Just started working as a network administrator? Do not want to be confused? Our article is useful to you. Have you heard how a time-tested administrator talks about network problems and mentions some levels? Maybe you were ever asked at work, which levels are protected and work if you are using an old firewall? To understand the basics of information security, you need to understand the principle of the hierarchy of the OSI model. Let's try to see the capabilities of this model.

A self-respecting system administrator must be well-versed in network terms.

OSI Network Model

Translated from English - the basic reference model of the interaction of open systems. More precisely, the network model of the stack of network protocols OSI / ISO. Introduced in 1984 as a conceptual framework, which divided the process of sending data on the World Wide Web into seven simple steps. It is not the most popular since the development of the OSI specification has been delayed. The TCP / IP protocol stack is more profitable and is considered the main model used. However, you have a huge chance to face the OSI model in the position of a system administrator or in IT.

Created many specifications and technologies for network devices. In such a variety is easy to get confused. It is the open systems interconnection model that helps network devices that use different communication methods to understand each other. Note that OSI is most useful for software and hardware manufacturers engaged in the design of compatible products.

Ask, what is the use for you? Knowledge of the multi-level model will give you the opportunity to freely communicate with employees of IT-companies, the discussion of network problems will not be oppressive boredom. And when you learn to understand at what stage the failure occurred, you can easily find the reasons and significantly reduce the range of your work.

OSI levels

The model contains seven simplified steps:

• Physical.
• Channel.
• Network.
• Transport.
• Session
• Executive.
• Applied.

Why does decomposition into steps simplify life? Each of the levels corresponds to a certain stage of sending a network message. All steps are sequential, which means that the functions are performed independently, there is no need for information about the work at the previous level. The only necessary component is a way to get data from the previous step, and how information is sent to the next step.

We turn to the direct acquaintance with the levels.

Physical level

The main task of the first stage is to send bits through physical communication channels. Physical communication channels are devices designed to transmit and receive information signals. For example, optical fiber, coaxial cable or twisted pair. Forwarding can also take place over the air. The first stage is characterized by the data transmission medium: protection from interference, bandwidth, characteristic impedance. The quality of the electrical final signals (the type of coding, voltage levels and signal transfer rate) are also set and applied to standard types of connectors, contact connections are assigned.

The physical phase functions are performed absolutely on every device connected to the network. For example, a network adapter implements these functions from the computer side. You may have come across the first step protocols: RS-232, DSL and 10Base-T, which determine the physical characteristics of the communication channel.

Link level

At the second stage, the abstract address of the device is associated with the physical device, the availability of the transmission medium is checked. Bits are formed into sets - frames. The main task of the link level is to identify and correct errors. For correct forwarding, special sequences of bits are inserted before and after the frame and the calculated checksum is added. When the frame reaches the addressee, the checksum of the already arrived data is calculated again; if it coincides with the checksum in the frame, the frame is considered correct. Otherwise, an error appears that is corrected through the retransmission of information.

The channel stage makes it possible to transfer information, thanks to a special link structure. In particular, buses, bridges, and switches operate through the data link layer protocols. The second step specification includes: Ethernet, Token Ring, and PPP. The functions of the channel stage in the computer are performed by network adapters and drivers for them.

Network layer

In standard situations, the channel stage functions are not enough for high-quality information transfer. Specifications of the second step can transfer data only between nodes with the same topology, for example, a tree. There is a need for the third stage. It is necessary to form an integrated transport system with a branched structure for several networks that have an arbitrary structure and differ in the method of data transfer.

If it is explained differently, the third step processes the Internet protocol and performs the function of a router: finding the best path for the information. A router is a device that collects data about the structure of interconnections and sends packets to a destination network (transit transmissions - hopes). If you encounter an error in the IP address, then this is a problem that occurred at the network level. Protocols of the third stage are divided into network, routing, or address resolution: ICMP, IPSec, ARP, and BGP.

Transport level

In order for the data to reach the applications and the upper layers of the stack, a fourth stage is necessary. It provides the necessary degree of reliability of information transfer. There are five classes of services of the transport stage. Their difference lies in the urgency, the feasibility of restoring interrupted communication, the ability to detect and correct transmission errors. For example, the loss or duplication of packages.

How to choose the service class of the transport stage? When the quality of the communication channels is high, the lightweight service will be an adequate choice. If the communication channels at the very beginning work unsafely, it is advisable to resort to an advanced service that will provide maximum opportunities for finding and solving problems (control of data delivery, delivery time-outs). Specifications of the fourth stage: TCP and UDP TCP / IP stack, Novell SPX stack.

The combination of the first four levels is called the transport subsystem. It fully provides the selected level of quality.

Session Level

The fifth stage helps in regulating the dialogues. It is impossible for the interlocutors to interrupt each other or speak synchronously. The session layer remembers the active side at a particular moment and synchronizes information, coordinating and maintaining connections between devices. Its functions allow you to return to the control point during a long transfer and not start all over again. Also at the fifth stage, you can terminate the connection when the exchange of information is completed. Session Layer Specifications: NetBIOS.

Representative level

The sixth stage is involved in the transformation of data into a universal recognizable format without changing the content. Since different formats are utilized in different devices, information processed at the representative level allows systems to understand each other, overcoming syntax and code differences. In addition, at the sixth stage, it is possible to encrypt and decrypt data, which ensures secrecy. Examples of protocols: ASCII and MIDI, SSL.

Application layer

The seventh stage in our list and the first, if the program sends data via the network. Consists of sets of specifications through which the user gets access to files, Web-pages. For example, when sending messages by mail, a convenient protocol is selected at the application level. The composition of the specifications of the seventh stage is very diverse. For example, SMTP and HTTP, FTP, TFTP or SMB.

You can hear somewhere about the eighth level of the ISO model. Officially, it does not exist, but the comic eighth stage appeared among the workers of the IT sphere. All because of the fact that problems can arise through the user's fault, and as you know, man is at the top of evolution, and that is the eighth level.

Having considered the OSI model, you were able to deal with the complex structure of the network and now you understand the essence of your work. Everything becomes quite simple when the process is broken apart!