The basic reference model for the interaction of open systems has. Reference model of interaction of open information systems. Reference Model for Open Systems Interconnection
Basic EMOS is the model adopted by ISO to describe general principles interactions information systems. EMWOS is recognized by all international organizations as the basis for the standardization of information network protocols.
In EMWOS, an information network is considered as a set of functions that are divided into groups called levels. The division into levels allows you to make changes to the means of implementing one level without restructuring the means of other levels, which greatly simplifies and reduces the cost of upgrading tools as technology develops.
EMOS contains seven levels. Below are their numbers, names and functions.
7th level - application (Application): includes tools for managing application processes; these processes can be combined to perform assigned tasks, exchange data with each other. In other words, at this level, those data that are to be transmitted over the network are determined and formatted into blocks. The level includes, for example, such means for the interaction of application programs as receiving and storing packages in "mail boxes" (mail-box).
6th level - representative (Presentation): data presentation functions (coding, formatting, structuring) are implemented. For example, at this level, the data allocated for transmission is converted from the EBCDIC code to ASCII, and so on.
Level 5 - session (Session): designed to organize and synchronize the dialogue conducted by objects (stations) of the network. At this level, the type of communication (duplex or half duplex), the beginning and end of tasks, the sequence and mode of exchange of requests and responses of interacting partners are determined.
4th level - transport (Transport): designed to manage end-to-end channels in the data transmission network; this layer provides communication between endpoints (as opposed to the next network layer, which provides data transfer through intermediate network components). The functions of the transport layer include multiplexing and demultiplexing (assembly-disassembly of packets), detection and elimination of errors in data transmission, implementation of the ordered service level (for example, the ordered speed and transmission reliability).
3rd level - network (Network): at this level, packets are formed according to the rules of those intermediate networks through which the original packet passes, and packets are routed, i.e. definition and implementation of routes along which packets are transmitted. In other words, routing is reduced to the formation of logical channels. A logical link is a virtual connection between two or more network layer entities that allows data to be exchanged between these entities. The concept of a logical channel does not necessarily correspond to some physical connection of data transmission lines between the connected points. This concept is 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 adversely affects the operation of the network.
2nd layer - channel (Link, data link layer): provides data exchange services between logical objects of the previous network layer and performs functions related to the formation and transmission of frames, detection and correction of errors that occur at the next, physical layer. A link layer packet is called a frame because a packet at previous layers may consist of one or more frames.
1st layer - physical (Physical): provides mechanical, electrical, functional and procedural means for establishing, maintaining and disconnecting logical connections between logical objects of the link layer; implements the functions of transferring data bits through physical media. It is at the physical level that the representation of information in the form of electrical or optical signals, the transformation of the waveform, the choice of parameters of the physical media for data transmission are carried out.
In specific cases, there may be a need to implement only a part of these functions, then, accordingly, only a part of the levels are available in the network. So, in simple (non-branched) LANs, there is no need for network and transport layer facilities. At the same time, the complexity of the link layer functions makes it expedient to divide it into two sublevels in the LAN: medium access control (MAC) and logical link control (LLC - Logical Link Control). The LLC sublayer, in contrast to the MAC sublayer, includes a part of the link layer functions that are not related to the characteristics of the transmission medium.
Data transmission over branched networks occurs using encapsulation/decapsulation of data chunks. Thus, a message that has arrived at the transport layer is divided into segments that receive headers and are transmitted to the network layer. A segment is usually referred to as a transport layer packet. The network layer organizes the transfer of data through intermediate networks. To do this, the 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 (i.e. encapsulation occurs). When transferring between nodes of an intermediate LAN, encapsulation of packets into frames is required with a possible breakdown of the packet. The receiver decapsulates the segments and reconstructs the original message.
network protocol information switching
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In the early 1980s, ISO recognized the need for a network model, on the basis of which telecommunications equipment suppliers could create networks that interacted with each other. In 1984, such a standard was released under the title "Reference Model for Open Systems Interconnection" (Open System Interconnect - OSI) or OSI/ISO.
The OSI reference model has become the main architectural model for messaging systems. When considering specific applied telecommunication systems, their architecture is compared with the OSI/ISO model. This model is best remedy for studying modern technology connections.
The OSI reference model divides the problem of transferring information between subscribers into seven smaller and therefore more easily solvable problems. The concretization of each task was carried out according to the principle of relative autonomy. Obviously, the autonomous task is easier to solve.
Each of the seven areas of the information transmission problem is assigned one of the levels of the reference model. The two lowest layers of the OSI reference model are implementedhardware and softwareprovision, the remaining five higher levels are usually implemented programmatic security. The OSI reference model describes how information flows through a medium (eg, metal wires) from a source application process (eg, voice) to a destination process.
As an example of an OSI type of communication, suppose that System A in Fig. 2.1 has information to send to System B. System A's application process communicates with System A's Layer 7 (top layer), which communicates with System A's Layer 6, which in turn communicates with System A's Layer 5, and so on up to System A's Layer 1 A. The task of Level 1 is to give (and also take) information to the physical environment. After information passes through the physical medium and is received by System B, it ascends through the layers of System B to reverse order(first Level 1, then Level 2, etc.) until it finally reaches the System B application process.
Each of the levels communicates with the higher and lower levels of the system. However, to perform the tasks inherent in the level, communication with the corresponding level of another system is necessary, i.e. the main task of Layer 1 of System A is to communicate with Layer 1 of System B; Layer 2 of System A communicates with Layer 2 of System B, and so on.
The OSI layer model excludes direct communication between the corresponding layers of different systems. Therefore, each layer of System A uses the services provided to it by adjacent layers to communicate with its corresponding layer of System B. The lower layer is calledservice provider, and the superior service user. The interaction of levels occurs in the so-calledpoint of service. The relationship between adjacent levels of a particular system is shown in Fig. 2.2.
Rice. 2.2. Interaction between the levels of a single system
The exchange of control information between the corresponding levels of different systems is carried out in the form of an exchange of special " headings ", added to the payload information. Typically, the header precedes the actual application information. Each lower layer of the transmitting system adds its header to the information block received from the higher layer with the necessary control information for the corresponding layer of the other system (Fig. 2.3).
Rice. 2.3. Formation of information blocks
The receiving system analyzes this control information and removes the corresponding header before passing the information block to the upper layer. Thus, the size of the information block increases as one moves from top to bottom through the layers in the transmitting system and decreases as one moves from bottom to top through the layers in the receiving system.
The OSI Reference Model is not an implementation of a network. It only defines the protocol functions of each layer.
2.2. Description of the layers of the OSI reference model
Each layer has a predetermined set of functions that it must perform in order to communicate.
Application layer(Layer 7) is the OSI layer closest to the user. It differs from the other layers in that it does not provide services to any of the other OSI layers. It provides services to application processes that are outside the scope of the OSI model. Examples of such application processes are speech processes, databases, word processors, and so on.
The application layer identifies and establishes the existence of intended communication partners, synchronizes collaborating application processes, and establishes and negotiates error recovery and information integrity management procedures. The application layer also determines whether sufficient resources are available for the intended communication. At this level, information is presented in the form of files, tables, databases, etc. objects
Representative level(layer 6) is responsible for ensuring that information sent from the application layer of one system is readable by the application layer of another system. If necessary, the presentation layer translates between a plurality of information presentation formats by using a common information presentation format.
The presentation layer is concerned not only with the format and presentation of actual user data, but also with the data structures that programs use. Therefore, in addition to transforming the format of the actual data (if necessary), the presentation layer agrees on the data transfer syntax for the application layer and, if necessary, performs data encryption and decryption.
session layer(layer 5) establishes, manages and terminates interaction sessions between application tasks. Sessions consist of a dialogue between two or more view objects. The session layer synchronizes the dialogue between the presentation layer objects and manages the exchange of information between them.
In addition, the session layer provides a means to send information, class of service, and exception notification of problems in the session, presentation, and application layers. Data at the session level is represented by blocks of a given length.
transport layer(level 4). The boundary between the session and transport layers can be represented as border between protocols of higher (application) levels and protocols of lower levels. While the application, presentation, and session layers are concerned with application matters, the four lower layers deal with data transport problems.
The transport layer provides data transport services, which saves higher layers from having to delve into its details. The function of the transport layer is to reliably transport data across the network. By providing reliable services, the transport layer provides mechanisms for establishing, maintaining, and orderly terminating links, systems for detecting and resolving transport faults, and controlling information flow (to prevent the system from being flooded with data from another system). At this level, information is represented in the form of messages exchanged between processes.
network layer (Layer 3) is an end-to-end layer that provides connectivity and route selection between two end systems.
Since two end systems that wish to communicate may be separated by a significant geographic distance and many subnets, the network layer is the domain of routing. Routing protocols select the best routes through a series of interconnected subnets. Traditional network layer protocols convey information along these routes. At the network level, information is represented by packets that contain address information for making a connection.
Link layer(Layer 2) (formally called data link layer) provides reliable data transit through the physical channel. In carrying out this task, the link layer deals with issues of physical addressing (as opposed to network or logical addressing), network topology, linear discipline (how the end system uses the network link), error notification, in-order delivery of data blocks, and information flow control. At the link layer, information is represented by blocks of bits, which are called frames or data packets. Packet boundaries are marked with flags, sequences of bits that do not occur in the data region. End-of-packet flag 01111110 for procedure HDLC shown in Figure 2.1 as a shaded area.
Physical layer(Layer 1) defines the electrical, mechanical, procedural and functional characteristics of establishing, maintaining and disconnecting a physical link between end systems. Physical layer specifications define characteristics such as voltages, timing parameters, physical information transfer rates, maximum information transfer distances, physical connectors, and other similar characteristics.
The physical environment in various telecommunications systems can be a variety of means from the simplest pair of wires to a complex transmission system of a synchronous digital hierarchy. At this level, information is represented in the form of electrical current signals, electromagnetic field or light energy.
L_02. Questions for self-examination.
1 Describe the form of data representation at the levels of the reference model.
2 Describe the functions of each level.
The world practice of creating systems has led to the need to develop standards for the entire range of issues of organizing network systems. In 1977, the ISO Committee on Computing and Information Processing proposed a description of the OSI reference model for open systems interaction, which was called the 7-layer model. At present, the model has received wide distribution and recognition, since it creates the basis for both the analysis of existing systems and the definition of new systems and standards.
The depicted levels, in full or in part, are present in any computer system and interact on a strict hierarchical basis, i.e. any level serves the level above and uses the services of the lower level.
Thanks to the standardization of the 7-layer model, any 2 network devices, subject to the standard, can interact, despite differences in design, functionality, and internal interfaces. Such interaction becomes possible for various models and classes of computers. The IEEE LAN Standards Committee has proposed that the physical environment be treated as layer 0.
Level 1 - Physical
Provides an interface between a device and a transmission medium. At the physical layer, a sequence of bits is transmitted through subscriber channels. Channel control is reduced to the selection of the beginning and end of the frame, the formation and reception of the signal, the analysis of the code sequence. Physical layer standards include X.21 recommendations that define the electrical, mechanical, functional, and procedural characteristics required for the physical interconnection of communication links.
Level 2 - Channel
Forms from the data transmitted by layer 1, the so-called. frames and their sequences, controls access to the transmission medium, detects and corrects errors. The physical and link layers define the characteristics of the channels and the frame transmission technique. Layer 2 protocols comply with the recommendations of X.25 CCTC and generally define the channel control procedure: duplex, half-duplex, simplex.
Level 3 - Network
Implements routing functions so that layer frames, called packets, can be sent over multiple links over one or more networks. This usually requires the network address to be included in the packet. The main task of the network protocol is to lay a set of logical channels (up to 4096) in each physical channel, increasing the efficiency of using the physical channel. The network layer can also handle errors. The transmission protocol standard contains recommendations X.25/3 of the CCTC.
Level 4 - Transport
It is done for users and performers of the transport service in open communication systems. The protocol frees the user from learning all the functions of switching, routing and information selection. provides end-to-end control over the movement of packets between these processes. An important role at the transport level is played by the window mechanism, which gives the sender the right to transfer several (up to 8) data blocks to the recipient without confirmation. At the end of the transmission, the recipient acknowledges the receipt of data blocks or reports errors in them. The procedure for executing this function is called the window mechanism. ECMA-72 transport protocol standard. Contains procedures of 5 classes.
Level 5 - Session
Provides the exchange of data blocks between the objects of the application layer. To this end, the protocol big number functions: 10 for organizing transmission and 3 for synchronizing interaction procedures. The ECMA-75 standard defines 4 classes of service: A–D.
Level 6 - Executive
Performs data interpretation. Character representation, page format, graphic coding are analyzed.
When managing the terminal screen, other functions are also implemented:
screen cleaning designation on the screen of the most important fields by means of flickering, etc.
The European Association of Computer Manufacturers has developed 4 interrelated standards ECMA-86 (basic principles for the 6th level of communication protocols). One ECMA-84 (Generalized Virtual Terminal Protocol). And one ECAM-88 standard (virtual terminal base class protocol).
Level 7 - Applied
Implements all functions that cannot be attributed to the lower level. At this level, ISO considers the following protocols:
FTAM - file transfer and management
JTM - job transfer and processing
VTSP - virtual terminal service
FTAM is based on the principle of virtual file storage, which provides a standard, computer-independent way of describing the structure of files and their characteristics.
JTM is based on remote input and output of information using external devices of various computers.
VTSP is designed to provide interaction between users located at terminals and application processes located in different computers.
The exchange of information in telecommunication networks is carried out according to certain, predetermined rules (standards). These rules are being developed by a number of international organizations.
Interaction in modern telecommunication networks is organized in accordance with the reference model for the interaction of open systems (EVOS), which was proposed in 1980 by the International Organization for Standardization ISO (ISO - International Organization for Standardization) for computer networks. open called systems that use the same communication protocols. Protocol - a set of rules that govern the interaction for the exchange of messages between independent devices or processes.
The general communication problem has two parts:
1) the first part concerns the communication network - the data transmitted over the network must arrive at the destination in the correct form and in a timely manner;
2) the second part - providing data recognition for further use - the functions of the user's terminal equipment.
All tasks solved for organizing user interaction are divided into seven groups - levels of the reference model (Figure 1.7).
Figure 1.7 - Reference model of open systems interaction
The three lower layers represent network services. Protocols that implement these layers must be provided in each network node. The top four layers represent services to end users and are associated with them, not with the network. The lower levels are used to route data from one user to another. The upper levels solve the problem of presenting data to the user in a form that he can recognize. The choice of seven levels is dictated by the following considerations:
1) it is essential to have enough layers so that each of them is not too complex in terms of protocol development;
2) it is desirable to have not too many levels so that their integration and descriptions do not become too complex;
3) it is desirable to choose natural boundaries so that related functions are collected at the same level.
In the reference model, the module of level n interacts with modules of only adjacent levels (n-1) and (n+1).
Model levels perform the following functions:
1) Physical layer provides the transmission of a sequence of bits in the form of signals of a certain physical nature at a rate corresponding to the channel capacity.
2) Link layer forms data blocks - frames, controls access to the transmission medium, detects and corrects errors.
3) network layer implements the routing function. Blocks of network layer data are called packets.
The physical, channel and network layers are network-dependent, and therefore their functioning varies depending on the type of communication network.
4) transport layer occupies a central place in the hierarchy of levels, ensures the interaction of processes in connected terminal devices and end-to-end control of the movement of packets between these processes. The presence of this level frees users from the extreme importance of learning all the functions of switching, routing and selection (selection) of data.
The four lower layers (physical, data link, network, transport) make up the transport network.
5) session layer ensures the maintenance of a dialogue between processes, performing the functions of organizing data transfer and synchronizing interaction procedures (Figure 1.8).
Figure 1.8 - An example of a dialogue in the network
6) Presentation Layer provides data interpretation. At this level, the syntax is implemented (character representation, page format, encoding, etc. are analyzed).
7) Application layer implements functions that are not assigned to previous levels. Application layer protocols give the appropriate meaning (semantics) of the exchanged information. The application level ensures the execution of all information and computing processes.
The multilevel organization of interaction gives rise to the extreme importance of modifying information at each level in accordance with the functions of the level (Figure 1.9).
Figure 1.9 - Interaction of levels
In each layer transmission, a block of data is received from the higher layer, control information is added to the data, and the block is transmitted to the lower layer. At the receiving end, each layer uses only the appropriate header without looking at the rest of the received data block. Therefore, the levels are independent and isolated from each other. This allows you to remove and replace protocols and programs of individual layers without affecting the rest of the model.
A multilevel organization ensures the independence of management at level n from the order of operation of the lower and upper levels:
Information channel management occurs regardless of the physical principles of the physical channel functioning;
Network management does not depend on ways to ensure the reliability of the information channel;
The transport layer interacts with the network as a single system that delivers messages to users;
An application process is created only to perform certain data processing functions without taking into account the network structure, route selection methods, type of communication channels, etc.
Users for organizing interaction rely on interaction service. Interaction between users is organized by session controls (layer 5), which operate on the basis of a transport channel that provides message transfer during the session. The transport channel created at layer 4 includes a communication network that organizes information channels between users (Figure 1.10).
Figure 1.10 - Organization of interaction between users
Reference model of interaction of open systems - concept and types. Classification and features of the category "Reference model of open systems interaction" 2017, 2018.
Solving the problem of transmitting messages over electrical communication systems imposes certain requirements on them. These requirements can be conditionally divided into two groups: requirements for the messaging process and requirements for technical means carrying out this process.
Among the requirements for technical means of electrical communication systems, the following are distinguished. First, the communication system must be able to increase its capabilities and eliminate unused capabilities. Systems that have this ability are called open systems. Secondly, various communication systems should have standardized and unified technical devices, which reduces the cost of their cost and operation. Thirdly, communication systems for various purposes must be capable of mutual exchange of messages.
These requirements gave rise to the need for a unified ideology for the design of communication systems. The International Telephony and Telegraphy Advisory Committee proposed such an ideology in the early 1980s by developing the Open Systems Interconnection Reference Model (OSIM).
In accordance with this model, the process of message transmission in communication systems is sequentially divided into fundamentally different operations. Each of these operations is assigned to its own level.
The levels are built according to the principle of a strict hierarchy: at the top level are the source and recipient of information - users of the communication system, at the bottom - the medium of propagation of electromagnetic waves. Highest level governs the work of the lower. Each level has its own technical device or an organizational unit of a communication system is a user or an official who ensures the functioning of the communication system. In some communication systems, some of these devices may be missing or not perform all the functions of a certain level.
There are 7 levels in EMOS: user, representative, session, transport, network, channel, physical (Fig. 1.4). The total set of funds from one user, performing operations at various levels, is called station.
At the user level, there are processes of processing information transmitted by the communication system. The executor of the functions of this level can be both a technical device (computer) and a person.
Presentation devices convert messages from a user-friendly presentation form to a user-friendly presentation form and vice versa. In particular, at this level, information is compressed, since it is always convenient for the communication system that the message occupies the smallest amount.
Session-level devices frame the transmitted message with service information so that the number of topological transmission options is as large as possible. The choice of the best option is carried out by devices of lower levels. Thus, this layer is responsible for organizing the communication session.
At the transport level, a decision is made to move this message to the user at the level of selecting the necessary communication networks. For this, the problem of inter-network addressing of messages and the problem of transferring messages between networks of various kinds, called the gateway problem, are solved.
At the network level, the problem of the best message delivery to the user within the framework of one communication network is solved. To do this, a subnet message movement route is selected, and the problem of intranet addressing of users is solved.
Link level devices protect transmitted messages from distortions that occur due to changes in signal parameters during propagation.
Physical layer devices provide the conversion of the transmitted message into signals and the restoration of the message from the received signal.
The rules by which devices of neighboring levels of one station interact are called an interface.
The rules by which devices of the same level at different stations interact are called a protocol.
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