## 4 Network Layer: The Main Building Block of Fixed Networks
### F1G PONs
#### 4.1.10 Outlook into Other Approaches for Access  
Despite the aforementioned PON standards, various other solutions have been proposed in the literature that incorporate different characteristics and, in many cases, outperform already standardized approaches. In particular, the wish list for a \"fresh\" PON standard, which will be able to dominate over the deployed ones, includes: cost-effectiveness, higher transportation capacity, greater coverage, higher number of supported users, scalability, flexible resource allocation and low power consumption. As a benchmark against these proposed, however, not-yet-standardized solutions, we use the TDM-PON, which is adopted by various deployed standards such as APON, BPON, GPON, EPON, XG(S)-PON and 10G-EPON. The potential solutions discussed in this section include UD-WDM-PON, Nyquist-WDM-PON, OFDMA-PON, CDMA-PON, coherent-PON, super-PON. The first three PON options have been proposed as an attempt to exploit the remaining available spectrum as efficiently as possible. Next, coherent-PON can increase the overall bit rate through the data transmission, both in amplitude and phase of the carrier signal. Finally, super-PON can increase the transmission distance through the use of optical amplifiers as well as reduce the network costs through central office consolidation. Figure 11: The wavelength chart of PONs (The deep red parts of the axes, uplink—downlink, denote occupied wavelengths). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Era** & **PON** & **Motivation for PON** & **QoS Support** & **Security** \\ \hline \multirow{4}{*}{\(\mathrm{F1G}\) (Narrowband-to-Wideband Era)} & \multirow{4}{*}{APON/ BPON} & ATM: Promising technology for broadband & SLA & \\  & & & & four traffic classes (priorities from \\  & & & & high to low): fixed, assured, \\  & & & legacy access systems & non-assured, best Effort & \\ \hline \multirow{4}{*}{\(\mathrm{F2G}\) (Wideband Era)} & \multirow{4}{*}{EPON} & Increase of Internet traffic & \multirow{4}{*}{802.1Q eight traffic classes (priorities from high to low):} & \multirow{4}{*}{AES encryption} \\  & & Low-cost Ethernet-LANs & network control, internetwork & \\  & & Need to fully support of all & control, voice (delay \(<\)10 ms), excellent effort, best effort, background & \\ \cline{1-1}  & & &  
_UD-WDM-PON_  
The ultra-dense WDM-PON, (UD)-WDM-PON, solution paves the way to the \"wavelength-to-the-user\" (WTTU) concept that is illustrated in Figure 12[43, 44]. This option shows similar advantages with TDM/WDM-PON in terms of diversity, scalability, higher capacity per user and coexistence with current standards, while it extends the spectral efficiency due to the ultra-dense WDM scheme. (UD)-WDM-PON can be realized with either coherent detection or employing narrow-selectivity filters. In the former case, which is essentially a special case of coherent-PON, a higher power budget can be attained, increasing the overall number of users and the transmission length, but with a significantly greater cost. In the latter case, low-cost components can be used, such as an ultra-dense AWG and direct detection receivers. The main drawback of this case is the significant losses due to the cascadeability of the tunable filters, which cannot achieve very narrow selectivity, extensive tunable range as well as required stability. _Nyquist-WDM-PON_  
The Nyquist-WDM-PON [45] is a solution which aims to minimize the overall number of transceivers. This is achieved by aggregating multiple transceivers onto a single hub transceiver. The hub transceiver uses coherent technology and multiple combined subcarriers based on the Nyquist criterion into a single optical wavelength, as it is illustrated in Figure 13. In this way, a 100G or a 400G wavelength can be divided into 4 \(\times\) 25 Gb/s or 16 \(\times\) 25 Gb/s subcarriers, respectively, or even into a combination of different number of subcarriers and line rates.