How the OSI model works. General Information on Network Technologies

Model Open Systems InterConnection (OSI) - This is a skeleton, foundation and base of all network entities. The model defines network protocols, distributing them on 7 logical levels. It is important to note that in any process, the control of the network transmission moves from the level to the level, consistently connected protocols on each of the levels.

Video: model OSI in 7 minutes

Lower levels are responsible for the physical transmission parameters, such as electrical signals. Yes - yes, the signals in the wires are transmitted using the presentation to the currents :) The currents are represented as a sequence of units and zeros (1 and 0), then the data is decoded and routing over the network. Higher levels cover requests related to data representation. Conditionally speaking, higher levels are responsible for network data from the user's point of view.

The OSI model was originally invented as a standard approach, architecture or pattern, which would describe the network interaction of any network application. Let's figure it out in more detail?

# 01: Physical (Physical) level

At the first level oSI models There is a transmission of physical signals (currents, light, radio) from the source to the recipient. At this level, we operate with cables, contacts in connectors, encoding units and zeros, modulation, and so on.

Among the technologies that live at the first level can be allocated the most basic standard - Ethernet. He is now in every home.

Note that not only electrical currents can act as a data carrier. Radio frequencies, light or infrared waves are also used everywhere in modern networks.

Network devices that refer to the first level are hubs and repeaters - that is, the "stupid" glands that can simply work with a physical signal, not attacked into its logic (not decoding).

# 02: Channel (Data Link) level

Imagine, we received a physical signal from the first level - physical. This is a set of voltages of different amplitudes, waves or radio frequencies. Upon receipt, in the second level, the transmission errors are checked and correct. At the second level, we operate with the concept of "frame", or as they say "Frame". Here the first identifiers appear - MAC addresses. They consist of 48 bits and look like this: 00: 16: 52: 00: 1f: 03.

Channel level complex. Therefore, it is consequently divided into two sublevels: logical channel management (LLC, Logical Link Control) and medium access control (Mac, Media Access Control).

This level lives such devices as switches and bridges. By the way! Ethernet standard is also here. It is comfortably located on the first and second (1 and 2) levels of the OSI model.

# 03: Network (Network) level

Go up! The network layer introduces the term "routing" and, accordingly, the IP address. By the way, to convert IP addresses in Mac - addresses and back is used aRP protocol.

It is at this level that traffic routing is happening as such. If we want to get to the site website, then we ship, get an answer in the form of IP addresses and substitute it in the package. Yes - yes, if at the second level we use the term frame / frame, as we said earlier, here we use the package.

From the devices here lives his majesty router :)

Process when the data is transmitted from the upper levels to the lower name encapsulation data, and when, on the contrary, upstairs, from the first, physical to the seventh, then this process is called decapsulation Data

# 04: Transport Level

The transport level, as can be understood from the name, provides data transmission over the network. Here are two main rock stars - TCP and UDP. The difference is that different transports are used for different categories of traffic. The principle is:

• Traffic is sensitive to loss - No problem, TCP (Transmission Control Protocol)! It provides control over data transfer;
• Lost - not scary - In fact, now, when you read this article, a couple of packages could be lost. But it does not feel for you as for the user. UDP (User Datagram Protocol) will suit you. And if it were telephony? Loss of packages is critical there, since the voice in real time will begin simply "Cair";

# 05: Session (session) level

Ask any network engineer to explain the session level. It will be difficult for him to do this, infa 100%. The fact is that in everyday work, the network engineer interacts with the first four levels - physical, channel, network and transport. The rest, or the so-called "upper" levels relate more to the work of software developers :) But we will try!

The session level is engaged in managing compounds, or simply, sessions. He breaks them. Remember mem about " There was no uniform break"? We remember. So, this fifth level tried :)

# 06 Presentation Level (Presentation)

On the sixth level there is a conversion of message formats, such as coding or compression. Here you live JPEG and GIF, for example. The same level is responsible for the transfer of flow to the fourth (transport level).

# 07 Application Level (Application)

On the seventh floor, at the very top of the iceberg, the level of applications lives! There are network services that allow us how to end users, surf the Internet. Look at what protocol do you have our own knowledge base? That's right, https. This guy from the seventh floor. Even here you live simple HTTP, FTP and SMTP.

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The model consists of 7 levels located above each other. Levels interact with each other (by "vertical") by interfaces, and can interact with a parallel level of another system (by "horizontal") using protocols. Each level can only interact with their neighbors and perform the functions only to it. More information can be found in the picture.

Applied (applications) Level (eng. Application Layer.)

The upper (7th) model level, ensures network interaction and user. The level allows user applications to have access to network services, such as databases handler, file access, email forwarding. Also responsible for the transfer of service information, provides applications for error information and forms requests to presentation level. Example: POP3, FTP.

Representative (presentation) (eng. Presentation Layer.)

This level is responsible for transforming protocols and encoding / decoding data. Application requests received from the level of applications, it converts to a transmission format over a network, and the data obtained from the network converts to the format that understands applications. At this level, compression / unpacking or encoding / decoding data can be performed, as well as redirects requests to another network resource if they cannot be processed locally.

The level 6 (representations) of the OSI reference model is usually an intermediate protocol to convert information from adjacent levels. This allows you to exchange between applications on heterogeneous computer systems transparent for applications. Presentation level provides code formatting and conversion. Code formatting is used to guarantee the application for processing information that would have meaning for it. If necessary, this level can translate from one data format to another. The view level is dealt not only with data formats and presentation, it is also engaged in data structures that are used by programs. Thus, level 6 provides the organization of data when sending them.

To understand how it works, imagine that there are two systems. One uses an extended binary exchange code for the ASCII formation code to represent data (most of the other manufacturers of computers are used). If these two systems need to exchange information, then the level of representations, which performs the transformation and translates between two different formats.

Another feature performed at the view level is encrypted data that applies in cases where it is necessary to protect the transmitted information from receiving unauthorized recipients. To solve this task, processes and codes that are at the view level must perform data conversion. At this level, there are other subprograms that compress texts and convert graphic images into bit streams, so that they can be transmitted over the network.

Presentation level standards also define ways of presenting graphic images. For these purposes, PICT format can be used - the image format used to transmit QuickDraw graphs between programs for Macintosh and PowerPC computers. Another representation format is a tagged format of JPEG image files.

There is another group of standards levels of representations, which determines the presentation of sound and film films. This includes the MPEG electronic music instruments interface, used to compress and encoding videos on CDs, storage in digitized and transmission with speeds up to 1.5 Mbps, and Session Layer.)

The 5th level of the model is responsible for maintaining a communication session, allowing applications to interact with each other for a long time. The level controls the creation / completion of the session, the exchange of information, task synchronization, the definition of data transfer and maintaining a session during applications' inactivity periods. Synchronization of the transmission is provided by the placement in the data flow of the control points, starting with which the process is renewed in disruption.

Transport level (eng. Transport Layer.)

The 4th model level is designed to deliver data without errors, loss and duplication in the sequence as they were transmitted. It does not matter what data are transmitted, from where and where, that is, it provides the transmission mechanism itself. Data blocks It shares into fragments, the size of which depends on the protocol, short combines into one, and the long breaks. Protocols of this level are designed to interact the point-to-point. Example: udp.

There are many classes of transport level protocols, ranging from protocols that provide only the main transport functions (for example, data transmission functions without confirmation of reception), and ending with protocols that guarantee delivery to the destination of several data packets in the proper sequence, multiplexing multiplexes of data streams, provide The data flow control mechanism and ensure the accuracy of the received data.

Some network-level protocols, called protocols without the installation of the connection, do not ensure that the data is delivered to the appointment in the order in which they were sent to the source device. Some transport levels cope with this by collecting data in the desired sequence before sending them to a session level. Multiplexing data (multiplexing) means that the transport level is able to simultaneously process multiple data streams (flows can also come from various applications) between two systems. The data flow control mechanism is a mechanism that allows you to adjust the amount of data transmitted from one system to another. Transport level protocols often have a data delivery control function, forcing the receiving data to send a confirmation to the transmitting data to receive data.

Network level (eng. Network Layer.)

The 3rd level of the OSI network model is designed to determine the data transfer path. Responsible for broadcasting logical addresses and names in physical, definition of shortest routes, switching and routing, tracking problems and congestion on the network. At this level, such a network device works like a router.

Network level protocols rout data from the source to the recipient and can be divided into two classes: protocols with a connection establishment and without it.

You can describe the operation of the protocols with the connection setup on the example of the usual phone. The protocols of this class begin to transfer data from a call or install the route of the packets from the source to the recipient. After that, starting the sequential data transmission and, then, at the end of the transmission, break the connection.

Protocols without the installation of the connection that send data containing full address information in each package, work similarly to the postal system. Each letter or package contains the address of the sender and the recipient. Next, each intermediate post office or network device reads address information and decides on data routing. A letter or data package is transmitted from one intermediate device to another until it is delivered to the recipient. Protocols without the installation of the connection do not guarantee the receipt of information to the recipient in the order in which it was sent. For the installation of data in the appropriate order when using network protocols without installing the connection, transport protocols are responsible.

Channel level (eng. Data Link Layer.)

This level is designed to ensure network interaction at the physical level and control over errors that may occur. The data obtained from the physical layer is packaged in frames, checks for integrity if errors need fixes (sends a repeated request of the damaged frame) and sends to the network level. The channel level can interact with one or more physical levels, controlling and managing this interaction. The IEEE 802 specification shares this level to 2 sublayers - Mac (Media Access Control) adjusts access to a shared physical environment, LLC (Logical Link Control) provides a network layer maintenance.

In programming, this level represents the network card driver, in operating systems there is a software interface for the interaction of a channel and network layer among themselves, this is not a new level, but simply implementing the model for a specific OS. Examples of such interfaces: ODI,

Physical level (eng. Physical Layer.)

The lowest level of the model is designed directly to transmit data stream. Carries out the transfer of electrical or optical signals to the cable or in the radio and, accordingly, their reception and conversion into data bits in accordance with the methods of coding digital signals. In other words, interfaces between the network media and the network device.

Sources

• Alexander Filimonov Building Multiservice Networks Ethernet, BHV, 2007 ISBN 978-5-9775-0007-4
• United Network Technology Guide // Cisco Systems, 4th Edition, Williams 2005 ISBN 584590787x

Wikimedia Foundation. 2010.

This model was developed in 1984 by the International Standard Organization, ISO, and the original is called Open Systems InterConnection, OSI.
Model of interaction of open systems (in fact - a network interaction model) is a standard for the design of network communications and involves a level approach to building networks.
Each level of the model serves the various stages of the interaction process. By dividing at levels, the OSI network model simplifies the joint operation of equipment and software. The OSI model shares network functions for seven levels: applied, presentation level, session, transport, network, channel and physical.

• Physical level (Physical Layer) - determines the method of physical connection of computers on the network. Functions of funds related to this level are the beaten conversion of digital data into signals transmitted via physical environment (for example, by cable), as well as the transmission of signals.
• Channel Level (Data Link Layer) - responsible for organizing data transfer between subscribers through the physical layer, therefore, at this level, addiction tools are provided, allowing to unambiguously identify the sender and recipient in the entire set of subscribers connected to the overall communication lines. The function of this level also includes the ordering of the transmission in order to parallel using one line of communication with several pairs of subscribers. In addition, the channels of the channel layer provide error checks that may occur when data transmission by physical level.
• Network level (Network Layer) - provides data delivery between network computers, which is a union of various physical networks. This level assumes the presence of a logical addressing tool, allowing to unambiguously identify the computer in the combined network. One of the main functions performed by the means of this level is the targeted data transmission to a specific recipient.
• Transport level (Transport Layer) - implements data transfer between two programs operating on different computers, while ensuring the lack of losses and duplication of information that may occur as a result of the errors of the lower levels. If the data transmitted through the transport level is exposed to fragmentation, the means of this level ensure the assembly of fragments in the correct order.
• Session (or session) level Session Layer - allows two programs to maintain long-term network interaction, called session (session) or session. This level controls the session establishment, information exchange and the end of the session. It is also responsible for identification, thus allowing certain subscribers to participate in the session, and ensures the work of security services in order to streamline access to session information.
• Presentation level (Presentation Layer) - interim transformation of the outgoing message data into a common format that is provided by the lower levels, as well as the reverse conversion of incoming data from a general format into a format that is clear by the receiving program.
• Applied level (Application Layer) - provides high-level network interaction functions, such as file transfer, sending messages by email, etc.

OSI model in simple language

The OSI model is an abbreviation from English Open System InterConnection, that is, an open system interaction model. Under open systems, you can understand network equipment (computers with network cards, switches, routers).
The OSI network model is a work scheme (or data exchange plan) for network devices. OSI also plays a role in creating new network protocols, as it serves as an interaction benchmark.
OSI consists of 7 blocks (levels). Each unit performs its unique role in the network interaction of various network devices.
7 levels of model OSI: 1 - physical, 2 - channel, 3 - network, 4 - transport, 5 - session, 6 - views, 7 applications.
At each level, the model has its own set of network protocols (data transmission standards), with which the network devices are exchanged.
Remember than the harder network device, the more opportunities it provides, but also more levels occupy, and as a result - it works slowly.

Network models. Part 1. OSI.

It is definitely better to start with the theory, and then, smoothly, move to practice. Therefore, first consider the network model (theoretical model), and then we will open the curtain on how the theoretical network model fits into the network infrastructure (on network equipment, computer computers, cables, radio waves, etc.).
So, network model - This is a model of interaction of network protocols. And the protocols in turn are the standards that determine how different programs will exchange data.
I will explain on the example: by opening any page on the Internet, the server (where the page opens) is sent to your browser data (hypertext document) via HTTP protocol. Thanks to the HTTP protocol, your browser, receiving data from the server, knows how to handle them, and successfully processes them, showing you the requested page.
If you are not yet aware that the page is on the Internet, I will explain in a nutshell: any text on a web page is enclosed in special tags, which indicate the browser what text size to use, its color, location on the page (left, right or in the center). This applies not only to the text, but also pictures, forms, active elements, and in general, the entire content, i.e. What is on the page. The browser, discovering tags, acts according to their instructions, and shows you the processed data that are enclosed in these tags. You yourself can see the tags of this page (and this text between the tags), for this go to your browser menu and select - view the source code.
We will not be very distracted, "Network Model" The desired topic for those who want to become a specialist. This article consists of 3 parts and for you, I tried to write not boring, clearly and briefly. For details, or receiving additional clarification, write out in the comments at the bottom of the page, and I will certainly help you.
We, as in the Cisco Network Academy, consider two network models: the OSI model and the TCP / IP model (sometimes it is called DOD), and at the same time and compare them.

Reference Network Model OSI

OSI is decrypted as Open System InterConnection. In Russian, it sounds as follows: Network model of interaction of open systems (reference model). This model can be called standard. It is this model that manufacturers of network devices are adhered when new products are developing.
The OSI network model consists of 7 levels, and it is customary to start the countdown from the bottom.
List them:
7. Application Level (Application Layer)
6. Executive Level or Presentation Level (Presentation Layer)
5. Session Level (Session Layer)
4. TRANSPORT LAYER
3. Network Level (Network Layer)
2. Data Layer (Data Link Layer)
1. Physical Level (Physical Layer)

As mentioned above, the network model is a model of interaction between network protocols (standards), here at each level and have its own protocols. List their boring process (and not for what), so it will be better to analyze everything on the example, because the digestibility of the material is much higher for examples;)

Applied level

Application or application level (Application Layer) is the topmost level of the model. He communicates with user applications with the network. These applications are all familiar to us: view web pages (HTTP), transfer and reception mail (SMTP, POP3), receiving and receiving files (FTP, TFTP), remote access (telnet), etc.

Representative level

Executive level or presentation level (Presentation Layer) - it converts data to the appropriate format. On the example, it is easier to understand: those pictures (all images) that you see on the screen are transmitted when sending a file in the form of small portions of units and zero (bits). So, when you send a photo by email, the SMTP application protocol sends a photo to the lower level, i.e. on the level of presentation. Where your photo is converted to a convenient view of data for lower levels, for example, bits (units and zolics).
It is in the same way when your friend starts to receive your photo, it will act as all the same units and zeros, and it is the level of presentation that converts bits into a full photo, for example JPEG.
This is how this level works with protocols (JPEG, GIF, PNG, TIFF), encoding (ASCII, EBDIC), music and video (MPEG), etc.

Session Level

Session Layer or Session Level (Session Layer) - as seen from the name, it organizes a communication session between computers. A good example will serve as audio and video conferencing, at this level is set, which codec will be encoded, and this codec must be present on both machines. An example is an example of the SMPP protocol (Short Message Peer-to-Peer Protocol), with the help of it you know well-known sms and USSD requests. And the last example: PAP (Password Authentication Protocol) is an old protocol to send a username and password to the server without encryption.
I will not say anything more about the session level, otherwise they will deepen in the boring features of the protocols. And if they (features) you are interested, write letters to me or leave a message in the comments with a request to reveal the topic in more detail, and the new article will not wait long ago;)

Transport level

Transport Layer - This level ensures reliability of data transfer from the sender to the recipient. In fact, everything is very simple, for example, you communicate with a webcam with your friend or teacher. Does you need reliable delivery of each bit of the transmitted image? Of course not, if you lose a few bits from streaming video, you will not even notice that, even the picture will not change (MB, the color of one pixel from 900,000 pixels will change, which will flash with a 24 frame rate per second).
And now we will give such an example: you send you a friend (for example, via mail) in the archive important information or program. You download to your computer this archive. Here, the reliability needs 100%, because If a couple of bit when downloading the archive is lost - you will not be able to unzip it, i.e. Extract the necessary data. Or imagine sending a password to the server, and on the way one bit lost - the password will already lose its kind and value will change.
Thus, when we look at the online videos, sometimes we see some artifacts, delays, noises, etc. And when we read the text from the web page - the loss (or scope) of the letters are not allowed, and when you download the programs - everything passes without errors.
At this level, I will allocate two protocols: UDP and TCP. UDP protocol (User Datagram Protocol) transmits data without connecting the connection, does not confirm the delivery of data and does not repeat. TCP Protocol (Transmission Control Protocol), which establishes a connection before transmission, confirms the delivery of data, does repeat, guarantees the integrity and correct sequence of downloadable data.
Consequently, for music, video, video conferencing and calls, we use UDP (we transmit data without checking and without delays), and for text, programs, passwords, archives, etc. - TCP (data transmission with confirmation of receipt is spent more time).

Network level

Network Level (Network Layer) - This level determines the path for which the data will be transmitted. And, by the way, this is the third level of the OSI network model, and there are such devices that are just called third-level devices - routers.
We all have heard about the IP address, this is the IP (Internet Protocol) protocol. The IP address is a logical address on the network.
At this level, there are quite a few protocols and all these protocols we will analyze in more detail later, in separate articles and on examples. Now it will only list a few popular.
As I heard everything about the IP address and the Ping command is the ICMP protocol.
Those most routers (with which we will continue in the future) use the protocols of this level to route packets (RIP, EIGRP, OSPF).
The whole second part of the CCNA (Exploration 2) on routing.

Channel Level

Data Link Layer - We need it to interact networks at the physical level. Probably, everyone heard about the MAC address, so it is a physical address. Channel level devices - switches, hubs, etc.
IEEE (Institute of Electrical and Electronics Engineers - Institute of Electrical Engineering Engineers and Electronics Engineers) Determines the channel level with two sublayers: LLC and Mac.
LLC - Logical Control Management (Logical Link Control), created to interact with the upper level.
MAC - Managing Access Control (Media Access Control), created to interact with the lower level.
I will explain on the example: in your computer (laptop, communicator) there is a network card (or some other adapter), so there is a driver for interacting with it (with the card). The driver is a certain program - the upper sublayer channel level, through which it is possible to contact the bottom levels, or rather with the microprocessor (iron) - the lower row of channel level.
Typical representatives at this level a lot. PPP (Point-to-Point) is a protocol to communicate two computers directly. FDDI (Fiber Distributed Data Interface) - Standard transmits data to a distance of up to 200 kilometers. CISCO Discovery Protocol is a proprietary (own) protocol owned by Cisco Systems, with it you can detect adjacent devices and obtain information about these devices.
The whole third part of the CCNA (Exploration 3) course on second-level devices.

Physical level

Physical Level (Physical Layer) is the lowest level directly transmitting data stream. Protocols we all are well known: Bluetooth, IrDA (infrared), copper wires (twisted pair, telephone line), Wi-Fi, etc.
Details and specifications wait in the following articles and in the course of CCNA. The entire first part of the CCNA (Exploration 1) is devoted to the OSI model.

Conclusion

So we disassemble the OSI network model. In the next part, we begin the TCP / IP network model, it is smaller and the same protocols. For successful tests of CCNA tests, you need to make a comparison and identify differences that will be done.

After a short reflection, I decided to place an article here from the site of the Networks. So that everything lay in one place.

And hello expensive friends again, today we will deal with the fact that the OSI network model is, why, in fact, it is intended.

As you probably understand, modern networks are very and very difficult, there are many different processes in them, hundreds of actions are performed. In order to simplify the process of describing this variety of network functions (and more importantly, simplify the process of further development of these functions) was made an attempt to structure them. As a result of the structuring, all functions performed by the computer network are divided into several levels, each of which is only responsible for a certain, highly specialized range of tasks. Here the network model can be compared with the structure of the company. The company is divided into departments. Each department performs its functions, but during operation contact with other departments.

Separating functions using a network model

The OSI network model is designed in such a way that the upstream levels of the network model used the lower levels of the network model, to transfer their information. Rules with which the model levels communicate are called network protocols. The network protocol of a certain level of the model can communicate either with its level protocols or with adjoining levels. Here, again, you can make an analogy with the work of the company. The company always has a clearly installed hierarchy, although not as strict as in the network model. Employees of one stage of the hierarchy perform orders received from employees of a higher level of hierarchy.

Interaction between the levels of the OSI network model

Each device running on the network can be represented as a system operating at the corresponding levels of the OSI model. Moreover, this device can use in its work, both all levels of the OSI model, and only some lower levels. Usually, when it is said that the device works at a certain level of the model, then it is implied that it works at this level of the network model and on all the levels below.

Work not some levels of the OSI network model

When two different network devices communicate with each other, they use protocols of the same levels of the network model, while the process of interaction is involved both the level protocols on which the interaction directly occurs and the necessary protocols of all underlying levels, as they are used to transmit data obtained from the upper levels.

Communication of two systems from the position of the OSI model

When transmitting information from the top-level network model to the lower level of the network model, some service information called the header is added to this useful information (not only the title is added to 2 levels). This process of adding service information is called encapsulation. When receiving (transmitting information from the lower level to the top), this service information is separated and the initial data obtaining. Such a process is called deethcapsulation. In essence, this process is very similar to the process of sending a letter by mail. Imagine that you want to send a letter to your friend. You write a letter - this is useful information. By sending it by mail, you pack it into an envelope, inscribing the address of the recipient on it, that is, add some title to useful information. In fact, it is encapsulation. Getting your letter, your friend degenerates it - that is, it breaks the envelope and takes out useful information from it - your letter.

Demonstration of the principle of encapsulation

The OSI model divides all functions performed in the interaction of systems on 7 levels: physical (Physical) - 1, channel (Data Link) -2, Network (Network) - 3, transport (Transport) - 4, session (session) -5, Executive (Presentation) -6 and applied (Application) - 7.

Open System Interaction Model

Briefly consider the purpose of each of the levels of the interaction model of open systems.

The application level is a point through which the applications communicate with the network (the entry point in the OSI model). With this level of the OSI model, the following tasks are performed: network management, system employment management, file transfer management, user identification by their passwords. Examples of these level protocols are: HTTP, SMTP, RDP and D.R. Very often, the application-level protocols are simultaneously performing the functions of representation and session protocols.

This level is responsible for data presentation format. Roughly speaking, it converts data from the application level to the format suitable for transmission over the network (well, and accordingly, transforming the information received from the network to the format is suitable for processing applications).

At this level, establishing, maintaining and managing the communication session between the two systems. This level is responsible for maintaining communication between systems for the entire period of time during which their interaction occurs.

The protocols of this level of the OSI network model are responsible for transmitting data from one system to another. At this level, large data blocks are divided into smaller blocks suitable for processing by a network layer (very small data blocks are combined into larger), these blocks are appropriately marked for their subsequent recovery on the receiving side. Also, when using the appropriate protocols, this level is able to ensure the control of the delivery of network layer packages. A data block that operates this level is commonly called segment. Examples of this level protocols are: TCP, UDP, SPX, ATP and D.R.

This level is responsible for routing (defining optimal routes from one system to another) data blocks of this level. The data block of this level is usually called a package. Also, this level is responsible for the logical addressing of systems (the very IP addresses), on the basis of which routing. You can attribute to the protocols of this level: IP, IPX, etc., to devices operating at this level - routers.

This level is responsible for the physical addressing of the network devices (MAC addresses), access control to the environment, as well as the correction of errors allowed by the physical level. The data block used on the channel level is customized with frame. These level include the following devices: Switches (not all), Bridges and D.R. Typical technology using this level is Ethernet.

Transmission of optical or electrical pulses by the selected transmission medium. All sorts of repeaters and hubs can be attributed to devices of this level.

The OSI model itself is not a practical implementation, it only suggests some set of rules for the interaction of system components. The practical example of the implementation of the network protocol stack is the TCP / IP protocol stack (as well as other less common protocol stacks).

Just started working as a network administrator? Do not want to be confused? Our article will come in handy. Have you heard a time-tested administrator talks about network problems and mentions some levels? Maybe you were ever asked at work, what levels are protected and work if you use the old firewall? To deal with 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.

Resperating system administrator must clearly understand the network terms

Translated from English - the basic reference model of interaction of open systems. More precisely, the network model of the OSI / ISO network protocols stack. Introduced in 1984 as a conceptual basis dividing the process of sending data to the World Wide Web on seven uncomplicated stages. It is not the most popular, since the development of the OSI specification has been delayed. 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 for the position of the system administrator or in the IT-sphere.

Many specifications and technologies have been created for network devices. This variety is easy to get confused. It is the model of interaction of open systems that helps to understand each other network devices using various methods of communication. Note that the most useful OSI for manufacturers of software and hardware engaged in design compatible products.

Ask what kind of benefit for you? Knowledge of a multi-level model will give you the possibility of free communication with IT employees, a discussion of network problems will not be an oppressive boredom. And when you learn to understand, at what stage a failure has happened, you can easily find reasons and significantly reduce the range of your work.

OSI levels

The model contains seven simplified stages:

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

Why is the decomposition of steps simplifies life? Each of the levels corresponds to a specific step of sending a network message. All steps are consistent, it means that the functions are executed independently, there is no need for information about working at the previous level. The only necessary component is a way to receive data from the previous step, and how information is sent to the subsequent step.

Let us turn to a direct acquaintance with the levels.

Physical level

The main task of the first stage is to forward bits through physical communication channels. Physical communication channels - devices created for transmission and reception of information signals. For example, fiber optic, coaxial cable or twisted pair. Shipment can pass through wireless. The first stage is characterized by a data transfer medium: protection against interference, bandwidth, wave resistance. The qualities of the electrical finite signals are also set (the type of coding, voltage levels, and signal transmission rate) and are summarized to standard connector types, contact connections are assigned.

The functions of the physical stage are performed completely on each device connected to the network. For example, a network adapter implements these functions from the computer. You could already encounter first step protocols: RS -232, DSL and 10BASE-T, which determine the physical characteristics of the communication channel.

Channel Level

At the second stage, the abstract address of the device with a physical device is associated, the availability of the transmission medium is checked. Bits are formed in kits - frames. The main task of the channel level is to identify and edit errors. For correct shipment before and after the frame, specialized bits sequences are inserted and the calculated checksum is added. When the frame reaches the destination, the checksum already arrived again, if it coincides with the checksum in the frame, the frame is recognized correct. Otherwise, an error is erupted via re-transmission of information.

The channel stage makes it possible to transmit information, thanks to the special link structure. In particular, through the channel level protocols, tires, bridges, switches are operating. The second step specification includes: Ethernet, token Ring and PPP. Functions of the channel stage in the computer are performed by network adapters and drivers to them.

Network level

In the standard situations of the channel functions, there is not enough for high-quality information transmission. Specifications of the second step can transmit data only between nodes with the same topology, for example, wood. The need for the third stage appears. It is necessary to form a combined transport system with an extensive structure for several networks that have an arbitrary structure and differing data transfer by the method.

If you explain differently, then the third step processes the Internet protocol and executes the function of the router: the search for the best way to information. The router is a device that collects data on the structure of firewalls and transmitting packets to the destination network (transit transmission - hops). If you encounter an error in the IP address, then this is a problem that occurred on the network level. The third stage protocols are divided into network, routing or address resolution: ICMP, IPSec, ARP and BGP.

Transport level

So that the data reached the applications and the upper stack levels, the fourth stage is required. It provides the desired 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 the interrupted communication, the ability to detect and correct the transfer errors. For example, loss or duplication of packages.

How to choose a transport stage service class? When the quality of communication channels of communication is high, adequate choice will be a lightweight service. If the communication channels at the very beginning work unsafe, it is advisable to resort to a developed service that will ensure maximum opportunities for finding and solving problems (data delivery, delivery time-outage). Fourth Stage Specifications: TCP / IP Stack UDP and UDP, Novell Stack SPX.

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

Session Level

The fifth stage helps in regulating dialogs. It is impossible for the interlocutors to interrupt each other or spoke synchronously. The session level remembers the active side at a specific point and synchronizes the information, harmonizing and maintaining connections between devices. Its functions allow you to return to the checkpoint during long shipment and not start all over again. Also at the fifth stage, you can stop the connection when the exchange of information is completed. Session Specifications: NetBIOS.

Representative level

The sixth stage participates in data transformation into a universal recognizable format without changing the content. Since various formats are utilized in different devices, information processed at the executive level allows systems to understand each other, overcoming syntactic and code differences. In addition, on the sixth stage, the possibility of encrypting and decryption data appears, which ensures secrecy. Examples of protocols: ASCII and MIDI, SSL.

Applied level

The seventh stage in our list and the first, if the program sends data through the network. It consists of sets of specifications through which the user, Web pages. For example, when sending messages by mail it is at the applied level that a convenient protocol is selected. The composition of the seventh stage specification 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 among the IT-sphere workers a comic eighth stage appeared. All due to the fact that problems may arise due to the fault of the user, and as you know, the person is at the top of the evolution, so the eighth level appeared.

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