Abstract
Originally, networks were engineered to provide only one type of service, i.e. either voice or data, so only one level of resiliency was requested. This trend has changed, and today’s approach in service provisioning is quite different. A Service Level Agreement (SLA) stipulated between users and service providers (or network operators) regulates a series of specific requirements, e.g., connection set-up times and connection availability that has to be met in order to avoid monetary fines. In recent years this has caused a paradigm shift on how to provision these services. From a “one-solution-fits-all” scenario, we witness now a more diversified set of approaches where trade-offs among different network parameters (e.g., level of protection vs. cost and/or level of protection vs. blocking probability) play an important role.
This chapter aims at presenting a series of network resilient methods that are specifically tailored for a dynamic provisioning with such differentiated requirements. Both optical backbone and access networks are considered. In the chapter a number of provisioning scenarios - each one focusing on a specific Quality of Service (QoS) parameter - are considered. First the effect of delay tolerance, defined as the amount of time a connection request can wait before being set up, on blocking probability is investigated when Shared Path Protection is required. Then the problem of how to assign “just-enough” resources to meet each connection availability requirement is described, and a possible solution via a Shared Path Protection Scheme with Differentiated Reliability is presented. Finally a possible trade off between deployment cost and level of reliability performance in Passive Optical Networks (PONs) is investigated. The presented results highlight the importance of carefully considering each connection’s QoS parameters while devising a resilient provisioning strategy. By doing so the benefits in terms of cost saving and blocking probability improvement becomes relevant, allowing network operators and service providers to maintain satisfied customers at reasonable capital and operational expenditure levels
TopIntroduction
Wavelength Division Multiplexing (WDM) enables optical networks to transport hundreds of wavelength channels through a single optical fiber, with a capacity that currently varies from 10 Gbit/s to 40 Gbit/s for each channel, and that is expected to reach 100 Gbit/s in the near future (Ray, 2010). Moreover, one single fiber cable consists of a large number of optical fibers, and an accidental single cable cut may lead to the interruption of a very large number of optical connections with the likely interruption of an enormous amount of services. For this reason it is extremely important to provide efficient survivability mechanisms in optical networks. With this regard a lot of work can be found in the literature that addresses the resiliency problem in both optical core (Mukherjee, 2006) and access networks (Chen, Mas Machuca, Wosinska & Jaeger, 2010; Yeh & Chi, 2007; Chan, Chan, Chen & Tong, 2003).
The term core refers to the backbone infrastructure of a network that usually interconnects large metropolitan areas, and may span across nations and/or continents (Figure 1). Usually interconnected in a mesh pattern the backbone nodes aggregate and transmit traffic from and to the peripheral areas of the network (i.e., the metro/access segment). The term access refers to the so called last mile or segment of a network where central offices (COs) and remote nodes (RNs) provide connectivity, using tree topologies, between the end users and the rest of the network infrastructure. Depending on the reach of the access segment core and access may or may not be interconnected via a metro infrastructure. With short reach access solutions (i.e., the CO is placed a few tens of kilometers from the end users) the traffic from the end users is aggregate at the metro level before being sent to the core. With long reach access solutions (i.e., the CO is more than one hundred kilometers from the end user) the traffic goes directly from the access into the core segment.
Figure 1. Telecom network hierarchy example: core, metro and access segment
Most of the attention was earlier devoted to reliability methods that were able to provide resiliency to all optical channels, or lightpaths, indistinctly. This was motivated essentially by the fact that in the absence of survivability mechanisms the first priority was to develop solutions that provide uninterrupted services in the case of network link or node failures. Another reason for this flat architecture was the nature of the services carried over the lightpaths. Historically, networks were engineered to provide only one type of service, i.e. either voice or data, so only one level of resiliency was needed.