Lecture#16

Estimation of the system’s dependability

The Bonch-Bruevich Saint-Petersburg State University of Telecommunications

Series of lectures “Telecommunication networks”

Instructor: Prof. Nikolay Sokolov, e-mail: sokolov@niits.ru

IntroductionStrictly speaking, dependability should be considered as one of quality aspects. Nevertheless, some specialists consider dependability as an independent term that has the same status as the quality. The dependability is the property of an object to retain, in a course of time, within specified limits values of all parameters, which characterize capability to perform required functions in predetermined for that object regimes and conditions of application, technical maintenance, repairs, storage and transportation. It is obvious, that there is no sense in speaking about object’s dependability during the time periods, when it is withdrawn from operation for execution of scheduled inspections, modernization and other procedures.

International Electrotechnical Commission: http://www.iec.ch (1)

Founded in 1906 with British scientist Lord Kelvin as its first president, the IEC has a long history of service to the market.

MissionThe International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes international standards for all electrical, electronic and related technologies. These serve as a basis for national standardization and as references when drafting international tenders and contracts.Through its members, the IEC promotes international cooperation on all questions of electrotechnical standardization and related matters, such as the assessment of conformity to standards, in the fields of electricity, electronics and related technologies.The IEC charter embraces all electrotechnologies including electronics, magnetics and electromagnetics, electroacoustics, multimedia, telecommunication, and energy production and distribution, as well as associated general disciplines such as terminology and symbols, electromagnetic compatibility, measurement and performance, dependability, design and development, safety and the environment.

International Electrotechnical Commission: http://www.iec.ch (2)

Objectives

The Commission's objectives are to:•meet the requirements of the global market efficiently •ensure primacy and maximum world-wide use of its standards and conformity assessment systems •assess and improve the quality of products and services covered by its standards •establish the conditions for the interoperability of complex systems •increase the efficiency of industrial processes •contribute to the improvement of human health and safety •contribute to the protection of the environment.

International Electrotechnical Commission: http://www.iec.ch (3)

TC 56 (Technical Committee) develops and maintains International Standards in the field of dependability.

Dependability is the ability of a product to do its job as and when required without encountering problems. Being dependable means the product has earned the user's trust and can be relied on to perform its intended task. Product dependability does not happen by itself but needs to be designed and built into the product. Properties of dependability include: •Reliability – how long the product can do its job without failing. •Maintainability – how easy it is to keep the product in good running condition. •Maintenance Support – the ability to have the product fixed when failure occurs at acceptable expense and timelines. These are factors of dependability that influence product performance. They are sometimes called dependability performance characteristics.

IEC, TC 56 (1)

IEC, TC 56 (2)

IEC, TC 56 (3)

Dependability vs cost

Dependability and type of service

Statistics of the dependability

Events and states chart

State of dependability

State of operability

State of nonoperability

Marginal state

Retirement

1

2

2

3

3

54

1. damage

2. failure

3. transition to marginal state

4. restoration

5. repair

Dependability tree

Functioning process of the object

t0X1 X2 X3

Y1 Y2

In the tenth lecture, considerations about quality of service presented in ITU-T Recommendation E.800 were stated. One of the important dependability indices in this recommendation is availability. Availability ratio A is evaluated by ratio of the time when device was in workable state, to the total time of observation.

Calculation of the A value (1)

{ }

{ } { }

jJ

j jJ J

YA

X Y

.

Downtime ratio is evaluated by ratio of the time, when device was in nonoperability state, to the total time of observation:

{ }

{ } { }

jJ

j jJ J

XA

X Y

.

It is obvious, that 1A A . It should be taken into consideration, that the time of scheduled (artificial) stops of the object is not included in any of the summands. Failures can be of different nature. Also degree of their influence on the functioning processes of infocommunication systems significantly differs. Classification of failures is shown on the next figure. Sense of each type of failures is defined by specific properties of the object.

Calculation of the A value (2)

Mean time between failures (MTBF) is the mean (average) time between failures of a system, and is often considered the "useful life" of the device i.e. not including 'infant mortality' or 'end of life' if the device is not repairable. Calculations of MTBF assume that a system is "renewed", i.e. fixed, after each failure, and then returned to service immediately after failure. The average time between failing and being returned to service is termed mean down time (MDT) or mean time to repair (MTTR).

MTBFA

MTBF MTTR

Classification of failures (1)

Classification of failures (2)

Intensity of failures

t

λ(t)

Sudden failures (infant

mortality region)

GradualFailures

(wear-out region)

t1 t2

λ(t) ≈ const

(steady-state region)

Reservation of infocommunication system on the level of access network

Wireless access

PC

Wireline access

Base station

Core Network

Calculation of the A2 valueLet’s assume, that availability ratio for utilized facilities of wire access equals to 1A . Let’s suppose, that required availability ratio between terminal and core network is set at the level 0A . Then it is necessary to specify availability ratio of wireless access facilities 2A .

It is obvious, that communications between terminal and core network will be absent in the case when both means of access have simultaneous failure. Probability of such event U is evaluated by obvious expression:

1 2(1 )(1 )U A A .

Consequently:

0 1 2 1 2 1 21 (1 )(1 )A A A A A A A .

From this expression it is easy to calculate desired value 2A guaranteeing observance of specified level for dependability of access networks:

0 12

11

A AA

A

.

Example of reserving for two sectors in access network

Wireless Access

Sector “B”Sector “A”

Node 1

Node 2

Node 5

Node 3

Node 4

Node 7

Node 6

Core Network

Switch

Forms of the maintenance

Dependability and network planning (1)

Dependability and network planning (2)

Examples of dependability analysis

A=p+p2–p3

A=?

If p=0.999, then A=0.999998001

A=2p5–5p4+2p3+2p2

A=?

If p=0.999, then A=0.999997998

If p=0.9, then A=0.981

If p=0.9, then A=0.978

Instructor: Prof. Nikolay Sokolov, e-mail: sokolov@niits.ru

Questions?

Estimation of the system’s dependability