A Survey on OFDM-Based Elastic Core Optical Networking <s> B. OFDM-Based Elastic Optical Network Architecture <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> B. OFDM-Based Elastic Optical Network Architecture <s> We consider the Routing and Spectrum Allocation (RSA) problem in an OFDM-based optical network with elastic bandwidth allocation. We asses the spectrum utilization gains of this flexible architecture compared to a traditional fixed-grid rigid-bandwidth WDM network. <s> BIB002
In the OFDM-based elastic optical network architecture, multiple data rate sub-wavelength or super-wavelength paths are realized through flexible granular grooming and switching in the spectrum domain, using data-rate/bandwidth-variable transponders and bandwidth-variable WXCs. The data-rate/bandwidth-variable transponder allocates just enough spectrum (subcarriers) to accommodate subwavelength traffic, known as spectrum slicing BIB001 . Several OFDM channels can be merged together into a super-channel, transporting a multiple of the capacity of an individual OFDM channel without spectrum guard band in between, as shown in Fig. 12(a) . Every WXC node on the optical path allocates a crossconnection with the appropriate spectrum bandwidth to create an appropriate-sized end-to-end optical path. When the traffic increases, the transmitter can increase the transmission capacity, and every WXC on the route expands the switching window, thus enabling data-rate/bandwidth-variable optical paths. Fig. 12(b) shows the architecture of the OFDM-based elastic optical network. The benefits of this OFDM-based elastic optical network architecture are as follows. 1) Supports flexible granularity service aggregation, enabling sub-wavelength, super-wavelength, and multiplerate data traffic accommodation, e.g., Gb/s to Tb/s. 2) Provides high spectrum efficiency through flexible spectrum allocation according to the transmitted data rate. It has been verified that the spectrum utilization of the elastic optical network improves by 5-95% compared to a fixed-grid WDM network, although the precise spectrum utilization improvement depends on the network topology and traffic pattern BIB002 . 3) Supports reach-adaptable line rate, as well as dynamic bandwidth expansion and contraction, through adjusting the number of subcarriers and modulation formats. 4) Supports energy-efficient operations to save power consumption by turning off some of the OFDM subcarriers when there is not enough traffic for transmission. 5) Enables optical network virtualization with the virtual link supported by OFDM subcarriers. As the OFDM-based elastic optical network offers finer bandwidth granularity than WDM and has a coarser granularity than optical packet switching (OPS), it is considered as a middle-term alternative to the as-of-yet immature OPS technology BIB001 . As a new and promising architecture, the OFDM-based elastic optical network has a variety of issues that need to be resolved including re-designing node devices; improving network planning, traffic engineering, and control plane technologies; as well as enhancing current standards. At the node level, novel data-rate/bandwidth-variable transponders and bandwidth-variable WXCs need to be developed. At the network level, flexible spectrum allocation and routing algorithms, traffic grooming approaches, survivability strategies, network virtualization, energy efficiency, and control plane technology need to be studied. Table II summarizes the current research in the aforementioned areas.