OSI Model

OSI Model

OSI model. Approach to the theory of network design.

Hello, my niece wants to talk about networks, and I’m doubting, because today’s topic is a bit abstract and exhausting. Following the concepts of the previous post, today’s topic is inevitable. I’ll make an effort to summarize it without losing the idea and thus ensure that my niece reads the text from beginning to end.


The open systems interconnection model, known as OSI, of the acronym “Open System Interconnection”, was published in 1984. In 1977 the International Organization for Standardization present in more than 170 countries began to analyze the existing connection models in order to find a common structure and develop a series of rules that allow all manufacturers to produce compatible models with each other.

In the OSI model, several layers were defined. In these layers, the theoretical functions necessary to obtain an independent, modular and standard communications network were specified. Although this model did not have a practical application in networks, it did work as a standard comparative framework for the interaction between protocols from different manufacturers and designers.

The model consists of seven defined layers with their main characteristics:

  • The physical layer is responsible for interacting with the network at the most basic level: electrical, optical, radiofrequency signals.
  • The data link layer ensures communication between computers within the network’s reach.
  • The network layer allows the sending of information outside the network.
  • The transport layer is responsible for sending information through the network.
  • The session layer manages the links created.
  • The presentation layer ensures the correct interpretation of the data.
  • The application layer It offers different protocols designed for specific connections according to the type of information sent.

The union of all the services of the different layers makes a communication network work globally and independent of the manufacturers.

OSI Model: Layer 1 – Physics

The physical layer manages the physical connection of the network equipment performs all the conversions, adaptations and encodings necessary to be transmitted, in other words, it translates the binary information that is in the form of electrical signals to signals that can be transmitted through the medium. selected. Not all physical media have the same properties, nor all protocols that use the physical medium will use the same signal characteristics to send the information. For this reason, the OSI reference model only lists some of the functions and conversions that may be needed and it will depend on each specific case how to send the information. For example, wireless systems use radiofrequency signals, fiber optic uses light pulses, etc.

OSI Model: Layer 2 – Data Link

The function of the data link layer is to send information without errors, between two machines that are directly connected. To ensure there is no transmission error, this layer incorporates error detection and correction services, as well as traffic flow control to ensure maximum efficiency in the transfer of information.

Being a theoretical model can be applied to different communications systems, for example, PPP, “Point-to-Point Protocol”, or HDLC, “High-level Data Link Control” although the best known protocol is Ethernet, which It is part of the IEEE family of protocols.

The OSI layer model allows the data link layer to perform the functions regardless of the physical medium used.

  • This layer provides a reliable link within the local network. For this, the procedures for establishing, controlling and terminating links are established. As an intermediate layer, it receives requests from its upper layer, the network layer and it is likely that the information received is larger than the bits that fit in a single layer two packet, so this layer must manage how to divide this information in the shipment and in the same way, unify this information once received.
  • This layer incorporates an error management and control system. It must be able to locate any error in the transfer to solve it by itself if the coding allows it or request the resending of the compromised information. Although each protocol establishes its own systems, error detection codes that are added to the information sent are generally used and ensure the reliability of communications to the upper layers.
  • This layer incorporates flow control systems. Since different teams with different capacities can coexist in the same network, the idea is to prevent a very powerful team from saturating the capabilities of a simpler team.

OSI Model: Layer 3 – Network

The network layer allows the connection and reliable transmission of information between devices that are located in different networks. This ensures that there is communication when there is no direct connection between computers.

This layer uses the services of the data link layer that is just below. In order to carry out this non-local communication, a system that assigns unique network addresses for each computer that uses that protocol is necessary. This address assignment allows communication between any of the nodes that make up that network. Here we find the routing protocols that are responsible for finding the most efficient way to send information between two nodes in a network. Some examples of this layer’s protocols would be “Internet Protocol” (IP), “Internet Control Message Protocol” (ICMP), “Internetwork Packet Exchange” (IPX), “Internet Group Management Protocol” (IGMP).

The protocols depend to a large extent on the format and distribution of the network addresses, so in the face of different network protocols we will find different routing

There are two large blocks of routing protocols:

Static routing protocols.

These protocols simply follow a map of routes preset by the network administrators, they are fast and easy to manage, but they tend to generate problems in case of failures not foreseen in the network.

Dynamic routing protocols.

They are those who learn the best routes helped by the network activity itself. They may have a preset table at boot time, but over time they are modified depending on the behavior of the network, either by the traffic they manage themselves or by the information they receive from other routing equipment.

OSI Model: Layer 4 – Transportation

The transport layer seeks to offer a system to transfer error-free information to the set of elements in the upper layers. In this layer some very useful concepts are added so that multiple applications can use connectivity.

The main point is to ensure that the communication has been made without errors. The transport layer will ensure that the information is delivered reliably. If for any problem this layer detects a transmission error, manages its repetition or simply discards the information, but does not deliver it to the higher levels.

The transport layer adds some flow control improvements over the network layer and manages to better manage congestion problems or partial service outages. Taking into account that network problems may arise that cause the information not to be received in the same order in which it was sent, this layer is responsible for delivering the information in the correct order.

It is currently unthinkable that a single application can work with network access. The usual thing is to have different applications with access to the information working simultaneously. For this, in layer four a differentiation of traffic flows is carried out through ports, so this layer can deliver the correct information to each top-level application without mixing, offering its service simultaneously to as many applications as they need. There are multiple transport protocols, for example “AppleTalk Transport Protocol” (currently deprecated), “Remote Desktop Protocol” RDP or “Stream Control Transmission” and the best known “Transmission Control Protocol” (TCP) and “Datagram Protocol of user ”(UDP).

OSI Model: Layer 5 – Session

The session layer facilitates session management mechanisms in communications between higher level applications.

Although we usually understand communications as a constant flow of information, the truth is that it is not always the case, not all applications require constant communication and base their operation on the management of the state of such communications. If a program needs to receive information over the network, before doing any work, it is most likely to check periodically if such information is already available and in the meantime perform other tasks. This non-continuous communication could be managed by the application itself but since it is recurring, these systems have been implemented for more general use in the session layer.

The management of simultaneous communications in both directions, “full-duplex”, or alternating, “half-duplex”. (In this case, before we can issue information we must ensure that it is our turn).

The session layer lets the application in use know if it can already send information or should still wait. Our application will be able to check if the sending of information has already finished before sending new data or if there has been a partial failure that can be solved. The most important feature of the session layer is the ability to manage the authentication and authorization of sending or receiving information using cryptographic techniques.

Some examples of this layer’s protocols could be “AppleTalk Session Protocol”, “Layer 2 Tunneling Protocol”, “Network Basic Input / Output System” better known as NetBIOS, or the H.245 IP telephony control protocol. As we see, this layer provides an improved transport layer system with new features.

OSI Model: Layer 6 – Presentation

The presentation layer is responsible for interpreting the data so that both ends of a network can work with the information.

At the time of the creation of the OSI model each manufacturer designed its own operating system and therefore used its own methods of storing information with its own coding.

Its main functions are the conversion of data to standardized formats and encodings, independent of the language or operating system used at each end.

Another important function is the compression of the data to ensure that the information occupies the least possible number of bits. The fact of saving bits facilitates the transmission of information and reduces the load of intermediate equipment. Using the security services of the lower layer, the presentation layer will handle the encryption of the information. This layer has functionalities closer to the application itself than to communications, but still it is responsible for ensuring correct connectivity regardless of both the applications and the operating system that we have on other computers. Some of the best known protocols within this layer would be “NetWare Core Protocol”, “External Data Representation”, “Apple Filing Protocol”.

OSI Model: Layer 7 – Application

The application layer is responsible for managing the information of the client’s applications. Here we will find different protocols that, using the capabilities of the lower layers, offer client programs different communication options designed for a specific task.

For example, the SMTP, POP and IMAP protocols are responsible for managing email. Other protocols are designed for efficient file transfer, such as FTP, SFTP or TFTP. HTTP or HTTPS protocols are often used for web browsing. We can use Telnet, SSH or RDP, ror connection to other remote devices. Peer-to-peer protocols such as BitTorrent also locate this layer. In general, any protocol that allows remote communication for a specific purpose is usually located in this layer.

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yottaByte · ICT | Communications and Information services. Maritime Technology. Project Management. ITIL. Cloud Computing. e-Learning made with love by Celica G. Di Ronco.
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