the variety of problems and tasks, it was decided to split this into different levels ( layers). In the OSI model, there are seven layers with specified requirements. On each layer each is an instance of the requirements.

The instances on transmitter and receiver must operate according to fixed rules, so that they agree on how the data are to be processed. 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 take advantage of a directly overlying instance. To provide the service itself uses an instance of the service of the immediately underlying instance. The real data flow is therefore vertical. The instances of a layer are then exactly interchangeable, if they can be exchanged both at the transmitter and the receiver.

The seven layers

The abstraction level of functionality increases from layer 7 to layer 1 from.

The OSI model at a glance ( see compared to the TCP / IP reference model):

Layer 7 - Application layer ( Application Layer)

Services, applications, and network management. The application layer provides functions for applications. This layer provides the connection to the lower layers. At this level, the data input and output takes place.

Layer 6 - Presentation Layer ( Presentation Layer)

The presentation layer (English Presentation Layer, and Data Presentation Layer, Data Provisioning level ) sets the system-dependent representation of the data (for example, ASCII, EBCDIC ) into an independent form and thus allows the syntactically correct data exchange between different systems. And functions such as data compression and encryption are to ensure that data sent from the application layer of a system that can be read by the application layer of another system for the layer 6, the presentation layer. If necessary, the presentation layer acts as a translator between different data formats, by making an understandable data format for both systems, the ASN.1 (Abstract Syntax Notation One) used.

Protocols and standards: ISO 8822 / X.216 ( Presentation Services ), ISO 8823 / X.226 ( Connection - Oriented Presentation Protocol), ISO 9576 ( Connectionless Presentation Protocol)

Layer 5 - Session Layer (Session Layer)

Layer 5 (English Session Layer, control of logical connections, including: session layer ) provides inter-process communication between two systems. Here we find, among other things, the protocol RPC (Remote Procedure Call). To resolve breakdowns of the meeting and similar problems, the session layer services for an organized and synchronized data exchange. For this purpose, recovery points, so-called fixed points ( check points ) was introduced, where the session can be synchronized after a failure of a transport connection again without the transfer must start from the beginning again.

Protocols and standards: ISO 8326 / X.215 (Session Service ), ISO 8327 / X.225 ( Connection - Oriented Session Protocol ), ISO 9548 ( Connectionless Session Protocol )

Layer 4 - Transport layer ( Transport Layer )

The responsibilities of the transport layer ( engl. Transport Layer, also: end-to- end control, transportation control) include the segmentation of the data stream and the congestion avoidance (English congestion avoidance ). The transport layer provides the application- oriented layers 5-7 uniform access, so that they do not need to take into account the characteristics of the communication network.

Five different service classes of different grades are defined in layer 4 and may be used by the upper layers, from the simplest to the most comfortable service with multiplexing mechanisms, error protection and error recovery procedures.

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

Layer 3 - Network layer (network layer)

The network layer (Network Layer, also: packet-level or network layer ) provides for line-oriented services for the switching of connections and packet-oriented services for the relaying of data packets. The data transfer in both cases across them across the entire communication network and includes the path search (routing ) between the network nodes. Since it is not always a direct communication between the sender and the destination is possible, packets must be forwarded by nodes that lie along the way. Next mediated packets do not reach the higher layers, but are provided with a new intermediate target and sent to the next node.

Among the most important tasks of the network layer include the provision of networked cross- addresses, routing, and the construction and updating of routing tables and the fragmentation of data packets. But the negotiation and ensuring a certain quality of service falls within the remit of the network layer.

In addition to the Internet Protocol include the NSAP addresses to this layer. Since a communication network of several sub- networks with different transmission media and protocols may be made, and the implementation functions are located in this layer, which are necessary for a redirect between the subnets.

Hardware on this layer: Routers, Layer 3 Switch ( brouter )

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

Layer 2 - Data Link Layer ( Data Link Layer)

Task of the data link layer ( engl. Data Link Layer, also: Section data link layer, data link layer, link layer, link-level, procedure level ) is to ensure a reliable, that is largely error-free transmission and to control access to the transmission medium. Serves dividing the Bitdatenstromes into blocks - referred to as frame or frame - and the addition of checksums in the channel coding. So bad blocks can be detected by the receiver and either discarded or even corrected; re requesting discarded blocks sees this layer but not before.

A " Flow Control " enables a receiver to dynamically controls the speed with which the other side is allowed to transmit blocks. The international engineering IEEE organization saw the need to regulate for local networks and to competing access to a transmission medium, which is not provided in the OSI model.

IEEE layer 2 is divided into two sub - layers (sub- layers ) LLC ( Logical Link Control, layer 2b) and MAC (Media Access Control layer 2a).

Hardware on this layer: Bridge, Switch ( multiport bridge)

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

Protocols and standards that are based on different layer -2 protocols and standards: HDLC, SDLC, DDCMP, IEEE 802.2 ( LLC), ARP, RARP, STP

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

Layer 1 - physical layer (Physical Layer)

The physical layer ( engl. Physical Layer) is the lowest layer. This layer provides mechanical, electrical and other functional tools are available to enable or disable them maintain and transmit bits over physical links. These may be, for example, electrical signals, optical signals ( optical fiber lasers), electromagnetic waves ( wireless network) or sound. The process they used is called the transmission-oriented method. Devices and network components that are associated with the physical layer, for example, the antenna and the amplifier, plug and socket for the network cable, the repeaters, the stroke, the transceiver, the tee and the terminating resistor is ( Terminator ).

The physical layer digital bit transmission is accomplished in a wired or leadless transmission path. Sharing a transmission medium can be carried out on said layer by means of static or dynamic multiplexing multiplexing. This requires not only the specifications of certain transmission media ( for example, copper cable, fiber optic cable, power supply ) and the definition of connectors other elements. Furthermore, it must be resolved at this level, on the way in which a single bit to be transmitted.

Thus, the following is meant: In computer networks today are mostly information transmitted in the form of bit or symbol sequences. In the copper cable and radio transmission, however, are modulated radio-frequency electromagnetic waves, the information carriers in the optical waveguide light waves of a certain wavelength or different. The information carrier know no bit strings, but can take much more different states than just 0 or 1 for each transmission type must therefore be established coding. This is done using the specification of the physical layer of a network.

Hardware on this layer: repeaters, hubs, cables, plugs, including

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

Example

General

The OSI reference model is often used when it comes to the design of network protocols and understanding their functions. On the basis of this model and network protocols have been developed, but these are almost only used in public communication technology, ie by major network operators such as Deutsche Telekom. In the private and commercial sector, the TCP / IP protocol family is mainly used. The TCP / IP reference model is tailored very specifically to the merger of networks ( Internetworking ).

The developed according to the OSI reference model, network protocols have in common with the TCP / IP protocol family that it is hierarchical models. However, there are significant conceptual differences: OSI defines the services determined exactly which shall provide each layer for the next higher. TCP / IP does not have such strict layers as the OSI concept. Neither the functions of the layers are well defined nor the services. It is allowed that a lower layer is used bypassing of intermediate layers directly from a higher layer. TCP / IP is therefore significantly more efficient than the OSI protocols. Disadvantage with TCP / IP is that it gives each its own network protocol for many small and micro services. OSI has, however, each set on its logs a large scope of services, which has very many options. Not all commercially available OSI software has implemented the full range. Therefore OSI profiles have been defined, which only contain a certain set of options each. OSI software from different vendors working together when the same profiles are implemented.

For the classification of communication protocols in the OSI model, see also:

The reference model for the telecommunications

The concept of the OSI model is derived from the data world is always transported data ( in form of data packets ). To map the telecommunications world on this model, the accessories were required. These additions take into account that in the telecommunications separate from the data streams signaling for connection establishment and termination is present, and that in the telecommunications equipment and facilities with the help of a management protocol configured remotely, monitored and suppressed. ITU- T has extended the OSI model by two additional protocol stacks for these additions and a generic reference model standardized ( ITU- T I.322 ). The three protocol stacks are referred to as

Each of these " plan " is in turn structured into seven layers according to OSI.

Standardization

The standardized reference model will be further developed in the ISO. The current status can be read in the ISO / IEC 7498-1:1994. The technical committee "Information Processing Systems " had set itself the goal of enabling information processing systems from different manufacturers to work together. Hence the name " Open Systems Interconnection " comes from.

At work in the context of the ISO and the Committee participated Open Communication Systems of DIN, which then took over the ISO standard as German industry norm in the original English version of the text. Also ITU- T took it: In a series of standards X.200, X.207, ... not just the reference model, but also the services and protocols of the individual layers are specified.

Other names for the model are the ISO / OSI model OSI reference model OSI reference model or 7 -layer model

Standardization documents:

  • ISO 7498-1, text identical with DIN ISO 7498, has the title Information technology - Open Systems Interconnection - Basic Reference Model: The basic model.
  • ITU- T X.200, X.207, ...

Analogy

The OSI model can be described by the following analogy from business life:

A company employee wants his business partner send a message. The employee is with the application process, which triggers the communication to equate. He speaks the message on a voice recorder. His assistant brings the news on paper. The wizard thus acts as the presentation layer. He then passes the message to the secretary, who administratively handles the sending of the message and thus represents the session layer. The house postal workers ( equal transport layer) brings the letter on the way. To this end, he clarifies with the network layer ( same post), which consist transmission paths, and selects the most suitable from. The postal worker brings the necessary endorsements on the envelope and passes it on to the distribution point, which corresponds to the data link layer. From there, the letter gets along with others in a means of transport such as truck or plane and after possibly several intermediate steps to the distribution point, which is responsible for the recipient.

On the receiver side, this process is run in reverse order, until the business partner finds spoken on a voice recorder the message.

This analogy does not point to the possibilities of error checking and repair, the OSI model provides, as these do not exist in the letter post.

Aphorisms

There are some mnemonics / computer science - mnemonics to the name of each OSI layers, which are often used for easier watchlist. Well sometimes one of the most popular sayings is " Please Do Not Throw Salami Pizza Away" (Physical Layer, Data Link Layer, etc.). A German version is " All the German students drink different kinds of beer " ( application layer, presentation layer, ...). Another very catchy mnemonic for the English version is "All priests drinking Tequila After the Sermon "

Satirical extension

Among IT professionals, the OSI model is sometimes extended to a non-existent eighth layer. Since in the model the last, seventh layer the user is closest, then one speaks of a problem on the OSI Layer 8, when its cause is suspected the user. Rarely financial, political and religious reservations in the eight to ten layers to be treated.

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