## 7 Passive Optical Network
### TWDM-PON Application
Moreover, each wavelength in a particular fiber can offer 10 Gbps speed. This enables throughputs of 40 Gbps and 80 Gbps on the fiber connection for 4 and 8 wavelengths, respectively. Consequently, the scheme comprises four or eight stacks of 10 Gigabit PONs (XG-PONs) with 4 or 8 wavelengths, respectively. Moreover, each XG-PON in the stack operates on different wavelengths [406]. Moreover, as previously mentioned, in Section 7.1.2, the XG-PON as well as GPON GEM frames are compatible with and can also be used in the TWDM PON scheme. As a result of system compatibility and support for coexistence with current PON architectures, the TWDM PON scheme is a feasible solution for the swift evolution of OAN [47, 405, 44]. Thus, the TWDM transmission scheme has been embraced for the NG-PON2 access technology. Also, TWDM can be implemented with an optional PtP WDM overlay extension in the NG-PON2. Consequently, DWDM scheme implementation permits NG-PON2 to provide multiple unshared PtP connections, likewise, the TDM scheme concurrently enables multiple PtMP connections. Therefore, NG-PON2 will allow the operators to suitably offer both fronthaul/backhaul and business services with PtP WDM overlay, through the exploitation of dedicated wavelengths. Besides, NG-PON2 will facilitate optical system evolution from the usually fixed broadband infrastructure to a converged platform for supporting both fixed as well as mobile broadband services [47, 408, 409, 413, 47]. Furthermore, with reference to the TWDM PON ONUs colorless tunable transceivers, three-wavelength channel tuning time classes have been well-defined for NG-PON2 by the physical media-dependent layer recommendation (ITU-T G.989.2). Moreover, different advanced schemes can be adopted by the associated wavelength-tunable devices to be able to support different classes. Regarding the recommended classes, the Class 3 solution uses the slowest tunable devices. Thus, it is appropriate in situations with occasional tuning processes or for applications in which short service disruption can be tolerated. Conversely, Class 1-based wavelength-tunable solutions offer the shortest tuning time. Therefore, the salient feature enables the solutions to be attractive for rendering DWBA solutions in the network. Moreover, with Class 1 employment, the transmission wavelengths of ONUs can be controlled dynamically by the OLT to enable wavelength hopping between the transmission periods [410]. Furthermore, it is remarkable that NG-PON2 employs G.989 standard that supports the TWDM scheme and uses a multi-wavelength access standard [411]. In addition, apart from being an advanced PON scheme with the potential for intense revolution in the providers' operational models, NG-PON2 also presents a flexible platform that can enhance their agility to various market demands in an unprecedented manner. Moreover, it can give cost-effective support to both the scale and volume of the existing gigabit broadband services. Besides, another relative advantage is that, it has more than enough capacity to support multi-gigabit bandwidth requirements of the 5G and beyond networks with an outstanding performance [406]. Although a TWDM-PON scheme proffers good bandwidth allocation among several customers, the realization of the low-jitter and low-latency requirements of some use cases could be challenging. Therefore, its employment for the NGN RAN transport network is contingent largely on different RAN use cases and deployment scenarios requirements [8]. In addition, as stated earlier, the NG-PON2 scheme uses multiple wavelengths, and consequently, tunable transceivers are required at the ONUs. Nonetheless, the condition may hinder its application because the current optical tunable transceivers are not cost-effective [386]. As a consequence of this, some network operators have been devising means of addressing this by considering provisional schemes to be adopted prior to the full NG-PON2 migration. This approach will allow them to have a seamless evolution with the slightest or no service disruption.