OSI reference model. The OSI model is easy! 7 tier model

The development of which was not related to the OSI model.

Layers of the OSI model

The model consists of 7 levels located one above the other. Layers interact with each other (vertically) through interfaces, and can interact with a parallel layer of another system (horizontally) through protocols. Each level can interact only with its neighbors and perform functions assigned only to it. More details can be seen in the figure.

OSI Model
Data type	Level	Functions
Data	7. Application layer	Access to online services
	6. Presentation Layer	Representation and encoding of data
	5. Session layer	Session management
Segments	4. Transport	Direct communication between endpoints and reliability
Packages	3. Networked	Route determination and logical addressing
Personnel	2. Channel	Physical addressing
bits	1. Physical layer	Working with media, signals and binary data

Application (Application) level (eng. application layer)

The top level of the model provides the interaction of user applications with the network. This layer allows applications to use network services such as remote access to files and databases, e-mail forwarding. It is also responsible for the transfer of service information, provides applications with information about errors and generates requests to presentation layer. Example: HTTP , POP3 , SMTP , FTP , XMPP , OSCAR , BitTorrent , MODBUS, SIP

Executive (Presentation layer) presentation layer)

This layer is responsible for protocol conversion and data encoding/decoding. It converts application requests received from the application layer into a format for transmission over the network, and converts data received from the network into a format understandable by applications. At this level, compression/decompression or encoding/decoding of data can be performed, as well as redirecting requests to another network resource if they cannot be processed locally.

Layer 6 (representations) of the OSI reference model is usually an intermediate protocol for converting information from neighboring layers. This allows communication between applications on dissimilar computer systems in a manner that is transparent to the applications. The presentation layer provides formatting and code transformation. Code formatting is used to ensure that an application receives information for processing that makes sense to it. If necessary, this layer can translate from one data format to another. The presentation layer deals not only with the formats and presentation of data, it also deals with the data structures that are used by programs. Thus, layer 6 provides for the organization of data during its transfer.

To understand how this works, imagine that there are two systems. One uses the EBCDIC Extended Binary Information Interchange Code, such as the IBM mainframe, for data representation, and the other uses the American Standard ASCII Information Interchange Code (used by most other computer manufacturers). If these two systems need to exchange information, then a presentation layer is needed to perform the transformation and translate between the two different formats.

Another function performed at the presentation level is data encryption, which is used in cases where it is necessary to protect transmitted information from being received by unauthorized recipients. To accomplish this task, the processes and code at the view layer must perform data transformations. At this level, there are other subroutines that compress texts and convert graphic images into bitstreams so that they can be transmitted over the network.

Presentation-level standards also define how graphics are presented. For this purpose, the PICT format, an image format used to transfer QuickDraw graphics between programs for Macintosh and PowerPC computers, can be used. Another presentation format is the tagged TIFF image file format, which is commonly used for high resolution bitmaps. The next presentation level standard that can be used for graphics is that developed by the Joint Photographic Expert Group; in everyday usage, this standard is simply referred to as JPEG.

There is another group of presentation level standards that define the presentation of sound and movies. This includes the Musical Instrument Digital Interface (MIDI) for the digital representation of music, developed by the Cinematography Expert Group, the MPEG standard, used to compress and encode videos on CD, store them digitally, and transfer at speeds up to 1.5 Mbps. /s, and QuickTime, a standard that describes audio and video elements for programs running on Macintosh and PowerPC computers.

The session layer session layer)

The 5th layer of the model is responsible for maintaining the communication session, allowing applications to interact with each other for a long time. The layer manages session creation/termination, information exchange, task synchronization, determination of the right to transfer data, and session maintenance during periods of application inactivity. Transmission synchronization is ensured by placing checkpoints in the data stream, from which the process is resumed if the interaction is broken.

The transport layer transport layer)

The 4th level of the model is designed to deliver data without errors, losses and duplication in the sequence in which they were transmitted. At the same time, it does not matter what data is transferred, from where and where, that is, it provides the transmission mechanism itself. It divides data blocks into fragments, the size of which depends on the protocol, combines short ones into one, and splits long ones. Example: TCP , UDP .

There are many classes of transport layer protocols, ranging from protocols that provide only basic transport functions (for example, data transfer functions without acknowledgment), to protocols that ensure that multiple data packets are delivered to the destination in the correct sequence, multiplex multiple data streams, provide data flow control mechanism and guarantee the validity of the received data.

Some network layer protocols, called connectionless protocols, do not guarantee that data is delivered to its destination in the order in which it was sent by the source device. Some transport layers deal with this by collecting data in the right order before passing it to the session layer. Multiplexing (multiplexing) data means that the transport layer is able to simultaneously process multiple data streams (streams may come from different applications) between two systems. A flow control mechanism is a mechanism that allows you to regulate the amount of data transferred from one system to another. Transport layer protocols often have the function of data delivery control, forcing the system receiving data to send acknowledgments to the transmitting side that data has been received.

You can describe the operation of protocols with the establishment of a connection using the example of a conventional telephone. Protocols of this class begin data transmission by invoking or setting the path of packets from source to destination. After that, the serial data transfer is started and then, at the end of the transfer, the connection is disconnected.

Connectionless protocols that send data containing full address information in each packet work similarly to the mail system. Each letter or package contains the address of the sender and the recipient. Next, each intermediate post office or network device reads the address information and makes a decision about data routing. A letter or data packet is transmitted from one intermediate device to another until it is delivered to the recipient. Connectionless protocols do not guarantee that information will arrive to the recipient in the order in which it was sent. The transport protocols are responsible for setting up the data in the appropriate order when using connectionless network protocols.

The network layer network layer)

The 3rd layer of the OSI network model is designed to determine the data transfer path. Responsible for translating logical addresses and names into physical ones, determining the shortest routes, switching and routing, monitoring network problems and congestion. A network device such as a router operates at this level.

Network layer protocols route data from a source to a destination.

Link layer data link layer)

This layer is designed to ensure the interaction of networks at the physical layer and control errors that may occur. It packs the data received from the physical layer into frames, checks for integrity, corrects errors if necessary (sends a repeated request for a damaged frame) and sends it to the network layer. The link layer can interact with one or more physical layers, controlling and managing this interaction. The IEEE 802 specification divides this level into 2 sublevels - MAC (Media Access Control) regulates access to the shared physical medium, LLC (Logical Link Control) provides network level service.

In programming, this level represents the network card driver, in operating systems there is a programming interface for the interaction of the channel and network levels with each other, this is not a new level, but simply an implementation of a model for a specific OS. Examples of such interfaces: ODI , NDIS

The physical layer physical layer)

The lowest level of the model is intended directly for the transfer of data flow. Carries out the transmission of electrical or optical signals to a cable or radio air and, accordingly, their reception and conversion into data bits in accordance with the methods of encoding digital signals. In other words, it provides an interface between a network carrier and a network device.

OSI model and real protocols

The seven-layer OSI model is theoretical, and contains a number of shortcomings. There were attempts to build networks exactly according to the OSI model, but the networks created in this way were expensive, unreliable, and inconvenient to use. Real network protocols used in existing networks are forced to deviate from it, providing unintended capabilities, so the binding of some of them to the OSI layers is somewhat arbitrary: some protocols occupy several layers of the OSI model, reliability functions are implemented at several layers of the OSI model.

The main defect of OSI is an ill-conceived transport layer. On it, OSI allows data to be exchanged between applications (introducing the concept port- application identifier), however, the possibility of exchanging simple datagrams (of the UDP type) is not provided in OSI - the transport layer must form connections, provide delivery, control the flow, etc. (of the TCP type). Real protocols implement this possibility.

TCP/IP family

The TCP / IP family has three transport protocols: TCP, which is fully OSI-compliant, providing verification of receipt of data, UDP, which corresponds to the transport layer only by the presence of a port, provides datagram exchange between applications, does not guarantee receipt of data, and SCTP, designed to eliminate some of the shortcomings of TCP and in which added some innovations. (There are about two hundred other protocols in the TCP/IP family, the best known of which is the service protocol ICMP , which is used internally to ensure operation; the rest are also not transport protocols.)

IPX/SPX family

In the IPX/SPX family, ports (called "sockets" or "sockets") appear in the IPX network layer protocol, enabling the exchange of datagrams between applications (the operating system reserves some of the sockets for itself). The SPX protocol, in turn, complements IPX with all other transport layer capabilities in full compliance with OSI.

For the host address, IPX uses an identifier formed from a four-byte network number (assigned by routers) and the MAC address of the network adapter.

DOD model

A TCP/IP protocol stack using a simplified four-layer OSI model.

Addressing in IPv6

Destination and source addresses in IPv6 are 128 bits or 16 bytes long. Version 6 generalizes the special address types of version 4 into the following address types:

Unicast is an individual address. Specifies a single node - computer or router port. The packet must be delivered to the node via the shortest route.
Cluster is the address of the cluster. Denotes a group of hosts that share a common address prefix (for example, attached to the same physical network). The packet must be routed to a group of nodes along the shortest path, and then delivered to only one of the members of the group (for example, the nearest node).
Multicast is the address of a set of hosts, possibly on different physical networks. Copies of the packet must be delivered to each node in the set using hardware multicast or broadcast capabilities, if possible.

As with IPv4, IPv6 addresses are divided into classes based on the value of the most significant few bits of the address.

Most of the classes are reserved for future use. The most interesting for practical use is the class intended for Internet service providers, called Provider-Assigned Unicast.

The address of this class has the following structure:

Each ISP is assigned a unique identifier that tags all networks it supports. Next, the provider assigns unique identifiers to its subscribers, and uses both identifiers when assigning a block of subscriber addresses. The subscriber himself assigns unique identifiers to his subnets and nodes of these networks.

A subscriber can use the subnetting technique used in IPv4 to further subdivide the subnet ID field into smaller fields.

The described scheme approximates the IPv6 addressing scheme to those used in territorial networks such as telephone networks or X.25 networks. The hierarchy of address fields will allow backbone routers to work only with the higher parts of the address, leaving processing of less significant fields to subscriber routers.

A minimum of 6 bytes must be allocated under the host ID field in order to be able to use LAN MAC addresses directly in IP addresses.

For compatibility with the IPv4 version of the addressing scheme, IPv6 has a class of addresses that have 0000 0000 in the high-order bits of the address. The lower 4 bytes of this class address must contain an IPv4 address. Routers that support both versions of addresses must provide translation when passing a packet from a network that supports IPv4 addressing to a network that supports IPv6 addressing, and vice versa.

Criticism

The seven-layer OSI model has been criticized by some experts. In particular, in the classic book UNIX. System Administrator's Guide" by Evi Nemeth and others write:

... While the ISO committees were arguing about their standards, the whole concept of networking was changing behind their backs and the TCP / IP protocol was being introduced around the world. …
…
And so, when the ISO protocols were finally implemented, a number of problems emerged:
These protocols were based on concepts that make no sense in today's networks.
Their specifications were in some cases incomplete.
In terms of their functionality, they were inferior to other protocols.
The presence of multiple layers has made these protocols slow and difficult to implement.
…
… Now even the most zealous supporters of these protocols admit that OSI is gradually moving towards becoming a small footnote in the pages of computer history.

The OSI (Open System Interconnection) model of open systems interaction is a set of standards for the interaction of network equipment with each other. It is also called the protocol stack. Designed to ensure that various network objects, regardless of manufacturer and type (computer, server, switch, hub, and even a browser that displays an html page) comply unified rules of work with data and could successfully carry out information exchange.

Network devices are different in function and "proximity" to the end user - a person or an application. Therefore, the OSI model describes 7 levels of interaction, each of which has its own protocols, indivisible portions of data, and devices. Let's analyze the principle of operation of the seven-layer OSI model with examples.

Network layers of the OSI model

Physical

Responsible for the physical transfer of data between devices over long and short distances. He describes types of signals and methods of their processing for different transmission media: wires (twisted pair and coaxial), optical fiber, radio link (wi-fi and bluetooth), infrared channel. The units of data at this level are bits converted into electrical impulses, light, radio waves, and so on. Also, the types of connectors, their pinout are fixed here.

Devices operating at the physical layer of the OSI Model (OSI Model): signal repeaters, concentrators (hubs). These are the least "intelligent" devices, the task of which is to amplify the signal or split it without any analysis and modification.

ducted

Being above the physical, it must “lower” correctly formatted data into transmission medium, having previously taken them from the top level. At the receiving end, the link-layer protocols "raise" information from physics, check the received for errors, and pass it up the protocol stack.

To implement the verification procedures, it is necessary, firstly, to segment the data for transmission into portions (frames), and secondly, to supplement them with service information (headers).

Also here for the first time the concept of an address pops up. Here, this is the MAC (English Media Access Control) address - a six-byte network device identifier required to indicate in frames as a recipient and sender when transmitting data within the same local segment.

Devices: network bridge (bridge), switch. Their primary difference from the "lower" devices is the maintenance of MAC address tables for their ports and the distribution / filtering of traffic only in the necessary directions.

network

Connects entire networks. decides global logistics challenges on data transfer between different segments of large networks: routing, filtering, optimization and quality control.

The unit of transmitted information is packets. Addressing nodes and networks is performed by assigning them 4-byte numbers - IP (English Internet Protocol) addresses, hierarchically organized, and allowing you to flexibly configure the mutual logical visibility of network segments.

There are also familiar symbolic node names, which are mapped to IP addresses by network layer protocols. Devices operating on this floor of the OSI model are routers (routers, gateways). Implementing in themselves all three first levels of the protocol stack, they unite different networks, redirect packets from one to another, choosing their route according to certain rules, maintain transmission statistics, and ensure security through filtering tables.

Transport

Transportation in this case is assumed to be logical (since 1 stage of the stack is responsible for the physical one): establishing a connection with the opposite node at the appropriate level, confirming the delivery of the received data, and controlling their quality. This is how the TCP (Transmission Control Protocol) protocol works. The transmitted portion of information is a block or segment.

To transfer streaming arrays (datagrams), the UDP (User Datagram Protocol) protocol is used.

Address - decimal number of the virtual software port of a particular workstation or server.

session

Manages the transfer process in terms of user access. Limits the connection (session) time of one node with another, controls access rights, synchronizes the beginning and end of the exchange.

Executive

The data received from below - from the session - must be correctly presented to the end user or application. Correct decoding, data decompression, if the browser saved your traffic - these operations are performed at the penultimate step.

Applied

Application or application layer. Surfing in a browser, receiving and sending mail, accessing other network nodes through remote access is the pinnacle of the OSI networking model.

An example of how the network model works

Consider a living example of the principle of the protocol stack. Let the computer user send a photo to a friend with a signature in the messenger. Going down the levels of the model:

On the applied a message is formed: in addition to the photo and text, information about the address of the message server is added to the package (the symbolic name www.xxxxx.com will turn into a decimal IP address using a special protocol), the recipient's identifier on this server, and possibly some other service information.
On the representative- a photo can be compressed if its size is large in terms of the messenger and its settings.
session track the user's logical connection to the server, his status. They will also control the data transfer process after it has begun, tracking the session.
On the transport data is divided into blocks. Service fields of the transport layer are added with checksums, error control options, etc. One photo can turn into several blocks.
On the network- blocks are wrapped with service information, which contains, among other things, the address of the sending host and the IP address of the message server. It is this information that will allow IP packets to reach the server, possibly across the whole world.
On the canal, IP packet data is packed into frames with the addition of service fields, in particular MAC addresses. The address of your own network card will be placed in the sender field, and the MAC of the default gateway will be placed in the recipient field, again from your own network settings (it is unlikely that the computer is on the same network with the server, respectively, its MAC is unknown, and the default gateway, for example, home router is known).
On the physical- bits from the frames will be translated into radio waves, and will reach the home router via the wi-fi protocol.
There, the information will rise along the protocol stack already up to the 3rd level of the router stack, then it will be packet forwarding to ISP routers. And so on, until on the messenger server, at the highest level, the message and the photo in their original forms get to the sender's personal disk space, then the recipient. And then a similar path of information will begin already to the addressee of the message, when he goes online and establishes a session with the server.

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 levels 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.

In the 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 the qualitative construction and configuration of any network, the system administrator must understand the principles of the OSI reference model, just so that you learn to understand the interaction of network applications, and in general 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 and others. 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 the data is, whether it's a word document or an email message.

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

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 and others.

Transport layer (4)- Responsible for the reliability of data transmission. It also splits the data into segments and reassembles them, as the data comes in different sizes. There are two well-known protocols of this level - these are TCP and UDP. The TCP protocol gives a guarantee that the data will be delivered in full, but 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, the physical transmission of 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 getting to 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.

In this article, we will understand what the OSI network model is, what levels it consists of, and what functions it performs. So, the subject of conversation is a certain model of interaction of standards that determine the sequence of data exchange, and programs.

The abbreviation OSI Open Systems Interconnection stands for Open Systems Interconnection Model. To solve the problem of compatibility of various systems, the standards organization released in 1983 the OSI model reference. It describes the structure of open systems, their requirements, and their interactions.

An open system is a system compiled according to open specifications that are available to everyone, and also meet certain standards. For example, the Windows operating system is considered an open system because it is based on open specifications that describe the operation of the Internet, but the initial codes of the system are closed.

The advantage is that it is possible to build a network of devices from different manufacturers, if necessary, replace its individual components. You can easily combine several networks into one whole.

According to the model we are considering, it is necessary that computer networks consist of seven levels. Because the model does not describe protocols defined by individual standards, it is not a network architecture.

Unfortunately, from a practical point of view, the open systems interaction model does not apply. Its peculiarity lies in mastering the theoretical issues of network interaction. That is why this model is used as a simple language for describing the construction of different types of networks.

Model levelsOSI

The basic structure is a system consisting of 7 levels. The question arises, what are the seven stages responsible for and why does the model need so many levels? All of them are responsible for a certain stage of the process of sending a network message, and also contain a certain semantic load. The steps are performed separately from each other and do not require increased control on the part of the user. Isn't it convenient?

The lower stages of the system, from the first to the third, manage the physical delivery of data over the network, they are called media layers.

The rest of the levels help ensure accurate delivery of data between computers on the network, they are called host machines.

Application is the closest level to the user. Its difference from others is that it does not provide services to other levels. Provides services to application processes that are outside the scope of the model, such as database transfer, voice, and more.

This stage is arranged relatively simpler than others, because apart from ones and zeros there are no other measurement systems in it, this level does not analyze information and that is why it is the lowest of the levels. It mainly carries out the transfer of information. The main load parameter is bit.

The main purpose of the physical layer is to represent zero and one as signals transmitted over a data transmission medium.

For example, there is a certain communication channel (CS), a message being sent, a sender and, accordingly, a recipient. The CS has its own characteristics:

Bandwidth, measured in bits / s, that is, how much data we can transfer per unit of time.
Delay, how long it will take before the message reaches the sender to the recipient.
The number of errors, if errors occur frequently, then the protocols should provide error correction. And if rare, then they can be corrected at higher levels, for example, at the transport level.

As a channel for transmitting information, the following are used:

Cables: telephone, coaxial, twisted pair, optical.
Wireless technologies such as radio waves, infrared radiation.
Satellite CS
Wireless optics or lasers are rarely used, due to low speed and a large amount of interference.

It is very rare for errors to occur in optical cables, since it is difficult to influence the propagation of light. In copper cables, errors occur, but rarely, and in a wireless environment, errors occur very often.

The next station the information will visit will remind customs. Namely, the IP address will be compared for compatibility with the transmission medium. It also identifies and corrects system deficiencies. For the convenience of further operations, the bits are grouped into frames - frame.

The purpose of the link layer is the transmission of messages over the CS - frames.

Tasksdata link

Find where in the bit stream the message starts and ends
Detect and correct errors when sending information
Addressing, you need to know which computer to send information to, because in a shared environment, basically, several computers connect
Provide consistent access to a shared environment so that at the same time, information is transmitted by one computer.

Errors are detected and corrected at the data link layer. If it is detected, the correctness of data delivery is checked, if it is incorrect, then the frame is discarded.

Error correction requires the use of special codes that add redundant information to the transmitted data.

Resending data, used in conjunction with the error detection method. If an error is detected in a frame, it is discarded and the sender resends the frame.

Detect and fix bugs

Practice has shown the effectiveness of the following methods, if a reliable medium for data transmission (wired) is used and errors rarely occur, then it is better to correct them at the top level. If errors occur frequently in the CS, then errors must be corrected immediately at the link level.

The functions of this stage in the computer are carried out by network adapters and drivers suitable for them. Through them, and there is a direct exchange of data.

Some of the protocols used at the link layer are HDLC using bus topology and others.

(NETWORK)

The stage resembles the process of distribution of information. For example, all users are divided into groups, and data packets diverge according to IP addresses, consisting of 32 bits. It is thanks to the work of routers at this instance that all network differences are eliminated. This is the so-called logical routing process.

The main task is to create composite networks built on the basis of network technologies of different link levels: Ethernet, MPLS. The network layer is the "backbone" of the Internet.

Purpose of the network layer

We can transfer information from one computer to another via Ethernet and Wi-Fi, then why need another layer? The link layer (CL) technology has two problems, firstly, the CL technologies differ from each other, and secondly, there is a scaling limitation.

What are the differences in link layer technologies?

A different level of service provided, some levels guarantee the delivery and the necessary order of messages. Wi-Fi just guarantees the delivery of the message, not.

Different addressing, by size, hierarchy. Network technologies may support broadcasting, ie. it is possible to send information to all computers in the network.

The maximum frame size (MTU) may differ, for example, in the Internet 1500, and in Wi-Fi 2300. How can such differences be negotiated at the network level?

You can provide a different type of service, for example, frames from Wi-Fi are received with an acknowledgment sent, and on Ethernet they are sent without acknowledgment.

In order to negotiate the difference in addressing, at the network level, global addresses are introduced that do not depend on the addresses of specific technologies (ARP for ) of the link layer.

Fragmentation is used to transfer data across multiple networks that have different frame sizes. Consider an example, the first computer transmits data to the second, through 4 intermediate networks, united by 3 routers. Each network has a different MTU.

The computer formed the first frame and transmitted it to the router, the router analyzed the frame size, and realized that it was impossible to transmit it completely through network 2, because its mtu2 was too small.

The router splits the data into 3 parts and transmits them separately.

The next router combines the data into one large packet, determines its size and compares it with network mtu 3. And it sees that one MTU3 packet cannot be transmitted entirely (MTU3 is larger than MTU2, but less than MTU1) and the router splits the packet into 2 parts and sends to the next router.

The last router combines the packet and sends the whole packet to the recipient. Fragmentation deals with the aggregation of networks and is hidden from the sender and receiver.

How is the scalability problem solved at the network layer?

Work is carried out not with individual addresses, as at the link level, but with blocks of addresses. Packets for which the path is not known are dropped rather than forwarded back to all ports. And a significant difference from the channel, the possibility of several connections between network level devices and all these connections will be active.

Network Layer Tasks:

Combine networks built by different technologies;
Provide quality service;
Routing, finding a path from the sender of information to the recipient, through intermediate network nodes.

Routing

Search for a way to send a packet between networks through transit nodes - routers. Let's look at an example of routing. The scheme consists of 5 routers and two computers. How can data be transferred from one computer to another?

Next time the data can be sent in a different way.

In the event of a breakdown of one of the routers, nothing terrible will happen, you can find a way around the broken router.

The protocols used at this stage are: Internet protocol IP; IPX, necessary for routing packets in networks, etc.

(TRANSPORT)

There is the following task, a packet arrives on a computer that is connected to a composite network, many network applications (web browser, skype, mail) are running on the computer, we need to understand which application needs to transfer this packet. The interaction of network applications is handled by the transport layer.

Transport Layer Tasks

Sending data between processes on different hosts. Providing addressing, you need to know for which process this or that packet is intended. Ensuring the reliability of information transmission.

Interaction modelopen system

Hosts are devices where useful user programs and network equipment, such as switches, routers, operate.

A feature of the transport layer is the direct interaction of one computer with the transport layer on another computer, at other levels the interaction goes along the links of the chain.

This layer provides an end-to-end connection between two interacting hosts. This layer is independent of the network, it allows you to hide the details of network interaction from application developers.

For addressing at the transport level, ports are used, these are numbers from 1 to 65535. Ports are written like this: 192.168.1.3:80 (IP address and port).

Transport Layer Features

Providing higher reliability, unlike the network that is used for data transmission. Reliable communication channels are used, errors in these CSs rarely occur, therefore, it is possible to build a reliable network that will be cheap, and errors can be corrected programmatically on hosts.

The transport layer guarantees the delivery of the data, it uses the confirmation from the recipient, if the confirmation is not received, the transport sends the data confirmation again. Message follow-up guarantee.

session layer (SESSION)

Session (session) is a set of network interactions aimed at solving a single task.

Now networking has become more complex and does not consist of simple questions and answers, as it used to be. For example, you load a web page to be displayed in the browser, you first need to download the web page text itself (.html), a style file (.css) that describes the design elements of the web page, download images. Thus, in order to complete the task of loading a web page, several, separate network operations must be implemented.

Session determines what will be the transfer of information between 2 application processes: half-duplex (one by one transmission and reception of data); or duplex (simultaneous transmission and reception of information).

Presentation layer(PRESENTATION)

Functions - to represent the data passed between application processes in the required form.

To describe this level, use the automatic translation of the network from various languages. For example, you dial a phone number, speak Russian, the network automatically translates into French, transfers information to Spain, where a person picks up the phone and hears your question in Spanish. This task has not yet been implemented.

To protect the data sent over the network, encryption is used: secure sockets layer, as well as transport layer security, these technologies allow you to encrypt data that is sent over the network.

Application layer protocols use TSL/SSL and can be identified by the s at the end. For example, https, ftps and others. If you see in the browser that the https protocol and a lock are used, this means that data is being protected over the network using encryption.

(APPLICATION)

It is necessary for interaction between network applications, such as web, e-mail, skype, etc.

In fact, it is a set of specifications that allow the user to enter the pages to find the information he needs. Simply put, the job of an application is to provide access to network services. The content of this level is very diverse.

Functionsapplication:

Solving problems, sending files; job and system management;
Identification of users by their login, e-mail address, passwords, electronic signatures;
Requests to connect with other application processes;

Video about all levels of the modelOSI

Conclusion

Analyzing problems using OSI network models will help you quickly find and fix them. No wonder the work on the draft program, capable of identifying shortcomings, while having a complex stepped device, was carried out for quite a long time. This model is actually a benchmark. Indeed, at one time, work was carried out with her to create other protocols. For example, . Today, they are used quite often.

Let's look at the purpose of the levels of the osi reference model in this article, with a detailed description of each of the seven levels of the model.

The process of organizing the principle of network interaction in computer networks is a rather complex and difficult task, therefore, to implement this task, we decided to use the well-known and universal approach - decomposition.

Decomposition- this is a scientific method that uses the division of one complex task into several simpler tasks - series (modules) interconnected.

Layered Approach:

all modules are divided into separate groups and sorted by levels, thereby creating a hierarchy;
modules of the same level to carry out their tasks sends requests only to the modules of the immediately adjacent lower level;
the principle of encapsulation is turned on - the level provides a service, hiding the details of its implementation from other levels.

The International Standards Organization (ISO, founded in 1946) was given the task of creating a universal model that clearly delineates and defines the various levels of interaction between systems, with named levels and giving each level its specific task. This model was named open systems interaction model(Open System Interconnection, OSI) or ISO/OSI model .

The Reference Model of Open Systems Interconnection (osi seven-level model) was introduced in 1977.

After the approval of this model, the interaction problem was divided (decomposed) into seven particular problems, each of which can be solved independently of the others.

Layers of the OSI Reference Model are a vertical structure where all network functions are divided between seven levels. It should be especially noted that strictly described operations, equipment and protocols correspond to each such level.

Interaction between levels is organized as follows:

vertically - inside a single computer and only with neighboring levels.
horizontally - logical interaction is organized - with the same level of another computer at the other end of the communication channel (that is, the network level on one computer interacts with the network level on another computer).

Since the seven-level osi model consists of a strict subordinate structure, any higher level uses the functions of the lower level, and recognizes in what form and in what way (i.e. through which interface) the data stream should be transmitted to it.

Let's consider how the transmission of messages over a computer network is organized in accordance with the OSI model. The application layer is the level of applications, that is, this level is displayed to the user in the form of the operating system used and the programs that are used to send data. At the very beginning, it is the application layer that forms the message, then it is transmitted to the representative layer, that is, it goes down the OSI model. The representative layer, in turn, parses the application layer header, performs the required actions, and adds its service information to the beginning of the message, in the form of a representative layer header, for the representative layer of the destination node. Then the message continues down, descends to the session layer, and it, in turn, also adds its service data, in the form of a header at the beginning of the message, and the process continues until it reaches the physical layer.

It should be noted that in addition to adding service information in the form of a header at the beginning of the message, levels can add service information at the end of the message, which is called "trailer".

When the message has reached the physical layer, the message is already fully formed for transmission over the communication channel to the destination node, that is, it contains all the service information added at the levels of the OSI model.

In addition to the term "data" (data), which is used in the OSI model at the application, presentation and session layers, other terms are used at other layers of the OSI model so that it is possible to immediately determine at which layer of the OSI model processing is performed.

In ISO standards, to denote a particular portion of data that protocols of different levels of the OSI model work with, a common name is used - a protocol data unit (Protocol Data Unit, PDU). To designate data blocks of certain levels, special names are often used: frame (frame), package (packet), segment (segment).

Physical layer functions

at this level, connector types and pin assignments are standardized;
determines how "0" and "1" are represented;
interface between a network carrier and a network device (transmits electrical or optical signals to a cable or radio air, receives them and converts them into data bits);
physical layer functions are implemented in all devices connected to the network;
physical layer equipment: hubs;
Examples of network interfaces related to the physical layer: RS-232C, RJ-11, RJ-45, AUI connectors, BNC.

Link Layer Functions

zero and one bits of the Physical layer are organized into frames - "frame". A frame is a piece of data that has an independent logical value;
organization of access to the transmission medium;
processing of data transmission errors;
determines the structure of links between nodes and ways of addressing them;
equipment operating at the data link layer: switches, bridges;
examples of protocols related to the link layer: Ethernet, Token Ring, FDDI, Bluetooth, Wi-Fi, Wi-Max, X.25, FrameRelay, ATM.

For a LAN, the link layer is divided into two sublayers:

LLC (LogicalLinkControl) - responsible for establishing a communication channel and for error-free sending and receiving data messages;
MAC (MediaAccessControl) - provides shared access of network adapters to the physical layer, determination of frame boundaries, recognition of destination addresses (for example, access to a common bus).

Network Layer Functions

Performs the functions:

determining the data transfer path;
determining the shortest route;
monitoring problems and congestion in the network.

Solves tasks:

transmission of messages over links with a non-standard structure;
harmonization of different technologies;
simplification of addressing in large networks;
creating barriers to unwanted traffic between networks.

Equipment operating at the network layer: router.
Types of network layer protocols:

network protocols (forwarding packets through the network: , ICMP);
routing protocols: RIP, OSPF;
address resolution protocols (ARP).

Transport layer functions of the osi model

provides applications (or application and session layers) data transfer with the required degree of reliability, compensates for the shortcomings of the reliability of lower levels;
multiplexing and demultiplexing i.e. collection and disassembly of packages;
protocols are designed for point-to-point interaction;
starting from this level, the protocols are implemented by the software of the end nodes of the network - the components of their network operating systems;
examples: protocols TCP, UDP.

Session Layer Functions

maintaining a communication session, allowing applications to interact with each other for a long time;
creating/ending a session;
information exchange;
task synchronization;
determination of the right to data transfer;
maintaining the session during periods of application inactivity.
transmission synchronization is ensured by placing checkpoints in the data stream, from which the process resumes in case of failures.

Presentation Functions

responsible for protocol conversion and data encoding/decoding. Application requests received from the application layer are converted into a format for transmission over the network, and data received from the network is converted into a format understandable by applications;
possible implementation:
compressing/decompressing or encoding/decoding data;
redirecting requests to another network resource if they cannot be processed locally.
example: SSL protocol(provides secret messaging for TCP/IP application layer protocols).

Application layer functions of the osi model

is a set of various protocols with the help of which network users gain access to shared resources, organize joint work;
provides interaction between the network and the user;
allows user applications to access network services such as database query handler, file access, e-mail forwarding;
responsible for the transfer of service information;
provides applications with error information;
example: HTTP, POP3, SNMP, FTP.

Network-dependent and network-independent layers of the seven-level osi model

According to their functionality, the seven layers of the OSI model can be classified into one of two groups:

a group in which the levels depend on the specific technical implementation of a computer network. The physical, link and network layers are network-dependent, in other words, these layers are inextricably linked to the specific network equipment used.
a group in which the levels are mainly focused on working with applications. Session, presentation and application levels - are focused on the applications used and practically do not depend on what kind of network equipment is used in a computer network, that is, network-independent.