**1. Introduction**

In recent years, with the proliferation of cloud computing and high-definition media streaming increasing the use of communications and information technology in photonic infrastructure. Fiber capacity crunch concerns are driving optical networking toward a spectral-efficiency-conscious design philosophy. Moreover, as the number of various high-capacity services increases such as video delivery service and data centers, transport network flexibility will become important, resulting in the demand of elastic transport optical networking [1–5]. Key methods for empowering the elastic/flexible optical system are: single SDN XCVR, BW increase, flexible spectrum approach, Multicore fiber using the SDM technology. Nonetheless, it would be cost-restrictive to convey an entirety set of advances.

Therefore, in the next generation optical communication, optical nodes will need to allocate resources in an elastic and effective way to professionally provision high and low data rate signals [6]. To represent this evolving network scenario, an elastic network is proposed and demonstrated, providing elastic resource allocation in spectrum as a means to address the disparity between required and allocated bandwidth. By using this technique it is possible to assign a customized bandwidth per channel depending on specific requirements. Elastic allocation in the spectral domain implies that the standard 50-GHz ITU grid is not used and a continuous spectrum can be allocated to accommodate high-capacity channels with large bandwidth requirements. Also, channels that require lower bandwidths can be accommodated more efficiently by using narrower channel spacing as long as the performance is not compromised. Next generation optical networking is expected to cope with a number of challenges, especially in terms of elastic optical nodes [7], as there is always a need of increasing flexibility in the allocation of spectral and temporal resources so that they are able to efficiently support on-demand services and functionality.

Apart from the obvious advances in capacity and performance, it is clear that with each progressive stage of evolution of the optical node, additional flexibility has been introduced, e.g. the introduction of wavelength granularity, the ability to add/drop individual wavelengths, reconfigurability, etc. Other types of functionality may also add flexibility to the system, e.g. wavelength conversion (WC), format conversion (FC), multicasting (MC), regeneration, etc. In [8] a broadcast and select node based on bandwidth-variable wavelength selective switches (BV-WSS) was proposed. However, the very nature of this architecture will restrict upgradeability and will limit support for evolving requirements and new functionalities, e.g. optical signal processing. However, in Reconfigurable Optical Add Drop Multiplexer (ROADM) architectures [9] it is difficult to introduce additional functionality due to the fact that several wavelengths are simultaneously switched over the same port. On the contrary, OXCs support additional functionality more naturally as wavelengths are split and switched individually. Thus, modules with the required functionality to operate on individual wavelengths can be positioned in the right place within the OXC. However, the requirement for a particular signal processing function is often uncertain, e.g. it may be required for some wavelengths at some time period and for other wavelengths at a different time period. Therefore, modules that provide a per-channel functionality are generally deployed for all wavelengths as there is no possibility of sharing them among several optical paths inside the OXC. A better solution would enable modules to be shared, thus improving modules' utilization and reducing the amount of modules required to satisfy a given demand for the offered functionality, i.e. better hardware utilization and efficiency.

WDM networks utilize routing and wavelength allocation (RWA) algorithms to find available resources for new requests. However, in elastic optical networks the problem is more difficult due to the new flexible spectrum allocation, where elastic spectrum bands rather than single wavelengths are considered. For new requests with specific bandwidth requirements, routing and spectrum allocation (RSA) algorithms need to identify sufficiently wide spectrum slots that are available from source to destination. Furthermore, as channels are added and removed, they leave behind noncontiguous slots of free spectrum. Although these fragments may add up to a considerable amount of bandwidth, new channel requests may be blocked due to the lack of sufficient contiguous spectrum [10]. Spectrum fragmentation may be prevented to some extent by introducing appropriate policies in RSA algorithms. Alternatively, techniques to defragment the spectrum may also be utilized, e.g. using wavelength conversion.

### *DOI: http://dx.doi.org/10.5772/intechopen.88354 All Optical Signal Processing Technologies in Optical Fiber Communication*

Another issue of optical node is that, it should manage transport of blend of suppliers' traffic facilities for legacy signals, core traffic and multiple format signals with variable bit rate. Thus, flexible optical nodes will ought to assign resources in an adaptable and proficient way to back a blend of super-channels and lower speed channels. The nodes' density will mainly influenced on the link interoperability between segments (i.e. core segment: need signal processing and to support superchannels, metro segments: might carry legacy 10 Gb/s [11] links).

To address increasing traffic growth, the most straightforward and economical way in which this can be done is to deploy additional 10G wavelengths. Thus, new 10G wavelengths are placed 50 GHz, according to the standard WDM grid, until the available bandwidth is exhausted. However, the maximum capacity that can thus be provided is 800 Gb/s (i.e. 80 × 10 G using only the C band), which is already in sufficient for heavily used backbone network links. Furthermore, providing additional capacity in this manner is highly inefficient in terms of the spectral resources that are consumed. The immediate solution to this problem is to deploy 100 G links, despite their higher cost compared to 10 × 10 G. 100 G is more spectrally efficient than 10G as it can fit in a standard 50-GHz WDM slot, advance modulation formats (like DP-QPSK), coherent detection, extensive use of Forward Error Correction (FEC) and electronic impairment mitigation. However, this solution is expected to be viable only for the short and medium terms. For super-channels at 400 Gb/s [12], 1 Tb/s [13] and beyond [14, 15] will occupy broader spectrum, require more complex multilevel modulation formats with higher OSNR and consequent shorter reach which neither fits within the existing ITU grid nor is supported by conventional optical network infrastructures. For instance, optical cross-connects and ROADMs allocate only discrete 50-GHz slots of bandwidth due to their internal WDM (de)multiplexers. Channels that require wider bandwidths are severely distorted if passed through such devices. Therefore, in order to efficiently support high-speed channels, a flexible bandwidth infrastructure is required.

In spite of the increasing popularity of elastic optical networks, there has been very little work focusing on elastic node architectures. Therefore it is critical to investigate the details of realizing optical networking solutions toward flexible and efficient allocation of network resources. Further, provide fully functional intelligent infrastructure for simultaneously supporting the switching and transport of combination of high-capacity super-channels and lower bit rate channels [16]. How elastic nodes support dynamic and on-demand provisioning of functionality, such as spectrum defragmentation, wavelength conversion, regeneration, grooming, format conversion, time multiplexing, etc. leaves the door open for future research.
