Reference model osi in short. Model of interconnection between critical systems (OSI). Butt of robotic hemlock model

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OSI reference model

For precision, the process of operation is carried out in the OSI reference model of separations on this level. This theoretical construction makes it easier to develop and comprehend complex concepts. The upper part of the OSI model has an extension that requires access to the edge resources, while the lower part contains the edge itself. In addition, as data flows from level to level down, protocols operate at these levels to step by step prepare them for transmission along the line. Having reached the target system, the data are pushed through the mountains, and the same protocols are completed in the same way, only in reverse order. U 1983 r International Organization for Standardization(International Organization for Standardization, ISO) і 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 the model of the division of boundary functions between 7 different levels (Fig. 1.7). It was reported that this seven-layer structure would become the basis for a new protocol stack, although it would never be implemented in commercial form. Instead, the OSI model is combined with other protocol stacks as an initial and advanced textbook. Most of the protocols that are popular these days appeared before the development of the OSI model, and its seven-level structure does not stink. Most often, one protocol combines the functions of two or several peer models, and the cordons of the protocols often do not correspond to the cordons of the OSI peers. The protean OSI model is deprived of a unique reference tool for monitoring edge processes, and professionals often associate functions and protocols with the same standards.

Data encapsulation

Essentially, the interoperability of protocols that operate at different levels of the OSI model is manifested in the fact that each protocol provides Heading(Header) or (in one type) trailer(Footer) to the information that we have taken away from the level, the grown-up matter. For example, the supplement generates up to the maximum resource. This will then go down the protocol stack. When it reaches the transport layer, the protocols of that layer add a header that consists of fields with information specific to the function of that protocol. The output signal itself becomes a data field for the transport level protocol (colored ones). Having added its header, the transport layer protocol passes the request to the middle layer. The edge layer protocol adds its own header to the transport layer protocol header. Thus, for the border level protocol, the output and header for the transport level protocol are the same. This entire structure is fundamentally important for the channel level protocol, which adds the title and trailer to it. In summary, this activity is plastic bag(Packet), ready for transmission along the border. Once the package reaches the destination, the process is repeated in reverse order. The protocol of the cutaneous layer of the stack (now from bottom to top) collects and displays the header equivalent to the protocol of the transmission system. When the process is completed, the output signal reaches all the destinations, in the same way as the generation. The process of adding headers to the input (Fig. 1.8), generated by the addon, is called data encapsulation(Data encapsulation). In essence, this procedure simulates the process of preparing a sheet for sending by mail. The entry is the same sheet, and the addition of headings is similar to the inserted sheet in the envelope, the writing of the address, stamping and authorization.

physical rhubarb

At the lowest level of the OSI model - physical(Physical) - the characteristics of the elements of the edge are determined - the edge core, the method of installation, the type of signals selected for transmission along the edge of double data. In addition, it is physically determined what type of edge adapter needs to be installed on your computer and what type of hub (as required). On a physical level, we are on the right with a copper or fiber-optic cable or with any drone-free connections. For a LAN, the specifications of the physical layer are directly related to the link layer protocol used at the network. Having selected a channel level protocol, you must choose one of the physical level specifications supported by that protocol. For example, the Ethernet link layer protocol supports a number of different physical layer options - one of two types of coaxial cable, twisted pair cable, or fiber optic cable. The parameters of each of these options are formed from numerical information about physical conditions, for example, to the type of cable and connectors, permissible length of cables, number of concentrators, etc. Doing so is necessary for the normal operation of the protocols. For example, in a long cable, the Ethernet system may not be able to detect packet collisions, and if the system is unable to detect errors, it cannot correct them, resulting in a waste of data. The channel level protocol standard does not cover all aspects of the physical level. The actions taken from them are counted literally. One of the most commonly used physical level specifications is described in the Commercial Building Telecommunications Cabling Standard, known as EIA/TIA 568A. The number of publications is complete American National Institutedarts(American National Standards Institute, ANSI), associationselectronics industries(Electronics Industry Association, EIA) і Association of industry producers(Telecommunications Industry Association, TIA). This document includes a description of cables for data transmission in industrial facilities, including the minimum distance from electromagnetic plants and other rules for laying cables. Today, cable laying in large areas is often entrusted to specialized companies. The hired contractor is responsible for being thoroughly familiar with EIA/TIA 568A and other similar documents, as well as operating procedures in place. Another communication element that appears on the physical level is the type of signal for transmitting data along the edge middle. 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 media may exhibit radio waves, infrared pulses, and other signals. Based on the nature of signals, on the physical level, a scheme for their transmission is established, i.e., a combination of electrical charges or light pulses, which is used to encode two types of information, such as generated by existing ranks. Ethernet systems have a stagnant signal transmission scheme, leading to Manchester code(Manchester encoding), and in Token Ring systems it is vikorized differentialManchester(Differential Manchester) scheme.

canal rhubarb

protocol channel(Data-link) level ensures the exchange of information between the hardware of the computer included in the network and the network software. It is ready to send data sent to the network by the network network protocol, and transmit data received by the system from the network to the network network. When designing and building a LAN, the link-level protocol is the most important factor in the choice of hardware and installation method. To implement the channel-level protocol, you need hardware and software: edge interface adapters (an adapter is a device that connects to the bus, which is called a edge interface board or just edge board yu); driver and harness adapter; edge cables (or other edge media) and additional equipment; Merezhevy concentrators (in some cases). Both fringe adapters and concentrators are separated for different channel-level protocols. Several hedging cables are also designed for specific protocols, as well as cables suitable for different protocols. Incredibly, today (as always) the most popular protocol of the channel level is Ethernet. Far beyond the new Token Ring, there are other protocols, for example, FDDI (Fiber Distributed Data Interface). The link level protocol specification requires three main elements to be included: the frame format (i.e., the header and trailer that is added to the edge level data before transmission to the edge); access control mechanism to the edge middle; one or more physical level specifications associated with this protocol.

frame format

The link-level protocol adds a header and trailer to the data captured by the edge-level protocol, converting them into frame(Frame) (Fig. 1.9). If we go back to the analogy with mail, the title and trailer are an envelope for sending a sheet. They contain the addresses of the sending system and the receiving system of the package. For LAN protocols like Ethernet and Token Ring, these addresses are 6-byte hexadecimal numbers assigned to the edge adapters at the factory. Signs, administrated to the address, vikoristovaya on other levels of the OSI model, are called appa military addresses(Hardware address) or MAC addresses (div. Below).

Note Protocols of different levels of the OSI model have different names for the structures they create by adding a header to the data that came with the original protocol. For example, what the channel-level protocol calls a frame will be a datagram for the edge-level. More common name for the structural unit of data on any level plastic bag.

It is important to understand that link-level protocols only ensure communications between computers on the same LAN. The hardware addresses in the header must always be placed on the computer on the same LAN, since the target system is located in a different area. Other important functions of the link-level frame are identification of the layer-level protocol generated by the data in the packet, and information for detecting errors. On the edge layer, different protocols can be used, and therefore, a code is included in the frame of the channel layer protocol, which allows you to install the very protocol of the edge layer, generating data in that packet. With this code, the link level protocol of the host computer overrides the data of the link level protocol of its edge layer. To identify benefits, the system transfers and calculates cycles Cue superfluous code(Cyclical redundancy check, CRC) of the koris navantazhenya and records it in the trailer frame. Having picked up the package, the whole computer adds the same calculations and equals the result with the trailer. If the results are correct, the information is transmitted without compromise. In the other case, the operator transmits that the package is used and does not receive it.

Controlling access to the middle

Computers on the LAN call for vikoryst to use the hidden medium. In this case, it is entirely possible to transfer data to two computers at the same time. In such situations, there is a kind of packet constriction, colony(Collision), when it is given neither in both packages to be destroyed. One of the main functions of the channel level protocol is access control to the edge middle (media access control, MAC), i.e. control over the transfer of data to each other from computers and reducing packet loss to a minimum. The access control mechanism to the middle is one of the most important characteristics of the link-level protocol. For Ethernet, to control access to the media, a mechanism is used to control collisions and detected collisions (Carrier Sense Multiple Access with Collision Detection, CSMA / CD). Some other protocols, such as Token Ring, rely on token passing.

Specifications of physical level

Channel layer protocols that are deployed on a LAN often support more than one layer middle, and the protocol standard includes one or more physical layer specifications. The channel and physical levels are closely linked, since the power of the edge middle is closely tied to how the protocol controls access to the middle. Therefore, we can say that in local layers, channel layer protocols also perform the functions of the physical layer. Global networks use channel-level protocols that do not include physical-level information, for example, SLIP (Serial Line Internet Protocol) and PPP (Point-to-Point Protocol).

Merezhevyi rhubarb

At first glance you might think that hemeshevy(Network) level duplicates the functions of the channel level. But it’s not like that: hemstone rhubarb is “provided” for through and through(End-to-end) communication, since channel level protocols only function between LANs. In other words, the edge tier ensures that the packet is transmitted from the output to the target system. Depending on the type of network, the sender and the hosts can be on the same LAN, on different LANs within the same network, or on LANs separated by thousands of kilometers. For example, if you connect to a server on the Internet, the next packets created by your computer must pass through dozens of barriers. Adapted to the network, the protocol of the channel level will change repeatedly, otherwise the protocol of the channel level will be deprived of the same thing in every way. The outermost core of the TCP/IP (Transmission Control Protocol/Internet Protocol) set of protocols and the most frequently used edge protocol is the IP (Internet Protocol). Novell NetWare has the most powerful hembark protocol IPX (Internetwork Packet Exchange), and in small Microsoft Windows environments the NetBEUI (NetBIOS Enhanced User Interface) protocol is widely used. Most of the functions assigned to the middle layer are derived from the capabilities of the IP protocol. Similar to the channel-level protocol, the edge-level protocol adds a header to the data as opposed to the main level (Fig. 1.10). The data element created by the boundary layer protocol is composed of the transport layer data and the boundary layer header and is called datagram(Datagram).

addressing

The header for the edge-level protocol, as well as the header for the link-level protocol, place fields with the addresses of the output and destination systems. However, in this category, the address of the target system lies at the end of the packet and can be subdivided into the address of the recipient in the header of the link-level protocol. For example, if you enter the address of the Web site in the address bar of your browser, in a package that is generated by your computer, as the address of the target system of the border level, the destination addresses of the additional Web server, then, as in the channel level of the target system, you enter the address of the router in Your LAN, which will provide access to the Internet. In IP, a powerful addressing system is used, so that it absolutely does not lie in the channel-level address. Each computer, in addition to the IP protocol, is manually or automatically assigned a 32-bit IP address, which identifies both the computer itself and the network in which it is located. With IPX, to identify the computer itself, a hardware address is used, and a special address is used to identify the network in which the computer is located. In NetBEUI, computers are separated by NetBIOS names and assigned to a skin system during installation.

fragmentation

The datagram of the channel level on the route must be made to pass through the anonymous border, colliding with specific authorities and the boundaries of various protocols of the channel level. One of these limits is the maximum packet size allowed by the protocol. For example, the size of a Token Ring frame can reach 4500 bytes, while the size of Ethernet frames cannot exceed 1500 bytes. If the datagram is large, formed in a Token Ring network, and transmitted over an Ethernet network, the Edge Ring protocol must break it up into fragments no larger than 1500 bytes in size. This process is called fragmentation(Fragmentation). During the fragmentation process, the edge-level protocol breaks the datagram into fragments, the size of which is consistent with the capabilities of the network-level protocol. The skin fragment becomes an independent package and extends the way to the entire hemline system. The output datagram is formed once all fragments have been reached. Sometimes, before reaching the entire system, fragments that contain a broken datagram must be fragmented again.

routing

routing(Routing) is the process of selecting, online, the most effective route for transmitting data from the sending system to the host system. In complex networks, for example, on the Internet or large corporate networks, often from one computer to another there are many paths. Merger designers specially create excess connections so that traffic that knows the way to the destination should be routed whenever one of the routers fails. Using additional routers, connect around the LAN that is connected to the Internet. The purpose of a router is to accept incoming traffic from one route and pass it on from a specific system to another. The Internet has two types of systems: kintsevi(End systems) crotch(Intermediate systems). The terminal systems are the distributors and carriers of packages. A router is an intermediate system. In edge systems, all of the OSI models are processed on all levels, so packets that are in the middle systems do not rise above the edge level. There, the router parses 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 destination, routers store tables with tracking information in memory. This information can be entered manually by the administrator or collected automatically from other routers using special protocols. The store of a typical element of the routing table includes the address of another network and the address of the router through which packets must reach this network. In addition, in the routing table element there is route metric - mental assessment of its effectiveness. If any system has a number of routes, the router selects the most efficient one and sends the datagram to the link level for transmission to the router designated in the table element with the shortest metric. In large networks, routing can be a very complex process, but most often it occurs automatically and unnoticed for the operator.

Identification to the transport layer protocol

So, just as the channel level header contains the edge level protocol, generating and transmitting data, the edge level header contains information about the transport level protocol, such as which data is removed. Accordingly, the host system transmits the input data via the transport layer protocol.

transport rhubarb

Functions that are specified by protocols transport(Transport) level, expand the functions of the border level protocols. Often these peer protocols, which are used to transmit data, create an interconnected pair, as seen in the TCP / IP application: TCP operates on the transport layer, IP - on the edge layer. Most protocol sets have two or more transport layer protocols that need to be added Various functions. An alternative to TCP is the UDP (User Datagram Protocol). The IPX protocol suite also includes a number of transport layer protocols, including NCP (NetWare Core Protocol) and SPX (Sequenced Packet Exchange). The difference between the transport layer protocols of the entire set lies in the fact that some of them are connection-oriented, and others are not. Systems that use the protocol, orientation towards connection(Connection-oriented), before transmitting data, exchange information in order to establish connections one by one. This ensures that the systems are turned on and ready for operation. The TCP protocol, for example, is connection oriented. If you connect to the Internet server with the help of a browser, the browser and the server are called three-step handmade(Three-way handshake). After this, the browser transmits to the server the address of the required Web site. Once the transfer of data is completed, the system will also sign the hand to attach the link. In addition, connection-oriented protocols create additional actions, for example, sending confirmation of packet removal, segmenting data, controlling flow, and also detecting and correcting errors. As a rule, protocols of this type are used to transfer large amounts of information that do not require a single bit of data, for example, data files or programs. Additional functions of connection-oriented protocols guarantee correct data transfer. Why are these protocols often called reliable(Reliable). Reliability in this case is a technical term and means that the delivery package is checked for the presence of damages, in addition, the dispatch system is notified about the delivery of the skin package. Few protocols of this type involve significant communication of essential data exchanged between two systems. First, additional information is transmitted when the connection is installed and completed. In other words, the header that is added to the packet by a connection-oriented protocol is completely larger in size than the header of a non-connection-oriented protocol. For example, the TCP/IP header takes 20 bytes, and the UDP header takes 8 bytes. protocol, not oriented toward connection(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 those who are ready to receive the data and whose current system is in the future. Some systems rely on protocols that are not connection-oriented, such as UDP, for short transactions that consist only of requests and output signals. The sender signal implicitly functions as a transmission acknowledgment signal.

Note Oriented and not oriented towards connected protocols, not only at the transport level. For example, the hemstone rhubarb is not oriented towards connection, since the reliability of the stink bond is ensured when placing it on the transport rhubarb.

Transport level protocols (both edge and channel levels) require information from other levels. For example, the TCP and UDP headers include port numbers that identify the source that originated the packet and the destination. on sessional(Session) there are significant differences between the actual protocols and the OSI model. There is no need to replace the lower levels of seeing session level protocols. The functions of this level are integrated into protocols that follow the functions of the representative and application levels. Transport, edge, channel and physical levels are responsible for the transmission of data along the edge. The protocols of the session and other rivals do not interfere with the relationship process. There are 22 services included in the session level, many of which define methods for exchanging information between systems included in the measure. The most important services for managing dialogue and sub-dialogue. The exchange of information between two systems is also called dialogue(Dialog). dialogue management(Dialog control) allows you to select the mode in which the systems will exchange notifications. There are two such modes: full-duplex(Two-way alternate, TWA) і binary(Two-way simultaneous, TWS). In full-duplex mode, two systems simultaneously transmit tokens along with data. You can transfer information only from a computer that has Danish moment there is a marker. This is how you get rid of this information in advance. The duplex model is more foldable. There are no markers in it; Both systems can transmit data at any time, even overnight. at the bottom of the dialogue(Dialog separation) is included in the data stream control points(Checkpoints), which allow you to synchronize the robot of two systems. The level of complexity of the dialogue depends on the mode in which it operates. In the half-duplex Lexne mode of the system, there is little synchronization, which is due to the exchange of information about the control cells. In duplex mode, the system will re-synchronize using the head/active marker.

representative rhubarb

on representative(Presentation) equals a single function: translation of syntax between different systems. Some computers may have different syntaxes. The representative rhubarb allows them to “find out” about the formal syntax for exchanging data. Once connected to the representative level, the systems exchange information about the syntaxes they use, and select the one they want to use during the session. Both systems, which take part in the union, have abstractsyntax(Abstract syntax) - this is the “real” form of the link. The abstract syntaxes of different computer platforms may differ. In the process of using the system, select the transfer syntaxtributes(Transfer syntax). The transmitting system transforms its abstract syntax into the syntax of data transmission, and the host system, upon completion of the transmission, does so. If necessary, the system can select a data transfer syntax with additional functions, for example, data compression or encryption.

applied rhubarb

Application level is the entry point through which programs gain access to the OSI model and edge resources. Most of the application level protocol provides services to access up to the limit. For example, the SMTP (Simple Mail Transfer Protocol) protocol most programs by e-mail Please contact us to send a notification. Other application level protocols, for example, FTP (File Transfer Protocol), are themselves programs. Application tier often includes the functions of session and representative tier. As a result, a typical protocol stack accommodates several protocols that operate at the application, transport, edge, and link levels.

In the border science, as in any field of knowledge, there are two principle approaches to the beginning: the approach from the hidden to the private and from the outside. Well, it’s not the case that in life people are victorious and approach in a clean manner, but nevertheless, at the beginning stages of the skin, they begin to choose for themselves one of the most important directions. For the higher school (at least the (post) Radyansky school) the first method is more characteristic, for self-enlightenment the other is most often: working with oneself as a person at the limit, depending on the hour of the time spent on one koristuvach to the nature of the administrative job, And I immediately wanted to marry you - and how , all over, all this damn thing is over?

Alas, these articles are not philosophical statements about the methodology of learning. I would like to introduce to the respect of the Pochatkivtsev rates then galley And smut, in any way, as coarse, you can dance to the most sophisticated private borders. Having understood the seven-layer OSI model and having started to “learn” its level in the technologies already known to you, you can easily collapse further in whatever fence you choose directly. The OSI model is the essence of the framework on which new knowledge about boundaries will be hung.

This model can also be recognized practically in any current literature on measures, as well as in the rich specifications of specific protocols and technologies. Not considering the need for a bicycle, I decided to publish lessons from the work of N. Olifer, V. Olifer (Center for Information Technologies) under the title “The role of communication protocols and functional significance the main types of ownership of corporate borders", which I respect most and most importantly in the publication on this topic .

chief editor

Model

Because the protocol is accepted by two mutual objects, in this case by two computers working in parallel, it does not mean that it is necessarily a standard. In practice, however, when implementing, it is possible to avoid violating standard protocols. These may be company, national or international standards.

The International Standards Organization (ISO) has developed a model that clearly identifies the different ways in which systems interact, gives them standard names, and specifies which robots are responsible for their work. This model is called the Open System Interconnection (OSI) model or the ISO / OSI model.

In the OSI model, interaction is divided into this level or balls (Fig. 1.1). Skin rhubarb is on the right with one important aspect of the relationship. Thus, the problem of mutual interaction is decomposed into 7 private problems, some of which may be affected independently from others. Skin rhubarb supports interfaces with higher levels of skin.

Small 1.1. Model of intercommunication between ISO/OSI critical systems

The OSI model describes only system interaction features that do not interfere with end devices. Add-ons implement their own communication protocols, extending to system features. Please note that the add-on can take on the functions of some of the upper levels of the OSI model, so, if necessary, it is necessary to switch directly to system functions, such as compiling functions lost the lower levels of the OSI model.

The addition of the terminal server can be used for system interaction not only for organizing dialogue with another application that is configured on another machine, but simply for terminating the services of another edge server su, for example, access to visible files, sending mail or to a friend on a shared printer.

Oh, please, don’t let the addition expand from the power supply to the application level, for example, to the file service. On this request, the application security program forms a notification in a standard format that contains service information (header) and, possibly, data transfer. Then this information is strengthened by the representative equal. The representative tier adds its header to the notification and passes the result down to the session tier, which in turn adds its header, etc. The implementation of protocols conveys the presence of not only the title, but also the ending. Once reached, the information reaches the lowest, physical level, which effectively transmits the connection along the lines.

If the message gradually arrives at another machine, it successively moves uphill from level to level. The skin tier analyzes, processes and displays the header of its tier, selects functions related to this tier and transmits the information to the tier.

The term “message” appears and there are other names that are used by mediators to designate the unit of data exchange. ISO standards for protocols of any level use the term “Protocol Data Unit” (PDU). By the way, the names frame, packet, datagram are often vikorized.

Features of ISO/OSI peer models

Physical level. This level deals with the transmission of data through physical channels, such as coaxial cable, twisted pair or fiber optic cable. To this extent, the characteristics of the physical media of data transmission, such as throughput, transmissibility, power support, and others, may vary. On this level, the characteristics of electrical signals are determined, such as those that are possible before the fronts of the pulses, the level of voltage and flow of the signal that is transmitted, the type of encoding, the speed of signal transmission. Moreover, the types of roses and the meaning of skin contact are standardized here.

The functions of the physical level are implemented in all devices connected to the limit. On the side of the computer, the physical level functions are connected to the edge adapter or the serial port.

An example of the physical level protocol can be the specification of 10Base-T Ethernet technology, which means, in a vicorized cable, an unshielded twisted pair of category 3 with a 100 Ohm cable support, an RJ-45 connector, a maximum power supply individual segment 100 meters, Manchester code for data submission on cables, and other characteristics of the medium and electrical signals.

Channel level. On the physical level, the beats are simply overpowered. In this case, it is not guaranteed that in certain areas in which communication lines are alternated (divided) into several pairs of mutually interacting computers, the physical transmission center may be occupied. Therefore, one of the tasks of specifying the channel level is checking the availability of the transmission middle. Other tasks of the channel level include the implementation of mechanisms for detecting and correcting damage. For this purpose, on the channel level, bits are grouped into sets called frames. The channel rhubarb ensures the correctness of the transfer of the skin frame by placing a special sequence of bits at the beginning and end of the skin frame to identify it, and also calculates the checksum, adding all the bytes to the frame in the same way and adding I'll check the sum up to the frame. When a frame arrives, it re-calculates the checksum of the removed data and equals the result with the checksum from the frame. Once the smell is avoided, the frame is considered correct and accepted. If the control amounts are not met, then a settlement is recorded.

Channel level protocols, developed in local layers, have a unique structure of links between computers and ways of addressing them. If the channel stream ensures the delivery of a frame between two nodes of a local network, it must work only in conjunction with a completely different topology of connections, the same topology itself, for which there will be splits. These typical topologies, which are supported by channel-level protocols of local networks, include the ground bus, ring and mirror. Applications of channel level protocols include Ethernet, Token Ring, FDDI, 100VG-AnyLAN protocols.

At local networks, channel level protocols are used by computers, bridges, switches and routers. In computers, the functions of the channel level are implemented by special forces edge adapters and their drivers.

In global networks, which rarely have a regular topology, the channel level ensures the exchange of information between two vessel computers that receive an individual line connection. Examples of point-to-point protocols (as such protocols are often called) include the PPP and LAP-B protocols.

Boundary level. This level serves to create a unified transport system that combines different principles of information transfer between terminal nodes. Let's take a look at the functions of the hemline level in the application of local hems. The channel level protocol of local networks ensures the delivery of data between any nodes along with the interconnection network. standard topology. This is an even stricter interconnection, which does not allow for the existence of measures with an uneven structure, for example, measures that combine a number of measures into a single measure, or high-confidence measures, in which there is an excessive connection Izh vuzla. In order, on the one hand, to preserve the simplicity of data transfer procedures for standard topologies, and on the other hand, to allow the selection of sufficient topologies, an additional hedging system is developed. On this level the concept of “merezha” is introduced. In this case, it is understood that the totality of computers connected to each other is based on one of the standard typical topologies and is used to transmit data using one of the channel-level protocols, the values ​​for which єї topologies.

Thus, in the middle, the delivery of data is regulated by the channel level, and the axis of data delivery between the boundaries is handled by the channel level.

The knowledge of the border region is usually called packages. When organizing the delivery of packages on the border level, the concept of "Measurement number". This type of address consists of the network number and the computer number in that network.

Networks connect to each other using special devices called routers. router- this device, which collects information about the topology of the inter-edge connections and on its base forwards packets of the edge level as appropriate. In order to transmit a message from a sender that is in one dimension to another that is in another dimension, you need to create a number of transit transfers (hops) between the boundaries, immediately reliable route. Thus, a route is a sequence of routers through which a packet passes.

The problem of choosing the best way is called routing And these have been resolved to the main tasks of the boundary level. This problem is complicated by the fact that the shortest route is never the shortest. Often the criterion when choosing a route is the hour of data transmission along that route, which depends on the capacity of communication channels and the intensity of traffic, which can change depending on the hour. Some routing algorithms need to be fixed until the destination changes, while others make decisions based on the average data for the current hour. The route selection may be based on other criteria, such as transmission reliability.

On the border level there are two types of protocols. The first type involves determining the rules for transmitting packets containing data from end nodes from the node to the router and between routers. The protocols themselves are of great concern, if we talk about the protocols of the border region. Another type of protocols, called routing information exchange protocols. Using these additional protocols, routers collect information about the topology of cross-border connections. Edge layer protocols are implemented by software modules of the operating system, as well as by software and hardware of routers.

The applications of the edge layer protocols are the TCP/IP IP stack inter-exchange protocol and the Novell IPX stack inter-exchange protocol.

Transport level. On the way from the driver to the delivery, packages can be damaged or destroyed. If you want some additional information, you need to know how to process the benefits, it’s important to know how to pay attention to the mother’s relationship with reliable connections. The job of the transport layer is to ensure that the appendages and upper layers of the stack—application and session layers—transfer data with the level of reliability they require. The OSI model defines five classes of service provided by the transport layer. These types of services vary depending on the type of service provided: terminology, the possibility of renewing a broken connection, the availability of multiplexing methods for connecting data between different application protocols via an underground transport protocol, and, above all, to the identification and correction of transfer issues, such as confusion, waste, and dubbing of packages.

The choice of service class of the transport level is determined, on the one hand, by the degree of reliability provided by the very supplements and protocols of the higher, lower transport levels, and on the other hand, take into account how reliable the entire system of transporting data to boundaries. So, for example, since the density of communication transmission channels is even high, and the likelihood of transfers not detected by protocols of lower levels is small, then it is reasonable to speed up one of the lighter services of the transport level, not burdened by numerical verifications, quotas and other methods of increasing reliability . Since the transport means are already unreliable, then it will completely expand to the most advanced transport service level, which will provide the maximum number of ways to identify and cancel the exemptions - with the help of the advance establishment of a logical 'Ednanny, delivery control is informed about additional control sums and cyclic numbering of packages , setting delivery timeouts, etc.

As a rule, all protocols, starting from the transport layer and above, are implemented by software of the edge nodes of the edge - components of their edge operating systems. As an example of transport protocols, we can cite the TCP and UDP protocols of the TCP / IP stack and the SPX protocol of the Novell stack.

Session layer. The session layer provides dialogue management to record which party is currently active, and also provides synchronization features. The rest allow you to insert checkpoints in subsequent transfers, so that each view can turn back to the remaining checkpoint, instead of starting all over again. In practice, these add-ons are victorious for session rhubarb, and they are rarely implemented.

Representation tier. This tier will ensure that the information sent by the application tier will be understood by the application tier in another system. If necessary, the submission process will convert the data formats into the original format of the presentation, and at the reception, obviously, the conversion will be completed. In this way, applied researchers can correct, for example, syntactic features in the presented data. On this level, encryption and decryption of data can be involved, so that the confidentiality of data exchange is ensured for all application services. An example of a protocol that works on the same level is the Secure Socket Layer (SSL) protocol, which ensures the secret exchange of messages for the protocols of the application layer of the TCP / IP stack.

Application level. Application level - it’s really just a set of different protocols, with the help of certain complex measures to control access to resources that are shared, such as files, printers or hypertext Web pages, and also organize your own I would like to ask the robot, for example, for additional electronic protocol poshti. One of these data, which is based on applied rhubarb, is called received (message) .

It is clear that there is a great variety of protocols at the applied level. Let's look at how applications would like the most extensive implementation of file services: NCP in the Novell NetWare operating system, SMB in Microsoft Windows NT, NFS, FTP and TFTP, which are part of the TCP / IP stack.

The OSI model represents, although even more importantly, one of the richest communication models. These models and the protocol stacks associated with them can be divided into a number of levels, their functions, message formats, services provided on the upper levels and other parameters.

Characteristics of popular communication protocol stacks

Also, the interaction between computers must be consistent with the previous rules for exchanging the required formats and their formats, in accordance with the protocols. A hierarchically organized set of protocols that communicate between network nodes is called a communication protocol stack.

There is a need to access the rich protocol stacks that are widely available at the edges. These stacks, including international and national standards, and company stacks, which expand the breadth of installations of both companies. Examples of popular protocol stacks include the IPX / SPX stack from Novell, the TCP / IP stack, which is used on the Internet and in many networks based on the UNIX operating system, the OSI stack of the International Organization for Standardization and the DECnet stack of Digital Equipment Corporation and other activities.

The combination of different communication protocol stacks has a lot to do with the barriers and their characteristics. In small areas, you can only use one stack. In large corporate networks, which combine different networks, usually a number of stacks are developed in parallel.

The communication installation implements protocols of the lower levels, which are standardized in the wider world, and lower protocols of the upper levels, and this is the basis for successful successful operation of the robot. From other virobniks. The variety of protocols supported by this and other communication devices is one of the most important characteristics of that device.

Computers implement communication protocols in the form of specific software elements of the edge operating system, for example, link-level protocols are usually found in the form of edge adapter drivers, and upper-level protocols These are the server and client components of edge services.

Knowing how to deal well with other operating systems is an important characteristic of communications proficiency. You can often read in advertisements for a fringe adapter or hub that it is designed specifically for working in a NetWare or UNIX fringe. This means that the vendors of the equipment optimized their characteristics entirely to these protocols, which are used in this operating system, or before this version of their implementation, which protocols are used in their OS. Due to the peculiarities of the implementation of protocols in different operating systems, as one of the characteristics of communication equipment, it is certified for the ability to work in the middle of a given OS.

On the lower levels - physical and channel - practically all stacks use the same protocols. The protocols Ethernet, Token Ring, FDDI and other protocols are well standardized, which allow the same equipment to be used at all levels.

The protocols of the edge and higher levels of other standard stacks vary greatly and generally do not correspond to the recommended ISO model of level distribution. However, in these stacks, the functions of the session and representative layers are most often combined with the application layer. This uncertainty is due to the fact that the ISO model appeared as a result of the suppression of already existing and actually corrupted stacks, and not out of nowhere.

OSI stack

The following is a breakdown of the OSI protocol stack and the OSI model. Since the OSI model conceptually defines the procedure for interaction between critical systems, decomposing the task into 7 levels, standardizes the meaning of the skin level and introduces standard names of levels, the OSI stack - without a set of very specific specifications What kind of protocols that create a convenient protocol stack. This protocol stack is supported by the US government in its GOSIP program. All computer connections that are installed in standard installations after 1990 must either directly support the OSI stack, or provide the means for transition to this stack in the future. Protely, the OSI stack is more popular in Europe rather than in the USA, since in Europe there are fewer old networks installed that use their own power protocols. In Europe, there is also a great demand for carbonate glass, since there are a large number of different countries here.

This is an international, independent standard from manufacturers. You can ensure mutual relations between corporations, partners and customers. This relationship is complicated by problems with addressing, data storage and security. All these problems in the OSI system are partial. OSI protocols generate great amounts of pain central processor They are more suitable for heavy-duty machines, rather than for personal computers. Most organizations are still planning to migrate to the OSI stack. Those who work directly in this regard can be called the US Department of the Navy and the NFSNET network. One of the largest vendors that supports OSI is AT&T. The Stargroup framework is entirely based on the OSI stack.

For good reason, the OSI stack, unlike other standard stacks, closely follows the OSI interconnection model, including specifications for all seven layers of the interoperability model. ii critical systems (Fig. 1.3).

Small 1.3. OSI stack

on The OSI stack supports Ethernet, Token Ring, FDDI protocols, as well as LLC, X.25 and ISDN protocols. These protocols will be discussed in detail in other sections of the guide.

services boundary, transport and session Rivniv It’s the same in the OSI section, but it’s a little wider. At the border level, protocols are implemented both without installation and with installations. The transport protocol of the OSI stack is consistent with the functions assigned to it in the OSI model, the front-end communication between edge services with and without installed connections, so that the wash the required service level regardless of the lower level. To ensure this, the transport system is important to ensure that the customer sets the required level of service. There are 5 classes assigned to the transport service, from the lowest class 0 to the highest class 4, which are divided by the level of resistance before the amendments and the ability to update the data after the amendments.

services applied level include file transfer, terminal emulation, directory services and mail. The most promising of them are directory service (X.500 standard), electronic mail (X.400), virtual terminal protocol (VT), file transfer, access and management (FTAM) protocol, transfer and robot management (JTM) protocol. The rest of the time, ISO concentrated its efforts on top-tier services.

X.400 recommendations indicate the minimum necessary set of services that merchants expect: access control, maintaining unique system identifiers, notifications about delivery and non-delivery For specified reasons, indication of the type of change of notification, indication of the change of notification, time indicators during transmission and delivery , select a delivery category (terminal, non-terminal, normal), multi-address delivery, delayed delivery (until the right time), configured for interaction with non-mail postal systems, for example, with telex and fax services, for pete about those delivered specifically notifications, distribution lists, which can have a built-in structure, and protection against unauthorized access, which are based on an asymmetric public key cryptosystem.

by recommendation X.500є development of standards for the global pre-survey service. The delivery process requires the known address of the owner, which is a problem when the size of the border is large; therefore, it is necessary to have a reporting service that helps to remove the addresses of the senders and owners. In layman's terms, the X.500 service is a distributed database of names and addresses. All contributors potentially have the right to enter this database, which contains a specific set of attributes.

Above the database of names and addresses for the current operation:

• reading - removing addresses from known names,
• ask - remove name by known address attributes,
• modification, which includes deleting and adding records in the database.

The main problems in implementing the X.500 recommendation are due to the sheer scale of the project, which claims to be a global research service. Therefore, software that implements X.500 recommendations is very cumbersome and has a high impact on hardware productivity.

protocol VT The main problem is the inconsistency of various terminal emulation protocols. Infection of a personal computer, including an IBM PC, for one-hour work with VAX, IBM 3090 and HP9000 computers requires the addition of three different programs for terminal emulation different types and analyze different protocols. If the host computer had in its warehouse a program that supported the ISO terminal emulation protocol, then you would need only one program that supports the VT protocol. Its ISO standard has extensively expanded terminal emulation functions.

File transfer is the greatest extension of a computer service. Access to files, both local and remote, is required for all add-ons - text editors, email, databases or remote launch programs. ISO transmits such a service in the protocol FTAM. Along with the X.400 standard, it is the most popular OSI stack standard. FTAM passes codes for localization and access to the file, and includes a set of directives for inserting, replacing, expanding, and clearing the file. FTAM also passes codes for manipulating the file as a whole, including creating, deleting, reading, opening, closing the file, and selecting its attributes.

Protocol for transfer and control of robots JTM Allows hackers to overpower robots that are responsible for Vikon on the host computer. The system of task management, which ensures the transfer of work, indicates the host computer, what kind of programs and files are responsible for Wikonian. The JTM protocol supports traditional batch processing, transaction processing, remote data entry, and access to distributed databases.

TCP/IP stack

The TCP/IP stack, also called the DoD stack and the Internet stack, is one of the most popular and promising communication protocol stacks. Since at this time there are major expansions in the UNIX OS, its implementation in the remaining versions of edge operating systems for personal computers (Windows NT, NetWare) is a significant change in the rapid increase in the number of installations of the TCP / IP stack.

A stack of developments from the US Department of Defense (DoD) initiative has more than 20 results in linking the experimental ARPAnet network with other satellite networks as a set of secret protocols for a variety of computing luval middle. The ARPA measure supported investigators and investigators in the military regions. The ARPA network of communications between two computers was based on the Internet Protocol (IP), which to this day is one of the main ones in the TCP / IP stack and appears in the name of the stack.

A major contribution to the development of the TCP/IP stack was made by Berkeley University, which implemented the stack protocols in its version of the UNIX OS. The widespread expansion of the UNIX OS led to a widespread expansion of the IP protocol and other stack protocols. This stack is home to the entire Internet information network, which is part of the Internet Engineering Task Force (IETF) and is a major contributor to the enhancement of stack standards, which are published in the form of RFC specifications.

Since the TCP/IP stack was fragmented before the ISO/OSI model of intercommunication between closed systems appeared, then, although it also has a rich structure, the similarity of the TCP/IP stack to the OSI model can be achieved intelligently.

The structure of the TCP / IP protocols is shown in Malyunka 1.4. TCP/IP protocols are divided into 4 levels.

Small 1.4. TCP/IP stack

Nainizchiy ( rhubarb IV ) - level of inter-interfaces - corresponds to the physical and channel levels of the OSI model. This level in the TCP / IP protocols is not regulated, but supports all popular standards of the physical and channel level: for local channels - Ethernet, Token Ring, FDDI, for global channels - wireless protocols on analog switching and video lines SLIP / PPP, as installed point-to-point connection via serial channels global measure, І territorial network protocols X.25 and ISDN. A special specification has also been developed, which means that ATM technology is used as a channel level transport.

Advent rhubarb ( rhubarb III ) - an inter-merge communication center that is responsible for transmitting data from various local networks, X.25 territorial networks, special communication lines, etc. as part of the main network network protocol (in in OSI model terms) the protocol is vicorized in the stack IP, Which was originally designed as a protocol for transmitting packets in warehouse networks, which consist of a large number of local networks, united by both local and global connections. Therefore, the IP protocol works well within the framework of a foldable topology, the rational availability of their subsystems and the careful use of the bandwidth of low-speed lines. The IP protocol is a datagram protocol.

All protocols related to the creation and modification of the routing table, such as protocols for collecting routing information, are included in the level of inter-interface communication. R.I.P.(Routing Internet Protocol) і OSPF(Open Shortest Path First), as well as the inter-boundary information protocol ICMP(Internet Control Message Protocol). The remaining purpose protocol is for exchanging information about beatings between the router and the gateway, the host system and the receiving system, in order to organize a return connection. With the help of special ICMP packets, information is provided about the impossibility of delivering a packet, about the delay in life or the difficulty of assembling a packet from fragments, about anomalous parameter values, about a change in the forwarding route and type of service, about system failure, etc.

Advent rhubarb ( rhubarb II) It is called the main one. On what level does the transmission control protocol function? TCP(Transmission Control Protocol) and koristuvach datagram protocol UDP(User Datagram Protocol). The TCP protocol provides virtual communication between remote application processes. The UDP protocol ensures the transmission of application packets using the datagram method, without installing a virtual connection, and thus requires less overhead than TCP.

Upper River ( rhubarb I) It is called applied. Over the years, the TCP / IP stack has accumulated a large number of protocols and application services. These include such widely used protocols as the FTP file copying protocol, the telnet terminal emulation protocol, the SMTP mail protocol, the use of electronic mail on the Internet and the Russian RELCOM service, and services for access to obtained information, such as WWW and many others. We will now report on some of them that are most closely related to the topics of this course.

protocol SNMP(Simple Network Management Protocol) is being developed to organize boundary management. The management problem is divided into two areas. The first task is related to the transfer of information. Protocols for transmitting certain information define the procedure for interaction between the server and the client program that runs on the administrator’s host. They indicate the formats of information that are exchanged between clients and servers, as well as the formats of names and addresses. Another task is related to the control of data. Standards regulate what sensitive data is stored and accumulated in gateways, the names of this data, and the syntax of these names. The SNMP standard has a specification for the network management information base. This specification, known as the MIB (Management Information Base), defines the data elements that the host or gateway is required to store, and the permissible operations on them.

File transfer protocol FTP(File Transfer Protocol) implements remote access to file. In order to ensure reliable transmission, FTP uses the established connection protocol - TCP - as a transport protocol. In addition to the file transfer protocol, FTP offers other services. So you can interact interactively with a remote machine, for example, you can unzip directories, FTP allows you to specify the type and format of data stored. Ok, FTP eliminates the authentication of clients. First, you must deny access to the file by submitting your name and password to the protocol.

At the TCP/IP station, the FTP protocol offers the widest range of services for working with files, and is also the most complex for programming. Add-ons that do not require all the capabilities of FTP can use another, more economical protocol - the simplest file transfer protocol TFTP(Trivial File Transfer Protocol). This protocol implements only file transfer, and as a transport, it is more simple than TCP, the protocol without installing a connection is UDP.

protocol telnet will ensure the transfer of bytes to a stream between processes, as well as between a process and a terminal. Most often, this protocol is used for emulation to a remote EOM terminal.

IPX/SPX stack

This stack is the original Novell protocol stack, which it developed for its NetWare network operating system back in the early 1980s. The Internetwork Packet Exchange (IPX) and Sequenced Packet Exchange (SPX) protocols that form the stack are direct adaptations of Xerox's XNS protocols, enhanced at a much lower level than IPX/SPX. The IPX/SPX protocols lead the majority of installations, and this is due to the fact that the NetWare OS itself occupies a leading position with a percentage of installations at a light scale of approximately 65%.

The Novell family of protocols and their conformity to the ISO / OSI model are presented in the small 1.5.

Small 1.5. IPX/SPX stack

on physical and channel levels Novell's framework supports all popular protocols of these competitors (Ethernet, Token Ring, FDDI and others).

on hemezhevoy rivni in the Novell section the protocol is running IPX, And also protocols for exchanging routing information R.I.P.і NLSP(Similar to the OSPF protocol of the TCP/IP stack). IPX is a protocol that handles power addressing and packet routing within Novell networks. IPX routing decisions are based on the address fields in the packet header, as well as on information found in the routing information exchange protocols. For example, IPX contains information supplied by either the RIP protocol or the NLSP (NetWare Link State Protocol) protocol for transmitting packets to a host computer or an external router. The IPX protocol supports only a datagram method of exchanging information, which saves computing resources. Also, the IPX protocol provides three functions: setting an address, setting a route, and sending data.

The transport layer of the OSI model in the Novell system is represented by the SPX protocol, which is used to transmit information about the installed connections.

on the upper applied, representative and sessional levels implement NCP and SAP protocols. protocol NCP(NetWare Core Protocol) is a protocol for interaction between the NetWare server and the workstation shell. This application level protocol implements a client-server architecture on the upper levels of the OSI model. For an additional function of this protocol, the workstation makes connections to the server, displays the server directories on local drive letters, and looks file system server, copies the downloaded files, changes their attributes, etc., as well as the relevant section border printer between work stations.

Servers and routers use SAP to disclose their services and network addresses. The SAP protocol allows third-party devices to continuously modify data about the services they provide at the same time. When starting the server, you need to use SAP to notify you about your services. When the server finishes its work, it will use SAP to ensure that all its services are completed.

The Novell and NetWare 3.x servers have wide-ranging SAP packages. SAP packages in a significant world are monitored by one of the main tasks of routers that connect to global communications, and filtering traffic of SAP packages and RIP packages.

The features of the IPX / SPX stack are due to the features of the NetWare OS, and the very orientation of its early versions (up to 4.0) to work in local small-sized networks that consist of personal computers with modest resources. Therefore, Novell needed protocols, the implementation of which required a minimal amount of RAM (located in IBM computers running MS-DOS 640 KB) and would quickly run on small processors oh, observable pain. As a result, the protocols of the IPX / SPX stack have until recently worked well in local networks and not so much in large corporate networks, as more global connections have become increasingly popular with wide-bandwidth packets, which are intensively exploited by several protocols of this stack ( for example, for installing a link between clients and servers).

This situation, as well as the fact that the IPX / SPX stack is owned by Novell and its implementation requires a license, has long limited its expansion only by NetWare layers. However, prior to the release of NetWare 4.0, Novell had made and continues to make major changes to its protocols aimed at making them suitable for use in corporate settings. The IPX/SPX stack is available not only in NetWare, but also in several other popular edge operating systems - SCO UNIX, Sun Solaris, Microsoft Windows NT.

NetBIOS/SMB stack

Microsoft and IBM have been working hard on edge-to-edge technologies for personal computers, and the NetBIOS/SMB protocol stack is their baby. NetBIOS features appeared in 1984 as a further extension of the standard functions of the basic input/output system (BIOS) of the IBM PC for the edge program PC Network from IBM, which on the application level (Fig. 1.6) was used for the implementation of edge services сів protocol SMB (Server Message Block).

Small 1.6. NetBIOS/SMB stack

protocol NetBIOS It operates on three levels of the model of interaction between critical systems: boundary, transport and session. NetBIOS can provide better service high level,The lower IPX and SPX protocols do not depend on the data before routing. Thus, NetBIOS is not a hedge protocol in the strict sense of the word. NetBIOS has a lot of core fringing functions that can be applied to the fringing, transport and session levels, however, packet routing is not possible, since the NetBIOS frame exchange protocol does not introduce such a concept as fringing. This demarcates the NetBIOS protocol with local boundaries that are not divided into submerges. NetBIOS supports both data exchange and installation communication.

protocol SMB, Compliant with the application and representative levels of the OSI model, it regulates the interaction of the workstation with the server. The SMB function includes the following operations:

• Session management. Creation and expansion of a logical channel between the workstation and the edge resources of the file server.
• File access. The work station can access the file server with requests for creating and deleting directories, creating, opening and closing files, reading and writing files, renaming and deleting files, searching for files, removing and setting file attributes, blocking writes iv.
• Service to a friend. The work station can place files in a file for each other on the server and retrieve information about the card for each other.
• Service notified. SMB supports simple transmission of notifications with upcoming functions: send simple notifications; send a broadcast message; send a message to the block; send a text to the notification block; send a message to the block; forward the name of the koristuvach; slid redirect; Untitle the car.

Through a large number of add-ons, such as API functions that are provided by NetBIOS, many edge operating systems have these functions implemented in the interface to their transport protocols. NetWare has a program that implements NetBIOS functions based on the IPX protocol, the NetBIOS program for Windows NT and the TCP/IP stack.

Why do we need this valuable knowledge? (Editorial)

As if one colleague once gave me a tricky meal. Well, you seem to know what the OSI model is... And why do you need it, what kind of knowledge is practical: why show off in front of dummies? It’s not true, the value of this knowledge is a systemic approach to the greatest number of practical power tasks. for example:

• troubleshooting (