Network layer detailed description. Open Systems Interconnection (OSI) model. Data flows through the OSI model

For a unified representation of data in networks with heterogeneous devices and software, the international organization for ISO standards (International Standardization Organization) has developed a basic communication model for open systems OSI (Open System Interconnection) . This model describes the rules and procedures for transferring data in various network environments when organizing a communication session. The main elements of the model are layers, application processes and physical means of connection. On fig. 1.10 shows the structure of the basic model.

Each layer of the OSI model performs a specific task in the process of transmitting data over the network. The base model is the basis for the development of network protocols. OSI divides communication functions in a network into seven layers, each of which serves a different part of the open systems interoperability process.

The OSI model only describes system-wide means of interaction, not end-user applications. Applications implement their own communication protocols by accessing system facilities.

Rice. 1.10. OSI model

If an application can take over the functions of some of the upper layers of the OSI model, then for communication it accesses directly the system tools that perform the functions of the remaining lower layers of the OSI model.

Interaction of layers of the OSI model

The OSI model can be divided into two different models, as shown in Fig. 1.11:

A horizontal model based on protocols that provides a mechanism for the interaction of programs and processes on different machines;

A vertical model based on services provided by neighboring layers to each other on the same machine.

Each layer of the sending computer interacts with the same layer of the receiving computer as if it were directly connected. Such a connection is called a logical or virtual connection. In fact, the interaction is carried out between adjacent levels of one computer.

So, the information on the sending computer must pass through all levels. Then it is transmitted over the physical medium to the receiving computer and again passes through all the layers until it reaches the same level from which it was sent on the sending computer.

In the horizontal model, two programs need a common protocol to exchange data. In a vertical model, adjacent layers communicate using Application Programming Interfaces (APIs).

Rice. 1.11. Computer Interaction Diagram in the Basic OSI Reference Model

Before being fed into the network, the data is broken into packets. A packet is a unit of information transmitted between stations on a network.

When sending data, the packet passes sequentially through all layers of the software. At each level, control information of this level (header) is added to the packet, which is necessary for successful data transmission over the network, as shown in Fig. 1.12, where Zag is the packet header, End is the end of the packet.

On the receiving side, the packet goes through all the layers in reverse order. At each layer, the protocol at that layer reads the packet's information, then removes the information added to the packet at the same layer by the sender, and passes the packet to the next layer. When the packet reaches the Application layer, all control information will be removed from the packet and the data will return to its original form.

Rice. 1.12. Formation of a package of each level of the seven-level model

Each level of the model has its own function. The higher the level, the more difficult the task it solves.

It is convenient to think of the individual layers of the OSI model as groups of programs designed to perform specific functions. One layer, for example, is responsible for providing data conversion from ASCII to EBCDIC and contains the programs necessary to perform this task.

Each layer provides a service to a higher layer, in turn requesting a service from the lower layer. The upper layers request a service in much the same way: as a rule, it is a requirement to route some data from one network to another. The practical implementation of the principles of data addressing is assigned to the lower levels. On fig. 1.13 provides a brief description of the functions of all levels.

Rice. 1.13. Functions of the OSI Model Layers

The model under consideration determines the interaction of open systems from different manufacturers in the same network. Therefore, it performs coordinating actions for them on:

Interaction of applied processes;

Data presentation forms;

Uniform data storage;

Network resource management;

Data security and information protection;

Diagnostics of programs and hardware.

Application layer

The application layer provides application processes with access to the interaction area, is the upper (seventh) level and is directly adjacent to application processes.

In fact, the application layer is a set of various protocols by which network users access shared resources such as files, printers, or hypertext Web pages, and also organize their joint work, for example, using the protocol Email. Special application service elements provide services for specific application programs such as file transfer and terminal emulation programs. If, for example, the program needs to send files, then the FTAM (File Transfer, Access, and Management) file transfer protocol will be used. In the OSI model, an application program that needs to perform a specific task (for example, update a database on a computer) sends specific data in the form of a Datagram to the application layer. One of the main tasks of this layer is to determine how an application request should be processed, in other words, what form the request should take.

The unit of data that the application layer operates on is usually called a message.

The application layer performs the following functions:

1. Performing various types of work.

File transfer;

Job management;

System management, etc;

2. Identification of users by their passwords, addresses, electronic signatures;

3. Determination of functioning subscribers and the possibility of access to new application processes;

4. Determining the sufficiency of available resources;

5. Organization of requests for connection with other application processes;

6. Transfer of applications to the representative level for the necessary methods for describing information;

7. Selection of procedures for the planned process dialogue;

8. Management of data exchanged between application processes and synchronization of interaction between application processes;

9. Determining the quality of service (delivery time of data blocks, acceptable error rate);

10. Agreement on the correction of errors and the determination of the reliability of data;

11. Coordination of restrictions imposed on the syntax (character sets, data structure).

These functions define the kinds of services that the application layer provides to application processes. In addition, the application layer transfers to application processes the service provided by the physical, link, network, transport, session and presentation layers.

At the application level, it is necessary to provide users with already processed information. This can be handled by system and user software.

The application layer is responsible for accessing applications to the network. The tasks of this level are file transfer, exchange postal messages and network management.

The most common top three layer protocols are:

ftp ( File Transfer Protocol) file transfer protocol;

TFTP (Trivial File Transfer Protocol) is the simplest file transfer protocol;

X.400 email;

Telnet work with a remote terminal;

SMTP (Simple Mail Transfer Protocol) is a simple mail exchange protocol;

CMIP (Common Management Information Protocol) common information management protocol;

SLIP (Serial Line IP) IP for serial lines. Protocol for serial character-by-character data transfer;

SNMP (Simple Network Management Protocol) simple network management protocol;

FTAM (File Transfer, Access, and Management) is a protocol for transferring, accessing and managing files.

Presentation layer

The functions of this level are the presentation of data transmitted between application processes in the desired form.

This layer ensures that the information passed by the application layer will be understood by the application layer in another system. If necessary, the presentation layer at the time of information transfer performs the conversion of data formats into some common presentation format, and at the time of reception, respectively, performs the reverse conversion. Thus, application layers can overcome, for example, syntactical differences in data representation. This situation can occur in a LAN with computers of different types (IBM PC and Macintosh) that need to exchange data. So, in the fields of databases, information should be presented in the form of letters and numbers, and often in the form of a graphic image. You need to process this data, for example, as floating point numbers.

The common data representation is based on the ASN.1 system, which is common for all levels of the model. This system serves to describe the structure of files, and also solves the problem of data encryption. At this level, data encryption and decryption can be performed, thanks to which the secrecy of data exchange is ensured immediately for all application services. An example of such a protocol is the Secure Socket Layer (SSL) protocol, which provides secure messaging for the application layer protocols of the TCP/IP stack. This layer provides data transformation (coding, compression, etc.) of the application layer into an information stream for the transport layer.

The representative layer performs the following main functions:

1. Generation of requests to establish interaction sessions between application processes.

2. Coordination of data presentation between application processes.

3. Implementation of data presentation forms.

4. Presentation of graphic material (drawings, drawings, diagrams).

5. Classification of data.

6. Sending requests to terminate sessions.

Presentation layer protocols are usually part of the protocols of the top three layers of the model.

Session layer

The session layer is the layer that defines the procedure for conducting sessions between users or application processes.

The session layer provides conversation control to keep track of which side is currently active, and also provides a means of synchronization. The latter allow you to insert checkpoints into long transfers so that in case of a failure, you can go back to the last checkpoint, instead of starting all over again. In practice, few applications use the session layer, and it is rarely implemented.

The session layer controls the transfer of information between application processes, coordinates the reception, transmission and issuance of one communication session. In addition, the session layer additionally contains the functions of password management, conversation control, synchronization and cancellation of communication in a transmission session after a failure due to errors in the lower layers. The functions of this layer are to coordinate communication between two application programs running on different workstations. It comes in the form of a well-structured dialogue. These functions include creating a session, managing the transmission and reception of message packets during a session, and terminating a session.

At the session level, it is determined what the transfer between two application processes will be:

Half duplex (processes will send and receive data in turn);

Duplex (processes will send data and receive them at the same time).

In half-duplex mode, the session layer issues a data token to the process that initiates the transfer. When the time comes for the second process to respond, the data token is passed to it. The session layer allows transmission only to the party that possesses the data token.

The session layer provides the following functions:

1. Establishment and completion at the session level of a connection between interacting systems.

2. Performing normal and urgent data exchange between application processes.

3. Managing the interaction of applied processes.

4. Synchronization of session connections.

5. Notification of application processes about exceptional situations.

6. Establishment of labels in the applied process, allowing, after a failure or error, to restore its execution from the nearest label.

7. Interruption in the necessary cases of the application process and its correct resumption.

8. Termination of the session without data loss.

9. Transmission of special messages about the progress of the session.

The session layer is responsible for organizing data exchange sessions between end machines. Session layer protocols are usually a component of the protocols of the top three layers of the model.

Transport Layer

The transport layer is designed to transfer packets through a communication network. At the transport layer, packets are divided into blocks.

On the way from the sender to the recipient, packets can be corrupted or lost. While some applications have their own error handling, there are some that prefer to deal with a reliable connection right away. The job of the transport layer is to ensure that applications or upper layers of the model (application and session) transfer data with the degree of reliability that they require. The OSI model defines five classes of service provided by the transport layer. These types of services differ in the quality of the services provided: urgency, the ability to restore interrupted communications, the availability of multiplexing facilities for multiple connections between different application protocols through a common transport protocol, and most importantly, the ability to detect and correct transmission errors such as distortion, loss and duplication of packets.

The transport layer determines the addressing of physical devices (systems, their parts) in the network. This layer guarantees the delivery of blocks of information to recipients and manages this delivery. Its main task is to provide efficient, convenient and reliable forms of information transfer between systems. When more than one packet is in processing, the transport layer controls the order in which the packets pass through. If a duplicate of a previously received message passes, then this layer recognizes this and ignores the message.

The functions of the transport layer include:

1. Network transmission control and ensuring the integrity of data blocks.

2. Detection of errors, their partial elimination and reporting of uncorrected errors.

3. Recovery of transmission after failures and malfunctions.

4. Consolidation or division of data blocks.

5. Granting of priorities at transfer of blocks (normal or urgent).

6. Transfer confirmation.

7. Elimination of blocks in deadlock situations in the network.

Starting from the transport layer, all higher protocols are implemented in software, usually included in the network operating system.

The most common transport layer protocols include:

TCP (Transmission Control Protocol) TCP/IP stack transmission control protocol;

UDP (User Datagram Protocol) is the user datagram protocol of the TCP/IP stack;

NCP (NetWare Core Protocol) basic protocol for NetWare networks;

SPX (Sequenced Packet eXchange) Novell Stack Sequenced Packet Exchange;

TP4 (Transmission Protocol) - class 4 transmission protocol.

Network Layer

The network layer provides the laying of channels connecting subscriber and administrative systems through a communication network, choosing the route of the fastest and most reliable way.

The network layer establishes communication in a computer network between two systems and provides the laying of virtual channels between them. A virtual or logical channel is such a functioning of network components that creates the illusion of laying the necessary path between the interacting components. In addition, the network layer informs the transport layer about errors that occur. Network layer messages are commonly referred to as packets. They contain pieces of data. The network layer is responsible for their addressing and delivery.

Laying the best path for data transmission is called routing, and its solution is the main task of the network layer. This problem is compounded by the fact that the shortest path is not always the best. Often the criterion for choosing a route is the time of data transfer along this route; it depends on the bandwidth of communication channels and traffic intensity, which can change over time. Some routing algorithms try to adapt to load changes, while others make decisions based on long-term averages. Route selection can also be based on other criteria, such as transmission reliability.

The link layer protocol provides data delivery between any nodes only in a network with an appropriate typical topology. This is a very strict limitation that does not allow building networks with a developed structure, for example, networks that combine several enterprise networks into a single network, or highly reliable networks in which there are redundant links between nodes.

Thus, within the network, data delivery is regulated by the link layer, but data delivery between networks is handled by the network layer. When organizing the delivery of packets at the network level, the concept of a network number is used. In this case, the recipient's address consists of the network number and the number of the computer on that network.

Networks are interconnected by special devices called routers. A router is a device that collects information about the topology of interconnections and, based on it, forwards network layer packets to the destination network. In order to transfer a message from a sender located in one network to a recipient located in another network, it is necessary to make a certain number of transit transfers (hops) between networks, each time choosing the appropriate route. Thus, a route is a sequence of routers that a packet traverses.

The network layer is responsible for dividing users into groups and routing packets based on the translation of MAC addresses into network addresses. The network layer also provides transparent transmission of packets to the transport layer.

The network layer performs the following functions:

1. Creation of network connections and identification of their ports.

2. Detection and correction of errors that occur during transmission through a communication network.

3. Packet flow control.

4. Organization (ordering) of sequences of packages.

5. Routing and switching.

6. Segmentation and consolidation of packages.

The network layer defines two kinds of protocols. The first type refers to the definition of rules for the transmission of packets with data of end nodes from a node to a router and between routers. It is these protocols that are usually referred to when talking about network layer protocols. However, another type of protocol, called routing information exchange protocols, is often referred to as the network layer. Routers use these protocols to collect information about the topology of interconnections.

Network layer protocols are implemented by software modules of the operating system, as well as software and hardware of routers.

The most commonly used protocols at the network layer are:

IP (Internet Protocol) Internet protocol, a network protocol of the TCP/IP stack that provides address and routing information;

IPX (Internetwork Packet Exchange) is an Internet packet exchange protocol designed for addressing and routing packets in Novell networks;

X.25 international standard for global packet-switched communications (this protocol is partially implemented at layer 2);

CLNP (Connection Less Network Protocol) is a network protocol without organizing connections.

Link layer (Data Link)

The information unit of the link layer are frames (frame). Frames are a logically organized structure into which data can be placed. The task of the link layer is to transfer frames from the network layer to the physical layer.

At the physical layer, bits are simply sent. This does not take into account that in some networks, in which communication lines are used alternately by several pairs of interacting computers, the physical transmission medium may be busy. Therefore, one of the tasks of the link layer is to check the availability of the transmission medium. Another task of the link layer is to implement error detection and correction mechanisms.

The link layer ensures that each frame is transmitted correctly by placing a special bit sequence at the beginning and end of each frame to mark it, and also calculates a checksum by summing all the bytes of the frame in a certain way and adding a checksum to the frame. When a frame arrives, the receiver again calculates the checksum of the received data and compares the result with the checksum from the frame. If they match, the frame is considered valid and accepted. If the checksums do not match, then an error is generated.

The task of the link layer is to take packets coming from the network layer and prepare them for transmission by fitting them into a frame of the appropriate size. This layer is required to determine where the block starts and ends, and to detect transmission errors.

This level defines the rules for the use physical layer network nodes. The electrical representation of data in the LAN (data bits, data encoding methods, and markers) is recognized at this and only at this level. Here, errors are detected and corrected (by requesting data retransmission).

The link layer provides the creation, transmission and reception of data frames. This layer services network layer requests and uses the physical layer service to receive and transmit packets. The IEEE 802.X specifications divide the link layer into two sublayers:

LLC (Logical Link Control) logical link control provides logical link control. The LLC sublayer provides services to the network layer and is concerned with the transmission and reception of user messages.

MAC (Media Assess Control) media access control. The MAC sublayer regulates access to the shared physical medium (token passing or collision or collision detection) and controls access to the communication channel. The LLC sublayer is above the MAC sublayer.

The data link layer defines media access and transmission control through a data transfer procedure over a link.

With large sizes of transmitted data blocks, the link layer divides them into frames and transmits frames as sequences.

Upon receipt of frames, the layer forms transmitted data blocks from them. The size of a data block depends on the transmission method, the quality of the channel through which it is transmitted.

In LANs, link-layer protocols are used by computers, bridges, switches, and routers. In computers, the functions of the link layer are implemented by the joint efforts of network adapters and their drivers.

The link layer can perform the following types of functions:

1. Organization (establishment, management, termination) of channel connections and identification of their ports.

2. Organization and transfer of personnel.

3. Detection and correction of errors.

4. Data flow management.

5. Ensuring the transparency of logical channels (transfer of data encoded in any way over them).

The most commonly used protocols at the link layer include:

HDLC (High Level Data Link Control) high-level data link control protocol for serial connections;

IEEE 802.2 LLC (Type I and Type II) provide MAC for 802.x environments;

Ethernet network technology according to the IEEE 802.3 standard for networks using bus topology and multiple access with carrier sniffing and collision detection;

Token ring network technology according to the IEEE 802.5 standard, using a ring topology and a token passing ring access method;

FDDI (Fiber Distributed Date Interface Station) IEEE 802.6 network technology using fiber optic media;

X.25 is an international standard for global packet-switched communications;

Frame relay network organized from X25 and ISDN technologies.

Physical Layer

The physical layer is designed to interface with the physical means of connection. The physical means of connection is the combination of the physical environment, hardware and software tools, which provides signal transmission between systems.

The physical medium is a material substance through which signals are transmitted. The physical medium is the foundation upon which the physical means of connection are built. Ether, metals, optical glass and quartz are widely used as physical media.

The Physical Layer consists of a Media Interface Sublayer and a Transmission Transformation Sublayer.

The first of them provides pairing of the data flow with the used physical communication channel. The second performs transformations related to the applied protocols. The physical layer provides the physical interface to the data channel and also describes the procedures for transmitting signals to and from the channel. At this level, the electrical, mechanical, functional and procedural parameters for physical communication in systems are defined. The physical layer receives data packets from the overlying link layer and converts them into optical or electrical signals corresponding to 0 and 1 of the binary stream. These signals are sent through the transmission medium to the receiving node. The mechanical and electrical/optical properties of the transmission medium are defined at the physical layer and include:

Type of cables and connectors;

Pin assignment in connectors;

Signal coding scheme for values 0 and 1.

The physical layer performs the following functions:

1. Establishment and disconnection of physical connections.

2. Transmission of signals in serial code and reception.

3. Listening, if necessary, channels.

4. Identification of channels.

5. Notification of the occurrence of faults and failures.

Notification about the occurrence of malfunctions and failures is due to the fact that a certain class of events is detected at the physical layer that interferes with the normal operation of the network (collision of frames sent by several systems at once, channel break, power failure, loss of mechanical contact, etc.). The types of service provided to the data link layer are defined by the physical layer protocols. Listening to the channel is necessary in cases where a group of systems is connected to one channel, but only one of them is allowed to transmit signals at the same time. Therefore, listening to the channel allows you to determine whether it is free to transmit. In some cases, for a clearer definition of the structure, the physical layer is divided into several sublevels. For example, the physical layer of a wireless network is divided into three sublayers (Figure 1.14).

Rice. 1.14. Wireless LAN physical layer

Physical layer functions are implemented in all devices connected to the network. On the computer side, physical layer functions are performed network adapter. Repeaters are the only type of equipment that only works at the physical layer.

The physical layer can provide both asynchronous (serial) and synchronous (parallel) transmission, which is used for some mainframes and minicomputers. At the Physical layer, an encoding scheme must be defined to represent binary values for transmission over a communication channel. Many local area networks use Manchester encoding.

An example of a physical layer protocol is the specification of 10Base-T Ethernet technology, which defines a category 3 unshielded twisted pair with a characteristic impedance of 100 ohms, an RJ-45 connector, a maximum length of a physical segment of 100 meters, a Manchester code for data representation, and other characteristics as the cable used. environment and electrical signals.

The most common physical layer specifications include:

EIA-RS-232-C, CCITT V.24/V.28 - Mechanical/Electrical Unbalanced Serial Interface;

EIA-RS-422/449, CCITT V.10 - mechanical, electrical and optical characteristics of a balanced serial interface;

Ethernet is an IEEE 802.3 network technology for networks using bus topology and multiple access with carrier sniffing and collision detection;

Token ring is an IEEE 802.5 network technology that uses a ring topology and a token passing ring access method.

Just started working as a network administrator? Don't want to be confused? Our article will help you. Have you heard a time-tested administrator talking about network problems and mentioning some levels? Have you ever been asked at work what layers are protected and work if you are using an old firewall? To understand the basics of information security, you need to understand the principle of the hierarchy of the OSI model. Let's try to see the possibilities of this model.

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

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

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

Ask, what is the use of this for you? Knowing the multi-level model will give you the opportunity to freely communicate with employees of IT companies, discussing network problems will no longer be an oppressive boredom. And when you learn to understand at what stage the failure occurred, you can easily find the causes and significantly reduce the range of your work.

OSI levels

The model contains seven simplified steps:

Physical.
Channel.
Network.
Transport.
session.
Executive.
Applied.

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

Let's move on to the direct acquaintance with the levels.

Physical layer

The main task of the first stage is the transfer of bits through physical communication channels. Physical communication channels are devices designed to transmit and receive information signals. For example, fiber optic, coaxial cable or twisted pair. The transfer can also go through wireless communication. The first stage is characterized by the data transmission medium: interference protection, bandwidth, wave impedance. The qualities of electrical final signals are also set (type of coding, voltage levels and signal transmission rate) and connected to standard types of connectors, contact connections are assigned.

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

Link layer

At the second stage, the abstract address of the device is associated with the physical device, and the availability of the transmission medium is checked. Bits are formed into sets - frames. The main task of the link layer is to detect and correct errors. For correct transmission, specialized bit sequences are inserted before and after the frame and a calculated checksum is added. When the frame reaches the destination, the checksum of the already arrived data is calculated again, if it matches the checksum in the frame, the frame is recognized as correct. Otherwise, an error occurs, which is corrected by retransmission of information.

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

network layer

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

To put it another way, the third step handles the Internet protocol and acts as a router: finding the best path for information. Router - a device that collects data on the structure of interconnections and transmits packets to the destination network (transit transfers - hops). If you encounter an error in the IP address, then this is a problem that has arisen at the network level. Protocols of the third stage are divided into network, routing or address resolution: ICMP, IPSec, ARP and BGP.

transport layer

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

How to choose a transport leg service class? When the quality of the communication transport links is high, a light service will be an adequate choice. If communication channels do not work securely at the very beginning, it is advisable to resort to a developed service that will provide maximum opportunities for finding and solving problems (data delivery control, delivery timeouts). Phase 4 specifications: TCP and UDP of the TCP/IP stack, SPX of the Novell stack.

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

session layer

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

Executive level

The sixth stage is involved in the transformation of data into a universal recognizable format without changing the content. Since different devices use different formats, information processed at the representational level allows systems to understand each other, overcoming syntactic and coding differences. In addition, at the sixth stage, it becomes possible to encrypt and decrypt data, which ensures secrecy. Protocol examples: ASCII and MIDI, SSL.

Application layer

The seventh stage on our list and the first if the program sends data over the network. Consists of sets of specifications through which the user, Web pages. For example, when sending messages by mail, it is at the application level that a convenient protocol is chosen. The composition of the specifications of the seventh stage is very diverse. For example, SMTP and HTTP, FTP, TFTP or SMB.

You may hear somewhere about the eighth level of the ISO model. Officially, it does not exist, but a comic eighth stage has appeared among IT workers. All due to the fact that problems can arise due to the fault of the user, and as you know, a person is at the pinnacle of evolution, so the eighth level appeared.

Having looked at the OSI model, you have been able to understand the complex structure of the network and now understand the essence of your work. Things get pretty easy when the process is broken down into parts!