Research Issues and Challenges in Elastic Optical Networks
The elastic optical network (EON) is a promising concept but its implementation remains some way off. There are several issues and challenges, which need further research to resolve [239,265]. This chapter addresses research issues and challenges faced by optical network researchers and shows some directions for further research.
Figure 13.1 summarizes the different areas demanding further work. In the following, we identify some interesting research opportunities for the EON.
Figure 13.1: Different research areas in elastic optical networks.
Innovative and sophisticated devices and components must be developed in order to achieve high capacity spectrum efficient elastic optical networks. Novel optical switching and filtering elements need to be developed in order to provide efficient client protocol data unit mapping procedures that extract the incoming client signal via client-specific physical coding sublayers and media access controller layers, high resolution and steep filtering performance, optimum modulation format for bandwidth variability and higher nonlinear impairment tolerance, etc.
One of the important challenges faced by the research community is to develop a new sliceable bandwidth-variable transponder, which supports sliceabil- ity, multiple bit rates, multiple modulation formats, and code rate adaptability. However, 100 Gb/s orthogonal frequency-division multiplexing (OFDM) transponders can adapt to lower bit rates. Therefore, fractional-bandwidth services may be provided with the use of the same device. This kind of technology and the use of optical integrated circuits may offer compact and cost-effective implementation. Similarly, the need for multiple temperature-stabilized, frequency controlled lasers can be improved by phase-locked carrier generation from a single laser source.
Recently, space division multiplexing (SDM) technology [266-268] has been incorporated into elastic optical networks in order to develop high-capacity, next- generation, and few-mode/multicore fiber infrastructures. The realization of this type of infrastructure should be enabled by the development of novel multidimensional spectral switching nodes, which can be fabricated by extending the designs of existing flexible spectrum selective switch (SSS) nodes, through the addition of advanced mode/core adapting techniques.
To achieve a long transmission reach, optical signals must be amplified at periodic regeneration points along the fiber span to compensate the power loss experienced in multi-core fiber. For regeneration, one technique is to demultiplex the SDM signals into multiple single-core fibers and then amplify the signals in each fiber using conventional single-core erbium-doped fiber amplifiers (EDFAs). The amplified signals are then recombined and injected back into the multi-core fiber span, which increases the system delay. Therefore, to develop a single-core power transient-suppressed EDFA (TS-EDFAs) is one of the challenging research goals that must be accomplished; a key issue is to reduce the time taken to adjust the operation point of the amplifier since a newly added signal may suddenly change the total power at the EDFA input.
The development of high-performance-sophisticated bandwidth variable transponders is essential in order to synchronize transmitter and receiver ends during hitless defragmentation. In addition, bandwidth-variable optical switches and filtering components need to be developed to execute efficient protocols and support finer granularity system. Without high-performance-sophisticated devices and components, it is impossible to perform hitless defragmentation for finely-granular systems and suppress fragmentation; regardless of developments hop retuning is not possible in finely-granular systems, such as 2.5 GHz systems, due to the lack of adequate filtering components. Therefore, development of hardware and optical devices is one of essential research topics, and must be emphasized.