A Survey on OFDM-Based Elastic Core Optical Networking <s> d) Bandwidth-Variable WXC Node Architecture <s> We demonstrated, for the first time, a novel spectrum-efficient elastic optical path network for 100 Gb/s services and beyond, based on flexible rate transceivers and variable-bandwidth wavelength crossconnects. <s> BIB001 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> d) Bandwidth-Variable WXC Node 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> BIB002 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> d) Bandwidth-Variable WXC Node Architecture <s> The trends, architecture, and performance of wavelength-selective switches (WSS) are analyzed in the context of their application to reconfigurable optical add/drop multiplexer (ROADM)-based optical networks. The resulting analyses define the requirements for the latest generation of ROADM systems and provide insight into the critical specifications of this technology. In addition, the current trends for WSS technology are reviewed in the context of synergies with the strengths of different switching technologies. <s> BIB003 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> d) Bandwidth-Variable WXC Node Architecture <s> A gridless dynamic multi-granular optical network supporting flexible spectrum allocation is proposed and experimentally demonstrated to efficiently accommodate high-speed traffic and increase channel density for lower speed traffic leading to improved network efficiency and scalability. <s> BIB004
A BV-WXC can be built employing bandwidth-variable WSSs mentioned above, using a broadcast-and-select architecture BIB002 [76] or a non-broadcast-and-select architecture BIB004 , as shown in Fig. 16 . A BV-WXC structure needs to support colorless (any service add/drop to any wavelength), directionless (any service add/drop to any direction), and contentionless (add/drop the same wavelength from different direction) features as described in  . In the broadcast-and-select architecture, the input signals are broadcast to all output ports and the appropriate channels are selected on each output port using the bandwidth-variable WSS. In the non-broadcast-and-select architecture, the input signals are demultiplexed by the bandwidth-flexible WSS, and then routed to different output ports using an optical crossconnect, and finally combined by a coupler at the output port. Optical splitters are used for providing multicast functionality. A six-node elastic optical network using broadcast-andselect WXCs and liquid crystal-based bandwidth-variable WSSs is demonstrated in BIB001 . The BV-WXC provides variable bandwidth in multiples of 50 GHz to accommodate elastic optical paths, as well as multicast functionality. A 44 LCoSbased WXC with variable channel bandwidth was experimentally demonstrated  . The channel bandwidth and center frequency can be tuned with minimum setting resolution of 1 GHz. A network experiment using non-broadcast-and-select bandwidth-variable WXC nodes was demonstrated in BIB004 , providing multiple data rate and flexible spectrum switching with 12.5 GHz granularity using LCoS-based WSS. Table IV summarizes current research and the capabilities of the bandwidth-variable WSSs. As noted in BIB004 [93], the choice of switching granularity is a compromise between operation complexity and flexibility. Standardization work on the elastic channel grid specification is on-going, and will be described in Section IV-D1. The first commercial bandwidth-variable WSS product is now available BIB003 [88], and will be further improved as the standards move forward.