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At what level of the osi model does switch operate. How the OSI model works

OSI reference model

For clarity, the network process in the OSI reference model is divided into seven layers. This theoretical construct makes fairly complex concepts easier to learn and understand. At the top of the OSI model is the application that needs access to network resources, at the bottom is the network environment itself. As data moves from layer to layer down, the protocols operating at those layers gradually prepare it for transmission over the network. Once it reaches the target system, the data moves up through the layers, with the same protocols performing the same actions, only in reverse order. In 1983 International Organization for Standardization(International Organization for Standardization, ISO) and Standardization sectortelecommunications of the International Telecommunications Union(Telecommunication Standardization Sector of International Telecommunication Union, ITU-T) published the document “The Basic Reference Model for Open Systems Interconnection”, which described a model for distributing network functions between 7 different levels (Fig. 1.7). This seven-layer structure was supposed to form the basis for a new protocol stack, but it was never implemented in commercial form. Instead, the OSI model is used with existing protocol stacks as a training and reference tool. Most of the protocols popular today predate the development of the OSI model, so they do not exactly conform to its seven-layer structure. Often, one protocol combines the functions of two or even several levels of the model, and the boundaries of the protocols often do not correspond to the boundaries of the OSI layers. However, the OSI model remains an excellent visual aid for examining network processes, and professionals often associate functions and protocols with specific layers.

Data Encapsulation

Essentially, the interaction of protocols operating at different levels of the OSI model is manifested in the fact that each protocol adds title(header) or (in one case) trailer(footer) to the information it received from the level above. For example, an application generates a request to a network resource. This request moves down the protocol stack. When it reaches the transport layer, protocols at that layer add their own header to the request, consisting of fields with information specific to the functions of that protocol. The original request itself becomes a data field (payload) for the transport layer protocol. After adding its header, the transport layer protocol passes the request to the network layer. The network layer protocol adds its own header to the transport layer protocol header. Thus, for a network layer protocol, the payload becomes the original request and the transport layer protocol header. This entire structure becomes the payload for the protocol link layer, which adds a title and trailer to it. The result of this activity is plastic bag(packet), ready for transmission over the network. When the packet reaches its destination, the process is repeated in reverse. The protocol of each subsequent layer of the stack (now from bottom to top) processes and removes the header of the equivalent protocol of the sending system. When the process is completed, the original request reaches the application it was intended for, in the same form in which it was generated. The process of adding headers to a request (Figure 1.8) generated by an application is called data encapsulation(data encapsulation). In essence, this procedure resembles the process of preparing a letter for sending by mail. The request is the letter itself, and adding headings is the same as putting the letter in an envelope, writing the address, stamping it, and actually sending it.

Physical layer

At the lowest level of the OSI model - physical(physical) - the characteristics of network equipment elements are determined - the network environment, installation method, type of signals used to transmit binary data over the network. In addition, the physical layer determines what type of network adapter needs to be installed on each computer and what kind of hub to use (if necessary). At the physical level, we are dealing with copper or fiber optic cable or some kind of wireless connection. In a LAN, the physical layer specifications are directly related to the data link protocol used on the network. Once you select a link layer protocol, you must use one of the physical layer specifications supported by that protocol. For example, the Ethernet link layer protocol supports several different physical layer options - one of two types of coaxial cable, any twisted pair cable, or fiber optic cable. The parameters of each of these options are formed from numerous information about the requirements of the physical layer, for example, the type of cable and connectors, the permissible length of cables, the number of hubs, etc. Compliance with these requirements is necessary for the normal operation of the protocols. For example, in a cable that is too long, the Ethernet system may not notice packet collisions, and if the system is unable to detect errors, it cannot correct them, resulting in data loss. Not all aspects of the physical layer are defined by the link layer protocol standard. Some of them are defined separately. One of the most commonly used physical layer specifications is described in the Commercial Building Telecommunications Cabling Standard, known as EIA/TIA 568A. It is jointly published American National Institute of Standarts(American National Standards Institute, ANSI), Associations fromelectronics industries(Electronics Industry Association, EIA) and Communications Industry Association(Telecommunications Industry Association, TIA). Included in this document detailed description cables for data transmission networks in industrial environments, including the minimum distance from sources of electromagnetic interference and other rules for laying cables. Today, cable laying in large networks is most often entrusted to specialized companies. The contractor hired should be thoroughly familiar with EIA/TIA 568A and other similar documents, as well as city building codes. Another communication element defined at the physical layer is the type of signal for transmitting data over the network medium. For cables with a copper base, this signal is an electric charge; for a fiber-optic cable, it is a light pulse. Other types of network environments may use radio waves, infrared pulses, and other signals. In addition to the nature of the signals, the scheme for their transmission is established at the physical level, i.e. the combination electric charges or light pulses, used to encode binary information that is generated by higher layers. Ethernet systems use a signaling scheme known as Manchester encoding(Manchester encoding), and in Token Ring systems it is used differentialManchester(Differential Manchester) scheme.

Data Link Layer

Protocol channel(data-link) level ensures the exchange of information between the hardware of a computer connected to the network and network software. It prepares data sent to it by the network layer protocol for sending to the network, and transmits data received by the system from the network to the network layer. When designing and building a LAN, the link layer protocol used is the most important factor in choosing equipment and how it is installed. To implement the link layer protocol, the following hardware and software are required: network interface adapters (if the adapter is a separate device connected to the bus, it is called a network interface card or simply a network card); network adapter drivers; network cables (or other network media) and ancillary connecting equipment; network hubs (in some cases). How network adapters, and hubs are designed for specific link-layer protocols. Some network cables are also tailored for specific protocols, but there are also cables that are suitable for different protocols. Of course, today (as always) the most popular link layer protocol is Ethernet. Token Ring is far behind, followed by other protocols such as FDDI (Fiber Distributed Data Interface). There are typically three main elements included in a link layer protocol specification: the frame format (i.e., the header and trailer added to the network layer data before transmission to the network); mechanism for controlling access to the network environment; one or more physical layer specifications used with a given protocol.

Frame format

The link layer protocol adds a header and trailer to the data received from the network layer protocol, turning it into frame(frame) (Fig. 1.9). Using the mail analogy again, the header and trailer are the envelope for sending the letter. They contain the addresses of the sending and receiving systems of the packet. For LAN protocols like Ethernet and Token Ring, these addresses are 6-byte hexadecimal strings assigned to network adapters at the factory. They, in contrast to the addresses used at other levels of the OSI model, are called appa military addresses(hardware address) or MAC addresses (see below).

Note Protocols at different layers of the OSI model have different names for the structures they create by adding a header to data coming from a higher protocol. For example, what a link layer protocol calls a frame would be a datagram to the network layer. A more general name for a structural unit of data at any level is plastic bag.

It is important to understand that link layer protocols provide communication only between computers on the same LAN. The hardware address in the header always belongs to a computer on the same LAN, even if the target system is on a different network. Other important functions link layer frame - identification of the network layer protocol that generated the data in the packet and information for error detection. The network layer can use different protocols, so the link layer protocol frame usually includes code that can be used to identify which network layer protocol generated the data in that packet. Guided by this code, the link layer protocol of the receiving computer forwards the data to the corresponding protocol of its network layer. To detect errors, the transmitting system calculates cyclical cue redundant code(cyclical redundancy check, CRC) of the payload and writes it to the frame trailer. Upon receiving the packet, the target computer performs the same calculations and compares the result with the contents of the trailer. If the results match, the information was transmitted without errors. Otherwise, the recipient assumes that the package is damaged and does not accept it.

Media access control

Computers on a LAN typically share a half-duplex network medium. In this case, it is quite possible that two computers will start transmitting data simultaneously. In such cases, a kind of packet collision occurs, collision(collision), in which data in both packets is lost. One of the main functions of the data link layer protocol is media access control (MAC), i.e., controlling the transmission of data by each computer and minimizing packet collisions. The media access control mechanism is one of the most important characteristics of a link layer protocol. Ethernet uses a mechanism with carrier sense and collision detection (Carrier Sense Multiple Access with Collision Detection, CSMA/CD) to control access to the medium. Some other protocols, such as Token Ring, use token passing.

Physical Layer Specifications

Link layer protocols used in LANs often support more than one network medium, and one or more physical layer specifications are included in the protocol standard. The data link and physical layers are closely related because the properties of the network medium significantly influence how the protocol controls access to the medium. Therefore, we can say that in local networks, link layer protocols also perform the functions of the physical layer. WANs use link layer protocols that do not include physical layer information, for example, SLIP (Serial Line Internet Protocol) and PPP (Point-to-Point Protocol).

Network layer

At first glance it may seem that network(network) layer duplicates some functions of the data link layer. But this is not true: network layer protocols are “responsible” for end-to-end(end-to-end) communications, while link layer protocols operate only within a LAN. In other words, network layer protocols completely ensure the transmission of a packet from the source to the target system. Depending on the type of network, the sender and recipient may be on the same LAN, on different LANs within the same building, or on LANs separated by thousands of kilometers. For example, when you communicate with a server on the Internet, packets generated by your computer pass through dozens of networks on their way to it. The link layer protocol will change several times to accommodate these networks, but the network layer protocol will remain the same all the way. The cornerstone of the TCP/IP (Transmission Control Protocol/Internet Protocol) protocol suite and the most commonly used network layer protocol is the Internet Protocol (IP). Novell NetWare has its own IPX (Internetwork Packet Exchange) network protocol, and on small networks Microsoft Windows Typically the NetBEUI (NetBIOS Enhanced User Interface) protocol is used. Most of the functions assigned to the network layer are determined by the capabilities of the IP protocol. Like a link layer protocol, a network layer protocol adds a header to the data it receives from a higher layer (Figure 1.10). A data element created by a network layer protocol consists of transport layer data and a network layer header and is called datagram(datagram).

Addressing

The network layer protocol header, like the link layer protocol header, contains fields with the addresses of the source and target systems. However, in this case, the destination system address belongs to the final destination of the packet and may differ from the destination address in the link layer protocol header. For example, when you type the address of a Web site into your browser's address bar, the packet generated by your computer specifies the address of the target network-level system as the address of the Web server, while at the link-layer the address of the router on your LAN that provides the Internet access. IP uses its own addressing system, which is completely independent of link-layer addresses. Each computer on an IP network is manually or automatically assigned a 32-bit IP address, identifying both the computer itself and the network on which it is located. In IPX, a hardware address is used to identify the computer itself, in addition, a special address is used to identify the network on which the computer is located. NetBEUI differentiates computers by the NetBIOS names assigned to each system during installation.

Fragmentation

Network layer datagrams must traverse multiple networks on their way to their destination, encountering the specific properties and limitations of various link layer protocols. One such limitation is the maximum packet size allowed by the protocol. For example, a Token Ring frame can be up to 4500 bytes in size, while Ethernet frames can be up to 1500 bytes in size. When a large datagram generated on the Token Ring network is transmitted to Ethernet network, the network layer protocol must break it into several fragments of no more than 1500 bytes in size. This process is called fragmentation(fragmentation). During the fragmentation process, the network layer protocol breaks the datagram into fragments, the size of which corresponds to the capabilities of the data link layer protocol being used. Each fragment becomes an independent packet and continues its path to the target network layer system. The source datagram is formed only after all fragments have reached the destination. Sometimes, on the way to the target system, the fragments into which the datagram is broken must be re-fragmented.

Routing

Routing routing is the process of selecting the most efficient route on the Internet for transmitting datagrams from a sending system to a receiving system. In complex internetworks, such as the Internet or large corporate networks, there are often several ways to get from one computer to another. Network designers deliberately create redundant links so that traffic can find its way to its destination even if one of the routers fails. Routers are used to connect individual LANs that are part of the Internet. The purpose of a router is to accept incoming traffic from one network and forward it to a specific system on another. There are two types of systems on internet networks: terminal(end systems) and intermediate(intermediate systems). End systems are senders and receivers of packets. A router is an intermediate system. End systems use all seven layers of the OSI model, while packets arriving at intermediate systems do not rise above the network layer. There, the router processes the packet and sends it down the stack for transmission to the next target system (Figure 1.11).

To correctly route the packet to the target, routers store tables with network information in memory. This information can be entered manually by the administrator or collected automatically from other routers using specialized protocols. A typical routing table entry includes the address of another network and the address of the router through which packets must travel to that network. In addition, the routing table element contains route metric - conditional assessment of its effectiveness. If there are multiple routes to a system, the router selects the most efficient one and sends the datagram to the data link layer for transmission to the router specified in the table entry with the best metric. In large networks, routing can be an unusually complex process, but most often it is done automatically and unnoticed by the user.

Transport Layer Protocol Identification

Just as the link layer header specifies the network layer protocol that generated and transmitted the data, the network layer header contains information about the transport layer protocol from which the data was received. Based on this information, the receiving system forwards incoming datagrams to the appropriate transport layer protocol.

Transport layer

Functions performed by protocols transport(transport) layer, complement the functions of network layer protocols. Often the protocols of these levels used for data transmission form an interconnected pair, as can be seen in the example of TCP/IP: the TCP protocol operates at the transport layer, IP at the network layer. Most protocol suites have two or more transport layer protocols that perform different functions. An alternative to TCP is UDP (User Datagram Protocol). The IPX protocol suite also includes several transport layer protocols, including NCP (NetWare Core Protocol) and SPX (Sequenced Packet Exchange). The difference between transport layer protocols from a particular set is that some are connection oriented and others are not. Systems using the protocol connection-oriented(connection-oriented), before transmitting data, they exchange messages to establish communication with each other. This ensures that systems are turned on and ready to go. The TCP protocol, for example, is connection-oriented. When you connect to an Internet server using a browser, the browser and the server first perform a so-called three-step handshake(three-way handshake). Only after this the browser transmits the address of the desired Web page to the server. When the data transfer is complete, the systems perform the same handshake to terminate the connection. In addition, connection-oriented protocols perform additional actions, such as sending a packet acknowledgment signal, segmenting data, controlling flow, and detecting and correcting errors. Typically, protocols of this type are used to transfer large amounts of information that must not contain a single bit of error, such as data files or programs. Additional features of connection-oriented protocols ensure correct data transfer. This is why these protocols are often called reliable(reliable). Reliability in this case is a technical term and means that every packet transmitted is checked for errors, and the sending system is notified of the delivery of each packet. The disadvantage of this type of protocol is the significant amount of control data exchanged between the two systems. First, additional messages are sent when communication is established and terminated. Second, the header added to the packet by a connection-oriented protocol is substantially larger than the header of a connection-less protocol. For example, the TCP/IP protocol header is 20 bytes, and the UDP header is 8 bytes. Protocol, not connection oriented(connectionless), does not establish a connection between two systems before data is transferred. The sender simply transmits information to the target system without worrying about whether it is ready to accept the data or whether the system even exists. Typically, systems resort to connectionless protocols such as UDP for short transactions consisting of only requests and response signals. The response signal from the receiver implicitly functions as a transmission acknowledgment signal.

Note Connection-oriented and connectionless protocols are not limited to the transport layer. For example, network layer protocols are usually not connection-oriented, since they rely on the transport layer to ensure communication reliability.

Transport layer protocols (as well as network and data link layers) usually contain information from higher layers. For example, the TCP and UDP headers include port numbers that identify the application that originated the packet and the application to which it is destined. On session(session) level, a significant discrepancy begins between the actually used protocols and the OSI model. Unlike lower layers, there are no dedicated session layer protocols. The functions of this layer are integrated into protocols that also perform the functions of the representative and application layers. The transport, network, data link and physical layers are responsible for the actual transmission of data over the network. Protocols of the session and higher levels have nothing to do with the communication process. The session layer includes 22 services, many of which define how information is exchanged between systems on the network. The most important services are dialogue management and dialogue separation. The exchange of information between two systems on a network is called dialogue(dialogue). Dialogue management(dialog control) consists of choosing the mode in which the systems will exchange messages. There are two such modes: half duplex(two-way alternate, TWA) and duplex(two-way simultaneous, TWS). In half-duplex mode, the two systems also transmit tokens along with the data. Information can only be transferred to a computer that has this moment there is a marker. This avoids message collisions along the way. The duplex model is more complicated. There are no markers in it; both systems can transmit data at any time, even simultaneously. Dividing dialogue(dialog separation) consists of inclusion in the data stream control points(checkpoints) that allow synchronizing the operation of two systems. The degree of difficulty of dividing the dialogue depends on the mode in which it is carried out. In half-duplex mode, systems perform minor synchronization by exchanging checkpoint messages. In full duplex mode, systems perform full synchronization using the master/active token.

Executive level

On representative The presentation layer performs a single function: syntax translation between different systems. Sometimes computers on a network use different syntaxes. The representative layer allows them to "agree" on a common syntax for exchanging data. When establishing a connection at the presentation layer, systems exchange messages about what syntaxes they have and select the one they will use during the session. Both systems involved in the connection have abstractsyntax(abstract syntax) is their “native” form of communication. The abstract syntaxes of different computer platforms may vary. During the system coordination process, a common transfer syntaxdata(transfer syntax). The transmitting system converts its abstract syntax into data transfer syntax, and the receiving system, upon completion of the transfer, does the opposite. If necessary, the system can select a data transfer syntax with additional functions, such as data compression or encryption.

Application layer

The application layer is the entry point through which programs access the OSI model and network resources. Most application layer protocols provide network access services. For example, using the SMTP (Simple Mail Transfer Protocol) protocol, most programs Email used to send messages. Other application layer protocols, such as FTP (File Transfer Protocol), are themselves programs. Application layer protocols often include session and presentation layer functions. As a result, a typical protocol stack contains four separate protocols that operate at the application, transport, network, and data link layers.

In the literature, it is most often customary to start describing the layers of the OSI model from layer 7, called application layer, at which user applications access the network. The OSI model ends with the 1st layer - physical, which defines the standards required by independent manufacturers for data transmission media:

type of transmission medium (copper cable, optical fiber, radio air, etc.),
type of signal modulation,
signal levels of logical discrete states (zeros and ones).

Any protocol of the OSI model must interact either with protocols at its layer, or with protocols one unit higher and/or lower than its layer. Interactions with protocols of one level are called horizontal, and with levels one higher or lower - vertical. Any protocol of the OSI model can perform only the functions of its layer and cannot perform functions of another layer, which is not performed in the protocols of alternative models.

Each level, with some degree of convention, corresponds to its own operand - a logically indivisible element of data, which at a separate level can be operated within the framework of the model and the protocols used: at the physical level the smallest unit is a bit, at the link level information is combined into frames, at the network level - into packets ( datagrams), on transport - into segments. Any piece of data logically combined for transmission - frame, packet, datagram - is considered a message. It is the messages in general view are operands of the session, presentation and application layers.

Basic network technologies include the physical and data link layers.

Application layer

Application layer (application layer; English application layer) - the top level of the model, ensuring the interaction of user applications with the network:

Allows applications to use network services:
- remote access to files and databases,
- forwarding email;
is responsible for transmitting service information;
provides applications with error information;
generates queries to the presentation layer.

Application level protocols: RDP, HTTP, SMTP, SNMP, POP3, FTP, XMPP, OSCAR, Modbus, SIP, TELNET and others.

Presentation layer

Often erroneously called the presentation layer, this layer provides protocol conversion and data encoding/decoding. Application requests received from the application layer are converted into a format for transmission over the network at the presentation layer, and data received from the network is converted into an application format. This layer can perform compression/decompression or encryption/decryption, as well as redirecting requests to another network resource if they cannot be processed locally.

The presentation layer is usually an intermediate protocol for transforming information from neighboring layers. This allows communication between applications on disparate computer systems in a manner transparent to the applications. The presentation layer provides code formatting and transformation. Code formatting is used to ensure that the application receives information to process that makes sense to it. If necessary, this layer can perform translation from one data format to another.

The presentation layer not only deals with the formats and presentation of data, it also deals with the data structures that are used by programs. Thus, layer 6 provides organization of data as it is sent.

To understand how this works, let's imagine that there are two systems. One uses EBCDIC, such as an IBM mainframe, to represent data, and the other uses ASCII (most other computer manufacturers use it). If these two systems need to exchange information, then a presentation layer is needed that will perform the conversion and translate between the two different formats.

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

Presentation layer standards also define how to present graphic images. For these purposes, the PICT format can be used - an image format used to transfer QuickDraw graphics between programs.

Another representation format is the tagged TIFF image file format, which is typically used for high-resolution raster images. The next presentation layer standard that can be used for graphic images is that developed by the Joint Photographic Expert Group; in everyday use this standard is simply called JPEG.

There is another group of presentation level standards that define the presentation of audio and film fragments. This includes the electronic musical instrument interface. Musical Instrument Digital Interface, MIDI) for the digital representation of music, the MPEG standard developed by the Motion Picture Experts Group, used to compress and encode videos on CDs, store in digitized form and transmit at speeds up to 1.5 Mbit/s, and QuickTime - a standard describing audio and video elements for programs running on Macintosh computers and PowerPC.

Presentation layer protocols: AFP - Apple Filing Protocol, ICA - Independent Computing Architecture, LPP - Lightweight Presentation Protocol, NCP - NetWare Core Protocol, NDR - Network Data Representation, XDR - eXternal Data Representation, X.25 PAD - Packet Assembler/Disassembler Protocol .

Session layer

Transport layer

Network layer

Data Link Layer

When developing protocol stacks at this level, the problems of error-resistant coding are solved. Such coding methods include Hamming code, block coding, Reed-Solomon code.

In programming, this level represents the driver network card, operating systems have software interface interaction of the channel and network layers with each other. This is not a new level, but simply an implementation of the model for a specific OS. Examples of such interfaces: ODI (English), NDIS , UDI .

Physical layer

Hubs, signal repeaters and media converters also operate at this level.

Physical layer functions are implemented on all devices connected to the network. On the computer side, the physical layer functions are performed by the network adapter or serial port. The physical layer refers to the physical, electrical, and mechanical interfaces between two systems. The physical layer defines such types of data transmission media as optical fiber, twisted pair, coaxial cable, satellite channel data transfers, etc. Standard types of network interfaces related to the physical layer are: V.35, RS-232, RS-485, RJ-11, RJ-45, AUI and BNC connectors.

When developing protocol stacks, synchronization and line coding problems are solved at this level. Such encoding methods include NRZ code, RZ code, MLT-3, PAM5, Manchester II.

Physical layer protocols:

OSI network model(open systems interconnection basic reference model - basic reference model of interaction open systems, abbr. EMVOS; 1978) - network stack model network protocols OSI/ISO (GOST R ISO/IEC 7498-1-99).

General characteristics of the OSI model

Due to the protracted development of the OSI protocols, the main protocol stack currently in use is TCP/IP, which was developed before the adoption of the OSI model and without connection with it.

By the end of the 70s, a large number of proprietary communication protocol stacks already existed in the world, including, for example, such popular stacks as DECnet, TCP/IP and SNA. This variety of internetworking tools has brought to the fore the problem of incompatibility between devices using different protocols. One of the ways to solve this problem at that time was seen as a general transition to a single protocol stack common to all systems, created taking into account the shortcomings of existing stacks. This academic approach to creating a new stack began with the development of the OSI model and took seven years (from 1977 to 1984). The purpose of the OSI model is to provide a generalized representation of network communication tools. It was developed as a kind of universal language for network specialists, which is why it is called the reference model. In the OSI model, the means of interaction are divided into seven layers: application, presentation, session, transport, network, link and physical. Each layer deals with a very specific aspect of how network devices interact.

Applications can implement their own communication protocols using a multi-level set of system tools for these purposes. It is for this purpose that an application program interface (API) is provided to programmers. In accordance with the ideal design of the OSI model, an application can make requests only to the topmost layer - the application one, however, in practice, many communication protocol stacks allow programmers to directly access services, or services, located below the layers. For example, some DBMSs have built-in tools remote access to files. In this case, the application does not use the system file service when accessing remote resources; it bypasses the upper layers of the OSI model and goes directly to those responsible for transporting messages across the network system tools, which are located at the lower levels of the OSI model. So, suppose an application on Host A wants to communicate with an application on Host B. To do this, Application A makes a request to an application layer, such as a file service. Based on this request, the application level software generates a message in a standard format. But in order to deliver this information to its destination, there are still many tasks to be solved, the responsibility for which lies with lower levels. After the message is generated, the application layer forwards it down the stack to the presentation layer. The presentation layer protocol, based on the information received from the application layer message header, performs the required actions and adds its own service information to the message - the presentation layer header, which contains instructions for the presentation layer protocol of the destination machine. The resulting message is passed down to the session layer, which, in turn, adds its header, etc. (Some protocol implementations place service information not only at the beginning of the message in the form of a header, but also at the end in the form of a so-called trailer.) Finally, the message reaches the lower, physical, level, which, in fact, transmits it along communication lines to the recipient machine. At this point, the message is “overgrown” with headings of all levels.

The physical layer places the message on the physical output interface of computer 1, and it begins its “journey” through the network (up to this point, the message was transmitted from one layer to another within computer 1). When a message over the network arrives at the input interface of computer 2, it is received by its physical layer and sequentially moves up from layer to layer. Each level analyzes and processes the header of its level, performing the appropriate functions, and then removes this header and passes the message to the higher level. As can be seen from the description, protocol entities of the same level do not communicate with each other directly; intermediaries are always involved in this communication - protocol tools of lower levels. And only the physical levels of different nodes interact directly.

OSI Model Layers

OSI model
Layer		)	Functions	Examples
Host layers	7. Application		Access to network services	HTTP, FTP, SMTP
	6. Presentation		Data representation and encryption	ASCII, EBCDIC, JPEG
	5. Session		Session management	RPC, PAP
	4. Transport	Segments/ Datagrams	Direct communication between endpoints and reliability	TCP, UDP, SCTP
layers	3. Network	Packets	Route determination and logical addressing	IPv4, IPv6, IPsec, AppleTalk
	2. Channel (data link)	Bits/ Frames	Physical addressing	PPP, IEEE 802.2, Ethernet, DSL, L2TP, ARP
	1. Physical	Bits	Working with transmission media, signals and binary data	USB, twisted pair, coaxial cable, optical cable

type of transmission medium (copper cable, optical fiber, radio air, etc.),
type of signal modulation,
signal levels of logical discrete states (zero and one).

Each level, with some degree of convention, corresponds to its own operand - a logically indivisible element of data, which at a separate level can be operated within the framework of the model and the protocols used: at the physical level the smallest unit is a bit, at the link level information is combined into frames, at the network level - into packets ( datagrams), on transport - into segments. Any piece of data logically combined for transmission - frame, packet, datagram - is considered a message. It is messages in general that are the operands of the session, representative and application levels.

Basic network technologies include the physical and data link layers.

Application layer

Application layer (application layer) - the top level of the model, ensuring the interaction of user applications with the network:

Allows applications to use network services:
- remote access to files and databases,
- forwarding email;
is responsible for transmitting service information;
provides applications with error information;
generates queries to the presentation layer.

Application level protocols: RDP, HTTP, SMTP, SNMP, POP3, FTP, XMPP, OSCAR, Modbus, SIP, TELNET and others.

Presentation layer

The presentation layer provides protocol conversion and data encoding/decoding. Application requests received from the application layer are converted into a format for transmission over the network at the presentation layer, and data received from the network is converted into an application format. This layer can perform compression/decompression or encryption/decryption, as well as redirecting requests to another network resource if they cannot be processed locally.

Another function performed at the presentation layer is data encryption, which is used in cases where it is necessary to protect transmitted information from access by unauthorized recipients. To accomplish this task, processes and code in the presentation layer must perform data transformation. There are other routines at this level that compress texts and convert graphics into bitstreams so they can be transmitted over a network.

Presentation layer standards also define how graphical images are represented. For these purposes, the PICT format can be used - an image format used to transfer QuickDraw graphics between programs.

There is another group of presentation level standards that define the presentation of audio and film fragments. This includes the Musical Instrument Digital Interface (MIDI) for the digital representation of music, the Motion Picture Experts Group's MPEG standard, used to compress and encode CD-ROM videos, store them in digitized form, and transmit at bit rates up to 1.5 Mbps, and QuickTime is a standard that describes audio and video elements for programs running on Macintosh and PowerPC computers.

Session layer

The session layer of the model ensures the maintenance of a communication session, allowing applications to interact with each other for a long time. The layer manages session creation/termination, information exchange, task synchronization, data transfer eligibility determination, and session maintenance during periods of application inactivity.

Session protocols: ADSP (AppleTalk Data Stream Protocol), ASP (AppleTalk Session Protocol), H.245 (Call Control Protocol for Multimedia Communication), ISO-SP (OSI Session Layer Protocol (X.225, ISO 8327)), iSNS (Internet Storage Name Service), L2F (Layer 2 Forwarding Protocol), L2TP (Layer 2 Tunneling Protocol), NetBIOS (Network Basic Input Output System), PAP (Password Authentication Protocol), PPTP (Point-to-Point Tunneling Protocol), RPC (Remote Procedure Call Protocol), RTCP (Real-time Transport Control Protocol), SMPP (Short Message Peer-to-Peer), SCP (Session Control Protocol), ZIP (Zone Information Protocol), SDP (Sockets Direct Protoco]) .

Transport layer

The transport layer of the model is designed to ensure reliable data transfer from sender to recipient. However, the level of reliability can vary widely. 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 proper sequence, multiplex multiple data streams, provide data flow control mechanism and guarantee the reliability of the received data. For example, UDP is limited to monitoring the integrity of data within one datagram, and does not exclude the possibility of losing an entire packet, or duplicating packets, or disrupting the order in which data packets are received; TCP provides reliable continuous data transmission, eliminating data loss or disruption of the order of its arrival or duplication; it can redistribute data, breaking large portions of data into fragments and, conversely, merging fragments into one packet.

Transport layer protocols: ATP (AppleTalk Transaction Protocol), CUDP (Cyclic UDP), DCCP (Datagram Congestion Control Protocol), FCP (Fiber Channel|Fiber Channel Protocol), IL (IL Protocol), NBF (NetBIOS Frames protocol), NCP ( NetWare Core Protocol), SCTP (Stream Control Transmission Protocol), SPX (Sequenced Packet Exchange), SST (Structured Stream Transport), TCP (Transmission Control Protocol), UDP (User Datagram Protocol).

Network layer

The network layer (lang-en|network layer) of the model is designed to determine the path of data transmission. Responsible for translating logical addresses and names into physical ones, determining the shortest routes, switching and routing, monitoring problems and congestion in the network.

Network layer protocols route data from source to destination. Devices (routers) operating at this level are conventionally called third-level devices (based on the level number in the OSI model).

Network layer protocols: IP/IPv4/IPv6 (Internet Protocol), IPX (Internetwork Packet Exchange), X.25 (partially implemented at layer 2), CLNP (connectionless network protocol), IPsec (Internet Protocol Security). Routing protocols - RIP (Routing Information Protocol), OSPF (Open Shortest Path First).

Data Link Layer

The data link layer is designed to ensure the interaction of networks at the physical level and control errors that may occur. It packs the data received from the physical layer, presented in bits, into frames, checks them for integrity and, if necessary, corrects errors (forms a repeated request for a damaged frame) and sends them to the network layer. The data link layer can communicate with one or more physical layers, monitoring and managing this interaction.

The IEEE 802 specification divides this layer into two sublayers: MAC (Media Access Control) regulates access to the shared physical medium, LLC (logical link control) provides network layer service.

Switches, bridges and other devices operate at this level. These devices are said to use Layer 2 addressing (based on the layer number in the OSI model).

Link layer protocols: ARCnet, ATM (Asynchronous Transfer Mode), Controller Area Network (CAN), Econet, IEEE 802.3 (Ethernet), Ethernet Automatic Protection Switching (EAPS), Fiber Distributed Data Interface (FDDI), Frame Relay, High-Level Data Link Control (HDLC), IEEE 802.2 (provides LLC functions to IEEE 802 MAC layers), Link Access Procedures, D channel (LAPD), IEEE 802.11 wireless LAN, LocalTalk, Multiprotocol Label Switching (MPLS), Point-to-Point Protocol (PPP), Point-to-Point Protocol over Ethernet (PPPoE), StarLan, Token ring, Unidirectional Link Detection (UDLD), x.25]], ARP.

In programming, this level represents the network card driver, in operating systems There is a software interface for interaction between the channel and network layers. This is not a new level, but simply an implementation of the model for a specific OS. Examples of such interfaces: ODI, NDIS, UDI.

Physical layer

Physical layer is the lowest level of the model, which defines the method of transferring data, presented in binary form, from one device (computer) to another. Various organizations are involved in compiling such methods, including: the Institute of Electrical and Electronics Engineers, the Electronics Industry Alliance, the European Telecommunications Standards Institute and others. They transmit electrical or optical signals into a cable or radio broadcast and, accordingly, receive and convert them into data bits in accordance with digital signal encoding methods.

Hubs]], signal repeaters and media converters also operate at this level.

Physical layer functions are implemented on all devices connected to the network. On the computer side, the physical layer functions are performed by the network adapter or serial port. The physical layer refers to the physical, electrical, and mechanical interfaces between two systems. The physical layer defines such types of data transmission media as optical fiber, twisted pair, coaxial cable, satellite data link, etc. Standard types of network interfaces related to the physical layer are:)

02.05.2020

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