Int. J. Communications, Network and System Sciences, 2011, 4, 720-726
doi:10.4236/ijcns.2011.411088 Published Online November 2011 (http://www.SciRP.org/journal/ijcns)
Analysis of Computer Network Reliability and Criticality:
Technique and Features
Iraj Elyasi-Komari1*, Anatoliy Gorbenko2, Vyacheclav Kharchenko2, Athanasios Mamalis3
1Technical & Engineering Faculty, Shoushtar Branch, Islamic Azad University, Khuzestan, Iran
2Aerospace University, Kharkov Aviation Institute, Kharkov, Ukraine
3National Technical University of Athens, Athens, Greece
Received July 26, 2011; revised August 19, 2011; accepted September 1, 2011
Abstract
The paper describes modern technologies of Computer Network Reliability. Software tool is developed to
estimate of the CCN critical failure probability (construction of a criticality matrix) by results of the
FME(C)A-technique. The internal information factors, such as collisions and congestion of switchboards,
routers and servers, influence on a network reliability and safety (besides of hardware and software
reliability and external extreme factors). The means and features of Failures Modes and Effects (Critical)
Analysis (FME(C)A) for reliability and criticality analysis of corporate computer networks (CCN) are
considered. The examples of FME(C)A-Technique for structured cable system (SCS) is given. We also
discuss measures that can be used for criticality analysis and possible means of criticality reduction. Finally,
we describe a technique and basic principles of dependable development and deployment of computer
networks that are based on results of FMECA analysis and procedures of optimization choice of means for
fault-tolerance ensuring.
Keywords: FME(C)A (Failure Modes and Effects (Criticality) Analysis), Computer Network Reliability,
Criticality, Corporate Computer Networks
1. Introduction
Lots of formalized dependability assessment techniques
based on failure criticality analysis (FME(C)A), con-
struction of the event and fault tree (FTA), emergency
situation analysis (HAZOP) [1,2], etc. has been devel-
oped during the last decade. The International Standard
[3] describes Failure Mode, Effects and Criticality Analy-
sis (FMECA), and gives guidance as to how they may be
applied to achieve various objectives by
 providing the procedural steps necessary to perform
an analysis;
 identifying appropriate terms, assumptions, criticality
measures, failure modes;
 defining basic principles;
 providing examples of the necessary worksheets and
other tabular forms.
FME(C)A is a methodology to identify and analyze
potential failure modes of the various parts of a system
and the effects these failures may have on the system.
The purpose of FME(C)A-technique is specification of
modes, sources and critical failure effects, including mul-
tiple and dependent failures, assessment of methods and
different means CCN fault-tolerance and safety ensuring.
It includes four main steps.
1) Analysis of a system structure and possible failures
of different systems.
2) Analysis of the failures modes and effects. As a re-
sult, the FMEA-table should be built.
3) Qualitative analysis of the failures criticality on the
base of their probability of occurrence and severity. As a
result, the criticality matrix should be built.
4) Identification of the most critical failures as those
that lie above the established criticality diagonal.
FME(C)A is used to identify, prioritize, and eliminate
potential failures from the system, design or process be-
fore they reach the customer FME(C)A is a technique to
“resolve potential problems in a system before they
occur”. However, this technique has to be adopted for the
system features.
The safety and fault-tolerance ensuring of CCN for
critical application (CA) (NPP I & C Systems, Airspace

I. ELYASI-KOMARI ET AL.721
Control Systems, Banking System, etc.) is an actual and
important problem. The use of FME(C)A-technique [3],
allows to identify the critical failures and failure effects
for CCNCA and other kinds of CCNs, to detect the
safety threats, to determine necessity of the redundancy
introduction and other means for enhancement a prob-
ability of accident-free failure effects.
The purpose of this paper is an analysis of features
of FME(C)A-technique application for corporate com-
puter networks that are the core of distributed informa-
tion and control systems (I&CS). The safety and fault-
tolerance ensuring of CCN for critical application (CA)
(NPP I&C Systems, Airspace Control Systems, Banking
System, etc.) is an actual and important problem. The use
of FME(C)A-technique [3], allows to identify the critical
failures and failure effects for CCNCA and other kinds
of CCNs, to detect the safety threats, to determine neces-
sity of the redundancy introduction and other means for
enhancement a probability of accident-free failure ef-
fects.
It is confirmed in publications that show method’s ap-
propriateness for security assessment using so-called
F(I)MEA (Failure (and Intrusion) Modes and Effects
Analysis)-technique and failure effects analysis from
recovery time view [4,5].
2. Features of FME(С)A-Technique
Application for CCN Dependability
Analysis
Application of methods of the analysis of a Mode and
consequences of failures FMEA, and also the analysis of
a Mode and Effects of critical failures—FME(C)A for
quality standard of reliability of complexes of critical
application allows to identify refusals and their Effects,
to determine necessity of introduction of reservation of
elements of system and the measures raising probability
of trouble-free operation [6,7].
The tasks of the reliability ensuring of computer net-
work based on the open standards and models (for ex-
ample, OSI or TCP/IP models) and used for critical ap-
plications according to COTS approach [8] are decided
at various layers of these models. The distinctive net-
work feature is that network failures are stipulated by
four basic causes:
 defects of the network hardware and software de-
signing and production;
 aging of the network physical components;
 objective and subjective external extreme factors
(EEF) such as seismic loads, electromagnetic distur-
bance (ED), human errors, hacking etc.;
 internal information factors which consist in periodic
increase of network traffic and, as a result, in conges-
tion of switchboards, routers and servers.
The network basic functional elements which may be
analyzed by using FME(C)A-technique are SCS, passive
and active telecommunication devices, such as hubs,
switchboards and routers, servers and workstations etc.
working at various layers of the OSI or TCP/IP models
and fallible in consequence of four causes mentioned
above. However, application of FME(C)A-technique for
evaluation of reliability and fault tolerance through traf-
fic overloads, unauthorized operations or human errors
requires a separate discussion and are not considered in
the given paper. Objects of FME(C)A are, as usual,
I&CS components—hardware and software components.
There is a modification of FME(C)A-method for soft-
ware—SFME(C)A [9]. In [10] it is proposed to apply
FME(C)A to hierarchical structures and correspond them
to hierarchy of FME(C)A-tables.
3. Results of Application
FME(C)A-Technique for CCN Reliability
Analysis
The classification of failure modes, causes, effects and
means of safety and fault-tolerance ensuring for the
network functional elements is obtained by using the
FME(C)A-format. The various means of safety and fault-
tolerance ensuring of the network hardware and software
are indicated in the last table column. The probability
and the severity for each failure mode of specified com-
puter network are determined on the basis of statistical
information or expert estimations. It allows to construct a
criticality grid, and with its help to execute a qualitative
analysis of CCN reliability, to determine a set of the
most critical failures and means for their recovery.
The using of FME(C)A-technique is shown on an ex-
ample of analysis of the National Airspace University
computer network. Figure 1 shows the university struc-
tured cabling system (SCS) [11], also ,for example ana-
lysis of the FME(C)A-table for , backbone subsystem for
which the FME(C)A-table was obtained (Table 1) and
the criticality matrix was constructed (Table 2).
Figure 2 shows an hierarchical approach to the
FME(C)A analysis of the computer network of the Na-
tional Airspace University “Kh.A.I.”.
4. Failures Criticality Analysis
The second step of FME(C)A technique is a criticality
analysis of all failure modes. It performs with the pur-
pose to explain the most serious failures and determine
ways in which criticality of this failures can be reduced
(Figure 3).
There are two common measures that are used for

I. ELYASI-KOMARI ET AL. 722
cd bd-3
bd-2
bd-4
bd-5
B1
B3
B2
B5
B4
B1-Rectorial building
B2-Aircraft building
B3-Engine building
B4-Laboratory building
B5-Radio engineering building
B6-Impulse building
cd
bd
-Campus Distributor
-Building Distributor
-Campus Backbone Cable
cbc-1
cbc-2
cbc-4
cbc-3
cbc-5
cbc
National Aerospace
U niv e r s ity
bd-1
Figure 1. University SCS backbone subsystem.
Table 1. Fragment of common FME(C)A—table of university SCS backbone subsystem.
SCS
element Failure Mode Failure Cause Failure Effect Failure Recovery Means
Means of fault-tolerance
provision
Probability
of failure
Criticality
level
Patch panel
connector damage
External extreme
factors (EEF);
aging; defect
Communication
disturbance
Connector
replacement
Connector
redundancy High Low
Patch panel
destruction EEF «» «»
Patch-panel
redundancy Lowest Mean
cd,
bc-1..bc-5
Distributor
destruction «» «»
Repair and
recovery
Distributor
redundancy Lowest High
Cable failure
(damage)
EEF; aging;
internal defect «» Cable replacement
Cable redundancy;
link path redundancy Low High
cbc-1..
cbc-5 Message
distortion
electromagnetic
disturbance (ED)
Short-term communication
disturbance;
loss of information
-
Electric cable screening;
maximum utilization
of optical fiber
Low Low
Table 2. Fragment of criticality matrix of university SCS backbone subsystem.
Probability of failure1
Lowest Low Mean High
High Destruction of the distributors
cd, bc-1..bc5
Failure (damage) of the
backbone cables cbc-1…cbc-5
Mean
Low Message distortion
Severity of
consequences2
Lowest Patch panel destruction Patch panel connector
damage
1The probability of failure is determined by the network service conditions; 2The weight of failure consequences is determined by destination and functions of
system elements, “weight” of failure effects and its influence on a system as a whole.
such analysis: 1) weight of failure consequences, and 2)
probability of failure occurrence. The failure criticality
defines by “weight” of failure effects on all system and
depends on function of faulty element. For computer net-
work it can be degree of connectivity decrease. The pro-
bability of failure occurrence is determined by the net-
work service conditions. It can be reduced by using
structured redundancy.
The critical failures are those, which are above the
criticality diagonal (see Figure 3). The criticality diago-
nal itself has to be set taking into account system reli-
ability requirements or system safety level. For example,

I. ELYASI-KOMARI ET AL.723
Figure 2. Mapping of assessed system hierarchy to hierar-
chy of FME(C)A—tables.
Probability of failure occurence
High
Mean
LowLowest
Lowest
High
Mean
Low
Weight of consequences
Failure
Criticality diagonal for systems
with higher reliability requirements
Area of critical failures
Criticality diagonal
Figure 3. Criticality matrix.
there are six different criticality diagonals in total that
can be set in the criticality matrix that is shown on Fig-
ure 3. The higher is the criticality diagonal the more
critical is the system.
In this paper we also propose to use an additional third
measure to assess failure criticality, which describes du-
ration of system nonoperability [12]. It is very important
for the computer and telecommunication systems where
the small amount of incorrect connections (due to incur-
rect routing) is allowed whereas the high availability of
the network is required.
This measure depends on recovery time that can be
reduced by using automated (computer-aided) recovery
means instead of manual operations or automatic (un-
manned) means instead of automated ones (Figure 4).
For the computer networks these means include dynamic
Figure 4. Failure criticality coordinate system.
routing which is more preferable than static one, the
spanning tree protocol against the manual recovery, etc.
5. Means of Failure Criticality Reduction
There are a lot of techniques that can be used for the
failure criticality reduction, like:
 Patch View System that control integrity of cabling
channels and patch-panels at the level of structured
cable system;
 Adapter Fault Tolerance (AFT) technology that pro-
vide hot sparing of network adapters;
 Adaptive Load Balancing (ALB), that allocate net-
work traffic between four server’s network adapters
and four switch ports as well as AFT;
 Fast Ether Channel (FEC) technology supporting
flexible channel capacity as well as AFT;
 Protocol of dynamic network reconfiguration Span-
ning Tree Protocol (STP);
 Protocols of dynamic rooting like OSPF and Cis-
coEIGRP that support load balancing.
Most of means mentioned above use redundancy of
the cabling channels, ports and network equipment.
Some technologies also provide possibility to increase
network throughput by using existing redundant roots
(like trunk technology) and allow automatic network
reconfiguration to isolate failures.
Thus, incorporating of different fault-tolerant mecha-
nisms together will provide possibility of complex and ef-
ficient failure criticality reduction. However, all existing
means have to be ranked taking into account their cost and
effectiveness as well as compatibility with another ones.
6. Dependable Development and Deployment
of Computer Networks
6.1. Using FMEA-Technique for Dependable
Network Development
To develop and deploy dependable computer networks

I. ELYASI-KOMARI ET AL. 724
the common FMEA-table and criticality matrix describe-
ing failures modes and effects have to be detailed taking
into account actual logical and physical architecture of
particular computer network as well as the set of network
hardware, communication protocols and application soft-
ware used (Figure 5).
Two different development strategies are possible. For
critical and business-critical applications it is necessary,
as a rule, to provide the required level of dependability at
the minimum cost, whereas for commercial applications
it is important to provide the maximum dependability at
the limited cost.
These goals can be achieved by solving optimization
problem, taking into account failures criticality, prob-
ability of occurrence and cost of fault-tolerance means,
their effectiveness and failures coverage. As a result the
particular computer network must be updated by using
chosen fault-tolerance means.
The principles proposed are in line with recent re-
search [13] where a functional failure mode, effects and
criticality analysis approach is proposed to address the
dependability optimization of large and complex systems.
6.2. The Principles of Dependable and Secure
Deployment of Computer Networks
Dependability and security of a computing system is its
ability to timely deliver service that can justifiability by
trusted [14]. The typical network faults are physical
faults of network equipment and communication media
(i.e. cabling system), configuration errors (e.g. errors in
static routing or firewall filtering rules or and security
policies), design faults, as a rule, of software components,
and interaction faults of physical (electromagnetic inter-
ference) or information nature (traffic congestions).
Fault and intrusion tolerance of computer networks,
their security and dependability as a whole could be im-
proved using the following principles.
1) Defense in depth and diversity (D & D). Defense in
depth implicates joint usage of existing intrusion and
fault-tolerance mechanisms at the different levels of the
network architecture (cabling systems, network equip-
ment, network technologies) and layers of the communi-
cation model (OSI or TCP/IP) to provide complex deci-
sion for dependability ensuring.
2) Adaptability and update (A & U). The essence of
this principle is in the dynamic changing of the network
architecture and diversity modes according to the ob-
served failures and intrusions. The intellectual monitor-
ing means for detection of failures and intrusions, their
analysis and the choice of better network configurations
could be used to achieve that.
7. Conclusions
CCN reliability and safety estimation is the complex task,
100/1000
Ethernet
Network architecture
(logical and physical)Specification of
network equipment
Network
Protocols
Failures & intrusions
criticality (cost) and
probability analysis
Analysis of cost,
effectiveness and
compatibility of
different means
Risk analysis,
optimization and
choice
Updating the
network specification,
architecture, set of
network equipment,
etc.
Computer Network
Network
technologies
Common
FMEA-tables
Detailed
FMEA-tables
Set of means for
fault & intrusion
tolerance provision
TCP
SONET
FDDI 100VG-
AnyLAN
xDSL IP OSPF
HTTP SNMP
UDP
ICMP
Figure 5. Using FMEA-technique for dependable web services development.

I. ELYASI-KOMARI ET AL. 725
which cannot be decided in isolation from application
area. It is stipulated that the internal information factors,
such as collisions and congestion of switchboards, routers
and servers, influence on a network reliability and safety
(besides of hardware and software reliability and external
extreme factors).
Computer networks are the complex systems which
contain a lot of elements. Therefore network failures are
unavoidable. In this case the risk and criticality analysis
[15], survivability and safety assessment [16] are more
actual tasks than evaluation of the probability of no-
failure operation.
As computer networks have a multilevel hierarchy the
network element failures, generally, have a dependent
character, i.e. the failure effects at one layer of the OSI
or TCP/IP models are the sources of new failures at suc-
ceeding layers. This feature of computer networks can be
taken into account by using layered analysis and repre-
sentation its results as a hierarchy of FME(C)A-tables. A
characteristic feature of active telecommunication de-
vices is that they contain not only hardware, but also
software components. For the software reliability and
safety qualitative analysis the Software ME(C)A-tech-
nique may be used [17].
The software tool is developed to estimate of the CCN
critical failure probability (construction of a criticality
matrix) by results of the FME(C)A-technique. This tool
consists of:
 database containing common FME(C)A-tables for the
network elements with an priori information;
 conversational procedure of FME(C)A-analysis and
evaluation of the specified network;
 procedure of automatic generation of criticality grids
and definition of the most critical network failures;
 procedure of an automatic choice of critical failure
recovery and fault-tolerance means.
This tool also may be extended by procedures for
network simulation and probabilistic assessment of re-
liability, safety and survivability. Directions of our future
researches are connected with analysis of multiply
failures during network development and maintenance
and cost-effective means of reducing failures criticality.
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