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Modelling and Analyzing Cost-Effective Dependability in Passive Optical Networks

Fernández, Álvaro Fernández
Doctoral thesis
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URI
http://hdl.handle.net/11250/2434357
Date
2017
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  • Institutt for informasjonssikkerhet og kommunikasjonsteknologi [2809]
Abstract
Lately, the proliferation of new applications and services as well as the

penetration of the Internet has created a growing need for the deployment

of broadband access networks. Mostly due to the high bandwidth they

offer, fiber-based solutions are well regarded to implement both present

and future broadband access networks. Hence, not only operators and

providers are upgrading their access networks to fiber, but also regulators

and governmental institutions are supporting its deployment to meet the

digital demands of our society. Particularly, Passive Optical Networks

(PONs) are one of the most important types of fixed fiber-based access

systems today, being widely deployed worldwide. Besides, PONs are

also key to enable Fiber-Wireless (FiWi) integration, serving as backhaul

networks. Being fiber-based, PONs offer high bandwidth, but also a set

of features, such as low-power consumption or easy management, that

make them very attractive as broadband access networks.

Nevertheless, real telecommunication networks are not failure free.

With society, industry and individuals becoming more dependent on

telecommunications networks, users also demand these networks to provide

dependable access to applications and services. In a dynamic business

ecosystem, being able to meet user demands is key to achieve a successful

network business. Further, in this context, dependability plays a major

role in the overall economics of a business. First, achieving dependability

entails a cost, typically due to the investment in fault tolerance. This

is especially a concern in access networks, which are more cost sensitive

than other parts of the network, due to the lower number of served users.

On the other hand, poor dependability has consequences that negatively

impact a business, from direct cost of failures and repairs, to bad publicity

and loss of reputation. It is then a challenge for operators to increase the

dependability of their access networks, hence reducing the consequences

of dependability, at an affordable cost. Consequently, this thesis aims at

addressing the trade-off between the cost of achieving dependability and

its consequences, i.e. cost-effective dependability, in PON deployments.

As a first step, this work focuses on modelling the PON deployment

area, which affects both sides of the trade-off, due to infrastructure

sharing, client clustering and design decisions. Especially, this suggestion

builds on a network geometric model, i.e. the Manhattan model, to

develop a closed formulation for the Capital Expenditures (CAPEX),

accounting for the cost of achieving dependability. Additionally, this

formulation also allows for calculating the failure impact (the number

of clients affected by a failure) and thus capturing failure dependencies among clients. With the physical framework provided by the Manhattan

model and the related equations, the CAPEX, asymptotic availability and

failure impact of both unprotected and protected PONs can be analyzed.

A central part of the thesis focuses on how to model the consequences

of dependability, covering penalties due to breached Service Level Agreements

(SLAs), cost of maintenance, buying of spare parts for repair and

loss of reputation. Based on the constraints of the Manhattan model,

a first approach proposes the use of Markov cost models to estimate

dependability consequences fully in monetary units, i.e. as Operational

Expenditures (OPEX). By developing the necessary equations, the objective

is to express dependability attributes (asymptotic availability and

failure impact) as expected OPEX. Thus, by combining CAPEX and

OPEX, a first analysis of cost-efficient dependability with respect to

hardware failures is performed, aiming at reducing the total cost.

Also, an important part of this thesis deals with introducing software

failures into the modelling of PON dependability. To do that, the software

failure intensity of a PON Optical Line Termination (OLT) is estimated

from empirical data by employing Duane’s model for software reliability

growth. Different types of software failures are characterized, and

integrated with hardware failures in the Markovian cost model. Further,

how to model hardware-software interaction with respect to imperfect

recovery of hardware faults due to software is also presented.

Then, with the insight provided in software failures, the approach

to model the consequences of dependability is further refined. Namely,

a risk assessment approach, considering the probability distribution of

the interval availability during a finite time frame, and thus the risk

it represents, is followed. Under this approach, the full probability

distribution of the OPEX can be computed, yet loss of reputation is

modelled through client dissatisfaction and large outages. Hence, the

probability mass function of the number of dissatisfied clients, as well as

scatter plots of the down time versus the failure impact. This approach

not only gives knowledge about the stochastic behaviour of these three

aspects, but also shows how different fault tolerance mechanisms modify

this behaviour and its associated risks.

Lastly, a final consideration of this work proposes how to protect

against software failures and assign available capacity to improve the

interval availability of a PON client. In essence, a policy to assign

available capacity to clients depending on their accumulated down time is

suggested. This policy modifies the distribution of the interval availability,

improving its expectation and reducing its variability, while allowing for

the implementation of differentiated dependability.
Publisher
NTNU
Series
Doctoral theses at NTNU;2017:34

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