1) characteristics of devices used in the network;

2) the network operating system used;

3) way physical connection network nodes via communication channels;

4) the method of propagating signals over the network.

60. For standard Ethernet technologies are used...

1) coaxial cable;

2) linear topology;

3) ring topology;

4) carrier sensing access;

5) forwarding the token

6) fiber optic cable;

61. List ways in which a workstation can be physically connected to the network?

1) using network adapter and cable outlet

2) using a hub

3) using a modem and a dedicated telephone line

4)using the server

62. Local networks are not allowed physically combine using...

1) servers

2) gateways

3) routers

4) concentrators

63. What is the main disadvantage of the ring topology?

1. high network cost;

2. low network reliability;

3. high cable consumption;

4. low noise immunity of the network.

64. For which topology is the statement true: “The failure of a computer does not disrupt the operation of the entire network”?

1) basic star topology

2) basic “bus” topology

3) basic “ring” topology

4) the statement is not true for any of the basic topologies

65. What is the main advantage of the star topology?

1. low network cost;

2. high reliability and controllability of the network;

3. low cable consumption;

4. good network noise immunity.

66. What topology and access method are used in Ethernet networks?

1) bus and CSMA/CD

2) bus and marker transfer

3) ring and marker transfer

4) bus and CSMA/CA

67. What network characteristics are determined by the choice of network topology?

1. equipment cost

2. network reliability

3. subordination of computers on the network

4. network expandability

68. What is the main advantage of the token passing access method?

no collisions
simplicity of technical implementation
low cost of equipment

Stages of data exchange in networks computer systems

1) data transformation in the process of moving from the top level to the bottom1

2) data transformation as a result of moving from the lower level to the upper ones3

3) transportation to the recipient computer2

70. Which protocol is the main one for transmitting hypertext on the Internet?

2) TCP/IP

3) NetBIOS

71. What is the name of a device that provides a domain name upon request based on an IP address and vice versa:

1) DFS server

2) host – computer

3) DNS server

4) DHCP server

72. The DNS protocol establishes correspondence...

1) IP addresses with switch port

2) IP addresses with domain address

3) IP addresses with MAC address

4) MAC addresses with domain address

73. What IP addresses cannot be assigned to hosts on the Internet?

1) 172.16.0.2;

2) 213.180.204.11;

3) 192.168.10.255;

4) 169.254.141.25

A unique 32-bit sequence of binary digits that uniquely identifies a computer on a network is called

1) MAC address

2) URL;

3) IP address;

4) frame;

Which (or which) identifiers are allocated in an IP address using a subnet mask

1) networks

2) network and node

3) node

4) adapter

76. For each server connected to the Internet, the following addresses are set:

1) digital only;

2) domain only;

3) digital and domain;

4) addresses are determined automatically;

77. At the network level of interaction of the OSI model...

1) erroneous data is retransmitted;

2) the message delivery route is determined;

3) programs that will carry out interaction are determined;

78. Which protocol is used to determine the physical MAC address of a computer corresponding to its IP address?

OSI model includes _____ levels of interaction

1) seven

2) five

3) four

4) six

80. What class of network does an organization with 300 computers need to register to access the Internet?

81. What distinguishes the TCP protocol from the UDP protocol?

1) uses ports when working

2) establishes a connection before transmitting data

3) guarantees delivery of information

82. Which of the following protocols are located at the network layer of the TCP/IP stack?

“UDC 621.396.6 RELIABILITY OF A LOCAL COMPUTING NETWORK RELIABILITY OF A LOCAL COMPUTING NETWORK BASED ON A THIN CLIENT AND WORKERS...”

Reliability and quality of complex systems. No. 4, 2013

UDC 621.396.6

RELIABILITY OF LOCAL COMPUTER NETWORK

S. N. Polessky, M. A. Karapuzov, V. V. Zhadnov

RELIABILITY OF A LOCAL COMPUTER NETWORK BASED ON THIN CLIENT AND WORKSTATIONS

BASED ON THIN CLIENT AND WORKSTATIONS

S. N. Polessky, M. A. Karapuzov, V. V. Zhadnov

The development of local area networks (LANs) faces two prospects: continue to design LANs, where the subscribers are traditional “workstations” (PCs), or instead of PCs, use so-called “thin clients” (hereinafter will be used as a synonym for “terminal terminals”). station").

Currently, the term “thin client” is increasingly used, when this term means a fairly wide range of devices and programs from the point of view of system architecture, which are united by a common property: the ability to work in terminal mode.

The advantage of a PC over a thin client is its independence from the presence of a working network - information will be processed even at the moment of its failure, since in the case of a PC, information is processed directly by the stations themselves.

In case of using the work thin client a terminal server is required. But at the same time, the thin client has a minimal hardware configuration, instead of hard drive To load a local specialized operating system (OS), DOM is used (DiskOnModule - a module with an IDE connector, flash memory and a chip that implements the logic of a regular hard drive, which is defined in the BIOS as a regular hard drive, only its size is usually 2–3 times smaller).

In some system configurations, the thin client loads the operating system over the network from the server using the PXE, BOOTP, DHCP, TFTP, and Remote Installation Services (RIS) protocols. Minimal use of hardware resources is the main advantage of a thin client over a PC.

In this regard, the question arises: what is better to use for LAN design in terms of reliability – thin client or traditional PC?

To answer this question, let’s compare reliability indicators standard scheme LAN built according to the “star” topology for two options for its implementation. In the first version, the LAN is built on the basis of thin clients, and in the second, on the basis of a PC. To simplify the assessment of LAN reliability indicators, consider a small corporate network of a department (enterprise), consisting of 20–25 standard devices.

Let us assume that the department under study is engaged in design work, using the appropriate software(BY). A typical PC-based LAN of such a department should contain workstations, a server, and a printer. All devices are connected to the network through a switch (see Fig. 1).

– – –

A typical LAN based on a thin client includes terminal stations, a server, a printer, and a terminal server, which provides users with access through the thin client to the resources necessary for work. All devices are connected to the network through a switch (Fig. 2).

Rice. 2. Connection diagram of devices in a LAN based on terminal stations

Let us formulate the failure criteria. To do this, it is necessary to determine which element failures are critical for the performance of specified network functions. Let there be 20 jobs allocated to a department (enterprise), and the workload of the department allows you to leave two jobs in reserve.

The remaining 18 workstations are used continuously throughout the working day (8 hours a day).

Based on this, the failure of more than two PCs (terminal stations) will lead to the failure of the entire LAN. A server failure, a failure of one of the terminal servers (for a thin client-only LAN), and a switch failure also result in a failure of the entire LAN. Printer failure is not critical, since the department's tasks are not directly related to its continuous use and therefore it is not taken into account when assessing reliability. The failure of the switching wire network is also not taken into account, since in both LAN implementation options the set of connections is almost the same, and the failure rate is negligible.

Failures of such PC elements as external storage device, monitor, keyboard, mouse, video card, motherboard, processor, cooling system, power supply, random access memory are critical for the PC and lead to its failure.

Taking into account the operating conditions of the LAN and failure criteria, we will construct reliability block diagrams (RSD) for different levels unbundling.

At the top level, a set of devices is considered, the SSN of which is a group “ serial connection» three blocks (switch, server, switching network) and a redundant group ( working group from terminals or workstations).

Structural diagrams of reliability are shown in Fig. 3 (for PC-based LAN) and in Fig. 4 (for LAN based on a thin client).

– – –

At the next level of disaggregation, a set of work/terminal stations is considered, the SSN of which is a group of “sliding redundancy n of m” of twenty blocks (18 main work/terminal stations are backed up by two stations, each of which can replace any failed main one).

At the lower level, a set of elements of a workstation is considered, the SCH of which is a group of “serial connection” of ten blocks (monitor, processor, RAM, hard drive, keyboard, mouse, power supply, motherboard, cooling system, video card).

Calculation of LAN reliability is carried out in two stages:

– firstly, the reliability of the elements separately is calculated (determined),

– secondly, the reliability of the LAN as a whole is calculated.

A typical diagram for calculating the reliability of a LAN, performed in IDEF0 notations, is presented in Fig. 5.

– – –

In Fig. Figure 6 shows a histogram constructed according to the data in Table. 1, which shows the distribution of average time between failures of the RS elements and the switch.

MTBF, thousand hours

– – –

In Fig. Figure 7 shows a histogram of the distribution of average time between failures components LAN.

MTBF, thousand hours Fig. 7. Histogram of distribution of average time between failures of LAN components Technological basis for increasing the reliability and quality of products

– – –

From the table 3 shows that the availability factor for a PC-based LAN is less than that of a similar thin client-based LAN. The mean time between failures for a thin client-based LAN is greater than that of a PC-based LAN, and the mean time to recovery is lower. The above comparison shows that the implementation of a LAN based on 20 terminal stations, two of which are in reserve, turns out to be more reliable than its implementation based on workstations.

Summarizing the results of the analysis, it can be argued that a more reliable type is a LAN based on terminal stations. From a practical point of view, this shows that the transition to creating a LAN based on a thin client is also advisable from a reliability standpoint.

The introduction of LANs consisting of terminal stations in combination with cloud software can significantly affect the increase in the level of automation, quality and reliability of enterprise operations.

References

1. GOST 27.009-89. Reliability in technology. Basic concepts. Terms and definitions. – M.: Publishing house of standards, 1990. – 37 p.

2. GOST R51901.14-2005 (IEC 61078:1991). Reliability block diagram method. – M.: Standartinform, 2005. – 38 p.

3. OST 4G 0.012.242-84. Methodology for calculating reliability indicators. – M., 1985. – 49 p.

5. Forecasting the quality of EMU during design: textbook. allowance / V.V. Zhadnov, S.N. Polessky, S.E. Yakubov, E.M. Gamilova. – M.: SINC, 2009. – 191 p.

6. Zhadnov, V.V. Assessing the quality of computer hardware components. / V.V. Zhadnov, S.N. Polessky, S.E. Yakubov // Reliability. – 2008. – No. 3. – P. 26–35.

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Introduction

local area network

Today there are more than 130 million computers in the world, and more than 80% of them are integrated into various information and computer networks, from small local networks in offices to global networks such as the Internet.

Network operating experience shows that about 80% of all information sent over the network is confined to one office. Therefore, so-called local area networks began to attract special attention from developers.

A local network is a collection of computers, peripheral devices (printers, etc.) and switching devices connected by cables.

Local area networks differ from other networks in that they are usually limited to a moderate geographic area (one room, one building, one neighborhood).

A lot depends on the quality and thoughtfulness of the execution of the initial stage of LAN implementation - on a pre-project examination of the document flow system of the enterprise or organization where it is planned to install a computer network. This is where such important network indicators as its reliability, range of functionality, service life, continuous uptime, service technology, operating and maximum network load, network security and other characteristics are laid down.

The worldwide trend towards connecting computers into networks is due to a number of reasons: important reasons, such as transmission acceleration information messages, the ability to quickly exchange information between users, receive and send messages without leaving the workplace, the ability to instantly receive any information from anywhere in the world, as well as exchange information between computers from different manufacturers running different software.

Such huge potential opportunities that a computer network carries, and the new potential rise that the information complex experiences at the same time, as well as the significant acceleration of the production process, do not give us the right not to accept this for development and not to apply it in practice.

1. Purpose of the work.

The goal of the work is to gain skills in developing the structure of local computer networks, calculating the main indicators that determine the operation of the network.

2. Theoretical part

2.1.The main goals of creating a local area network (LAN).

The constant need to optimize the distribution of resources (primarily information) periodically confronts us with the need to develop a fundamental solution to the issue of organizing an information and computer network on the basis of an existing computer park and software package, meeting modern scientific and technical requirements, taking into account growing needs and the possibility of further gradual development of the network in connection with the emergence of new technical and software solutions.

We can briefly highlight the main advantages of using a LAN:

Resource Sharing

Resource sharing allows for economical use of resources,

for example, control peripherals such as laser printers from all connected workstations.

Data separation.

Data sharing provides the ability to access and manage databases from peripheral workstations that require information.

Software separation

Software separation makes it possible simultaneous use centralized, previously installed software.

Processor resource sharing

By sharing processor resources, it is possible to use computing power to process data by other systems on the network.

Basicsclear definitions and terminology

A local area network (LAN) is a high-speed communication line between data processing hardware in a limited area. A LAN can combine personal computers, terminals, minicomputers and mainframes computers, printing devices, speech processing systems and other devices -

Network devices (ND) are specialized devices designed to collect, process, convert and store information received from others network devices, workstations, servers, etc.

The main component of a local area network is a local area network workstation (LANW), i.e., a computer whose hardware capabilities allow it to exchange information with other computers.

A local area network is a complex technical system, which is a combination of hardware and software, since simply connecting devices, however, does not mean the possibility of their working together. For effective communication various systems Requires appropriate software. One of the main functions operational support LAN is about maintaining such communications.

The rules of the system - how the system polls and must be polled - are called protocols.

Systems are called similar if they use the same protocols. When using different protocols, they can also work in communication with each other using software that performs mutual protocol conversion; LANs can be used to communicate not only with PCs. They can link video systems, systems telephone communication, production equipment and almost anything that requires high-speed data exchange. Several local area networks can be connected through local and remote connections in the internetworking mode.

Personal computers are networked mainly for sharing programs and data files, message transmission (mode email) and for sharing resources (printing devices, modems, and internetworking hardware and software). In this case personal computers are called local area network workstations.

Modern local area network technology makes it possible to use various types cables on the same network, and also seamlessly connect different LAN equipment, such as Ethernet, Archnet, and Token-ring, into one network.

Forcottages solved when creating a LAN

When creating a LAN, the developer faces a problem: with known data on the purpose, list of LAN functions and the basic requirements for a set of hardware and software LAN tools, build a network, that is, solve the following problems:

determine the LAN architecture: select the types of LAN components;

evaluate LAN performance indicators;

determine the cost of the LAN.

In this case, the rules for connecting LAN components based on network standardization and their limitations specified by the manufacturers of LAN components must be taken into account.

The LAN configuration for an automated control system significantly depends on the characteristics of a particular application area. These features come down to the types of information transmitted (data, speech, graphics), the spatial location of subscriber systems, the intensity of information flows, acceptable delays of information during transmission between sources and recipients, the volume of data processing in sources and consumers, characteristics subscriber stations, external climatic, electromagnetic factors, ergonomic requirements, reliability requirements, LAN cost, etc.

Defining the Network Topology

Let's consider topology options and the composition of local area network components.

The topology of a network is determined by the way its nodes are connected by communication channels. In practice, 4 basic topologies are used:

star-shaped (Fig. 1, a, 1, b);

ring (Fig. 2);

tire (Fig. 3);

tree-like or hierarchical (Fig. 4).

AK - active concentrator PC - passive concentrator Fig. 4. Hierarchical network with hubs.

The selected network topology must correspond to the geographic location of the LAN network, the requirements established for the network characteristics listed in Table. 1.

Table 1. Comparative data on LAN characteristics.

Selecting the type of communication media. twisted pair

The cheapest cable connection is a twisted two-wire connection, often called a twisted pair. It allows you to transfer information at speeds of up to 10 Mbit/s, is easily expandable, but is not secure against wireless communications. The cable length cannot exceed 1000 m at a transmission speed of 1 Mbit/s - The advantages are low price and hassle-free installation. To increase the noise immunity of information, shielded twisted pair is often used, i.e., twisted pair placed in a shielding sheath, similar to the screen of a coaxial cable. This increases the cost of twisted pair and brings its price closer to the price of coaxial cable,

Coaxial cable

Coaxial cable has an average price, has good noise immunity and is used for communication over long distances (several kilometers). Information transmission speeds range from 1 to 10 Mbit/s, and in some cases can reach 50 Mbit/s - Coaxial cable is used for basic and broadband information transmission,

Broadband coaxial cable

Broadband coaxial cable is immune to interference and is easy to extend, but its price is high. The information transmission speed is 500 Mbit/s. When transmitting information in the base frequency band over a distance of more than 1.5 km, an amplifier, or a so-called repeater (repeater), is required. Therefore, the total distance when transmitting information increases to 10 km. For computer networks with a bus or tree topology, the coaxial cable must have a terminating resistor (terminator) at the end.

Ethernet cable

Ethemet cable is also a 50 ohm coaxial cable. It is also called thick Ethernet or yellow cable.

Due to its noise immunity, it is an expensive alternative to conventional coaxial cables. The maximum available distance without a repeater does not exceed 500 m, and the total distance Ethernet networks- about 3000 m. The Ethernet cable, due to its backbone topology, uses only one load resistor at the end.

Cheapernet - cable

Cheaper than an Ethernet cable is a Cheapernet cable connection or, as it is often called, thin Ethernet. It is also a 50 ohm coaxial cable with an information transfer rate of ten million bps. When connecting Cheapernet cable segments, repeaters are also required. Computer networks with Cheapernet cable are low cost and minimum costs when building up. Network cards are connected using widely used small-sized bayonet connectors (CP-50). No additional shielding is required. The cable is connected to the PC using T-connectors. The distance between two workstations without repeaters can be a maximum of 300 m, and the total distance for a network on a Cheapernet cable is about 1000 m. The Cheapernet transceiver is located on network card and how to galvanic isolation between adapters, and to amplify an external signal.

Fiber optic lines

The most expensive are optical conductors, also called fiberglass cable. The speed of information dissemination through them reaches several gigabits per second. The permissible distance is more than 50 km. There is virtually no external interference. This is currently the most expensive LAN connection. They are used where electromagnetic interference fields occur or information transmission over very long distances is required without the use of repeaters. They have anti-frizz properties, since the branching technique in fiber optic cables is very complex. Fiber optic conductors are combined into a LAN using a star connection.

Selecting the type of set constructionand by the method of information transmission

Local Token Ring Network

This standard was developed by IBM. Unshielded or shielded twisted pair (UPT or SPT) or optical fiber is used as the transmission medium. Data transfer speed 4 Mbit/s or 16 Mbit/s. The Token Ring method is used as a method for controlling a station's access to the transmission medium. The main provisions of this method:

Devices are connected to the network using a ring topology;

All devices connected to the network can transmit data only after receiving permission to transmit (token);

At any given time, only one station in the network has this right.

You can connect computers in a network using a star or ring topology.

Arcnet local network

Arknet (Attached Resource Computer NETWork) - simple, inexpensive, reliable and quite flexible architecture local network. Developed by Datapoint Corporation in 1977. Subsequently, the license for Arcnet was acquired by SMC (Standard Microsistem Corporation), which became the main developer and manufacturer of equipment for Arcnet networks. The transmission medium used is twisted pair, coaxial cable (RG-62) with a characteristic impedance of 93 Ohms and fiber optic cable. The data transfer rate is 2.5 Mbit/s. When connecting devices, Arcnet uses bus and star topologies. The method of controlling station access to the transmission medium is the Token Bus. This method provides the following rules:

At any given time, only one station in the network has this right;

Basic operating principles

Each byte is transmitted to Arcnet using a special ISU (Information Symbol Unit) package, consisting of three service start/stop bits and eight data bits. At the beginning of each packet, the initial AB separator (Alert Burst), which consists of six service bits, is transmitted. The start delimiter acts as a packet preamble.

There are two topologies that can be used in an Arcnet network: star and bus.

Ethernet LAN

The Ethernet specification was proposed by Xerox Corporation in the late seventies. Later, Digital Equipment Corporation (DEC) and Intel Corporation joined this project. In 1982, the Ethernet specification version 2.0 was published. Based on Ethernet, the IEEE Institute developed the IEEE 802.3 standard. The differences between them are minor.

Basic operating principles:

At the logical level, Ethernet uses a bus topology;

All devices connected to the network have equal rights, i.e., any station can start transmitting at any time (if the transmitting medium is free);

Data transmitted by one station is available to all stations in the network.

Selectop network operating system

The wide variety of types of computers used in computer networks entails a variety of operating systems: for workstations, for department-level network servers and enterprise-level servers as a whole. They may have different performance requirements and functionality, it is desirable that they have a compatibility property that would allow different operating systems to work together. Network operating systems can be divided into two groups: department-scale and enterprise-scale. Departmental or workgroup operating systems provide a range of network services, including file, application and printer sharing. They must also provide fault tolerance properties, for example, work with RAID arrays, support cluster architectures. Departmental network OSes are typically easier to install and manage than enterprise network OSes, have fewer functionalities, provide less data protection, and have weaker interoperability with other types of networks, as well as poorer performance. An enterprise-scale network operating system must first of all have the basic properties of any enterprise products, including:

scalability, that is, the ability to work equally well in a wide range of different quantitative characteristics of the network,

compatibility with other products, that is, the ability to work in a complex heterogeneous Internet environment in a plug-and-play mode.

The corporate network OS must support more complex services. Like a workgroup network OS, an enterprise network OS must allow users to share files, applications, and printers, and do so for more users and data volume and with more high performance. In addition, an enterprise-scale network OS provides the ability to connect disparate systems - both workstations and servers. For example, even if the OS runs on an Intel platform, it must support UNIX workstations running on RISC platforms. Likewise, the server OS running on a RISC computer must support DOS, Windows, and OS/2. An enterprise-scale network OS must support multiple protocol stacks (such as TCNR, IPX/SPX, NetBIOS, DECnet and OSI), providing easy access to remote resources, convenient service management procedures, including agents for network management systems.

An important element of an enterprise-scale network OS is a centralized help desk, which stores data about users and shared network resources. This service, also called a directory service, provides a user with a single logical sign-on to the network and provides a convenient means of viewing all the resources available to him. Administrator, if there is a centralized network help desk, freed from the need to create a repeating list of users on each server, which means freed from a large amount of routine work and potential errors when determining the composition of users and their rights on each server. An important property of the help desk is its scalability, ensured by the distributed database of users and resources.

Network OSes such as Banyan Vines, Novell NetWare 4.x, IBM LAN Server, Sun NFS, Microsoft LAN Manager and Windows NT Server can serve as an enterprise operating system, while NetWare 3.x, Personal Ware, Artisoft LANtastic is more suitable for small workgroups.

The criteria for choosing an enterprise-scale OS are the following characteristics:

Seamless multi-server network support;

High efficiency of file operations;

Possibility of effective integration with other operating systems;

Availability of a centralized, scalable help desk;

Good development prospects;

Effective work of remote users;

A variety of services: file service, print service, data security and fault tolerance, data archiving, messaging service, various databases and others;

Various transport protocols: TCP/IP, IPX/SPX, NetBIOS, AppleTalk;

Supports a variety of end user operating systems: DOS, UNIX, OS/2, Mac;

Support network equipment Ethernet, Token Ring, FDDI, ARCnet standards;

Availability of popular application interfaces and mechanisms for calling remote procedures RPC;

Ability to interact with the network monitoring and management system, support for SNMP network management standards.

Of course, none of the existing network OSs fully meets the listed requirements, so the choice of a network OS is usually made taking into account the production situation and experience. The table shows the main characteristics of popular and currently available network operating systems.

Determining the reliability of LAN operation. 2.4.1. Pindicators of LAN operation reliability

In general, reliability is the ability of a technical device or product to perform its functions within limits. permissible deviations within the space of.

The reliability of a product is laid down at the design stage and significantly depends on such criteria as the choice of technical and technological specifications and the compliance of the adopted design solutions with world standards. The reliability of the LAN is also affected by the literacy of personnel at all levels of network use, the conditions of transportation, storage, installation, setup and testing of each network node, and compliance with the operating rules of the equipment.

When calculating and assessing the reliability of a computer network, the following terms and definitions will be used:

Performance is the state of a product in which it is capable of performing its functions within the established requirements.

Failure is an event in which the performance of a product is disrupted.

Malfunction is a condition of a product in which it does not meet at least one requirement of the technical documentation.

Running time - the duration of operation of a product in hours or other time units.

MTBF, or mean time between failures, is the average operating time of a repaired product between failures.

The probability of failure-free operation is the probability that a product failure will not occur in a given period of time.

Failure rate is the probability of failure of a non-repairable product per unit time after at this moment time.

Reliability is the property of a product to remain operational for some operating time.

Durability is the property of a product to remain operational to its limiting state with interruptions for maintenance and repair.

Resource - operating time of a product to its limit state, specified in the technical documentation.

Service life is the calendar duration of operation of the product to its limit state, specified in the technical documentation.

Maintainability - availability of a product for its maintenance

and repairs.

Reliability is a complex property that includes such properties as:

performance;

preservation;

maintainability;

durability.

The main property described by quantitative characteristics is performance.

Loss of performance - failure. Failures of an electrical product can mean not only electrical or mechanical damage, but also that its parameters go beyond acceptable limits. In this regard, failures can be sudden and gradual.

The occurrence of sudden failures in a device are random events. These failures can be independent, when the failure of one element in the device occurs independently of other elements, and dependent, when the failure of one element is caused by the failure of others. The division of failures into sudden and gradual is conditional, since sudden failures can be caused by the development of gradual failures.

Main quantitative characteristics of reliability (operability):

probability of failure-free operation during time t: P(t);

probability of failure during time t: Q(t)= 1 - P(t);

failure rate X(t) - indicates the average number of failures occurring per unit time of product operation;

average time until failure of a product T (the inverse value of the failure rate).

The actual values of the specified characteristics are obtained from the results of reliability tests. In calculations of time to failure / is considered a random variable, therefore the apparatus of probability theory is used.

Properties (axioms):

Р(0)=1 (operation of functional products is considered);

lim t _ >00 P(t)=O (operability cannot be maintained indefinitely);

dP(t)/dt<0 (в случае если после отказа изделие не восстанавливается).

During the service life of a technical device, three periods can be distinguished, the failure rates in which vary differently. The dependence of the failure rate on time is shown in Fig. 5.

Fig.5. Typical curve of changes in X(t) during the service life (life) of the product.

I - stage of running-in dX(t)/dt<0

II - stage of normal operation X(t)-const

III - aging stage dX(t)/dt>0

In the first period, called the running-in period, structural, technological, installation and other defects are identified, so the failure rate can increase at the beginning of the period, decreasing as it approaches the period of normal operation.

The period of normal operation is characterized by sudden failures of constant intensity, which increases during the wear period.

During the wear period, the failure rate increases over time as the product wears out.

Obviously, the main period should be the period of normal operation, and the other periods are the periods of entry and exit from this period.

Axiom 3 is valid for non-recoverable elements (microcircuits, radioelements, etc.). The process of operating restored systems and products differs from the same process for non-repairable ones in that, along with the flow of failures of product elements, there are stages of repair of failed elements, i.e. there is a flow of element recovery. For restored systems, the third property of reliability characteristics is not satisfied: dP(t)/dt<0. За период времени At могут отказать два элемента системы, а быть восстановленными - три аналогичных элемента, а значит производная dP(t)/dt>0.

When configuring computer networks, they operate with such a concept as the mean time between failures of a particular network element (Tn).

For example, if 100 products were tested during the year and 10 of them failed, then Tn will be equal to 10 years. Those. It is assumed that after 10 years all products will fail.

A quantitative characteristic for the mathematical determination of reliability is the failure rate of a device per unit time, which is usually measured by the number of failures per hour and is denoted by the X symbol.

The mean time between failures and the mean time to restore operability are related to each other through the availability factor Kg, which is expressed in the probability that the computer network will be in an operational state:

Thus, the availability coefficient Kg of the entire network will be determined as the product of the partial availability coefficient Kri. It should be noted that the network is considered reliable when Kg > 0.97.

Example of reliability calculationand local area network

A local area network usually includes a set of user workstations, workstation network administrator (one of the user stations can be used), server core (a set of hardware server platforms with server programs: file server, WWW server, database server, mail server etc.), communications equipment (routers, switches, hubs) and structured cabling system (cable equipment).

Calculation of LAN reliability begins with the formation of the concept of failure of a given network. To do this, we analyze the management functions that are performed at the enterprise using this LAN. Functions that cannot be violated are selected, and the LAN equipment involved in their implementation is determined. For example: during the working day, of course, it should be possible to call/record information from the database, as well as access to the Internet.

For a set of such functions according to the structural electrical diagram LAN equipment is determined, the failure of which directly violates at least one of specified functions, and a logical diagram for calculating reliability is drawn up.

This takes into account the number and working conditions of repair and restoration teams. The following conditions are generally accepted:

Recovery is limited - i.e. At any given time, more than one failed element cannot be restored, because there is one repair team;

the average recovery time of a failed element is set either based on permissible interruptions in the operation of the LAN, or from technical capabilities delivery and inclusion in the work of this element.

Within the framework of the above approach to calculation, the reliability calculation scheme, as a rule, can be reduced to a series-parallel circuit.

Let us set as a criterion for LAN failure the failure of equipment included in the network core: servers, switches or cable equipment. We believe that the failure of user workstations does not lead to a failure of the LAN, and since the simultaneous failure of all workstations is an unlikely event, the network continues to function in the event of individual failures of workstations.

Fig.6. Diagram of LAN elements for calculating total reliability.

Let us assume that the local network under consideration includes two servers (one provides access to the Internet), two switches and five cable fragments related to the core of the network. The failure rates and recovery rates for them are given below.

Thus,

1) the failure rate of the entire network L is 6.5 * 10 - 5 1/h,

2) the average time between failures of the entire TN network is approximately 15.4 thousand hours,

3) the average TV recovery time is 30 hours.

The calculated values of the corresponding readiness are presented in table. 4:

The availability factor of the entire network is

Calculation of LAN operating efficiency

To determine the network functioning parameters, control points are selected and justified. For these selected points, information is collected and parameters are calculated:

request processing time - calculation of the time interval between the formation of a request and receipt of a response to it, performed for selected basic services.

response time in a loaded and unloaded network - calculation of the performance indicator of an unloaded and unloaded network.

frame transmission delay time - calculation of the delay time of link-level frames of selected main network segments.

determination of real throughput - determination of real throughput for routes of selected main network nodes.

analytical calculation of reliability indicators - analytical assessment of the possible failure rate and mean time between failures.

availability factor - analytical calculation of the degree of readiness (average recovery time) of a LAN.

Let us assume that the network between two users is organized according to the scheme presented in Fig. 7.

Work order

To complete the work you need:

a) repeat the safety rules when working with computer equipment;

b) study the lecture materials on the "" courses, as well as the theoretical part of these guidelines;

c) choose a semi-hypothetical enterprise or organization and study it existing system document flow from the point of view of automation. Propose a new document flow system based on the use of computer networks, evaluate the advantages and disadvantages of the existing and proposed systems (speed, cost, topology, changes in the wage fund, etc.);

d) calculate numerical indicators new system document flow: network reliability, MTBF, availability factor, time of message delivery to the addressee, time of receiving a message delivery receipt;

e) in accordance with the requirements given in section 5, prepare a laboratory report;

g) defend laboratory work by demonstrating to the teacher:

1) laboratory report;

2) understanding of the basic principles of organizing a local computer network;

3) theoretical knowledge on the quantitative parameters of the operation of a computer network.

When preparing for a self-test defense, it is recommended to answer test questions, given in section 5.

4. Report requirements

The laboratory report must contain:

a) title page;

b) task condition;

c) justification for the development of a LAN and calculations for the proposed network topology;

d) comments and conclusions on the work done.

References

1. Guseva A.I. Working in local networks NetWare 3.12-4.1: Textbook. - M.: "DIALOG-MEPhI", 1996. - 288 p.

2. Lorin G. Distributed computing systems:. - M.: Radio and communication, 1984. - 296 p.

4. Frolov A.V., Frolov G.V. Local networks of personal computers. Using IPX, SPX, NETBIOS protocols. - M.: "DIALOG-MIFI", 1993. - 160 p.

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6.7 Calculation of network reliability

The designed LAN is mounted on the basis of finished products, and the mean time between failures is taken from the data provided by the equipment manufacturers.

The reliability of an element (system) is understood as its ability to perform specified functions with a given quality for a certain period of time under certain conditions. A change in the state of an element (system), which entails the loss of a specified property, is called a failure. Transmission systems are repairable systems in which failures can be repaired.

One of the central provisions of the reliability theory is that failures are considered in it as random events. The time interval from the moment the element (system) is turned on until its first failure is a random variable called “failure-free operation time”. The cumulative distribution function of this random variable, which is (by definition) the probability that the failure-free operation time will be less than t, is denoted q(t) and has the meaning of the probability of failure in the interval 0...t. The probability of the opposite event - failure-free operation during this interval - is equal to

р(t) = 1 - q(t), % (3)

A measure of the reliability of elements and systems is the failure rate l(t), which is the conditional probability density of failure at moment t, provided that there were no failures before that moment. There is a relationship between the functions l(t) and р(t)

During normal operation (after running-in, but before physical wear occurs), the failure rate is approximately constant. In this case

Thus, a constant failure rate characteristic of a period of normal operation corresponds to an exponential decrease in the probability of failure-free operation over time.

Consequently, the average time between failures during normal operation is inversely proportional to the failure rate

Let us evaluate the reliability of our system, consisting of many different types of elements. Let p1(t), p2(t),…, pr(t) be the probabilities of failure-free operation of each element on the time interval 0...t, r be the number of elements in the system. If failures of individual elements occur independently, and the failure of at least one element leads to failure of the entire system (this type of connection of elements in reliability theory is called sequential), then the probability of failure-free operation of the system as a whole is equal to the product of the probabilities of failure-free operation of its individual elements

where is the system failure rate, h-1;

Failure rate of the i-th element, h-1.

The average time of failure-free operation of the system tcr.syst., h, is found by the formula

The main characteristics of the reliability of restored elements and systems include the availability factor

where tav is the average recovery time of the element (system).

It corresponds to the probability that an element (system) will be operational at any given time.

The methodology for calculating the main characteristics of LAN reliability is as follows: calculating the failure rate and the average time between path failures.

In accordance with the expression, the LAN failure rate, h-1, is determined as the sum of the failure rates of network nodes (VPN router, three servers, 10 workstations) and cable

where are the failure rates of a PC, router, server, one meter of cable, respectively, h-1;

Number of PCs, routers, servers

L - cable length, km.

We determine values for individual devices using reference books and operating conditions.

As a result we get:

4,77*10-5*10+5,26*10-5*1+4,02*10-5*3+4,28*10-7*0,1=2,69*10-4 (11)

Let's calculate the average uptime of a LAN using the formula

The probability of failure-free operation of a LAN during a given period of time t1 = 24 hours (day), t2 = 720 hours (month) at 2.69 * 10-4 h-1 is found by the formula:

At t = 24 hours (days)

At t =720 h (month)

Calculation of useful network bandwidth

A distinction must be made between useful and total throughput. Under useful throughput refers to the speed of information transmission, the volume of which is always slightly less than the transmitted information, since each transmitted frame contains official information, guaranteeing its correct delivery to the addressee.

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Reliability and safety

One of the initial goals of creating distributed systems, which include computer networks, was to achieve greater reliability compared to individual computers.

It is important to distinguish several aspects of reliability. For technical devices, reliability indicators such as mean time between failures, probability of failure, and failure rate are used. However, these indicators are suitable for assessing the reliability of simple elements and devices that can only be in two states - operational or inoperative. Complex systems consisting of many elements, in addition to states of operability and inoperability, may also have other intermediate states that these characteristics do not take into account. In this regard, a different set of characteristics is used to assess the reliability of complex systems.

Availability or availability refers to the percentage of time that a system can be used. Availability can be improved by introducing redundancy into the system structure: key elements of the system must exist in several copies so that if one of them fails, the others will ensure the functioning of the system.

For a system to be considered highly reliable, it must at least have high availability, but this is not enough. It is necessary to ensure the safety of data and protect it from distortion. In addition, data consistency (consistency) must be maintained, for example, if several copies of data are stored on several file servers to increase reliability, then their identity must be constantly ensured.

Since the network operates on the basis of a mechanism for transmitting packets between end nodes, one of the characteristic characteristics of reliability is the probability of delivering a packet to the destination node without distortion. Along with this characteristic, other indicators can be used: the probability of packet loss (for any reason - due to a router buffer overflow, due to a mismatch checksum, due to the lack of a workable path to the destination node, etc.), the likelihood of corruption separate bit transmitted data, the ratio of lost packets to delivered ones.

Another aspect of overall reliability is safety(security), that is, the system’s ability to protect data from unauthorized access. This is much more difficult to do in a distributed system than in a centralized one. In networks, messages are transmitted over communication lines, often passing through publicly accessible premises in which means of listening to the lines may be installed. Another vulnerability may be personal computers left unattended. In addition, there is always the potential threat of hacking network security from unauthorized users if the network has access to global networks general use.

Another reliability characteristic is fault tolerance. In networks, fault tolerance refers to the ability of a system to hide the failure of its individual elements from the user. For example, if copies of a database table are stored simultaneously on several file servers, then users may simply not notice the failure of one of them. In a fault-tolerant system, the failure of one of its elements leads to a slight decrease in the quality of its operation (degradation), and not to a complete stop. So, if one of the file servers fails in the previous example, only the database access time increases due to a decrease in the degree of query parallelization, but in general the system will continue to perform its functions.

How the reliability of a local network is increased. Increasing the reliability of a computer network

“UDC 621.396.6 RELIABILITY OF A LOCAL COMPUTING NETWORK RELIABILITY OF A LOCAL COMPUTING NETWORK BASED ON A THIN CLIENT AND WORKERS...”

RELIABILITY OF LOCAL COMPUTER NETWORK

RELIABILITY OF A LOCAL COMPUTER NETWORK BASED ON THIN CLIENT AND WORKSTATIONS

BASED ON THIN CLIENT AND WORKSTATIONS

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6.7 Calculation of network reliability