1. IntroductionNetwork reliability of optical networks has been a major issue for network operatorsdue to the huge quantity of data that may be lost when a single failure occurs. How-ever, the impact that a failure has on the operational cost has been mostly disre-garded. Nowadays, the continuous and rapid network evolution combined with theexistence of new operators making the market more agressive and competitive haveforced network manufacturers and operators to keep a close eye on the costs. Theobjective is to minimize the total cost of ownership (TCO) so that the manufacturerscan better sell their new products and network operators have larger margins to offerservices with more competitive prices.
The TCO has two main factors: capital expenditures (CAPEX) and operationalexpenditures (OPEX). Although the processes that are included in each factor havenot been standardized, the most accepted definition is that CAPEX copes with theexpenses related to the network infrastructure (fiber, nodes, interfaces, required floorspace, etc.), while OPEX includes the cost factors of the network operation such aspower, personnel involved in the network operation, and the maintenance of runningservices, testing equipment to detect failures, etc. The first technoeconomic studiesfocused on CAPEX, which are easier to evaluate. However, it has been shown recentlythat OPEX is a very important factor of TCO, even up to 85% [1,2], which indicatesthat OPEX should not be disregarded.
OPEX can be divided in two groups of costs [3]: (i) network related costs and (ii) ser-vice related costs. The former includes the costs related to the operation of the net-work such as the reparation of failures, maintenance of the equipment, etc., while thelatter comprises the costs associated with the operation of the services such as theestablishment and the release of services, the surveillance of the service quality, etc.The impact the network reliability has on OPEX is mostly related to two processes:the failure reparation process, which is a network related cost, and the service resto-ration process, which is a service related cost. In other words, when a failure occursand at least one service is disrupted, two processes are triggered: the service restora-tion process aiming at restoring the service depending on the protection schemeagreed at the service level agreement (SLA) (called as many times as services are dis-rupted) and the failure reparation process aiming at locating and repairing the failednetwork component. This paper aspires to evaluate the impact that different protec-tion schemes and the network component availability have on the operation costs.
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This paper is structured as follows: Section 2 introduces the OPEX process costclassification and the identification of the processes related to the reliability, whichare failure reparation and service restoration. Section 3 presents the failure repara-tion cost problem, including a description of the network platforms that are evaluated,the cost and availability parameters, etc. Sections 4 and 5 show the case study resultson the cost dependence for different component availabilities and protection schemes.Section 6 concludes the paper.
2. OPEX Processes Directly Related to ReliabilityIn this study, OPEX has been classified in two cost groups: the network related costsand the service related costs. The costs associated to each group depend on whetherthey are assigned to the network or to the services running on the network. For thispurpose, the first step was to model the network and service costs, which are bothbased on their life cycles shown in Figs. 1(a) and 1(b), respectively.
2.A. Failure ReparationAfter the long-term planning and initial installation and configuration, the networkoperates until the operator decides its final dismantling. Network maintenance per-forms monitoring actions, testing, as well as proactive maintenance procedures. Dur-ing the network operation, several actions may be taken in parallel such as theupgrade of hardware and/or software, service migrations between different platforms,and the reparation of possible failures that may occur in the network.
The process of failure reparation consists of several subprocesses:
• Identification and location of the failure based on the alarms received by the net-work manager from the equipment, possible complaints by clients, etc.
• Check availability of: (i) personnel with required knowledge, (ii) equipmentrequired for the failure reparation and testing, and (iii) transportation such as car,van, etc. In this work it has been assumed that there is an available stock of sparematerial, and the cost of storing and buying it is outside the scope of this study. Thereason is that some operators consider the stock equipment as part of CAPEX.
• Reparation of the failure: once personnel has traveled to the failure location, theyrepair the failure (in some cases, replacing equipment is required). Depending on thefailure, different times and personnel are needed, as shown in Section 4.
• Testing that the failure has been successfully repaired and the signal quality hasan acceptable level.
• Updating databases.
The cost of the process of failure reparation has been shown to be very high [4], andhence, one of the objectives of this study shown in Section 4 is the evaluation of thecost variation with respect to different parameters such as the number of employeesinvolved in the process and their location within the network, the availability of thenetwork components, etc.
2.B. Service RestorationWhen a failure occurs in the network, one or several services are interrupted. Basedon the SLA associated with each of these services, different restoration mechanisms
Fig. 1. (a) Network and (b) service life cycles.
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can be triggered, which will obviously also depend on the protection capabilities thatthe platform can offer. The mechanisms compared in our study are the following:
• Protected services: these services have reserved up to twice the capacity theyneed, establishing up to twice the number of needed connections. The so-called 1+1protection is the fastest and most reliable, and therefore is used by services, which arevery sensitive to losses and delays.
• Restorable services: this protection scheme aims at restoring the service once thefailure occurs and the service is disrupted. It is more capacity efficient than protectedservices since it does not reserve any extra capacity, but the restoration time is a bitlonger since it requires more process steps after the failure occurrence, and it mayrequire the human action of restoring the service [either through the graphical userinterface (GUI) or through command lines depending on the platform].
• Unrestorable services: these services do not have any associated restoration, thatis, these services wait until the failure is repaired. Then, the service is automaticallyrestored. The average restoration time is 8 h, and during this time there is no serviceto be maintained.
In following studies, statistical multiplexing will be also covered.The process associated with the service restoration when needed is called service
restoration. Its associated cost depends on the actions required by the human man-ager. These actions differ from mechanism to mechanism and from platform to plat-form, depending on the existing control and management capabilities. In this studythree different actions are considered:
• Typing command lines is the most consuming action and requires a deep knowl-edge on the network management system.
• Typing values or keywords in a GUI.• Selecting one of the several alternatives offered at the GUI. This is the less time-
demanding action.• Automatic restoration is disregarded here, because no operational cost needs to be
assigned to this service restoration action.
The cost associated with each of these actions depends on the time required to dothem and the salary of the personnel executing it.
Summarizing, the two processes related with the network reliability are the failurereparation network process and the service restoration process, which are going to bedescribed and studied in detail in Sections 3 and 5, respectively.
3. Failure Reparation Cost Study of Different PlatformsThis section presents the failure reparation problem and its cost evaluation on differ-ent platforms. We assumed Ethernet services configured and managed in differentnetworks.
3.A. Studied PlatformsFour different platforms with different control and management capabilities havebeen studied [5]. The capabilities are not directly related to the technology, but to theabilities of the network management system controlling the network. The studiedplatforms are:
• First-generation synchronous digital hierarchy (SDH) with manual configurationcapabilities, which is denoted as manual. This is the case of the first SDH networks,where every virtual connection (VC), which is point to point (P2P), has to be estab-lished through command lines and every node has to be configured separately. Even iftraditional SDH platforms are usually not able to produce Ethernet services, weincluded the platform as a reference for configuring and repairing services.
• Next generation SDH platform with Ethernet over SDH [virtual concatenation(VCAT), generic framing procedure (GFP), and link capacity adjustment scheme(LCAS)] support and with automatic configuration capabilities, which is denoted asE/SDH. However, this platform typically does not yet support multipoint VCs, andtherefore we assume for our case study to provide P2P VCs only.
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• Ethernet platform with multiprotocol label switching (MPLS) based control plane,which is denoted as E-MPLS. There are first implementations of carrier-grade Ether-net solutions, which are controlled by MPLS based L2 control functions. Practical con-figuration experiences with testbed implementations of the anticipated platformsaccompanied our modeling and case studies. Therefore, we specified two variants ofthe configuration model in an E-MPLS platform. The first variant uses the MPLS con-trol functions. The second variant’s configuration management additionally allowsusing some predefined service profiles of typical services to facilitate and speed up theestablishment of new services. Hence, we will distinguish E-MPLS with profile andE-MPLS without profile platforms (Table 1).
The availability values of these platforms rely on the same physical layer (i.e., sametype and quality of fiber, amplifiers, etc.) but have different end equipment (eitherSDH or Ethernet nodes) with different availability values. These values have beenobtained based on the mean time to repair (MTTR) and the mean time between fail-ures (MTBF) given by Verbrugge et al. [6]. The availability �q� of a component can bedefined as the probability of finding the component in an operating state at any time[7] and can be expressed as
q =MTBF − MTTR
MTBF.
Although the MTTR definition [8] includes the traveling time, the availability val-ues provided by manufacturers consider the MTTR as pure reparation time withoutany traveling time penalty. The real component availability will obviously depend onwhere the network component is installed and the management policy of the operator,which decides when to send technicians to repair the failure, depending on their avail-ability. Hence, our study relies on the availabilities provided by the manufacturers,which are higher values than the real ones, once the component is being used in thereal network. Subsection 3.C shows the comparison between the initial availabilityvalues with the ones obtained for the equipment installed in the German17 network[9], once the employee location problem has been solved.
The general link model, as shown in Fig. 2, consists of transponders and multiplex-ers at both ends of the link and as many amplifiers as required in the link (we assumespans of 80 km). Hence, the availability of a link of l km can be computed as shown inTable 2.
In this study, not only are link failures considered but also node failures. The nodeavailabilities depend on the node technology of every platform, as shown in Table 3.The number of persons required to repair a node failure is one and to repair a linkfailure is three.
Table 1. Mapping Rate of Services onto VCs for the Different PlatformsWhere M is the Multipoint Degree
Unprotected Service Full Protected Service
P2P P2MP MP2MP P2P P2MP MP2MP
Manual 1 M M�M−1� 2 2M 2M�M−1�E/SDH 1 M M�M−1� 2 2M 2M�M−1�
E-MPLS 1 1 1 2 2 2
Fig. 2. General link model used in the overall availability computation, where OLAstands for on-line amplifier.
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3.B. Failure Reparation ProblemThe problem studied in this paper aims at finding the number of employees and theirlocations in the network so that their associated costs together with the penalties thatshould be paid for long unrestored services is minimized. Hence, the minimum ofCostel+Costv should be found, where Costel is the cost associated to the employees �e�involved in the failure reparation and to the locations �l� required to host them, andCostv is the cost associated to the penalties that should be paid for SLA violations �v�.Obviously, the less personnel and locations, the longer the reparation times, andhence, more SLA violations and higher Costv.
The problem has been divided into two subproblems:
1. Optimal locations: this subproblem finds the best locations that minimize the dis-tance to any possible failure that may occur. It is assumed that locations are placed atthe network nodes, where network operators may have some buildings or floor space.Hence, the subproblem is solved as many times as nodes are in the network �N� andfinds the best locations when having anywhere from 1 to N locations. For example,when having one location, the best location is the node which has the minimum–maximum distance to any other node in the network, i.e., the node that is closer to itsfurthest node.
2. Optimal number of employees at each location: this subproblem allocates forevery scenario offered by the previous subproblem, the minimum number of employ-ees so that the minimum cost Costel+Costv is achieved.
The costs considered in this study are as follows:
• Employee cost: we assume that an employee works 8 h per day. Hence, threeemployees are required to cover 24 h. The salary is considered to be an average of 1.62basic salaries (BS). BS corresponds to how much a base employee costs to the com-pany taking into account his salary, housing, the required equipment, the insurance,etc. Since it has been shown by Crawford [10] that failures during nights and week-ends are less frequent than during working hours because most of the failures arerelated to the human intervention at working hours, we have assumed that duringthe weekdays there are more employees (and cheaper) than during nights and week-ends, and hence, we can consider the same average salary for all the employees. Dis-tinction between different failure times will be covered in further studies.
• Locations cost: the cost associated to a new location is assumed to be 2 BS forevery ten employees at the location, which includes costs such as parking slots, officefloor space, cleaning staff, etc.
• SLA violation penalties: the SLA violation penalties depend on the type of service;the more protection the service has, the more penalties should be paid. In our study,we have distinguished between protected, restorable, and unrestorable services aspresented in Subsection 2.B. The penalties follow a step increasing function, wherethe steps have a different depth depending on the service as shown in Fig. 3: 15 min,
Table 2. Computation of the Overall Availability of a Link of l kmsa
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3 h, and 12 h for protected, restorable, and unrestorable services, respectively. Itshould be taken into account that when a failure occurs, protected and restorable ser-vices are usually restored within an agreed time. Only in some cases the restorationmay not be possible; e.g., protection switch problem for protected services, no availablesecondary path for restorable services, etc. Hence, we have assumed that the protec-tion mechanisms fail 0.1% of the time, and the restoration mechanisms fail 5% of thetime. Table 4 shows the values of the services considered in this study. The networksupports 500,000 services.
3.C. Cost Evaluation ResultsThese platforms have been applied to the Germany17 topology shown in Fig. 4(a).This network consists of 17 nodes and 26 links of different lengths. Based on theavailability of their components, the optimal number of locations and employees fromthe cost point of view have been computed. The total cost Costel+Costv has been plot-ted in Fig. 5(a) for the different number of locations where personnel responsible for
Table 4. Distribution of Types of Services and Their Values
ServiceOverall
PercentageFailure of Restoration
Mechanism
Protected 80% 0.1%Restorable 14% 5%
Unrestorable 6% N.A.
Fig. 3. SLA penalties in function of the service disruption time.
Fig. 4. (a) Germany network topology considered in the study, and (b) service lifemodel.
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failure reparation are placed. For the Germany17 network, the optimal number oflocations is five, which are: Berlin, Bremen, Dusseldorf, Frankfurt, and Stuttgart, andthe cost is minimized (125,487 BS).
Based on the five locations solution, the updated average MTTR, which includes theaverage travel distance, has been computed, and the real network component avail-ability has been compared with the one delivered by the manufacturers. The compari-son is depicted in Table 5, which shows a decrease between 0.0181% and 0.0389%,depending on the platform.
4. Impact of the Variation of the Network Component AvailabilitiesIn this section, the sensitivity of the cost to the network component availabilities ispresented. The SDH platform scenario has been considered with a variation of theircomponents’ availability from 100% to 99.99975%, 99.9995%, 99.99925%, and 99.999%of their initial value. For each case, the total failure reparation cost, including person-nel, location, and SLA violation penalties, have been computed and plotted inFig. 5(b). It can be seen that for the case of having five locations with personnelresponsible for the failure reparation, the cost can be duplicated when the availabilityof the components decreases to a 99.9995%. This increase in cost is due to the factthat there are more failures, and hence, more personnel is required and more SLA vio-lations penalties should be paid. Another conclusion is that the cost, when having theinital availabilities (100% case) and only one location, is comparable with the cost ofdecreasing the availabilities to 99.99975% and having three locations, which meansthat the higher number of failures due to the lower availability can be compensatedby adding two more locations and reducing the travel time (and hence, increase theavailability of the personnel to repair new failures).
5. Impact of Different Protection Mechanisms to the Service CostThis section presents the study of the impact that different protection mechanisms ondifferent platforms have on the total service cost. Different types of services have beenconsidered: P2P, point to multipoint (P2MP), and multipoint to multipoint (MP2MP).Services are mapped onto one or several VCs, depending on the type of service, theprotection scheme, and the platform. The mapping rates for every possible combina-tion are shown in Table 1.
Table 5. Comparison between Network Component Availabilitiesa
aWithout traveling times �by manufacturers�.bWith traveling times TT �by operator once the equipment is installed in the real network�.
Fig. 5. (a) Total failure reparation cost including location, personnel, and SLA violationpenalties for different number of locations. (b) Cost sensitivity study of the availabilityon the total cost of the failure reparation problem for different locations and the optimalnumber of employees at each of these locations.
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To take into account an average life cycle of a service, several actions have beenconsidered [all of them related to the processes of Fig. 4(b) with their associated prob-abilities, as shown in Table 6]. The cost associated to each protection mechanism isrelated to the actions required by the human manager and the time he/she needs toexecute them, as mentioned at the end of Section 2. The cost associated to each actionare: BS/480 for command lines, BS/960 for GUI typing, and BS/1920 for GUI menuselection.
We also should take into account that the number of services and VCs that can bemanaged through a GUI will depend on the platform and its manufacturer. In ourcase it has been considered that the E/SDH platform can monitor twice the number ofVCs than the manual platform, whereas the E-MPLS platform can monitor threetimes more services/ Ethernet Virtual Connections (EVCs) than in the manual plat-form.
Another difference between the platforms is the salary of the employees. In general,the newer the platform, the more expensive the employee since he/she should havefollowed different training courses, and there are less available employees with theright knowledge. Hence, assuming that the BS is paid to the employees of the manualplatform, 10% more is paid to the employees of the other two platforms.
Based on all these assumptions, the cost impact of the different protection schemesfor the different platforms have been studied and plotted in Fig. 6. Simulations weredone once the service life model shown in Fig. 4(b) was implemented as a Markovchain in MATLAB as shown in [3]. Simulations were run to establish 95% confidenceintervals (CI), which are smaller than 10% of the average value. Based on these fig-ures, it can be seen that the cost of the different services on the E-MPLS platform iscomparable due to its capability to offer multipoint VCs. On the contrary, the cost ofmultipoint services on the other two platforms increases with the number of VCs asso-ciated with the service. It can also be concluded the impact that manual commandsrequired in the manual platform have on the cost when compared with the GUI facili-ties offered by the E/SDH platform. The graph also shows the impact that protectedservices have on the cost for the different platforms; for E-MPLS it is negligible,whereas for manual it is more than twice than the unrestorable services.
6. ConclusionThis work has studied in detail the cost of the failure reparation in telecommunicationnetworks from different aspects. First, the problem and definition of network and ser-vice reparation has been distinguished and presented. The failure reparation costoptimization problem has been introduced and the proposed method has been appliedto the Germany17 network, which resulted in five locations as the optimal solution.Furthermore, the impact that the locations have on the network component availabil-ity has been studied, and in this particular case, it has been shown to be between
Table 6. List of Actions Related to a Service and Their Probabilities
Service Configuration Failure Probability 0.0001 Service Change Probability 0.01Failed Action Probability 0.0001 VC Change Probability 0.01
Software Failure Probability 0.0001 VC Release Probability 0.01
Fig. 6. Different service type cost comparison on different platforms.
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0.0181% and 0.0389% lower than provided by manufacturers. The impact that theavailability variation has on the total reparation cost has also been studied, and it hasbeen shown that this variation has an important impact on the final failure repara-tion cost. In general terms, a higher number of failures due to the lower availabilitycan be compensated by adding more locations and reducing the travel time (andhence, increase the availability of the personnel to repair new failures). Last, but notleast, the impact that different platforms with their corresponding component avail-abilities and the different protection schemes have on the service cost has beenshown.
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