A Survey on OFDM-Based Elastic Core Optical Networking <s> 2) Routing and Spectrum Allocation Algorithm <s> The sustained growth of data traffic volume calls for an introduction of an efficient and scalable transport platform for links of 100 Gb/s and beyond in the future optical network. In this article, after briefly reviewing the existing major technology options, we propose a novel, spectrum- efficient, and scalable optical transport network architecture called SLICE. The SLICE architecture enables sub-wavelength, superwavelength, and multiple-rate data traffic accommodation in a highly spectrum-efficient manner, thereby providing a fractional bandwidth service. Dynamic bandwidth variation of elastic optical paths provides network operators with new business opportunities offering cost-effective and highly available connectivity services through time-dependent bandwidth sharing, energy-efficient network operation, and highly survivable restoration with bandwidth squeezing. We also discuss an optical orthogonal frequency-division multiplexing-based flexible-rate transponder and a bandwidth-variable wavelength cross-connect as the enabling technologies of SLICE concept. Finally, we present the performance evaluation and technical challenges that arise in this new network architecture. <s> BIB001 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 2) Routing and Spectrum Allocation Algorithm <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> 2) Routing and Spectrum Allocation Algorithm <s> We propose a dynamic routing and frequency slot assignment algorithm for SLICE networks that employ distance adaptive modulation. We verify that the spectrum utilization penalty that stems from non-uniform bandwidth allocation is marginal. <s> BIB003 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 2) Routing and Spectrum Allocation Algorithm <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> BIB004
To establish an elastic optical path that uses a flexible (i.e., non-uniform) spectrum width to accommodate multi-data rate services, the traditional routing and wavelength assignment (RWA) algorithms of WDM networks are no longer directly  applicable. New routing and spectrum allocation (RSA) algorithms are needed BIB001 BIB002 . In RSA, the wavelength-continuity constraint of traditional WDM networks is transformed into a spectrum-continuity constraint. A connection requiring a certain capacity should be satisfied by assigning a number of contiguous subcarrier slots BIB002 . To route different subcarriers through WXCs and have acceptable signal performance at the receiver side, adjacent OFDM channels need to be separated by spectrum guard bands, each of which normally occupies an integral number of subcarrier spectrum widths (see Section IV-C3). In addition, the adaptive modulation feature of O-OFDM technology gives the flexibility of choosing modulation formats and adapting the signal spectrum width according to the transmission distance. Recently, there are an increasing number of research works investigating solutions to the RSA problem of elastic optical networks under both static and dynamic traffic scenarios, including considerations of the distance-adaptive modulation technology. Due to the non-uniform spectrum usage, some spectrum utilization penalty may exist, leaving fragmented spectrum in OFDM links that is difficult to use. Therefore, spectrum allocation policies to minimize fragmentation have been proposed BIB003 , and spectrum defragmentation algorithms to reconfigure the network have been studied BIB004 .