Abstract—This paper presents the principles of designing

resilient optical transport networks including failure scenarios, survivability architectures, routing and wavelength assignment problem and implementation approaches. Furthermore, the paper addresses current and future research challenges including covering multiple simultaneous failures, QoS-based RWA, robustness and future demand uncertainty accommodation, and quality of service issues in deployment of resilient backbones for next generation telecommunication networks.

Index Terms—Optical networks, resilient systems, failure scenarios, principles and state-of-the art.

I. INTRODUCTION HE optical networks are prone to failures and unexpected failures of network components such as link and node

failures at optical layer can potentially lead to a catastrophic loss of data and revenues producing an unacceptable deterioration in the delivered quality of service (QoS). Therefore, one of the most important optical network design issues is survivability [1], which is the ability of a network to provide continuous service at an acceptable level in the presence of different failure scenarios [2, 3]. Some reasons of designing resilient optical transport networks could be summarized as; 1) The increasing demands of end-users of client networks to have reliable communications and services with fair quality [4]. 2) The unexpected failures of network components such as

link and node failures at the optical layer may result in multiple failures at client layers.

3) The aggregation bandwidths of the order of several Tb/s per fiber, using dense-wavelength-division-multiplexing (DWDM) technique, causes enormous data and revenue loss in the event of network’s failure.

4) Fault tolerance and traffic restoration at optical transport layer have several advantages, such as shorter restoration time, efficient resource utilization, and protocol transparency, over that at the client layer.

Yousef S. Kavian is with Electrical Engineering Department, Faculty of Engineering, Shahid Chamran University, Ahvaz, Iran (e-mail: y.s.kavian@scu.ac.ir ).

II. PRINCIPLES The survivable networks employ redundant resources like spare ligthpaths and switches to cover the failures which increase the network cost planning.

A. Failure Models and Redundancy The network failures are classified in three types; transient,

temporary and permanent failures which affect the network redundancy. The researches on survivability in DWDM networks focus on permanent failures employing permanent redundancies like permanent redundant bandwidth and spare switches for failure covering. Three main considering failure models are; The Single Link Failure Model

This is the more realistic and probable failure model for DWDM networks [5], where the fiber may cut by workers, fire, earthquake, changing environmental condition like temperature, humidity. In this model the fiber cut in a link of the network is repaired before another fiber cut assumed to occur in the network. The link failures are assumed to be independent of each other and the probability of a single link failure is the same for all links and; it has uniform distribution function.

The Node Failure Model

The node (OXC, OADM, transmitter, receiver, and amplifier) failure model is also a real issue that needs to be considered in designing fault tolerant backbone networks [6]. While the probability of a node failure is generally much smaller than a link failure due to the built-in redundancy of many network equipments but node failure is still possible and will cause server service disruption.

B. Survivability Architectures The hierarchy of optical mesh transport network protection

and restoration options is illustrated in Fig. 1.

Protection and Restoration Architectures Two main survivability architectures called protection and

restoration provide the basis of the different schemes to the design of fault tolerant optical transport networks. Protection architectures [7] establish working and spare lightpaths for arrival requests during network configuration before network

Resilient Optical Networks: Principles and State-of-the-Art

Yousef S. Kavian

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operation, whereas restoration architectures [8] use network state variables to assign spare lightpaths in the event of failure. Whilst the former is static and offers a faster restoration time, the latter is dynamic with greater bandwidth efficiency. Furthermore, the restoration architecture is more flexible against network configuration changes and may cover multiple link and node failures. Link, path and sub-path survivability

Both protection and restoration architectures can be further divided into link, path and sub-path survivability schemes. In link survivability scheme, the traffic is rerouted only around the failed link. In path survivability scheme [9], the working and the spare lightpaths must be link disjoint so in the event of failure on working lightpath the traffic is rerouted through its spare lightpath. Whilst the former provides efficient utilization of redundant resources and lower propagation delay, the latter provides lower switching delay. In the sub-path scheme [10] the whole network is divided into different domains where a ligthpath segment in one domain must be protected by the resources in the same domain. Compared to path protection, sub-path protection can achieve high scalability and faster recovery time for a modest sacrifice in resource utilization [11, 12]. Dedicated and shared protection

Furthermore the protection architecture is classified into two major categories, dedicated protection and shared

protection. In dedicated protection backup resources along spare lightpath are especially dedicated for that lightpath and can not be utilized by other spare ligthpaths, but in shared segment [13, 14] and shared path [15] protection schemes redundant resources can be shared between several spare ligthpaths. Dedicated protection is faster than shared protection, but shared protection is more efficient for resource utilization. The problem of survivable light path provisioning using shared path protection in optical mesh networks employing WDM was investigated in [16].

Shared mesh WDM optical networks with partial wavelength conversion capability has been considered in [17]. In addition in [18] the state of the art progress in developing both shared path protection (SPP) and segment shared protection (SSP) were reviewed.

C. Routing and Wavelength Assignment Problem The routing and wavelength assignment (RWA) problem

lies at the heart of designing the wavelength-routed DWDM optical transport networks [19] where a variety of services including voice, data and video are multiplexed together and routed from origin to destination over a common infrastructure called lightpaths or an all- optical communication channels without any optical-electronic-optical conversion and buffering at the intermediate nodes, known as wavelength routing. Establishment of a lightpath

Protection

Link

Restoration

Link

Sub-path

Path

Dedicated

Shared

Path

Dedicated

Shared

Sub-path

Dedicated

SharedFailure Recovery Schemes

Fig. 1. The survivability hierarchy in optical transport network

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consists of choosing a route and assigning wavelengths known as an NP-hard problem. Static and Dynamic RWA

The RWA problem is called static where the demand matrix is known and static and a set of lightpaths should be established between demand node pairs while the objective is network resource optimization like minimizing the number of allocated wavelength channels. The static RWA problem is employed in protection architectures. The RWA problem is dynamic when the traffic pattern is dynamic and changes duration network operation, and the objective is network resource optimization like minimizing the number of allocated wavelength channels and minimizing blocking ratio [20, 21].

D. Implementing Approaches

Heuristics and ILP approaches The integer linear programming (ILP) approach has been

extensively employed for modeling different survivability schemes. The survey papers [22], [23] and [24] may be referred to in order to present an indication of the body of work in the literature. The ILP approach works well for small problems with linear objective functions and constraints, and for small networks where the number of decision variables increases dramatically by increasing the size of networks. However, the limitations of ILP models have led to the introduction of a diverse range of heuristic algorithms. The Intelligent Techniques

The other successful approaches are based on intelligent techniques especially genetic algorithms. A genetic algorithm (GA) is a stochastic global search method based on natural biological evolution and genetics [25] that may be employed to solve optimization problems that are not well suited for standard optimization algorithms, including those where the objective function is discontinuous or non differentiable. The complexity of problem, the abilities of genetic algorithm to search solution for NP-hard problems in large-scale search space [26], and the successful background of genetic algorithms in designing communication networks [27] lead to provide genetic algorithm based models for solving RWA problems in designing the survivable DWDM optical transport networks. Encoding the working and spare lightpaths into chromosomes, creating next generation using crossover, mutation and selection operators, population initializing, developing termination rules and construction the fitness function are the main problems for implementation a GA approach for designing survivable DWDM optical transport networks [28].

III. CURRENT AND FUTURE RESEARCH CHALLENGES

A. The Multiple-link Failure

Another failure scenario that should be considered in next generation high speed telecommunication networks is multiple near simultaneous link failures where , for example, a new failure would happen before repairing former occurred failure in the network or more than one link be affected in the network during a natural earthquake disaster [29]-[31]. Here for the case of M-failure the network topology should be (M+1)-connected. For example in case of 2-link failure, the network should be 3-connected that means it needs to establish three link-disjoint paths between each node pair. As shown in Fig. 2, during design of a survivable network, in addition to setting up a working lightpath, (4-6), between each original and destination node pair, two link-disjoint spare lightpath,(4-5-6),(4-3-2-1-6), needs to be established between the node pair, too. The diverse spare ligthpaths are employed as the means to restore the traffic demand when two links affected in the network.

B. QoS based RWA The resource reservation routing algorithms provide

lightpaths which minimize the bandwidth (wavelength) utilization and they may not be suitable to accommodate the QoS requirements [32]-[33]. The effect of QoS requirements, propagation delay optimization, on solutions of RWA problem is compared with resource (bandwidth) optimization. The topology of the network has been shown in Fig. 3 that

Fig. 2. A survivable network: double link failures covering

Fig. 3. Resource reservation and QoS-based RWA

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includes 6 nodes and 8 links. Table I describes the weight of connecting links in fiber kilometers. The demand is a connecting request between node pair (2, 6) for 3 wavelengths bandwidth. The results are tabulated in Table II. The adjusted delay on each link was assumed to be 1 ms/km. The resource reservation based RWA tried to establish bandwidth efficient lightpath where the QoS based RWA found ligthpath with low latency time. In reservation based RWA the number of allocated wavelengths are 6 wavelengths where the propagation delay of the request is 24ms. In QoS based RWA the number of allocated wavelengths are 9 wavelengths where the propagation delay of the request is 17ms. Consequently, the QoS based RWA tries to establish lightpath with lower latency time by extending network resource utilization that is assured through bandwidth trading.

C. Robustness and Accommodation of demand uncertainty Rising Internet usage is producing increased traffic

uncertainty that must be accommodated in optical core design optimization. The delivery of the required quality of service (QoS) by client networks such as Internet network providers faces uncertain environment capacity problems where demand volume changes extensively over different periods of network operation. Therefore, existing network capacity is unable to accommodate the demands at all times due to the natural variability of unpredictable loads in periods of high demand. Planning for robust DWDM transport networks capable of meeting future uncertain demands is, therefore, a necessity [34]-[37].

D. The Quality of Service (QoS) Issues The quality of service (QoS) refers to the ability of a

network to enforce preferential treatment through classification.

TABLE I THE WEIGHT OF CONNECTING LINKS IN KILOMETERS

Link Length (km) (1,2) 12 (1,6) 12 (2,3) 4 (3,4) 3 (3,5) 8 (4,5) 7 (4,6) 10 (5,6) 14

TABLE II RESOURCE RESERVATION AND QOS-BASED RWA

Objective Lightpath

Path/Wavelength Lightpath Properties (Bandwidth, Delay)

Bandwidth 2-1-6/( 321 ,, λλλ ) (6,24)

Propagation Delay 2-3-4-6/( 321 ,, λλλ ) (9,17)

The QoS of a network can be studied in the context of any combination of properties of a service delivered to the end users including availability, delay, jitter, loss ratio and throughput [38, 39]. Although survivability of DWDM networks improves the client network’s QoS by tolerance against component failures and avoiding losing data and revenue but also the survivability parameters including restoration time, restoration bandwidth, restoration granularity, and call acceptance ratio will affect the delivering QoS. The QoS at optical layer is differentiated to differentiated reliability, reliable connection, quality of protection, and quality of recovery [40]. The important role of quality of service (QoS) in deployment of a resilient DWDM backbone for global networks requires critical design-phase planning optimization. In general, QoS requirement extends network resource utilization that is assured through bandwidth trading [41].

IV. CONCLUSION The survivability is an important and critical issue that

must be taken in deployment of DWDM optical transport networks as backbones for reliable high speed next generation telecommunication networks due to their ability to carry large traffic volumes. This paper reviewed some major issues of designing fault tolerant DWDM optical transport networks for service provision at acceptable level of performance in the presence of failures. Furthermore, the paper addressed some current and future research challenges in deployment of resilient DWDM backbones for next generation telecommunication networks.

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