For the convenience of modernization, complex information systems are made as open as possible, that is, adapted to make changes to some part of the system while keeping the rest of the parts unchanged. With regard to computer networks, the implementation of the concept of openness has led to the emergence of the reference model for the interconnection of open systems (EMVOS), proposed by the International Organization for Standardization (ISO - International Standard Organization). This model describes the general principles, rules, agreements that ensure the interaction of information systems and are called protocols.

The information network in EMVOS is considered as a set of functions (protocols), which are subdivided into groups called levels. It is the division into levels that makes it possible to make changes to the means of implementation of one level without restructuring the means of other levels, which greatly simplifies and reduces the cost of upgrading the means as technology develops.

There are seven levels of EMVOS.

At the physical level, information is presented in the form of electrical or optical signals, the transformation of the signal form, the choice of parameters of physical media for data transmission, and the transfer of information through physical media is organized.

At the link level, data is exchanged between neighboring network nodes, i.e., nodes directly connected by physical connections without other intermediate nodes. Note that link layer packets are commonly referred to as frames.

At the network level, packets are formed according to the rules of those intermediate networks through which the original packet passes, and packets are routed, that is, the definition and implementation of routes along which packets are transmitted. In other words, routing is about the formation of logical channels. A logical channel is a virtual connection of two or more network layer objects, in which data exchange between these objects is possible. The concept of a logical channel does not necessarily correspond to the physical connection of data lines between the connected points. This concept was introduced to abstract from the physical implementation of the connection. Another important function of the network layer after routing is to control the load on the network in order to prevent congestion that negatively affects the operation of the network.

The transport layer provides communication between endpoints (in contrast to the previous network layer, which provides data transmission through intermediate network components). The functions of the transport layer include multiplexing and demultiplexing (assembling / disassembling messages into packets at endpoints), detecting and eliminating errors in the transmitted data, setting the required level of services (for example, ordered speed and transmission reliability).


At the session level, the type of communication (duplex or half-duplex), the start and end of tasks, the sequence and mode of exchange of requests and responses of interacting partners are determined.

The presentation level implements the functions of data presentation (encoding, formatting, structuring). For example, at this level, data allocated for transmission is converted from one code to another, in particular, for the purpose of encryption.

At the application level, the data that is to be transmitted over the network is determined and formatted into messages.

In specific cases, there may be a need to implement only a part of the named functions, then, accordingly, the network will contain only a part of the levels. So, in simple (unbranched) LANs, there is no need for means of the network and transport layers. At the same time, the complexity of the link layer functions makes it expedient to divide it into two sublevels in the LAN:

channel access control (MAC - Medium Access Control);

Logical Link Control (LLC)

The LJLC sublayer, in contrast to the MAC sublayer, includes some of the link layer functions that do not depend on the characteristics of the transmission medium.

Data transmission over branched networks occurs when using encapsulation / decapsulation of data chunks. So, a message arriving at the transport layer is divided into segments, which receive headers and are transmitted to the network layer.

A segment is commonly referred to as a transport layer packet. The network layer organizes the transfer of data through intermediate networks. For this, a segment can be divided into parts (packets) if the network does not support the transmission of segments as a whole. The packet is supplied with its own network header (that is, the segment is encapsulated into a network layer packet). When transferring between nodes of an intermediate LAN, it is required to encapsulate packets in frames with a possible packet breakdown. The receiver decapsulates the segments and recovers the original message.

ISO 7498 standard

This standard has a triple heading "Information and Computing Systems - Open Systems Interconnection - Reference Model". It is commonly referred to in short as the Open Systems Interconnection Reference Model. The publication of this standard in 1983 summed up many years of work by many well-known telecommunications companies and standards organizations.

The main idea underlying this document is to break down the process of information interaction between systems into levels with clearly delineated functions.

The advantages of the layered organization of interaction lie in the fact that such an organization ensures the independent development of level standards, modularity of hardware and software development for information-computing systems, and thereby contributes to technical progress in this area.

In accordance with ISO 7498, seven levels (layers) of information interaction are distinguished:

  1. Application layer
  1. Presentation layer
  1. Session level
  1. Transport layer
  1. Network layer
  1. Link layer
  1. Physical layer

The information interaction of two or more systems, thus, is a set of information interactions of level subsystems, and each layer of the local information system interacts only with the corresponding layer of the remote system.

Protocol a set of algorithms (rules) for the interaction of objects of the same level is called.

Interface a set of rules is called, in accordance with which interaction with an object of a given level is carried out.

The process of putting a fragmented block of data from one level into blocks of data from another level is called encapsulation.

Layer hierarchy, protocols and stacks

A hierarchically organized set of protocols, which is sufficient for organizing the interaction of nodes in a network, is called communication protocol stacks.

Communication protocols can be executed both in software and hardware. Lower-layer protocols are most often implemented by a combination of software and hardware, while higher-layer protocols are usually implemented purely in software.

A software module that implements a protocol is often referred to as a protocol for brevity. In this case, the relationship between a protocol - a formally defined procedure and a protocol - a software module that performs this procedure, is similar to the relationship between an algorithm for solving a certain problem and a program that solves this problem.



The same algorithm can be programmed with varying degrees of efficiency. Likewise, a protocol can have multiple software implementations. Based on this, when comparing protocols, it is necessary to take into account not only the logic of their work, but also the quality of software solutions. In addition, the efficiency of the interaction of devices in the network is influenced by the quality of the entire set of protocols that make up the stack, in particular, how rationally functions are distributed between protocols of various layers and how well the interfaces between them are defined.

Protocols are organized not only by computers, but also by other network devices, such as hubs, bridges, switches, routers, etc. In general, computers in a network are not connected directly, but through various communication devices. Depending on the type of device, it requires certain built-in tools that implement a particular set of protocols.

TCP / IP Model Levels

Internet layer

All these requirements led to the choice of a packet-switched network model, which was based on the connectionless Internet layer. This layer, called the Internet layer or internetwork layer, is the foundation of the entire architecture. Its task is to provide the ability for each host to send packets on any network that will independently travel to their destination (for example, on another network). They may not arrive in the order in which they were sent. If the order of departure is required, the higher levels perform this task. Note that the word "Internet" is used here in its original sense, even though this layer is present on the Internet.



Here you can see an analogy with the postal system. A person can drop several international letters in a mailbox in one country, and with luck, most of them will be delivered to the correct addresses in other countries. The letters will probably pass through several international postal locks along the way, but this will remain a mystery to the correspondents. Each country (that is, each network) may have its own stamps, preferred envelope sizes and shipping rules that are invisible to users of the postal service.

Internet layer defines the official packet format and protocol called IP (Internet Protocol). The job of the Internet Protocol is to deliver IP packets to their destinations. The main aspects here are the choice of the package route and avoidance of blockage of the transport arteries. Therefore, it can be argued that the Internet layer of the TCP / IP model is functionally close to the network layer of the OSI model. This correspondence is shown in Fig.

Transport layer

The layer above the Internet layer of the TCP / IP model is usually called the transport layer. It is designed so that peer entities on both receiving and transmitting hosts can communicate in a manner similar to the transport layer of the OSI model. At this level, two end-to-end protocols must be described. The first, TCP (Transmission Control Protocol), is a reliable connection-oriented protocol that allows a byte stream to be delivered from one machine to any other machine on the internetwork without error. It splits the input stream of bytes into individual messages and transfers them to the Internet layer. At the destination, the receiving TCP process collects an output stream from the received messages. In addition, TCP enforces flow control so that the fast sender does not flood the slow receiver with information.

The second protocol in this layer, UDP (User Data Protocol), is an unreliable connectionless protocol that does not use TCP sequential flow control, but provides its own. It is also widely used in one-time client / server requests and applications where responsiveness is more important than accuracy, such as voice and video transmission. The relationship between IP, TCP, and UDP is shown in Fig. 1.18. Since the inception of the IP protocol, this protocol has been implemented in many other networks.

Application level

There is no session or presentation layer in the TCP / IP model. These levels were simply not needed, so they were not included in the model. Experience with the OSI model has proven that this point is correct: most applications need little of them.

Above the transport level is application level... It contains all high-level protocols. Older protocols include Virtual Terminal Protocol (TELNET), File Transfer Protocol (FTP), and Email Protocol (SMTP), as shown in the diagram. The virtual terminal protocol allows a user to log into and work on a remote server. File Transfer Protocol provides an efficient way to move information from machine to machine. E-mail was originally a form of file transfer, but later a special protocol was developed for it. Many other protocols have been added over the years, such as DNS (Domain Name Service) to translate hostnames to network addresses, NNTP (Network News Transfer Protocol), HTTP, the protocol used to create pages on the World Wide Web, and many others.

Host network layer

The TCP / IP reference model does not detail what lies below the Internet layer. All that is reported is that the host connects to the network using some protocol that allows it to send IP packets over the network. This protocol is not defined in any way and can vary from host to host and from network to network. Books and articles on the TCP / IP model rarely discuss this issue.

There are seven levels of EMVOS.

Workstation equipment in automated design and control systems.

As a means of data processing in modern CAD systems,

They use workstations, servers, personal computers. The use of mainframes, including supercomputers, is uncharacteristic, since they are expensive and their performance ratio - the price is significantly lower than that of servers and many workstations. AWPs are created on the basis of workstations or personal computers.

Typical composition of AWP devices: computers with one or more microprocessors, disk, RAM and cache memory and buses serving

For the interconnection of devices; input-output devices, including at least a keyboard, mouse, display; additionally, the AWS may include a printer, scanner, plotter (plotter) and some other peripheral devices.

Depending on the purpose, there are AWPs for the designer, AWP for the technologist, AWP for the project manager, etc. They may differ in the composition of peripheral devices, in the characteristics of the computer. In the workstation of the designer (graphic workstations), raster monitors with color tubes are used. Digitizers, scanners, printers, plotters can be part of the AWS or shared by users of several workstations as part of a local area network.

Peripherals.

To enter graphic information from existing documents into CAD, digitizers and scanners are used. The digitizer is used for manual input. It has the appearance of a drawing board, the cursor moves along its electronic board, on which the viewfinder and the keypad are located. The cursor is electromagnetically coupled to a grid of wires in the electronic board. When the button is pressed at a certain position of the cursor, information about the coordinates of this position is entered into the memory. Thus, manual splitting of drawings can be carried out. For automatic input of information from existing text or graphic documents, scanners of a flatbed or broaching type are used. Optical reading method. The scanning head houses fiber-optic self-focusing lenses and photocells. Resolution in different models ranges from 300 to 800 dpi (this parameter is often referred to as dpi). The read out information is in raster form, the scanner software represents it in one of the standard formats, for example, TIFF, GIF, PCX, JPEG, and for further processing it can perform vectorization - transfer of graphic information into vector form, for example, into DXF format. Printers and plotters are used to display information. The first of them are focused on obtaining small-format documents (A3, A4), the second - on the output of graphic information on large-format media. Typical resolution of printers and plotters is 300 dpi, currently increased to 720 dpi. In modern devices, control

It is carried out by built-in microprocessors. Typical output time for A1 format monochrome image is within 2 ... 7 minutes, color - 2 times longer.

Software components. Requirements for mathematical models and numerical methods in CAD

MO analysis includes mathematical models, numerical methods, algorithms for performing design procedures. The components of ML are determined by the basic mathematical apparatus specific to each of the hierarchical levels of design. At the micro level, typical mathematical models are represented by partial differential equations along with boundary conditions. These models, called distributed, include many equations of mathematical physics. The objects of study here are the fields of physical quantities, which is required when analyzing the strength of building structures or machine-building parts, studying processes in liquid media, modeling the concentrations and fluxes of particles in electronic devices, etc. , phases of the state of aggregation) in practically used models of the microlevel cannot be large due to the complexity of the computational nature. It is possible to drastically reduce computational costs in multicomponent environments only by applying a different approach to modeling based on the adoption of certain assumptions. The assumption, expressed by the discretization of space, allows you to go to macro-level models.

Macro-level models, also called lumped ones, are systems of algebraic and ordinary Differential equations, since only time remains the independent variable here. Simplification of the description of individual components (parts) allows you to study models of processes in devices, instruments, mechanical assemblies, the number of components in which can reach several thousand. In cases where the number of components in the system under study exceeds a certain threshold, the complexity of the system model at the macro level again becomes excessive. Therefore, taking the appropriate assumptions, they go to the functional-logical level. At this level, the apparatus of transfer functions is used to study analog (continuous) processes or the apparatus of mathematical logic and finite automata, if the object of research is a discrete process, i.e. a process with a discrete set

finally, to study even more complex objects, examples of which can be industrial enterprises and their associations, computing systems and networks, social systems and other similar objects, the apparatus of the queuing theory is used, it is possible to use some other approaches, for example, Petri nets. These models are at the system level of modeling.

The main requirements for medical equipment are the requirements for adequacy, accuracy, and efficiency. The model always only approximately reflects some properties of the object. Adequacy takes place if the model reflects the given properties of the object

with acceptable accuracy. Accuracy is understood as the degree of correspondence between the Assessments of the properties of the same name of the object and the model. Economy (computational efficiency) is determined by the cost of resources required to implement the model. Since CAD uses mathematical models, further we will talk about the characteristics of mathematical models, and efficiency will be characterized by the cost of computer time and memory. Adequacy is assessed by a list of reflected properties and areas of Adequacy. Adequacy area is an area in the parameter space, within which the model errors remain within acceptable limits.

Technical support structure. Requirements for technical support.

CAD hardware includes various hardware tools used to perform automated

Design, namely computers, peripheral devices, network equipment, as well as the equipment of some auxiliary systems (for example, measuring) that support the design.

The technical means used in CAD should provide:

1) implementation of all necessary design procedures for which there is the appropriate software;

2) interaction between designers and computers, support for an interactive mode of operation;

3) interaction between team members working on a common project. The first of these requirements is met if there are computers and systems in CAD with sufficient performance and memory capacity. The second requirement relates to the user interface and is fulfilled by including convenient means of data input-output in the CAD system and, above all, devices for the exchange of graphic information. The third requirement stipulates the integration of CAD hardware into a computer network.

As a result, the general structure of TO CAD is a network of nodes interconnected by a data transmission medium. Nodes (stations

Data) are the workplaces of designers, often called automated workstations (AWS) or workstations

(WS - Workstation), they can also be large computers (mainframes), separate peripherals and measuring devices. It is in the AWP that there should be means for the interface between the designer and the computer. With regard to computing power, it can be distributed among various nodes of the computer network.

The data transmission medium is represented by data transmission channels, consisting of communication lines and switching equipment.

In each node, it is possible to distinguish data terminal equipment (DTE), which performs certain design work, and data channel termination (DCE) equipment, intended for communication of the DTE with the environment.

Data transmission. For example, a personal computer can be regarded as a DTE, and a network card plugged into the computer can be regarded as a DCE. A data link is a means of two-way data exchange that includes a DCE and a communication line. A communication link refers to the portion of the physical medium used to propagate signals in a specific direction; examples of communication lines are coaxial cable, twisted pair of wires, fiber-optic communication line (FOCL). Closely related is the concept of a channel (communication channel), which is understood as a means of one-way data transmission. An example of a communication channel can be a frequency band allocated to one transmitter in radio communication. In a certain line, several communication channels can be formed, each of which carries its own information. It is said that the line is split between several channels.

Reference Model for Open Systems Interconnection.

With regard to computer networks, the implementation of the concept of openness has led to the emergence of the Reference Model for Open Systems Interconnection (EMVOS), proposed by the International Organization for Standardization (ISO - International

Standard Organization). This model describes the general principles, rules, agreements that ensure the interaction of information systems and are called protocols. The information network in EMVOS is considered as a set of functions (protocols), which are subdivided into groups called levels. It is the division into levels that makes it possible to make changes to the means of implementation of one level without restructuring the means of other levels, which greatly simplifies and reduces the cost of upgrading the means as technology develops.

There are seven levels of EMVOS.

At the physical level, information is presented in the form of electrical or optical signals, the signal form is converted, the parameters of the physical media are selected, and information is transmitted through the physical media.

At the link level, data is exchanged between neighboring network nodes, i.e., nodes directly connected by physical connections without other intermediate nodes. Note that link layer packets are commonly referred to as frames.

At the network level, packets are formed according to the rules of those intermediate networks through which the original packet passes, and the packets are routed, that is, the definition and implementation of routes along which the packets are transmitted.

The transport layer provides communication between endpoints (in contrast to the previous network layer, which provides data transmission through intermediate network components). The functions of the transport layer include multiplexing and demultiplexing (assembly-disassembly of messages into packets at endpoints).

At the session level, the type of communication (full-duplex or half-duplex), the start and end of tasks, the sequence and mode of exchange of requests and responses of interacting partners are determined.

At the representative level, data presentation functions (coding, formatting, structuring) are implemented.

At the application level, the data that is to be transmitted over the network is determined and formed into messages.

Lesson number 14 "Digitalization and integration of communication networks"

1. Stages of development of technologies for building TCS

In modern computerized communications, practically all "functional" stuffing is determined by a program stored in memory that controls the operation of a microprocessor. In this case, it is almost impossible to associate individual functions with individual electronic components.

A sequence of commands executed by a computer according to a given program is usually presented in the form of an algorithm, the image of which is in the form of functional modules arranged vertically. Such an image really resembles a building and allows for the lost visualization of information transformation processes in modern communications to use the analogy of the construction concept of "architecture".

Another example of the use of new analogies from the field of construction is the expression “ technologies for building TCS", In which the meaning is not invested in the construction of stationary objects or the deployment of field communication facilities, but in the choice of a certain interconnected functional content of the network elements in the form of one or another" architecture ".

Specific network technologies (network building technologies) actually determine the rules by which the network operates for a given (external control system) time, spending resources corresponding to these technologies and fulfilling the current communication requirements of users (subscribers) in certain (usually interfering) conditions the impact of the external environment.

It should be noted that users, in principle, are not interested in what technologies are implemented in the network. Users need the result of the network's activity in the form of communication services provided by the network under certain (interfering) conditions and for a certain payment for resources (or in exchange for allocated resources). But the relationship between the quality of services, permissible external conditions and consumed resources (described by external functional characteristics), just depends on the technologies used in the network (described by internal functional characteristics).

The result of the merger of the branches of information processing and exchange was the emergence of information networks that implement the entire set of information processes for the processing and transmission of information.

Information network(IS) is a complex technical system distributed in space, which is a functionally related set of software and hardware for processing and exchanging information and consisting of geographically distributed information nodes (information processing subsystems) and information transmission channels connecting these nodes.

Generally, the functional architecture of the IS can be represented in the form of a three-level conceptual model.

The first level (internal) describes the functions and rules of interconnection when transferring various types of information between geographically remote subscriber systems through physical communication channels (transmission) and is implemented transport network(previously, similar functions were performed by the primary communication network).

The second level (intermediate) describes the functions and rules for the exchange of information in the interests of the interconnection of application processes of various subscriber systems and is implemented telecommunication network representing a single

infrastructure for the exchange of various types of information in the interests of users of the information network (previously, various secondary communication networks performed similar functions).

The third level (external) is formed by a set of application processes located in geographically remote subscriber systems, which are consumers of information and perform its meaningful processing. The third level, supplementing the first and second with the specified information processing functions, forms the external appearance information network.

2. Reference Model for Open Systems Interconnection

The information process of user interaction in the IS begins and ends outside the network itself and includes a number of nested stages, one of which is the implementation of the interconnection process by the telecommunications network in the interests of the interaction of information processes that implement meaningful processing of formalized messages when solving a particular applied problem.

The specified interconnection process can also be represented as a sequence of various and, as a rule, multiple functional transformations of information messages in various network elements from one digital form to another and from one type of physical (electrical) signals to others.

For different telecommunication networks, created at different times by different manufacturers, the grouping of these functional transformations is different. The number of allocated stages and functions of the interconnection process also differs, which are often combined within a particular functional architecture of the IS (TCS) into separate levels or layers. Currently, there are a number of different architectures that have become de facto or de jure international open (generally accepted) standards.

An example of the most famous and detailed architecture is the seven-level open systems interconnection reference model(EMVOS),

proposed by the International Organization for Standards. This architecture is focused on describing the implementation of only interconnection functions in the interaction of IPs that perform the functions of meaningful information processing in geographically distributed IS nodes (therefore, we will call this architecture the TCS architecture, and not the IS architecture).

In the English-language designation of EMVOS, sometimes it is emphasized that this model belongs open systems interconnection(OSI) (OSI - Open System Interconnection) to the development of ISO (ISO - International Standards Organization) as

It should be noted that in the Russian-language literature, the abbreviation EMVOS is often deciphered as a reference model of "interaction", not "interconnection" of open systems, which is a consequence of inaccurate translation of the word

"Interconnection".

The main Russian standard that defines the principles of the architecture of the interconnection of open systems is GOST 28906-91 “Information processing systems. Interconnection of open systems. Basic reference model ". This standard has been prepared by the direct application of ISO 7498–84, ISO 7498–84 Add. 1 and fully corresponds to them. Similar recommendations

The concept of "openness" of systems means mutual recognition and support of the relevant interconnection standards and is not associated with their specific implementation and with the used technical (software) means.

3. Connections

Transport

4. Physical environment

Rice. 1. The main elements of EMVOS and their relationship with the architecture of the IS

The EMVOS is based on four elements shown in Fig. 1, according to which the components of application processes, called application logical objects (hereinafter for brevity - logical objects), implement the processes of interconnection of open systems through established connections through the OSI environment, which is understood as a set of interacting real open systems together with the physical environment for OSI intended for transfer of information between them. The physical medium for OSI is usually digital transmission channels of various physical nature.

The reference model within the OSI environment, in addition to the connection option (using permanent or switched virtual circuits), also provides for the connectionless option, which corresponds to the datagram mode of the packet switched network (without the use of virtual circuits). In general, the great variety and complexity of interconnection functions have led to the need for their hierarchical division into groups (layers, levels) within the framework of an open system and the creation of a multi-level architecture of telecommunication networks.

Level organization of EMVOS

Any multi-level organization of specialized systems is deliberately redundant and ineffective for specific conditions of use, but it greatly simplifies the construction of open systems (general use) designed to work in diverse conditions and consisting of many elements that consistently perform the functions of individual levels, being developed by various independent manufacturers.

When deciding where the boundaries between the levels should be drawn

and how many levels should be, the EMVOS developers relied on certain layering principles, the main of which are the following:

the number of levels should not be too large; the boundary between the levels should be drawn where the service description

is the simplest, the number of cross-border operations is minimal and a suitable standard interface already exists;

separate levels should be created to perform specific functions that differ in the processes or technical solutions that implement them;

levels should be formed from easily localized functions with the provision of the possibility of updating them independently of the functions of neighboring levels;

for each level, you should create interfaces only with the higher and lower levels;

it is possible to form sublevels within one level in the case when it is required by specific types of services (the possibility of bypassing sublevels should be provided).

Guided by these principles, seven levels were identified in EMVOS, usually listed from top to bottom:

7 level - application layer;

6 level - data presentation or presentation layer; 5th level - session layer;

4th level - transport layer;

3rd level - network layer;

Level 2 - data link layer; Level 1 - physical layer.

The description of the EMVOS levels is based on a number of formalized concepts, listed below with brief explanations:

Protocol is a set of rules for the interaction of equal logical objects (various open systems).

Interlayer interface- a set of rules for interaction of logical objects of neighboring levels when providing N-services to objects of the (N +1) -layer.

The main functions of all levels are:

choice of protocol; connection establishment and termination;

multiplexing and splitting connections; transmission of normal (normal) data; urgent (extraordinary) data transmission (with priority);

data flow control (latency, speed and size of PBUs); segmentation (assembly) or blocking (release) of data; organization of data sequence (numbering); error protection (correction, detection and reset and / or retry);

routing (addressing and distribution of data streams).

Differences in the composition of these functions and their quantitative parameters for individual levels are the hallmarks of real network technologies that do not fully coincide with EMVOS.

Often, a hierarchically organized set of protocols of different layers of specific network technologies is called a protocol stack.

An important task for quantifying the results of the provision of N-services is the task service quality parameters, the main of which are:

information transmission delay parameters; information distortion parameters; information loss parameters; incorrect addressing parameters;

parameters of protection against unauthorized access.

Basically, these parameters are probabilistic (averaged or boundary). Determining the relationship of these parameters with the parameters of the protocols, available resources and interfering conditions is the main task in assessing the quality of network technologies.

The values ​​of the quality of service parameters of the lower layers affect the values ​​of the quality of service parameters of the upper layers. Ultimately, the values ​​of the parameters of the quality of service of the upper level determine the quality of service (QoS - Quality of Service) provided by the communication network in the form of specific network services.

Open System Interconnection (OSI) Reference Model

Protocols

Applied

Representative

Session

Transport

Link level

data (channel)

Physical

Transmission medium (copper cable, optical cable, radio)

Physical layer

The Physical layer deals with the transmission of bits over physical communication channels, such as, for example, coaxial cable, twisted pair,

fiber optic cable or digital area channel. This level is related to the characteristics of physical data transmission media, such as bandwidth, noise immunity, characteristic impedance, and others. At the same level, the characteristics of electrical signals transmitting discrete information are determined, for example, the steepness of the pulse edges, the voltage or current levels of the transmitted signal, the type of coding, and the signal transmission rate. In addition, the types of connectors and the purpose of each contact are standardized here.

Physical layer functions are implemented in all devices connected to the network. On the computer side, the physical layer functions are performed by a network adapter or serial port.

An example of a physical layer protocol is the l0-Base-T specification of Ethernet technology, which defines an unshielded twisted pair cable of category 3 with a characteristic impedance of 100 Ohm, an RJ-45 connector, a maximum length of a physical segment of 100 meters, a Manchester code for representing data in cable, as well as some other characteristics of the environment and electrical signals.

Link layer

At the physical layer, bits are simply transferred. This does not take into account that in some networks in which communication lines are used (shared) alternately by several pairs of interacting computers, the physical transmission medium may be busy. Therefore, one of the tasks of the Data Link layer is to check the availability of the transmission medium. Another task of the data link layer is to implement error detection and correction mechanisms. To do this, at the data link layer, bits are grouped into sets called frames. The link layer ensures the correctness of the transmission of each frame by placing a special sequence of bits at the beginning and end of each frame to extract it, and also calculates the checksum, processing all the bytes of the frame in a certain way and adding the checksum to the frame. When a frame arrives over the network, the receiver computes the checksum of the received data again and compares the result with the checksum from the frame. If they match, the frame is considered correct and accepted. If the checksums do not match, then an error is recorded. The link layer can not only detect errors, but also correct them by retransmitting damaged frames. It should be noted that the error correction function is not mandatory for the link layer, therefore, it is not available in some protocols of this layer, for example, in Ethernet and frame relay.

V data link-layer protocols used in local networks have a certain structure of connections between computers and methods of their addressing. Although the link layer ensures the delivery of a frame between any two nodes of the local network, it does this only in a network with a completely defined topology of links, exactly the topology for which it was designed. Common bus, ring, and star topologies supported by LAN link-layer protocols include common bus, ring, and star, as well as structures derived from them using bridges and switches. Examples of link layer protocols are Ethernet, Token Ring, FDDI, l00VG-AnyLAN.

V local area networks link layer protocols are used by computers,

bridges, switches and routers. In computers, link layer functions are implemented jointly by network adapters and their drivers.

V wide area networks, which rarely have a regular topology, the data link layer often provides the exchange of messages only between two neighboring computers connected by a single link. Examples of protocols Point-to-point (as such protocols are often called) can be the widely used PPP and LAP-B protocols. In such cases, network layer facilities are used to deliver messages between end nodes across the entire network. This is how X.25 networks are organized. Sometimes in wide area networks, it is difficult to isolate link layer functions in their pure form, since in the same protocol they are combined with network layer functions. Examples of this approach are ATM and frame relay protocols.

V In general, the data link layer is a very powerful and complete set of functions for transferring messages between network nodes. In some cases, link layer protocols turn out to be self-sufficient vehicles and can allow application layer protocols or applications to work directly on top of them, without involving the means of the network and transport layers. For example, there is an implementation of the SNMP network management protocol directly over Ethernet, although by default this protocol runs over the IP network protocol and the UDP transport protocol. Naturally, the use of such an implementation will be limited - it is not suitable for composite networks of different technologies, for example, Ethernet and X.25, and even for such a network,

v which in all segments uses Ethernet, but between the segments there are loop-like connections. But in a two-segment Ethernet network connected by a bridge, the implementation of SNMP above the data link layer will be quite workable.

Nevertheless, to ensure high-quality transport of messages in networks of any topologies and technologies, the functions of the link layer are not enough, therefore, in the OSI model, the solution of this problem is assigned to the next two levels - network and transport.

Network layer

The Network layer serves to form a single transport system that unites several networks, and these networks can use completely different principles for transferring messages between end nodes and have an arbitrary structure of connections. The functions of the network layer are quite diverse. Let's start examining them on the example of combining local networks.

Data link-layer protocols of local area networks ensure data delivery between any nodes only in a network with an appropriate typical topology, for example, a hierarchical star topology. This is a very severe limitation that does not allow building networks with a developed structure, for example, networks that combine several enterprise networks into a single network, or highly reliable networks in which there are redundant connections between nodes. It would be possible to complicate the link-layer protocols to maintain loop-like redundant links, but the principle of separation of duties between the layers leads to a different solution. In order, on the one hand, to preserve the simplicity of data transfer procedures for typical topologies, and on the other hand, to allow the use of arbitrary topologies, an additional network layer is introduced.

At the network level, the term network itself is endowed with a specific meaning. In this

In this case, a network is understood as a set of computers interconnected in accordance with one of the standard typical topologies and using one of the link layer protocols defined for this topology for data transmission.

Within the network, data delivery is provided by the appropriate link layer, but the network layer is responsible for the delivery of data between networks, which supports the ability to choose the correct route for transmitting messages even if the structure of connections between the constituent networks has a character different from that adopted in the link layer protocols. Networks are interconnected by special devices called routers. A router is a device that collects information about the topology of internetworking and, based on it, forwards the network layer packets to the destination network. To transfer a message from a sender located in one network to a recipient located in another network, you need to make a certain number of transit transfers between networks, or hops (from hop - a jump), each time choosing a suitable route. Thus, a route is a sequence of routers through which a packet passes.

The problem of choosing the best path is called routing, and its solution is one of the main problems of the network layer. This problem is compounded by the fact that the shortest path is not always the best. Often the criterion for choosing a route is the time of data transmission along that route; it depends on the bandwidth of the communication channels and the traffic intensity, which can change over time. Some routing algorithms try to adapt to changes in load, while others make decisions based on averages over time. Route selection can be carried out according to other criteria, for example, transmission reliability.

In the general case, the functions of the network layer are broader than the functions of transferring messages over links with a non-standard structure, which we have now examined using the example of combining several local networks. The network layer also solves the problem of harmonizing different technologies, simplifying addressing in large networks, and creating reliable and flexible barriers to unwanted traffic between networks.

Network layer messages are commonly referred to as packets. When organizing the delivery of packets at the network level, the concept of "network number" is used. In this case, the recipient's address consists of the upper part - the network number and the lower part - the node number in this network. All nodes of one network must have the same upper part of the address, therefore the term "network" at the network level can be given another, more formal definition: a network is a collection of nodes whose network address contains the same network number.

At the network level, two kinds of protocols are defined. The first type - network protocols (routed protocols) - implement the forwarding of packets through the network. These are the protocols that are commonly referred to when talking about network layer protocols. However, another type of protocol is often referred to as the network layer, called routing information exchange protocols, or simply routing protocols. Routers use these protocols to collect information about the topology of interconnection. Network layer protocols are implemented by operating system software modules, as well as by software and hardware of routers.

At the network level, there are still other types of protocols that are responsible for mapping the node address used at the network level to the local network address. These protocols are often called Address Resolution Protocol, ARP. Sometimes they are referred not to the network level, but to the channel level, although the subtleties of the classification do not change their essence.

Examples of network layer protocols are the TCP / IP stack IP internetworking protocol and the IPX stack internetworking protocol.

Transport layer

On the way from sender to receiver, packets can be garbled or lost. While some applications have their own error handling facilities, there are some that prefer to deal with a reliable connection straight away. The Transport layer provides applications, or the top layers of the stack — application and session — to transfer data with the degree of reliability they require. The OSI model defines five classes of service provided by the transport layer. These types of services are distinguished by the quality of the services provided: urgency, the ability to restore an interrupted connection, the availability of multiplexing facilities for multiple connections between different application protocols via a common transport protocol, and most importantly, the ability to detect and correct transmission errors such as distortion, loss and duplication of packets.

The choice of the class of service of the transport layer is determined, on the one hand, by the extent to which the problem of ensuring reliability is solved by the applications themselves and protocols higher than the transport levels, and on the other hand, this choice depends on how reliable the data transport system is. a network provided by the layers below the transport - network, data link and physical. So, for example, if the quality of communication channels is very high and the probability of errors that are not detected by lower-level protocols is small, then it is reasonable to use one of the lightweight services of the transport layer, not burdened with numerous checks, acknowledgments and other methods of increasing reliability. If the vehicles of the lower levels are initially very unreliable, then it is advisable to turn to the most developed service of the transport level, which works using the maximum means for detecting and eliminating errors - by means of preliminary establishment of a logical connection, control of message delivery by checksums and cyclic numbering of packets. , setting delivery timeouts, etc.

As a rule, all protocols, starting from the transport layer and higher, are implemented by the software of the end nodes of the network - the components of their network operating systems. Examples of transport protocols include the TCP and UDP protocols of the TCP / IP stack and the SPX protocol of the Novell stack.

The protocols of the lower four levels are generally called network transport or transport subsystem, since they completely solve the problem of transporting messages with a given level of quality in composite networks with arbitrary topology and various technologies. The other three upper levels solve the problem of providing application services based on the existing transport subsystem.

Session level

The Session layer provides control of the dialogue: it fixes which of the parties is currently active, provides synchronization means. The latter allow breakpoints to be inserted into long passes so that in the event of a failure, you can go back to the last breakpoint rather than starting over. In practice, few applications use the session layer, and it is rarely implemented as separate protocols, although the functions of this layer are often combined with the functions of the application layer and implemented in a single protocol.

Representative level

The Presentation layer deals with the form of presentation of information transmitted over the network, without changing its content. Due to the presentation layer, the information transmitted by the application layer of one system is always understood by the application layer of the other system. With the help of this layer, application protocols can overcome syntactic differences in data representation or differences in character codes such as ASCII and EBCDIC. At this level, data encryption and decryption can be performed, thanks to which the secrecy of data exchange is ensured for all application services at once. An example of such a protocol is Secure Socket Layer (SSL), which provides secure messaging for the application layer protocols of the TCP / IP stack.

Application level

The Application layer is really just a collection of different protocols by which network users can access shared resources such as files, printers, or hypertext Web pages, and organize their collaboration, for example, using the e-mail protocol. The unit of data that the application layer operates on is usually called a message.

There is a very wide variety of application services. Let's take as an example at least a few of the most common file service implementations: 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 generalized structure of any software or information system can be represented, as noted above, by two interacting parts:

  • functional part that includes application programs that implement the functions of the application area;
  • environment or system part that ensures the execution of applied programs.

Closely related to this separation and interconnection are two sets of standardization issues:

  1. standards of interfaces for interaction of application programs with the IS environment, application program interface (API);
  2. standards of interfaces for interaction of the IS itself with an external environment for it (External Environment Interface - EEI).

These two groups of interfaces define the specifications of the external description of the IS environment - architecture, from the point of view of the end user, the IS designer, the application programmer who develops the functional parts of the IS.

The specifications of the external interfaces of the IS environment and the interfaces of interaction between the components of the environment itself are precise descriptions of all the necessary functions, services and formats of a certain interface.

The totality of such descriptions is Open Systems Interconnection (OSI) Reference Model... This model has been in use for over 30 years and grew out of the SNA (System Network Architecture) proposed by IBM. The Open Systems Interconnection Model is used as the basis for the development of many ISO IT standards. The publication of this standard summed up many years of work by many well-known standardizing organizations and manufacturers of telecommunications equipment.

In 1984, the model received the status of the international standard ISO 7498, and in 1993 an expanded and supplemented edition of ISO 7498-1-93 was published. The standard has a composite title "Information and Computing Systems - Interconnection (Interaction) of Open Systems - Reference Model". The short name is the Open Systems Interconnection / Basic Reference Model (OSI / BRM).

The model is based on dividing the computing environment into seven levels, the interaction between which is described by the corresponding standards and provides communication between the levels, regardless of the internal structure of the level in each specific implementation (Fig. 2.6).


Rice. 2.6.

The main advantage of this model is a detailed description of connections in the environment from the point of view of technical devices and communication interactions. However, it does not take into account the relationship with regard to the mobility of the application software.

The advantages of the "layered" organization of the interaction model are that it provides independent development level standards, modularity of development of hardware and software of information and computing systems and thereby contributes to technical progress in this area.

In the ISO 7498 standard, seven levels (layers) of information interaction are distinguished, which are separated from each other by standard interfaces:

  1. application layer (application layer)
  2. presentation layer
  3. session (session level)
  4. transport
  5. network
  6. channel
  7. physical.

In accordance with this, the information interaction of two or more systems is a set of information interactions of level subsystems, and each layer of the local information system interacts, as a rule, with the corresponding layer of the remote system. Interaction is carried out using appropriate communication protocols and interfaces. In addition, using encapsulation techniques, you can use the same software modules at different levels.

Protocol is a set of algorithms (rules) for the interaction of objects of the same name levels of different systems.

Interface is a set of rules according to which interaction with an object of a given or another level is carried out. The standard interface may be referred to as a service in some specifications.

Encapsulation is the process of putting fragmented blocks of data from one level into blocks of data from another level.

When dividing the environment into levels, the following general principles were observed:

  • do not create too many small partitions, as this complicates the description of the interaction system;
  • to form a level from easily localized functions, this, if necessary, allows you to quickly rebuild the level and significantly change its protocols to use new solutions in the field of architecture, software and hardware, programming languages, network structures, without changing the standard interfaces of interaction and access;
  • place similar functions on the same level;
  • create separate levels for performing such functions, which clearly differ in the actions that implement them or technical solutions;
  • to draw the boundary between the levels in a place where the description of services is the smallest, and the number of interactions across the border (border crossing) is minimized;
  • to draw the boundary between the levels in such a place where at a certain moment the corresponding standard interface should exist.

Each layer has a protocol specification, i.e. a set of rules governing the interaction of peer processes at the same layer; and a list of services that describe the standard interface to the higher layer. Each layer uses the services of the lower layer, each lower one provides services to the higher one. Let's give a brief description of each layer, noting that in some descriptions of the OSI model, the numbering of the layers can go in reverse order.

Level 1 is the application layer or application layer (Application Layer). This level is associated with application processes. Layer protocols are designed to provide access to network resources and user application programs. At this level, the interface with the communication part of the applications is defined. An example of application layer protocols is Telnet, which allows a user to access a "host" (a host, one of the main elements in a multi-machine system, or any device connected to a network that uses TCP / IP protocols) in remote terminal mode.

The application layer performs the task of providing various forms of interaction between application processes located in various systems of the information network. To do this, he performs the following functions:

  • description of forms and methods of interaction of applied processes;
  • performing various types of work (job management, file transfer, system management, etc.);
  • identification of users (interaction partners) by their passwords, addresses, electronic signatures;
  • definition of functioning subscribers;
  • announcement of the possibility of access to new application processes;
  • determining the adequacy of available resources;
  • sending requests to connect to other application processes;
  • submission of applications to the representative level for the necessary methods of describing information;
  • selection of procedures for the planned dialogue of processes;
  • management of data exchanged by application processes;
  • synchronization of interaction of application processes;
  • determination of the quality of service (delivery time of data blocks, admissible error rate, etc.);
  • agreement to correct errors and determine the accuracy of the data;
  • negotiation of restrictions imposed on syntax (character sets, data structure).

The application layer is often divided into two sublevels. The upper sub-layer includes network services. Bottom - contains standard service elements that support the operation of network services.

Level 2 - Presentation Layer. At this level, information is converted to the form in which it is required for the execution of application processes. The presentation layer provides for the encoding of data issued by application processes and the interpretation of the transmitted data. For example, algorithms for converting the data representation format for printing - ASCII or KOI-8 are performed. Or, if a display is used to visualize data, then this data according to a given algorithm is formed in the form of a page that is displayed on the screen.

The representative level performs the following main functions:

  • selection of the image of representations from possible options;
  • changing the image of the presentation into a given virtual image;
  • conversion of data syntax (codes, symbols) to standard;
  • defining the data format.

Level 3 - the session layer or the session layer (Session Layer). At this level, sessions are established, maintained, and terminated between representative application objects (application processes). As an example of a session layer protocol, consider the Remote Procedure Call (RPC) protocol. As the name suggests, this protocol is designed to display the results of a procedure execution on a remote host. During this procedure, a session connection is established between applications. The purpose of this connection is to service requests that arise, for example, when a server application interacts with a client application.

The session layer provides interaction with the transport layer, coordinates the reception and transmission of data of one communication session, contains the functions of password management, calculating the payment for the use of network resources, etc. This level provides the following functions:

  • establishment and termination at the session level of the connection between partners;
  • performing normal and urgent data exchange between application processes;
  • synchronization of the work of session connections;
  • notification of application processes about exceptional situations;
  • setting labels in the application process that allow, after a failure or error, to restore its execution from the nearest label;
  • interruption, if necessary, of the application process and its correct resumption;
  • termination of the session without loss of data;
  • transmission of special messages about the course of the session.

Layer 4 is the Transport Layer. The transport layer is designed to control the flow of messages and signals. Flow control is an important function of transport protocols, since this mechanism makes it possible to reliably provide data transfer over networks with a heterogeneous structure, while the route description includes all components of the communication system that ensure data transfer all the way from the sender's devices to the receiving devices of the recipient. Flow control consists in obligatory waiting of the receiver for acknowledgment of reception of the specified number of segments by the receiver. The number of segments that the transmitter can send without acknowledging their receipt from the receiver is called a window.

There are two types of transport layer protocols - sharding protocols and datagram protocols. Segmenting transport layer protocols break the original message into transport layer data blocks - segments. The main function of such protocols is to ensure the delivery of these segments to the destination and recovery of the message. Datagram protocols do not segment a message; they send it in one packet along with address information. A data packet, called a Datagram, is routed on address switching networks or sent over a local area network to an application or user.

The transport layer can also negotiate the network layers of various incompatible networks through special gateways. The considered level determines the addressing of subscriber systems and administrative systems. The main task of the transport layer is the use of virtual channels laid between interacting subscriber systems and administrative systems for transferring data blocks in packets. The main functions performed by the transport layer:

  • control of the transfer of data blocks and ensuring their integrity;
  • detection of errors, their partial elimination, reporting of uncorrected errors;
  • restoration of transmission after failures and malfunctions;
  • enlargement or downsizing of data blocks;
  • granting of priorities when transferring blocks;
  • transmission of confirmations of transmitted data blocks;
  • elimination of blocks in case of deadlocks in the network.

In addition, the transport layer can recover data blocks lost at the lower layers.

Layer 5 is the Network Layer. The main task of the network layer protocols is to determine the path that will be used to deliver data packets when the upper layer protocols work (routing). In order for the packet to be delivered to any given host, this host must be assigned a network address known to the transmitter. Host groups that are geographically grouped together form networks. To simplify the routing task, a host's network address is composed of two parts: the network address and the host address. Thus, the task of routing is split into two - finding a network and finding a host in this network. The following functions can be performed at the network level:

  • creation of network connections and identification of their ports;
  • detection and correction of errors occurring during transmission through the communication network;
  • packet flow control;
  • organization (ordering) of sequences of packets;
  • routing and switching;
  • segmentation and bundling of packages;
  • return to its original state;
  • choice of types of service.

Layer 6 is the data link layer or data link layer. The purpose of the link layer protocols is to provide data transmission in the transmission medium over a physical carrier. A start signal of data transmission is generated in the channel, the start of transmission is organized, the transmission itself is performed, the correctness of the process is checked, the channel is disconnected in case of failures and recovery after the elimination of the malfunction, a signal is generated at the end of the transmission and the channel is put into standby mode.

Thus, the link layer can perform the following functions:

  • organization (establishment, management, termination) of channel connections and identification of their ports;
  • transfer of data blocks;
  • detection and correction of errors;
  • data flow control;
  • ensuring transparency of logical channels (transmission of data encoded in any way through them).

At the data link layer, data is transmitted in blocks called frames. The type of transmission medium used and its topology largely determine the type of transport layer protocol frame that should be used. When using the topology "common bus" (Common Bus) and "one-to-many" (Point-to-Multipoint), the means of the link layer protocol define the physical addresses with the help of which data exchange in the transmission medium will be carried out and the procedure for accessing this medium ... Examples of such protocols are Ethernet (where applicable) and HDLC. Transport protocols, which are designed to operate in a point-to-point environment, do not define physical addresses and have simplified access procedures. An example of this type of protocol is PPP.

Layer 7 is the Physical Layer. The physical layer protocols provide direct access to the data transmission medium for the protocols of the data link and subsequent layers. Data is transmitted using the protocols of this layer in the form of bit sequences (for serial protocols) or groups of bits (for parallel protocols). At this level, the set of signals exchanged between systems, the parameters of these signals (time and electrical) and the sequence of signal generation during the data transfer procedure are determined.

The physical layer performs the following functions:

  • establishes and disconnects physical connections;
  • transmits a sequence of signals;
  • "listens" to channels if necessary;
  • performs channel identification;
  • notifies about the occurrence of malfunctions and failures.

In addition, at this level, requirements are formulated for the electrical, physical and mechanical characteristics of the transmission medium, transmitting and connecting devices.

Network-dependent and network-independent levels. The above functions of all levels can be attributed to one of two groups: either to functions focused on working with applications regardless of the network device, or to functions that depend on the specific technical implementation of the network.

The top three layers - application, representative, and session - are application-oriented and practically do not depend from the technical features of building a network. The protocols of these layers are not affected by any changes in network topology, equipment replacement, or migration to another network technology.


Rice. 2.9.

Interface standardization provides complete transparency of interaction regardless of how the layers are arranged in specific implementations (services) of the model.