## 7 Passive Optical Network
### TWDM-PON Application  
The TDM-PON scheme employment in the 4G networks presents a cost-effective solution for channel sharing among the cell sites. This is realized through diverse time slots allocation for some cell sites. Nonetheless, with the mobile network evolution and the related defined use cases such as uRLLC, eMBB, and mMTC, the TDM-PON solution might not be an appropriate transport network for the 5G and beyond networks. Consequently, a hybrid TWDM-PON solution offer relative advantages that can help in supporting the network demands while simultaneously enhancing the system performance. In the TWDM-PON scheme, apart from the time slots that are normally allocated dynamically, wavelength resources can be allotted flexibly between the DUs. Furthermore, the presented virtualized and centralized PON BS can extensively facilitate the system energy saving capability. Similarly, the virtualized scheme offers several benefits like better network reliability, excessive handover reduction, and low handover delay. In consequence, advantages such as cell-edge user throughput enhancement, cost-saving, and improved mobility management can be observed [144; 404; 405]. In general, the related multiple wavelengths and the wavelength tenability potentials offer TWDM-PON the edge over the TDM PONs in enhancing the network functionalities [404; 405]. Moreover, OFDM-PON is adifferent and attractive PON scheme. It is noteworthy that, an OFDM-PON solution offers a comparable capability for sharing bandwidth resources dynamically as observed in the TWDM scheme. Conversely, the OFDM options in which direct detection (DD) schemes are adopted normally offer relatively poor performance concerning the network reach. Correspondingly, OFDM variants that employ coherent detection schemes are relatively uneconomical [8]. In addition, among its contemporaries like Optical Code Division Multiplexed PON (OCDM-PON), standard WDM-PON, and OFDM-PON, that can support 40 Gbps or even more (80 Gbps) aggregated bandwidth, the Full-Service Access Network (FSAN) community has selected TWDM PON as the most important broadband solution. Moreover, the TWDM PON can support a long reach of 40 km while offering high capacity with a 1:64 splitting ratio. Consequently, based on its salient features, the TWDM PON solution can address the current and future broadband services demands [143, 405, 406, 407]. Moreover, apart from the fact that a typical TWDM PON system offers support for multiple wavelengths, the associated wavelengths can also efficiently coexist on a shared ODN infrastructure through a WDM scheme. In addition, another salient benefit is that each wavelength can effectively serve multiple ONUs via the TDM access scheme. Based on the ITU-T recommendation, 4-8 wavelengths in the L band (1590-1610 nm) and the C band (1520-1540 nm), can be utilized for the DS and US transmissions, respectively, over the TWDM PON system. Moreover, each DS wavelength is capable of operating at 10 Gbps, whereas each US wavelength functions at either 2.5 Gbps or 10 Gbps data rate [144, 405]. Furthermore, a notable feature of the TWDM PON solution is that colorless tunable transceivers are used at its ONUs to facilitate selective transmission/reception of any related US/DS wavelengths through a wavelength pair. Consequently, the ONU inventory concern can be checked with TWDM PON implementation. Fundamentally, apart from the fact that its TRx's appealing characteristics facilitate ease of network deployment and inventory management, the TWDM PON solution can also offer effective support for load balancing in the system. In addition, the employed dynamic wavelength and bandwidth allocation (DWBA) algorithm enables flexible exploitation of large bandwidth. Note that the TWDM PON (NG-PON2) scheme is the industry's first multi-wavelength access standard that supports up to 4 wavelengths with further increment to 8 wavelengths in the future. Therefore, the approach offers a pay-as-you-grow wavelength solution on each fiber.