A Survey on OFDM-Based Elastic Core Optical Networking <s> e) RSA for Time-Varying Traffic <s> We study the problem of bandwidth fragmentation in links that comprise rings and meshes in SONET networks. Fragmentation is a serious challenge for network operators since it creates "holes" in the transport pipe causing new demands to be rejected, in spite of sufficient bandwidth being available. Unlike the well-studied, general fragmentation problem, link defragmentation is "hard" and novel due to some unique constraints imposed by the SONET standard. Since a defragmentation operation typically occurs on a network carrying live traffic, in addition to the "quality" of the output, any link defragmentation algorithm has to avoid traffic hit and also optimize the "cost" of reorganizing circuits. We propose an algorithm called mobipack that is optimal in its defragmentation quality. Moreover, we demonstrate via extensive simulations, that it also achieves its goal in a hitless manner with only marginal additional cost over the optimal, making it extremely attractive to use in practice <s> BIB001 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> e) RSA for Time-Varying Traffic <s> Orthogonal Frequency Division Multiplexing (OFDM) has been recently proposed as a modulation technique for optical networks, due to its good spectral efficiency and impairment tolerance. Optical OFDM is much more flexible compared to traditional WDM systems, enabling elastic bandwidth transmissions. We consider the planning problem of an OFDM-based optical network where we are given a traffic matrix that includes the requested transmission rates of the connections to be served. Connections are provisioned for their requested rate by elastically allocating spectrum using a variable number of OFDM subcarriers. We introduce the Routing and Spectrum Allocation (RSA) problem, as opposed to the typical Routing and Wavelength Assignment (RWA) problem of traditional WDM networks, and present various algorithms to solve the RSA. We start by presenting an optimal ILP RSA algorithm that minimizes the spectrum used to serve the traffic matrix, and also present a decomposition method that breaks RSA into two substituent subproblems, namely, (i) routing and (ii) spectrum allocation (R+SA) and solves them sequentially. We also propose a heuristic algorithm that serves connections one-by-one and use it to solve the planning problem by sequentially serving all traffic matrix connections. To feed the sequential algorithm, two ordering policies are proposed; a simulated annealing meta-heuristic is also used to obtain even better orderings. Our results indicate that the proposed sequential heuristic with appropriate ordering yields close to optimal solutions in low running times. <s> BIB002 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> e) RSA for Time-Varying Traffic <s> We introduce the network defragmentation problem for FWDM networks, formulate it, and propose heuristics. The network defragmentation process consolidates the available spectrum significantly while minimizing the number of interrupted connections. <s> BIB003 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> e) RSA for Time-Varying Traffic <s> We propose a general policy to allocate subcarriers to time-varying traffic in a flexible OFDM optical network. We compare the OFDM network performance to that of a fixed-grid WDM network using simulations. <s> BIB004
The data-rate/bandwidth-variable concept of OFDM-based networks brings immense flexibility by supporting time-varying traffic with a transmission rate that fluctuates over time BIB002 . Network planning of the time-varying traffic models can be done by spectrum reservation and resource sharing among different connections. For example, connections that have complementary transmission rates in time (i.e., when the rate of a connection increases, the opposite tends to happen for another connection), could be served by shared spectrum slots. An alternative approach is to assign each connection a guaranteed transmission rate, and a probabilistic model for exceeding this rate, where spectrum reservation and sharing between connections could be performed BIB002 . e) Network Defragmentation RSA Under a dynamic traffic scenario, connection setup and release may lead to fragmentation throughout the network, by separating the available spectrum into small non-contiguous spectrum bands. This problem will increase the blocking probability of incoming connection requests, as there may be insufficient contiguous spectrum available. Moreover, because network traffic evolves over time, the allocated optical routes and spectrum might not be optimal in terms of spectral utilization. Therefore, it is desirable for network operators to periodically reconfigure the optical paths and spectrum; this is referred to as network defragmentation. Unlike the defragmentation operation in SONET/SDH networks, where each link is defragmented independently BIB001 , defragmentation in an elastic optical network requires the optical path to be reconfigured as a whole. Since defragmentation is performed in a live network, the key requirement is that it should have no (i.e., hitless) impact on existing services. Normally, bridge-and-roll operations, where an alternative connection is first setup and used to carry a copy of the signal (bridge phase) after which the original connection is broken down (roll phase) are performed during defragmentation to guarantee that the service disruption time remains below a certain threshold (e.g., 50 ms in SONET/SDH). The defragmentation algorithm should also give a step-by-step transition sequence to migrate the connections and minimize the number of moved connections to reduce service disruption BIB001 . Also, further research is necessary to explore the conditions of when to trigger the defragmentation operation, incorporating metrics such as blocking rate, resource utilization, network capacity, etc. An ILP formulation of the defragmentation problem of FWDM networks was proposed in BIB003 to provide the optimal defragmentation with considerations of the spectrum continuity and the non-overlapping spectrum constraints. Two heuristic algorithms, namely, the Greedy-Defragmentation algorithm and the Shortest-Path-Defragmentation algorithm were also provided for large-scale networks, with the objective of maximizing the consolidation of the spectrum as well as minimizing the number of service interruptions. Table V summarizes current research on RSA in elastic optical networks. Although considerable progress has been made in this area, we point out that more research is needed on dynamic RSA problems, especially to deal with time-varying traffic models to exploit the data-rate and bandwidth flexibility offered by elastic optical networks. A general policy to allocate subcarriers to time-varying dynamic traffic is proposed in BIB004 . Two types of subcarriers BIB003 are defined for a connection, one type is allocated subcarriers for the connection to provide guaranteed service, the other type is reserved subcarriers that could be allocated whenever the service traffic exceeds the allocated capacity, and will be released when traffic decreases. A dynamic algorithm to allocate additional subcarriers to an existing connection was given, and numerical studies are performed to determine the appropriate amount of subcarriers to be reserved.