▷ Osi model: what it is and what it is used for

In this article we will try to define in detail what the OSI model is. Despite the fact that the network model used in local area networks does not theoretically coincide with this communication model, they do have many characteristics of its own. In addition, we must bear in mind that this varies depending on the different network topologies used especially in business environments and large companies. What the OSI model intends is that we understand in a standardized way the different levels of communication.

Index of contents

Currently we always have the construction of standardized models for different aspects of our environment. We see this more sharply in telecommunications protocols between machines. Standardization is necessary for an environment in which there are a large number of networks and types of machines connected to them, not to mention the large number of telecommunications operators that exist in the market.

An example of this is the model proposed by ISO, this has been key to achieving precisely the development of these communications among a multitude of elements that are essentially totally different from each other. Let us now see in detail its main points of interest.

What is the OSI model

The OSI model was developed back in 1984 by the ISO organization (International Organization for Standardization). This standard pursued the ambitious objective of interconnecting a system of different origin so that this could exchange information without any type of impediment due to the protocols with which they operated in their own way according to their manufacturer.

The OSI model is made up of 7 layers or levels of abstraction. Each of these levels will have their own functions so that together they are able to achieve their final objective. Precisely this separation into levels makes possible the intercommunication of different protocols by concentrating specific functions at each level of operation.

Another thing to keep in mind is that the OSI model is not the definition of a topology or a network model in itself. Nor does it specify or define the protocols used in communication, since they are implemented independently of this model. What OSI really does is define their functionality to achieve a standard.

The levels of which the OSI model is composed are:

Types of service

The OSI model establishes the two basic types of service that exist for telecommunications:

• With connection: it is necessary to establish a connection through a circuit first to exchange information. One type of communication with connection is the telephone, both mobile and fixed. No connection: to send or receive information it is not necessary to establish a circuit. The message is sent with a destination address and it will arrive as quickly as possible, but not necessarily ordered. A typical example is sending emails.

Concepts and terminology used in the OSI model

To talk about OSI we must also know different terms that are directly related to it. If they did not we would understand many of the concepts of the model.

System

It is the physical element where the model is applied. It is the set of physical machines of various kinds that, connected, are capable of transferring information

Model

A model helps define a structure along with a series of functions that the telecommunications system will perform. A model does not provide the definition of how a telecommunications network should be implemented, but only defines what the standard procedure for exchanging information should be.

level

It is a set of specific functions to facilitate communication grouped into an entity that in turn is related to both a lower level and a higher level.

Interactions between levels are called primitives, and can be prompts, responses, requests, or confirmations. Each level has these characteristics:

• Each level is designed to perform specific functions. When we need to implement certain functions to the network, we will apply the level that corresponds to these functions. Each of these levels is related to the previous and subsequent levels on the abstraction scale. Obtains data from the lower level and provides these to the higher level Each level contains services that are independent of practical implementation Limits must be established for each level as long as they ensure the flow of information between each one

Function or Algorithm

It is a set of instructions that are related to each other so that, through input stimuli (arguments), it produces certain outputs (outputs).

OSI layers

Basic operation

Now we have to talk about the seven levels established by the OSI communication standard. Each of these levels will have their own functions and protocols that will work to communicate with other levels.

The protocols of each level communicate with their counterparts or peers, that is, their own protocol located at the other end of the communication. In this way, other protocols of other levels will not have influence.

To establish the information flow, the originating machine sends the information that will depart from the most superficial layer to the physical layer. Then in the destination machine the flow will reach this physical layer and rise to the most superficial layer that exists.

In addition, each level works independently of the others, if need know the operation of the other levels. In this way each one is modifiable without influencing the others. For example, if we want to add a physical equipment or a network card, this will only influence the layer that controls these devices.

The levels can be divided into two groups, those that are network oriented and those that are application oriented.

Network-oriented OSI levels

These levels are responsible for managing the physical section of the connection, such as establishing communication, routing it and sending

Layer 1: Physics

This level deals directly with the physical elements of the connection. It manages the procedures at the electronic level so that the string of information bits travels from the transmitter to the receiver without any alteration.

• Defines the physical transmission medium: twisted pair cables, coaxial cable, waves, and fiber optics Manages electrical signals and transmits bit stream Defines the characteristics of materials such as connectors and voltage levels

Some standards related to this level are: ISO 2110, EIA-232, V.35, X.24, V24, V.28

Layer 2: Data Link

This level is in charge of providing the functional means to establish the communication of the physical elements. It deals with the physical routing of data, access to the medium and especially the detection of errors in transmission.

This layer builds the bit frames with the information and also other elements to control that the transmission is done correctly. The typical element that performs the functions of this layer is the switch or also the router, which is responsible for receiving and sending data from a transmitter to a receiver

The best known protocols for this link are IEEE 802 for LAN connections and IEEE 802.11 for WiFi connections.

Layer 3: Red

This layer is responsible for identifying the routing between two or more connected networks. This level will allow the data to arrive from the transmitter to the receiver, being able to make the necessary switching and routing for the message to arrive. Due to this, it is necessary that this layer knows the topology of the network in which it operates.

The best known protocol that does this is IP. We also find others like IPX, APPLETALK or ISO 9542.

Layer 4: Transportation

This level is in charge of transporting the data found within the transmission packet from the origin to the destination. This is done independently of the type of network that the lower level has detected. The information unit or PDU before seen, we also call it Datagram if it works with the UPD protocol oriented towards connectionless sending, or Segment, if it works with the protocol TCP oriented towards connection.

This layer works with logical ports such as 80, 443, etc. In addition, it is the main layer where sufficient quality must be provided so that the transmission of the message is carried out correctly and with the user's requirements.

Application-oriented OSI levels

These tiers work directly with applications that request lower tier services. It is in charge of adapting the information so that it is understandable from the point of view of a user, through an interface and a format.

Layer 5: Session

Through this level, the link between the machines that are transmitting information can be controlled and kept active. This will ensure that once the connection is established, it is maintained until the transmission ends.

It will be responsible for mapping the session address that the user enters to pass them to transport addresses that the lower levels work with.

Layer 6: Presentation

As its name suggests, this layer is responsible for the representation of the transmitted information. It will ensure that the data that reaches users is understandable despite the different protocols used in both a receiver and a transmitter. They translate a string of characters into something understandable, so to speak.

This layer does not work with message routing or links, but is responsible for working with the useful content that we want to see.

Layer 7: Application

This is the last level, and in charge of allowing users to execute actions and commands in their own applications such as a button to send an email or a program to send files using FTP. It also allows communication between the rest of the lower layers.

An example of the application layer can be the SMTP protocol for sending emails, FTP file transmission programs, etc.

Data Entities in the OSI model

It is an element that processes information in an open system to apply it to certain functions. In this case, it will try to process information for its exchange between machines. A process consists of:

• Service access point (SAP): place where each layer finds the services of the layer just below Interface Data Unit (IDU): block of information that one layer passes to a lower layer Data unit of the protocol (N-PDU): information packets that carry the information that is intended to be sent over the network. This information will be divided and composed of a header that carries control information. This information is exchanged between two entities that belong to the same level in different places. Service Data Unit (SDU): Each IDU consists of an Information Field for Interface Control (ICI) and another field with Information with Network Information (SDU). A n-level SDU represents the n + 1 level PDU, thus n + 1-PDU = n-SDU

Graphically it could be represented as follows:

Data transmission process in the OSI model

Let us now see how the layers of the OSI model work in the transmission of data.

1. The application layer will receive the message from the user. The message is located in the application layer. This layer adds an ICI header to it to form the application layer PDU and it is renamed IDU. Now go to the next layer The message is now located in the presentation layer. This layer adds its own header to it and it is transferred to the next layer The message is now in the session layer and the previous procedure is repeated again. The physical layers are then sent In the physical layers the packet will be properly addressed to the receiver When the message reaches the receiver each layer removes the header that its approved layer has placed to transmit in message Now the message reaches the application layer of the destination to be delivered to the user understandably

This concludes our article on the OSI model

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