What is the OSI model and what is its function


about OSI model

In the following article we are going to take a basic look at what the OSI model is and what its function is. East reference model of Open Systems Interconnection (OSI, Open System Interconnection) was released in 1984, and was the descriptive network model created by ISO (International Organization for Standardization). The OSI model is nothing more than a standard for communication protocols. RED. These protocols are communication rules used to connect two or more computers. What the OSI model does is group these protocols into specific groups or layers.

this standard pursued the ambitious goal of interconnecting systems of different origin, so that they could exchange information without any type of impediment, due to the protocols with which they operated according to their manufacturer. The OSI model is made up of 7 layers or levels of abstraction. Each of these levels will have its own functions, so that together they are able to reach their final goal. Precisely this separation in levels makes possible the intercommunication of different protocols, by concentrating specific functions in each level of operation.

As I said, each layer of the OSI model has a specific function and communicates with the layers above and below. The protocols will be responsible for communication between teams, so that a host can interact with a different one, layer by layer.

It must be remembered that OSI is a theoretical reference model, that is, a useful standard for systems from different manufacturers and/or companies to communicate optimally. One thing to keep in mind is that the OSI model is not the definition of a Topology nor a network model itself. What OSI really does is define their functionality to achieve a standard.. This model also does not specify or define the protocols that are used in the communication, since these are implemented independently.

The 7 layers of the OSI model

the OSI model stack

This architecture addresses the problem of electronic communication with a method of 7 layers or levels. The highest level information 7 layer, is where you work with application data, and these are encapsulated and transformed until they reach the Layer 1, or lower level, which manages pure bits to be transmitted to a physical medium (electrical signals, radio waves, pulses of light…).

Physical Layer (1 level)

This is the lowest layer of the OSI model, and takes care of the network topology and the global connections of the equipment to the network. It refers both to the physical medium and to the way in which information and networks are transmitted. The physical level or physical layer (1 level) is where the transformations that are made to the sequence of bits to transmit them from one place to another are performed.

This layer It is responsible for transmitting the bits of information through the medium used for transmission. It also deals with the physical properties and electrical characteristics of the various components. In addition, you will be in charge of mechanical aspects of connections and terminals, including the interpretation of electrical/electromagnetic signals.

Guided Media

The Physical Layer (1 level) is responsible for the physical connections of the equipment to the network, both in terms of the physical environment (guided media and unguided media), at medium characteristics (type of cable or its quality; type of standardized connectors, etc …) already the way information is transmitted.

The physical layer receives a stream of bits and tries to send it to the destination, and it is not its responsibility to deliver them error-free, since this responsibility falls to the data link layer. The physical layer provides services to the data link, with the objective that it provides services to the network layer.

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Data Link Layer (Layer 2)

This layer deals with physical addressing, medium access, error detection, ordered frame distribution, and flow control. It is responsible for the reliable transfer of information through a data transmission circuit. this layer receives requests from the network layer and uses the services of the physical layer.

Any transmission medium must be capable of providing error-free transmission, that is, reliable data transit over a physical link. To achieve this, you have to mount information blocks (called frames in this layer), provide them with a link layer address (MAC address), manage error detection or correction, and deal with flow control between teams. Therefore, this layer must create and recognize the limits of the frames, as well as solve the problems derived from the deterioration, loss or duplication of these information blocks.

parts of a plot

You can also include some traffic regulation mechanism, with which to avoid the saturation of a receiver that is slower than the transmitter.

The main functions of this layer are: initiation, termination and identification, segmentation and blocking, octet and character synchronization, frame delineation and transparency, error control, flow control, fault recovery and management, as well as communication coordination.

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Net Cloak (Level 3)

This is a level or layer that provides connectivity and path selection between two host systems, which may be located on geographically distinct networks. The units of data are called packets, and they can be classified into routable protocols and routing protocols. Offers services at the higher level (transport layer) and is supported by the data link layer, that is, use its functions.

The main task of the data link layer is to take a data transmission and transform it into an error-free one for the network layer.. It accomplishes this function by splitting the input data into dataframes (that you do not plot), and transmitting the frames sequentially to process the status frames it sends to the destination node.


To accomplish your task, can assign unique network addresses, interconnect different subnets, route packets, use congestion control and error control.

The job of the network layer is to get data from the source to the destination, even when the two are not directly connected. Routers work at this layer, although they can act as a layer 2 switch in certain cases, depending on the function assigned to it. What's more firewalls act on this layer mainly, to discard machine addresses.

Here is done the logical addressing of the terminal equipment, which is assigned an IP address.

Some network layer protocols are: IP, OSPF, IS-IS, ICMP, ICMPv6, IGMP.

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Transport Layer (Level 4)

This layer It is responsible for transporting the error-free data from the source machine to the destination machine., regardless of the type of physical network you are using.

The ultimate goal of the transport layer is provide an efficient and reliable service to users, which are typically application layer processes. To achieve this goal, this layer uses the services provided by the network layer. The transport layer hardware or software that handles the transport is called transportation entity.

network cables

This is the first layer that carries out end-to-end communication., and this condition will already be maintained in the upper layers.

Its basic function is to accept the data sent by the upper layers, divide it into small parts (segments) if necessary, and pass them to the network layer. In the case of the OSI model, it is also ensured that they arrive correctly at the other side of the communication. Another feature to note is that must isolate the upper layers from the different possible implementations of network technologies in the lower layers.

in this layer connection services are provided for the session layer, which will ultimately be used by network users when sending and receiving packets. The Internet has two main transport layer protocols, one connectionless (UDP), and one connection-oriented (TCP). These services will be associated with the type of communication used, which may be different depending on the request made to the transport layer.

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Session Layer (Level 5)

The session layer emerges as a way to organize and synchronize dialogue and control data exchange. Its mission is to organize the connection between both end systems., which is why it is also called the communication layer. A session allows the ordinary transport of data, as the transport layer does, but also provides enhanced services that are useful in some applications.

This layer is the one in charge of maintaining and controlling the link established between two computers that are transmitting data of any kind. What's more provides the mechanisms to control the dialogue between the applications of the end systems.

East Level 5 offers several services that are crucial for communication, as they are:

  1. Dialogue Control. It can be simultaneous in both directions (full-duplex) or alternated in both directions (half duplex)
  2. Grouping Control. With this it is achieved that two communications are not made at the same time.
  3. Recovery (checkpoints). These serve so that if a transmission interruption occurs, it can be resumed from the last verification point and not from the beginning.

Therefore, The service provided by this layer is the ability to ensure that, given a session established between two machines, it can be carried out for the operations defined from start to finish, resuming them in case of interruption.. In many cases, session layer services are partially or totally expendable.

The protocols that work in the Session layer are: RPC Protocol (remote procedure call), SCP (Secure Copy) and ASP (APPLE TALK session protocol).

Firewalls act on this layer, to block access to the ports of a computer.

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Presentation Layer (Level 6)

The purpose of the presentation layer is to take care of the representation of the information, so that although different computers may have different internal representations of characters (ASCII, Unicode, EBCDIC), numbers, sound or images, the data arrives in a recognizable way. Data is transported locally in standardized formats

This layer is the first to work more on the content of the communication than on how it is established. It deals with aspects such as the semantics and syntax of the transmitted data, since different computers may have different ways of handling them.

We can summarize this layer as the one in charge of handling the abstract data structures, and of carrying out the data representation conversions necessary for the correct interpretation of the same. In a nutshell he is a translator.

Layer 6 serves three main functions. These functions are: data formatting, data encryption and data compression.

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Application Layer (Level 7)

This layer offers applications (user or not) the possibility of accessing the services of the other layers, and defines the protocols used by applications to exchange data, such as email (POP and SMTP), database managers or a file server (FTP). There are as many protocols as there are different applications, since new applications are continually being developed, the number of protocols is constantly growing.

some application layer protocols

In this layer the connection for the other levels is established and the functions are prepared for the applications. Contains the applications visible to the user. It should be noted that the user does not normally interact directly with the application level. It usually interacts with programs that in turn interact with the application level.

Between the popular generic protocols include:

  1. HTTP (HyperText Transfer Protocol) for access to web pages.
  2. (FTPFile Transfer Protocol) for file transfer.
  3. SMTP (Simple Mail Transfer Protocol) for sending and distributing emails.
  4. POP (Post Office Protocol)/IMAP, for email retrieval.
  5. SSH (Secure SHell) mainly remote terminal.
  6. Telnet to access remote computers. Although it has fallen into disuse due to its insecurity, since the keys travel unencrypted over the network.

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With the intention of To facilitate learning and memorizing the names of the layers that make up the OSI model, there is a simple rule that consists of memorizing them as a mnemonic: FERTSPA. This in English would sound similar to First Spa (first spa in Spanish):


  • Fphysical
  • Elink
  • Red
  • Ttransportation
  • Session
  • Ppresentation
  • Aapplication

In short, it can be said that The OSI Stack is a model based on 7 layers or levels of abstraction. Each of the layers has its own functions, to together define a communication standard where hardware and different protocols can interact.

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  1.   Jesus said

    Thanks for the input! it never hurts to remember the OSI model