EN 61907:2010

SLOVENSKI STANDARD
01-april-2010
1DYRGLOD]DWHKQLþQRQDþUWRYDQMH]DJRWRYOMLYRVWLNRPXQLNDFLMVNHJDRPUHåMD,(&

Guidance on communication network dependability engineering (IEC 61907:2009)
Zuverlässigkeit von Telekommunikationsnetzen (IEC 61907:2009)
Recommandation pour l'ingénierie de la sûreté de fonctionnement des réseaux de
communication (CEI 61907:2009)
Ta slovenski standard je istoveten z: EN 61907:2010
ICS:
21.020 =QDþLOQRVWLLQQDþUWRYDQMH Characteristics and design of
VWURMHYDSDUDWRYRSUHPH machines, apparatus,
equipment
33.040.40 Podatkovna komunikacijska Data communication
omrežja networks
35.110 Omreževanje Networking
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 61907
NORME EUROPÉENNE
February 2010
EUROPÄISCHE NORM
ICS 21.020; 33.040.40
English version
Communication network dependability engineering
(IEC 61907:2009)
Ingénierie de la sûreté de fonctionnement Zuverlässigkeit
des réseaux de communication von Kommunikationsnetzen
(CEI 61907:2009) (IEC 61907:2009)

This European Standard was approved by CENELEC on 2010-02-01. CENELEC members are bound to comply
with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard
the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the Central Secretariat or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and notified
to the Central Secretariat has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus,
the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia,
Spain, Sweden, Switzerland and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: Avenue Marnix 17, B - 1000 Brussels

© 2010 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61907:2010 E
Foreword
The text of document 56/1339/FDIS, future edition 1 of IEC 61907, prepared by IEC TC 56,
Dependability, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as
EN 61907 on 2010-02-01.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN and CENELEC shall not be held responsible for identifying any or all such patent
rights.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2010-11-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2013-02-01
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 61907:2009 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
[1] IEC 60300-1 NOTE  Harmonized as EN 60300-1.
[2] IEC 60300-2 NOTE  Harmonized as EN 60300-2.
[3] IEC 60300-3-1 NOTE  Harmonized as EN 60300-3-1.
[4] IEC 60300-3-3 NOTE  Harmonized as EN 60300-3-3.
[5] IEC 60300-3-11 NOTE  Harmonized as EN 60300-3-11.
[6] IEC 60300-3-12 NOTE  Harmonized as EN 60300-3-12.
[7] IEC 60812 NOTE  Harmonized as EN 60812.
[8] IEC 61025 NOTE  Harmonized as EN 61025.
[9] IEC 61078 NOTE  Harmonized as EN 61078.
[10] IEC 61165 NOTE  Harmonized as EN 61165.
__________
- 3 - EN 61907:2010
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.

NOTE  When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD
applies.
Publication Year Title EN/HD Year

IEC 60050-191 - International Electrotechnical Vocabulary - -
(IEV) -
Chapter 191: Dependability and quality of
service
IEC 60300-3-15 - Dependability management - EN 60300-3-15 -
Part 3-15: Application guide - Engineering of
system dependability
IEC 61907 ®
Edition 1.0 2009-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Communication network dependability engineering

Ingénierie de la sûreté de fonctionnement des réseaux de communication

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
XB
CODE PRIX
ICS 21.020; 33.040.40 ISBN 2-8318-1072-8
– 2 – 61907 © IEC:2009
CONTENTS
FOREWORD.4
INTRODUCTION.6
1 Scope.7
2 Normative references .7
3 Terms, definitions and abbreviations .7
3.1 Terms and definitions .7
3.2 Abbreviations .10
4 Overview of communication network dependability .10
4.1 Network dependability framework .10
4.2 Network life cycle and evolution process .11
5 Network dependability implementation.12
5.1 Dependability engineering applications.12
5.2 Network technology consideration .21
5.3 Network service functions consideration.22
5.4 Network performance consideration.23
5.5 Integrity of network data and information .26
5.6 Quality of service (QoS) .27
6 Network dependability assessment and measurement .30
6.1 Network dependability analysis .30
6.2 Network dependability fault insertion test .31
6.3 Measurement of network dependability attributes .32
6.4 Network dependability assessment methods .32
7 Quality of service measurement.34
7.1 QoS measurement overview.34
7.2 Generic user-oriented QoS parameters and requirements .35
Annex A (informative) Generic communication network model and related concepts .37
Annex B (informative) Network life cycle and evolution process .46
Annex C (informative) Criteria for establishing network security services .56
Bibliography.60

Figure A.1 – A generic communication network model .37
Figure A.2 – A process model on interactions of areas of influence.38
Figure A.3 – OSI reference model.39
Figure A.4 – QoS, dependability and network performance relationships .41
Figure B.1 – Network evolution process incorporated in the network life cycle stages .46

Table 1 – A matrix for capturing user’s QoS data .36
Table A.1 – Examples of dependability influencing factors affecting network
technology .42
Table A.2 – Examples of dependability influencing factors affecting network service
functions.
Table A.3 – Examples of dependability influencing factors affecting delivery of network
performance .44
Table A.4 – Examples of dependability influencing factors affecting integrity of
network data and information and provision of QoS .45

61907 © IEC:2009 – 3 –
Table C.1 – Four basic types of vulnerability causes.57

– 4 – 61907 © IEC:2009
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
COMMUNICATION NETWORK DEPENDABILITY ENGINEERING

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61907 has been prepared by IEC technical committee 56:
Dependability.
The text of this standard is based on the following documents:
FDIS Report on voting
56/1339/FDIS 56/1350/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

61907 © IEC:2009 – 5 –
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – 61907 © IEC:2009
INTRODUCTION
Communication networks are today growing in complexity to meet diverse market demands
and public communication needs; networks such as mobile phones, e-commerce, intranet and
Internet services.
At the same time, communication technologies are developing rapidly to provide efficient
network services and dependable performance needed in worldwide communications. The
essential communication services such as information exchange, data processing and
network connections enable public and private communications work to be carried out cost-
effectively. Business and private sectors greatly depend on these communication services
that have become pivotal in their daily routines. A key factor in ensuring network performance
and network service functions is dependability.
Network dependability is the ability of a network to perform as and when required and to meet
users’ communication needs for continuous network performance and service operation. From
a user’s perspective, dependability infers that the provision of network service functions is
trustworthy and capable of performing the desirable service upon demand. Network
dependability is characterized by its performance attributes including availability of network
performance and quality of service.
The network concept is an extension of the systems concept, addressing a common
framework for the interaction of network elements and interoperability of service functions that
together achieve specific communication objectives.
The network requires specific performance characteristics in order to deliver both its service
functions and communication services. Network dependability engineering is a specific risk-
based technical discipline intended to deal with the diverse applications and deployment of
essential communication services. Unlike the system life cycle where system retirement exists,
a network seldom reaches retirement. A network evolves with time to accommodate
innovative feature applications and provision of continual communication service needs. The
network life cycle is evolutionary and has to address technology convergence issues and
renewal processes as well as characterize specific dependability attributes to meet network
performance objectives. The need for network dependability standardization is essential to
achieve cost-effective development and implementation of communication networks.
Communication network dependability provides important performance attributes for network
equipment developers and suppliers, network integrators and providers of network service
functions who are mainly concerned with global competitive environments. The primary
reason is that dependability can seriously impact revenue generation and affect return-on-
investments. Users of network service functions and communication services rely heavily on
network functions and reliable services that guarantee network security and uninterrupted
network connections for voice, video and data transmission.
This International Standard provides a generic framework for communication network
dependability. The communication network includes telecommunications networks, Internet
and intra-networks utilizing information technology. This standard describes the influence of
dependability attributes and their impact on network performance. It provides the criteria and
methodology for network technology designs, security service functions, dependability
assessment and quality of service evaluation. This is to guide engineering and
implementation processes for realization of network dependability performance objectives.
This standard constitutes part of a framework of standards on system aspects of
dependability by extending the system dependability concepts of IEC 60300-3-15 for network
applications, and to support IEC 60300-1 and IEC 60300-2 on dependability management.
The network performance and communication services in this standard are referenced in the
International Telecommunication Union Telecommunication standardization sector (ITU-T)
series of recommendations.
61907 © IEC:2009 – 7 –
COMMUNICATION NETWORK DEPENDABILITY ENGINEERING

1 Scope
This International Standard gives guidance on dependability engineering of communication
networks. It establishes a generic framework for network dependability performance, provides
a process for network dependability implementation, and presents criteria and methodology
for network technology designs, performance evaluation, security consideration and quality of
service measurement to achieve network dependability performance objectives.
This standard is applicable to network equipment developers and suppliers, network
integrators and providers of network service functions for planning, evaluation and
implementation of network dependability.
2 Normative references
The following referenced documents are indispensable for the application of this standard. For
dated references, only the edition cited applies. For undated references, the latest edition of
the referenced document (including any amendments) applies.
IEC 60050-191, International Electrotechnical Vocabulary – Chapter 191: Dependability and
quality of service
IEC 60300-3-15, Dependability management – Part 3-15: Application guide – Engineering of
system dependability
3 Terms, definitions and abbreviations
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050-191 and the
following apply.
3.1.1
communication network
system of communication nodes and links that provides transmission of analog or digital
signals
EXAMPLES Telecommunications networks, Internet, intranet, extranet, Wide Area Networks (WAN), Local Area
Networks (LAN) and computer networking utilizing information technology.
NOTE 1 A network has its boundary. All nodes at the network boundary are called ends. In some applications, the
term “node” is used instead of “end” as a communication access point to the network, as well as for
interconnections between the transmission links.
NOTE 2 A “backbone” communication network consists of core network and high-speed transmission lines
(national or international), connecting between major switching network nodes (interconnection of transmission
lines) at various locations in a country or region.
3.1.2
(network) dependability
ability to perform as and when required to meet specified communication and operational
requirements
– 8 – 61907 © IEC:2009
3.1.3
(network) availability
ability to be in a state to perform as and when required, under given conditions, assuming that
the necessary external resources are provided
NOTE 1 Availability depends on the combined characteristics of the reliability, maintainability, and recoverability
of the item, and generally, on the maintenance support performance.
NOTE 2 Given conditions would include aspects that affect reliability, maintainability and maintenance support
performance.
3.1.4
(network) reliability
ability to perform as required for a given time interval, under given conditions
NOTE 1 Given conditions would include aspects that affect reliability, such as: mode of operation, stress levels,
environmental conditions.
NOTE 2 Reliability may be quantified using appropriate measures such as meantime to failure, or the probability of
no failure within a specified period of time.
3.1.5
(network) maintainability
ability to be retained in, or restored to, a state in which it can perform as required under given
conditions of use and maintenance
NOTE 1 ”Given conditions of use” may include storage.
NOTE 2 ”Given conditions of maintenance” include the procedures and resources to be used.
NOTE 3 Maintainability may be quantified using such measures as, mean time to restoration, or the probability of
restoration within a specified period of time.
3.1.6
maintenance support
resources to conduct maintenance according to a given maintenance concept and policy
3.1.7
maintenance support performance
ability of an organization to complete specified network maintenance support upon demand
under given conditions
NOTE The “given conditions” include those related to the maintenance organization, and to the conditions under
which the item is used, maintained, and relevant maintenance policies and procedures.
3.1.8
(network) recoverability
ability to recover from a failure, without corrective maintenance
NOTE 1 The ability to recover may or may not require external actions.
NOTE 2 Recoverability may be quantified using such measures as, mean time to restoration, or the probability of
restoration within a specified period of time.
3.1.9
(network) element
subsystem or component of a communication network
EXAMPLES Terminals, nodes, links and switches.
NOTE 1 A network element may involve human input to perform its service function.
NOTE 2 Network nodes and access points are connected by network links.

61907 © IEC:2009 – 9 –
3.1.10
(network) link
electrical, wireless or optical connection between network nodes
3.1.11
(network) performance
ability to provide the service functions related to communications between users
[ITU Recommendation I.350]
NOTE Network dependability performance refers to the ability of the network to provide or demonstrate the
dependability attributes in network operation to achieve network objectives and quality of service requirements.
3.1.12
(network) management
application of organized processes and resources to manage the performance, configuration,
accounting, fault, and security activities
3.1.13
(network) service function
program or application that interacts with the network users or within the network
infrastructure to transmit or exchange data and information in the network
NOTE A network service function may consist of hardware and software elements, and may involve human
interactions for realizing a specific function.
3.1.14
network services
provision of network service functions and communication services to the network users
NOTE 1 Communication services are the network services subscribed by the end-users.
NOTE 2 A bearer service is a communication service function that allows transmission of user-information signals
between user-network interfaces.
3.1.15
quality of service
collective effect of service performance that determines the degree of satisfaction of a user of
the service
3.1.16
network failure
loss of network ability to perform its function as required
NOTE The failure may be due to, for example, equipment failure, natural disasters or human-caused disturbance.
3.1.17
network fault
state characterized by the inability of the network to perform its function as required
NOTE 1 In the context of network operation, a fault may be natural due to an abnormal condition, or malfunction
resulting in a network element failure, or induced by external means such as fault insertion.
NOTE 2 A degraded state in network performance is a situation where one or more performance characteristics
do not conform to requirements.
3.1.18
service provider
organization that provides communication network services
EXAMPLES Telephone companies, data carriers, mobile services, Internet service providers, and cable television
operators.
– 10 – 61907 © IEC:2009
NOTE Network carrier or common carrier is an organization that transports a product or service using its facilities,
or those of other carriers, and offers services to the general public. The term communication carrier refers to
various telephone companies that provide local, long distance or value added services.
3.1.19
user
party that employs the services of a service provider for direct network access
NOTE 1 A user may be a source or recipient of user information, or both.
NOTE 2 In some circumstances, a user of a communication service is also known as a subscriber.
3.1.20
(network) integrity
ability to ensure that the data throughput contents are not contaminated, corrupted, lost or
altered between transmission and reception
3.2 Abbreviations
E2E End-to-End
FMEA Failure Mode and Effects Analysis
FTA Fault Tree Analysis
IP Internet Protocol
ITU-T International Telecommunication Union Telecommunication standardization sector
LAN Local Area Network
NGN Next Generation Network
OSI Open Systems Interconnection
QoS Quality of Service
RBD Reliability Block Diagram
SLA Service Level Agreement
TCP/IP Transmission Control Protocol/Internet Protocol
WAN Wide Area Network
4 Overview of communication network dependability
4.1 Network dependability framework
A communication network consists of network elements such as switches, transmitters,
computers, telephones and other devices connected by communication facilities for
exchanging information electronically. The connections can be permanent via cable or
temporary by means of telephone or Internet links. The transmission medium can be physical
(e.g. fibre optics) or wireless (e.g. satellite). It covers many technologies such as radio,
television, telephone, data communication and computer networking utilizing information
technology.
61907 © IEC:2009 – 11 –
The nature of network operation is multi-faceted and ever expanding and evolving such as
doing business on-line via the Internet. The infrastructure connecting different types of
communication systems and interacting networks is extremely complex. The integration of
disparate networks and systems demands evolving establishment of interfaces and protocols
for interoperability to attain viable network connectivity and service performance. Enterprise
marketing and e-commerce pressures often dictate development of new techniques that
change the ground rules in business and service applications.
Network related activities are unique in assuring dependable performance and quality of
service. The challenge is to provide network solutions that link network designs and service
function applications to realize the relevant dependability attributes. Network dependability
infers that the network performance is able to maintain information integrity and capable of
delivering the network service functions to satisfy user expectations as well as the service
provider’s needs. The strategic framework is to select and implement appropriate network
elements for the network configuration to achieve dependability and integrity in network
performance.
Annex A presents a generic communication network model and related concepts.
Network dependability and network service functions are influenced by the dependability
attributes relevant to their specific applications in various network life cycle stages. These
dependability attributes include availability, reliability, maintainability, maintenance support
performance, recoverability and integrity performance characteristics, incorporated in the
network designs and implemented in the network service functions for network operation and
maintenance.
The communication network dependability framework consists of six main interacting areas of
influence:
a) the application of dependability engineering;
b) the utilization of network technology;
c) the delivery of network performance;
d) the deployment of network service functions;
e) the integrity of network data and information;
f) the provision of quality of service.
These six interacting areas of influence form the basis for network dependability management.
Implementation of network dependability processes and methods are described in Clause 5.
4.2 Network life cycle and evolution process
A network evolves and changes with time to meet the dynamic network applications and
network service needs in communications. The network evolution process is reflected in the
system life cycle stages described in IEC 60300-3-15 and modified for network life cycle
application to include the renewal process. The following presents the network life cycle
stages:
• concept/definition stage to identify network operating scenario;
• design/development stage to determine technology applications and develop network
service functions;
• realization/integration stage to realize network operation, verification and validation to
assure network performance;
• operation/maintenance stage to sustain network operation and provision of network
services;
• enhancement stage:
– 12 – 61907 © IEC:2009
– to optimize and improve network performance by technology evolution and service
functions convergence;
– to upgrade or renew continuing network services, and launch new features in network
applications;
• retirement/decommissioning stage to discontinue obsolete service functions.
Annex B describes the network life cycle and evolution process.
5 Network dependability implementation
5.1 Dependability engineering applications
5.1.1 Management of network dependability
Dependability is a technical discipline that is managed by engineering principles and practices.
The dependability management systems [1] and guidelines [2] are used in this standard for
formulation of dependability management strategies and general application of technical
approaches for implementation of dependability elements and tasks. Additional management
processes are introduced to address network specific management issues. Dependability
management involves project planning, resource allocation, dependability task assignments,
monitoring and assurance, measurement of results, data analysis and continual improvement.
Dependability activities should be conducted in conjunction with other technical disciplines to
attain the necessary synergistic effects and add values to the project outcomes. Project
tailoring is emphasized for cost-effective management of network projects. Where applicable,
life cycle cost analysis [4] and risk assessment [11] should be used for resource allocation
and optimization for evaluation of acquisition and ownership costs.
From a communication network management perspective, dependability management should
be an integral part of the network management process with relevant dependability activities
to support development and implementation of network service functions throughout the life
cycle and network evolution. Network management is the execution of the set of management
processes for planning, controlling, allocating, deploying, coordinating, maintaining and
monitoring the resources of a network.
From an engineering perspective, a network can be viewed as a complex system consisting of
multiple interconnecting systems. The technical processes in IEC 60300-3-15 are used to
describe the network life cycle stages for project implementation. The network life cycle
stages are modified to accommodate the process descriptions of network evolution and
technology convergence, optimization and renewal, obsolescence and retirement. Network
management process and dependability related activities for each network life cycle stage are
presented in Annex B.
5.1.2 Network dependability attributes
The basic set of network dependability attributes is derived from a network performance
perspective. Other attributes may be added for specific applications.
a) Availability
Network availability reflects the users’ requirements from three aspects:
– the network has the ability to perform as and when required;
– the network service functions should not be affected by network faults;
– the network can recover as quickly as possible when a network fault affects its service
functions.
b) Reliability
—————————
Figures in square brackets refer to the Bibliography.

61907 © IEC:2009 – 13 –
Network reliability can be improved by using redundancy technology and protection
mechanisms. For a network with high reliability requirements, the protection of multipoint
faults should be considered. The manifestation of faults becomes more critical for complex
network operation, especially with the continued usage of obsolete equipment that may
have compatibility problems.
c) Maintainability
Maintainability reflects the ease of restoring a network to an operating state following a
fault, or from a degraded state of network performance operation. The incorporation of
redundancy in design and fault management capability into the network can affect
maintainability performance. For example, remote fault management where fault
identification and corrective action are carried out by remote intervention, can shorten
maintenance time.
d) Maintenance support performance
Maintenance support performance is dependent on the provision and management of
resources to perform maintenance activities. Network dependability design requires an
approach to reduce the complexity of network equipment for maintenance, easy access for
active maintenance during network operation, standardized maintenance procedures,
network fault identification and traceability, and perfected spare parts management system.
This approach applies to hardware and software as well as to human functions for
maintenance support performance and design consideration.
e) Recoverability
Recoverability in service performance is dependent on the design of network architecture,
fault-tolerant protection mechanisms, access for maintenance and self-healing features
incorporated into the network functions. The means of achieving restoration may be
automatic or by external actions.
f) Integrity
Integrity infers that the network is stable and robust, and able to maintain consistency in
performance and use. Network integrity provides security and protection for information
transfer in network performance and service functions.
5.1.3 Network failures and faults
5.1.3.1 Network failure criteria
Network failure criteria should be established during the concept and definition stage and
continually reviewed throughout the lifecycle of the network in order to permit classification
and updating of network failures.
Failure criteria should be determined by the network service provider, based on industry
standards and relevant data, including the network users’ inputs, so that all conceivable
conditions can be covered. This information is used in:
• specification of dependability requirements to the network equipment suppliers;
• reaching agreements (such as SLA) of the network dependability performance attributes
with the users of the network services.
The following should be considered when establishing failure criteria.
a) Network node and link failure: The failure of network nodes and links includes total failure
and partial failure. Total failure is the loss of functionality of the entire network for a period
of time. Partial failure is the loss of functionality of part of the network.
b) Quality of service (QoS) degradation failure: Degradation failure is due to an unacceptable
level of QoS in the provision of network service performance experienced by the users.
Network service providers routinely gather QoS data through measurements and surveys
to determine the level of QoS.

– 14 – 61907 © IEC:2009
5.1.3.2 Network failure classification
Most network failures can be classified as to their probable causes so that failure analysis
results can be categorized for network dependability improvement.
• Facility-related network failure – This is related to failures of individual network elements
whose failure rate data can be used for predicting the corresponding node and link
dependability parameters.
• Traffic-related network failure – This is related to the operation scenarios of network
service traffic, which changes according to the network users’ usage demands. Network
design should consider users’ usage demands and plan network traffic capacity to satisfy
most operation scenarios to avoid network service performance degradation.
• Disaster-related network failure – This is related to failures that are man-made or caused
by natural disaster. Sometimes the network facilities in the disaster area may be totally
destroyed.
• Security-related network failure – This is related to network failures caused by insufficient
security to prevent unauthorized intrusion, such as hacking, cyber attacks or sabotage
activities.
• Scheduled, activity-related network failure – This is related to failures that occur during
scheduled network maintenance activities, such as software upgrade causing the network
to be temporarily out-of-service.
• Human factor-related network failure –This is related to failures due to human error,
misused or negligence, such as network design errors, configuration errors, or mistakes in
management of software versions.
5.1.3.3 Network fault conditions
A network failure represents an event in network operation and maintenance where the
occurrence of an anomaly or malfunction in performance is experienced. A fault condition
related to the failure occurrence describes the fault state or status of the fault condition
observed. In practice, the cause of actual network failures is often unknown at its initial
observation of incident reporting. The incident report captures the observed symptoms of the
fault state or fault conditions on the network’s inability to perform its function. The complexity
of the network operations, the involvements of diverse service functions, and the time
limitation to restore network services often prevent further investigation of the failure event
situation. Priority under such fault conditions is to reroute, reboot and recover the failed
service functions in order to restore normal network services. Network incident reporting
should capture the failure symptom in order to initiate appropriate follow-up action and
determine the cause of failure where practicable. Relevant information, captured by incident
reporting during network operation and maintenance, provides an important source of failure
data for implementing an effective network fault management system.
5.1.3.4 Network fault management system
The network fault management system is a network service support system. It facilitates the
reporting of failure event data and capturing the observed fault conditions to support
management and correction of network faults. The fault management system consists of a set
of supporting functions to detect, isolate and also compensate, where applicable,
environmental changes as well as correct network malfunctions. The system is designed and
deployed to support network service restoration for tracing and identifying faults, performing
diagnostics tests, correcting faults, activating alarms and reporting error conditions,
maintaining error logs and providing error detection notifications (see 5.1.5.3). Network fault
management system is an essential enabling mechanism to support network service functions.

61907 © IEC:2009 – 15 –
5.1.4 Establishment of network dependability requirements
5.1.4.1 Network dependability requirements identification
Information gathering helps identify market needs of the new or upgraded network. The
objective is to determine the commercial and technical requirements for network development
and service enhancement. Information can come from various sources.
a) Information on upgrading an old network – Network design could use the existing network
as a basis for upgrade or enhancement, such as expansion of existing service facilities
and capability to meet growing market needs. This would avoid extensive resource
investments on building a completely new network. The enhancement incorporates new
functional features and provides additional services and capacities to augment and
complement the old network service functions.
b) Service functions and performance information – These are the network service functions
required by the network service provider; they include existing applications (e.g. telephone
service) and new applications (e.g. internet service), as well as requirements for network
performance, capacity and other network service features.
c) Customized information – This type of information covers, for example, the requirements
of end-users for customized services provided by the communication network. For
example, users may ask for a videophone service.
d) User suggestions and complaints – Useful suggestions and re
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