## 4 Network Layer: The Main Building Block of Fixed Networks
### _Ofdm-Pon_
The operation of a typical coherent transmitter and receiver used to generate and receive a QAM signal, respectively, is illustrated in Figure 16. In practice, simplified coherent reception techniques are proposed that can trade physical layer performance for a lower cost [53]. Figure 15: OCDMA-PON architecture. Super-PON  
Super-PON is a PON variant which can exploit WDM, wavelength routing and optical amplification in order to increase the number of users and the distance between OLT and ONU. In a typical PON, the number of end users for a link length of 20 km is limited to about 64. In super-PON, due to the significantly improved power budget, the link length can be increased to 50 km and the number of end users can be increased to 1024 (Figure 17). The reach extension from 20 to 50 km also enables a CO consolidation, which leads to a reduction of the overall number of powered sites and, as a consequence, leads to a decrease of the overall network operational costs. The impact of network consolidation in a network with a 50 km radius is depicted in Figure 18. As is evident, the number of COs, in this case, can be reduced from nine down to only one. For the majority of European cities, this CO reduction can lead to about 25% lower infrastructure building costs compared with XGS-PON [56]. In this direction, super-PON will be defined in ITU-T recommendation G.9807.3. Figure 16: Illustration of (**a**) transmitter and (**b**) receiver employed for coherent transmission. Figure 17: Scalability and reach of (**a**) typical PON and (**b**) super-PON [56]. Figure 18: Architecture used to cover an access network with a 50 km radius using (**a**) typical PON and (**b**) super-PON [56]. Exploiting oDAC to exceed 1 Tb/s rate  
Integrated photonics can provide more compact and cost-attractive component solutions in the long run. Recently, the direct digital to optical domain (D/O) conversion has been studied as an attempt to replace the electronic digital to analog converter (eDAC) intermediaries [57, 58]. EDACs are key conversion interfaces from the digital to the analog domain. In high-speed digital optical transmitters, the D/O translation typically comprises power consuming and costly eDAC-based data conversion, followed by analogue optical modulation. In a conventional optical system, eDACs are used to process multiple inputs of bitstreams in order to create analog multilevel signals. From the viewpoints of performance, energy efficiency, complexity and cost reduction, it would be advantageous to adopt direct D/O conversion and eliminate the eDAC intermediaries. This would be particularly useful in future generations along the roadmap of ultra-high-speed photonic interconnects, as eDAC technology is becoming more expensive, making increasingly challenging to keep scaling up in data rates, while keeping power dissipation at reasonable levels and maintaining the required effective number of bits (ENOB) performance. Over the last decade, the predominant activity in the oDAC research area has pertained to \"serial\" structures based on partitioning of the electrodes of the basic Mach-Zehnder modulator (MZM) (Figure 19). The so-called segmented Mach-Zehnder modulator (SEMZM) [57] breaks up the MZM electrodes pair along the two MZM waveguides into multiple (shorter) electrically isolated segments. Each modulation segment amounts to an elementary 1-bit modulation gate driven by a separate binary signal. The summation of the optical phases induced by the array of 1-bit segments along the device generates a multilevel output optical signal. As an alternative approach, a \"parallel\"-based design employing electro-absorption modulators (EAMs) instead of MZMs has been proposed in [59]. The compact size of the EAMs and their ease of integration with silicon photonics make them a really lucrative option.