The OSI Reference Model is a 7-level network hierarchy created by the International Standards Organization (ISO). The presented model in Fig.1 has 2 different models:

• a horizontal protocol-based model that implements the interaction of processes and software on different machines
• a vertical model based on services implemented by neighboring layers to each other on the same machine

In the vertical - neighboring levels change information using APIs. The horizontal model requires a common protocol for exchanging information at the same level.

Picture 1

The OSI model only describes system methods interactions implemented by the OS, software, etc. The model does not include end user interaction methods. AT ideal conditions applications must access the upper layer of the OSI model, but in practice, many protocols and programs have methods for accessing lower layers.

Physical layer

At the physical level, data is represented as electrical or optical signals corresponding to 1s and 0s of the binary stream. The transmission medium parameters are defined at the physical layer:

• type of connectors and cables
• pin assignment in connectors
• signal coding scheme 0 and 1

The most common types of specifications at this level are:

• - unbalanced serial interface parameters
• — balanced serial interface parameters
• IEEE 802.3 -
• IEEE 802.5 -

At the physical level, you can not delve into the meaning of the data, since it is represented in the form of bits.

Link layer

This channel implements the transport and reception of data frames. The layer implements network layer requests and uses physical layer for receiving and transmitting. The IEEE 802.x specifications divide this layer into two sublayers: logical link control (LLC) and medium access control (MAC). The most common protocols at this level are:

• IEEE 802.2 LLC and MAC
• ethernet
• token ring

Also at this level, detection and correction of transmission errors is implemented. At the link layer, the packet is placed in the data field of the frame - encapsulation. Error detection is possible with different methods. For example, the implementation of fixed frame boundaries, or a checksum.

network layer

At this level, network users are divided into groups. It implements packet routing based on MAC addresses. The network layer implements the transparent transmission of packets to the transport layer. At this level, the boundaries of networks are erased. different technologies. work at this level. An example of the network layer is shown in Fig. 2. The most common protocols:

Drawing - 2

transport layer

At this level, information flows are divided into packets for transmission at the network level. The most common protocols of this layer are:

• TCP - Transmission Control Protocol

session layer

At this level, the organization of information exchange sessions between terminal machines takes place. At this level, the definition active side and session synchronization is implemented. In practice, many other layer protocols include session layer functionality.

Presentation Layer

At this level, data is exchanged between software on different operating systems. At this level, information transformation (compression, etc.) is implemented to transfer the information flow to the transport level. Layer protocols are used and those that use the higher layers of the OSI model.

Application layer

The application layer implements the application's access to the network. The layer manages file transfer and network management. Protocols used:

• FTP/TFTP - File Transfer Protocol
• X 400 - Email
• telnet
• CMIP - information management
• SNMP - Network Management
• NFS - network file system
• FTAM - file transfer access method

Ecology of consciousness: There are different levels of consciousness, in accordance with different levels of perception of the surrounding world. Each level of development of consciousness is a certain level of the ability to love and show love.

There are different levels of consciousness, in accordance with different levels of perception of the surrounding world. Each level of development of consciousness is a certain level of the ability to love and show love.

1. At the first level of perception of the world there are people for whom material acquisitions are the meaning of life. The lowest manifestation of this level is when a person wants only to receive, not wanting to give anything in return. Unfortunately, modern media aimed at pulling and keeping a person at this level of anthropocentrism, when each person considers himself the center of the universe and seeks to exploit the resources of the planet of all those around him for his own selfish pleasure. Now the efforts of all means mass media aimed specifically at ensuring that people see the meaning of their existence in acquisition, and build relationships, mainly only on the basis of sexual intimacy.

2. Those who have risen above selfish aspirations and find happiness in achieving their creative goals are the obvious movers of progress. They make great discoveries, live for art, build bridges across the English Channel, introduce Newest technologies and strive to change outer life society for the better. Such individuals can attract significant material resources due to the fact that they regard money not as a source of personal pleasure, but as an opportunity for the implementation of creative goals.

If the first level unites those whose meaning of life is collecting things, then the people of creativity are at the second stage. Due to the fact that money for them is not an end, but a means, they have a strong internal energy which allows them to achieve success in life and live it more vividly and richly.

3. Creative people gradually come to the understanding that the path to happiness and prosperity lies not only through external changes in society and in more- through the development of universal values like mercy, kindness, justice and simplicity, which are the basis for the beginning of the spiritual life. Those who sincerely strive to develop sublime qualities of character and always try to benefit others find peace and joy in pure and sublime relationships. The nobility of the soul is the main hallmark people at this level and above.

4. Spiritual development is accompanied by the natural disappearance of laziness and an increase in responsibility. A person with exalted qualities of character always strives to benefit others. He perfects his talent inner world, and all their abilities in the spirit of service to society. At this level, a person understands the importance of doing duty. Sincerely and disinterestedly fulfilling his duties, a person acquires those qualities that are the basis for a stable life. material prosperity and rapid spiritual progress.

Almost all oriental cultures based on this reliable foundation of selfless service to society. Bushido - ancient culture samurai is the living embodiment of achieving internal and external harmony through the disinterested fulfillment of duty. The very word "samurai" in translation means "servant". A true samurai is a perfect servant, impeccably in control of his feelings and devoid of even a shadow of self-interest.

Commitment to the dharma - disinterested performance of one's duties was also the essence of culture ancient india and the Vedic worldview in general. Selfless service corresponds to the original nature of the soul and therefore brings peace and inner joy, which are the cause and effect of further spiritual enlightenment. The higher spiritual level of a person, the more disinterested he is, the less he is interested in wealth, but the more accessible it becomes.

5. People at this level consider spiritual development as main goal their lives, and act in such a way that each of their actions brings benefit to others.

6. Spiritual exaltation through self-sacrifice is a state of mind when a person wishes happiness for others more than for himself, and through this rises to an even higher level of holiness. At this level, Love for all living beings induces a person to sacrifice his own interests for the sake of the spiritual uplift of others. Almost all the founders of the true world religions acted from the mindset inherent in this level.

7. Having reached the highest level of development, a person loses the dual perception of the world and acquires the qualification to return to spiritual world. Such a person sees only love, considering everyone around him much better than himself. For such a person, the concepts of enemies, grief and evil no longer exist, since each of his actions naturally brings love and happiness to the whole world.

As consciousness develops, selflessness develops in a person and laziness disappears, and the fulfillment of his duties gives him more and more pleasure. When a person is focused only on personal gain and his own pleasures, the process of work does not bring him any special joy, since he is focused only on the result - getting money. But at higher levels of consciousness, the same activity becomes a reward in itself, and work is perceived as a hobby. published

This article is dedicated to the reference network seven-layer OSI model. Here you will find the answer to the question why system administrators need to understand this network model, all 7 layers of the model will be considered, and you will also learn the basics of the TCP / IP model, which was built on the basis of the OSI reference model.

When I started to get involved in various IT technologies, I started working in this area, of course, I didn’t know about any model, I didn’t even think about it, but a more experienced specialist advised me to study, or rather, just understand this model, adding that “ if you understand all the principles of interaction, it will be much easier to manage, configure the network and solve all kinds of network and other problems". I, of course, obeyed him and began to shovel books, the Internet and other sources of information, at the same time checking on the existing network whether this is all true in reality.

AT modern world the development of network infrastructure has reached such a high level that without building even a small network, an enterprise ( including and small) will not be able to simply exist normally, so system administrators are becoming more and more in demand. And for high-quality construction and configuration of any network, System Administrator must understand the principles of the OSI reference model, just so that you learn to understand the interaction of network applications, and indeed the principles of network data transfer, I will try to present this material in an accessible way even for novice administrators.

OSI network model (open systems interconnection basic reference model) is an abstract model of how computers, applications, and other devices interact on a network. In short, the essence of this model is that the ISO organization ( International Organization for Standardization) developed a standard for network operation so that everyone could rely on it, and there was compatibility of all networks and interaction between them. One of the most popular network interaction protocols, which is used all over the world, is TCP / IP and it is built on the basis of the reference model.

Well, let's go directly to the levels of this model, and first, get acquainted with the general picture of this model in the context of its levels.

Now let's talk in more detail about each level, it is customary to describe the levels of the reference model from top to bottom, it is along this path that interaction takes place, on one computer from top to bottom, and on the computer where data is received from bottom to top, i.e. data passes through each level sequentially.

Description of the levels of the network model

Application layer (7) (application layer) is the starting point and at the same time the end point of the data you want to transfer over the network. This layer is responsible for the interaction of applications over the network, i.e. Applications communicate at this level. This is the highest level and you need to remember this when solving problems that arise.

HTTP, POP3, SMTP, FTP, TELNET other. In other words, application 1 sends a request to application 2 using these protocols, and in order to find out that application 1 sent a request to application 2, there must be a connection between them, and it is the protocol that is responsible for this connection.

Presentation layer (6)- this layer is responsible for encoding the data so that it can then be transmitted over the network and converts it back accordingly so that the application understands this data. After this level, the data for other levels becomes the same, i.e. no matter what kind of data it is, be it word document or message Email.

The following protocols work at this level: RDP, LPP, NDR other.

Session layer (5)– is responsible for maintaining the session between data transfers, i.e. the duration of the session differs, depending on the data being transmitted, so it must be maintained or terminated.

The following protocols work at this level: ASP, L2TP, PPTP other.

Transport layer (4)- Responsible for the reliability of data transmission. It also splits the data into segments and collects them back, since the data is different size. There are two well-known protocols of this level - these are TCP and UDP. The TCP protocol guarantees that data will be delivered to in full, and the UDP protocol does not guarantee this, which is why they are used for different purposes.

Network layer (3)- it is intended to determine the path that the data should take. Routers work at this level. He is also responsible for: translating logical addresses and names into physical ones, determining a short route, switching and routing, and monitoring network problems. It works at this level. IP protocol and routing protocols like RIP, OSPF.

Link layer (2)- it provides interaction at the physical level, at this level are determined MAC addresses network devices, errors are also monitored and corrected here, i.e. re-request the corrupted frame.

Physical layer (1)- this is directly the conversion of all frames into electrical impulses and vice versa. In other words physical transmission data. Work at this level concentrators.

This is what the whole data transfer process looks like from the point of view of this model. It is a reference and standardized and therefore other network technologies and models are based on it, in particular the TCP / IP model.

TCP IP Model

TCP/IP model slightly different from the OSI model, to be more specific, in this model some layers of the OSI model have been combined and there are only 4 of them here:

• Applied;
• Transport;
• Network;
• Channel.

The picture shows the difference between the two models, and also shows once again at what levels the well-known protocols work.

It is possible to talk about the OSI network model and specifically about the interaction of computers on the network for a long time and it will not fit in one article, and it will be a little incomprehensible, so here I tried to present, as it were, the basis of this model and a description of all levels. The main thing is to understand that all this is really true and the file that you sent over the network just goes through " huge» path before reaching the end user, but it happens so fast that you don’t notice it, thanks in large part to advanced network technologies.

I hope all this will help you understand the interaction of networks.

When computers are connected over a network, a lot of operations are performed to ensure the transfer of data from computer to computer. The user working with some application, in general, does not care what happens and how. It simply has access to another application or computer resource located on another computer on the network. In reality, all transmitted information goes through many stages of processing. First of all, it is divided into blocks, each of which is supplied with control information. The received blocks are made in the form of network packets, these packets are encoded, transmitted using electrical or light signals over the network in accordance with the selected access method, then the data blocks enclosed in them are restored from the received packets, the blocks are combined into data that become available to another application. This is, of course, a very simplified description of the ongoing processes. Some of these procedures are implemented only in software, others in hardware, and some operations can be performed both by software and hardware.

To streamline all the procedures performed, to divide them into levels and sublevels that interact with each other, just the network models are called upon. These models allow you to correctly organize the interaction of both subscribers within the same network, and a variety of networks on various levels. The so-called reference model for information exchange of the open system OSI (Open System Interchange) is currently the most widely used. Under the term "open system" in this case is understood as a system that is not closed in itself, having the ability to interact with some other systems (as opposed to a closed system).

4.1. OSI Reference Model

The OSI model has been proposed international organization ISO (International Standards Organization) standards in 1984. Since then, it has been used (more or less strictly) by all manufacturers of network products. Like any universal model, the OSI model is quite cumbersome, redundant and not very flexible, so the real network facilities offered various firms, do not necessarily adhere to the accepted separation of functions. However, familiarity with the OSI model allows you to better understand what is happening on the network.

All network functions in the model are divided into 7 levels (Fig. 4.1). At the same time, higher levels perform more complex, global tasks, for which they use lower levels for their own purposes, and also manage them. The purpose of the lower layer is to provide services to the higher layer, and the higher layer does not care about the details of the performance of these services. Lower levels perform simpler, more specific functions. Ideally, each level interacts only with those that are next to it (above it and below it). Top level matches applied task, the currently running application, the lower one - direct transmission of signals over the communication channel.

The functions included in the levels shown in Figure 4.1 are implemented by each network subscriber. At the same time, each level on one subscriber works as if it has a direct connection with appropriate level another subscriber, that is, there is a virtual connection between the same levels of network subscribers. Subscribers of the same network have real communication only at the lowest, first, physical level. In the transmitting subscriber, information passes through all levels, from the top to the bottom. At the receiving subscriber, the received information makes Return trip: from the lower level to the upper (Fig. 4.2).

Consider more features different levels.
Application layer(Application), or application layer, provides services that directly support user applications, such as file transfer software, database access, e-mail, server registration service. This layer manages the other six layers.
Executive level(Presentation), or data presentation layer, defines and converts data formats and their syntax into a form convenient for the network, that is, it performs the function of a translator. It also encrypts and decrypts data, and, if necessary, compresses them.
session layer(Session) controls the conduct of communication sessions (that is, establishes, maintains and terminates communication). The same level recognizes the logical names of subscribers, controls the access rights granted to them.
transport layer(Transport) ensures the delivery of packets without errors and losses, in the desired sequence. Here, the transmitted data is divided into blocks placed in packets, and the received data is restored.
network layer(Network) is responsible for addressing packets and translating logical names to physical network addresses (and vice versa), as well as choosing the route along which the packet is delivered to its destination (if there are several routes on the network).
link layer, or the data link control layer, is responsible for the formation of packets standard view, including the initial and final control fields. Here, network access control is performed, transmission errors are detected, and erroneous packets are resent to the receiver.
Physical layer(Physical) - this is the lowest level of the model, which is responsible for encoding the transmitted information into signal levels accepted in the transmission medium, and reverse decoding. It also defines the requirements for connectors, connectors, electrical matching, grounding, protection against interference, etc.

Most of the functions of the two lower levels of the model (1 and 2) are usually implemented in hardware (part of the functions of level 2 are software driver network adapter). It is at these levels that the transmission rate and network topology, the exchange control method and the packet format are determined, that is, what is directly related to the type of network (Ethernet, Token-Ring, FDDI). More high levels do not work directly with specific hardware, although levels 3,4 and 5 may still take into account its features. Levels 6 and 7 have nothing to do with hardware at all. They simply will not notice the replacement of network equipment with another one.

In level 2 (channel), two sublevels are often distinguished.

The upper sublevel (LLC - Logical Link Control) manages the logical link, that is, establishes a virtual communication channel (part of its functions are performed by the network adapter driver program). The lower sublevel (MAC - Media Access Control) provides direct access to the information transmission medium (communication channel). It is directly connected to the network equipment.
In addition to the OSI model, there is also the IEEE Project 802 model, adopted in February 1980 (hence the number 802 in the title), which can be considered as a modification, development, refinement of the OSI model. The standards defined by this model (the so-called 802 specifications) are divided into twelve categories, each of which is assigned a number.
802.1 - network aggregation.
802.2 - logical link control.
802.3 - local area network with CSMA/CD access method and tina topology (Ethernet).
802.4 - local area network with "bus" topology and token
access.
802.5 - local area network with "ring" topology and token access.
802.6 - metropolitan area network (MAN).
802.7 is a broadcast technology.
802.8 is fiber optic technology.
802.9 - integrated networks with the ability to transmit voice and data.
802.10 - network security.
802.11 is a wireless network.
802.12 - local area network with centralized access control based on request priorities and star topology (100VG-Any LAN).
The 802.3, 802.4, 802.5, 802.12 standards directly refer to the MAC sublayer of the second (link) layer of the OSI reference model. The remaining 802 specifications decide general issues networks.

The modern IT world is a huge branching structure that is difficult to understand. To simplify understanding and improve debugging, a modular architecture was used at the design stage of protocols and systems. It is much easier for us to find out that the problem is in the video chip when the video card is a separate device from the rest of the equipment. Or notice a problem in separate area networks than to shovel the entire network entirely.

A separate layer of IT - a network - is also built in a modular way. The network functioning model is called the network model of the basic reference model of interaction open systems ISO/OSI. Briefly - the OSI model.

The OSI model consists of 7 layers. Each level is abstracted from the others and knows nothing of their existence. The OSI model can be compared to the structure of a car: the engine does its job, creating torque and giving it to the gearbox. The engine absolutely does not care what happens next with this torque. Will he turn the wheel, caterpillar or propeller. Just like a wheel, it doesn't matter where this torque comes from - from the engine or the crank that the mechanic turns.

Here it is necessary to add the concept of payload. Each level carries a certain amount of information. Some of this information is serviceable for this level, for example, an address. The IP address of the site does not carry any information for us. useful information. We only care about the cats that the site shows us. So this payload is carried in that part of the layer called the protocol data unit (PDU).

Layers of the OSI Model

Let's take a closer look at each layer of the OSI Model.

1 level. Physical ( physical). Load unit ( PDU) here is a bit. In addition to ones and zeros, the physical level knows nothing. Wires, patch panels, network hubs (hubs that are now difficult to find in the networks we are used to), network adapters work at this level. It is the network adapters and nothing more from the computer. The network adapter itself receives a sequence of bits and passes it on.

2nd level. Channel ( data link). PDU - frame ( frame). Addressing appears at this level. The address is the MAC address. The link layer is responsible for the delivery of frames to the destination and their integrity. In the networks we are used to, the ARP protocol works at the data link layer. Second-level addressing works only within one network segment and knows nothing about routing - this is handled by a higher level. Accordingly, devices operating on L2 are switches, bridges and a network adapter driver.

3rd level. Network ( network). PDU package ( packet). The most common protocol (I won’t talk about the “most common” further - an article for beginners and they, as a rule, do not encounter exotic) here is IP. Addressing occurs by IP addresses, which consist of 32 bits. The protocol is routable, that is, a packet is able to get to any part of the network through a certain number of routers. Routers work on L3.

4th level. Transport ( transportation). PDU segment ( segment)/datagram ( datagram). At this level, the concepts of ports appear. TCP and UDP work here. Protocols of this layer are responsible for direct communication between applications and for the reliability of information delivery. For example, TCP is able to request a retransmission of data in case the data was received incorrectly or not all. TCP can also change the data transfer rate if the receiving side does not have time to accept everything (TCP Window Size).

The following levels are only "correctly" implemented in the RFC. In practice, the protocols described in next levels work simultaneously at several layers of the OSI model, so there is no clear separation between the session and presentation layers. In this regard, the main stack currently used is TCP / IP, which we will talk about below.

Level 5 session ( session). data PDU ( data). Manages a communication session, information exchange, rights. Protocols - L2TP, PPTP.

6th level. Executive ( presentation). data PDU ( data). Presentation and encryption of data. JPEG, ASCII, MPEG.

7th level. Applied ( application). data PDU ( data). The most numerous and varied level. It runs all high-level protocols. Such as POP, SMTP, RDP, HTTP, etc. The protocols here do not have to think about routing or guaranteeing the delivery of information - these are handled by lower layers. At level 7, only implementations are needed concrete action, for example, receiving an html code or an email message to a specific recipient.

Conclusion

The modularity of the OSI model allows you to quickly find problem areas. After all, if there is no ping (3-4 levels) to the site, there is no point in digging into the overlying layers (TCP-HTTP) when the site is not displayed. Abstracting from other levels, it is easier to find an error in the problematic part. By analogy with a car - we do not check the candles when we pierced the wheel.

The OSI model is a reference model - a kind of spherical horse in a vacuum. Its development took a very long time. In parallel with it, the TCP / IP protocol stack was developed, which is actively used in networks at the present time. Accordingly, an analogy can be drawn between TCP/IP and OSI.