## 4 Network Layer: The Main Building Block of Fixed Networks
### F1G PONs
#### 4.1.6 Fqc--2010-2018 (Ultra-Broadband Era)  
**NG-PON2--ITU-T Rec. G.989.x:** The 40 Gb/s capable next-generation PON 2 (NG-PON2) was standardized (2015) not only to increase the PON scalability and the transmission rate (at 40 Gb/s), but mainly to offer backward compatibility with the TDM-PONs: GPON, XG-PON, EPON and 10G-EPON. These are achieved through wavelength division multiplexing (WDM) both in the downlink and the uplink [11,27]. Instead of using one wavelength per direction, the use of w (>1) wavelengths increases the transmission rate by w, while the user may have ample bandwidth by getting the entire wavelength bandwidth from the ONU/ONT. In addition, NG-PON2 supports legacy services (such as POTS, T1 and E1) using emulation [28]. Moreover, the introduction of multiple wavelengths made the study of PONs more challenging and attractive. For instance: (a) blocking analysis could be performed in case that the number of available wavelengths in the PON is less than the required number of wavelengths by the ONUs/ONTs [29]; or (b) dynamic wavelength allocation (DWA) among ONUs/ONTs could be introduced for an efficient use of the available wavelengths according to the needs [30], as well as for power saving in PONs (see paragraph 4.1.8., method of \"Shutdown Wavelength(s)\"). (c) The DWA strongly depends on the characteristics of the offered traffic and therefore several features of traffic could be investigated [31,32]. Needless to say, the cost of such a WDM-PON is high, due to the need to install variable wavelength laser transmitters, so that each ONU/ONT can transmit to any of the wavelengths supported by the network (colorless ONU/ONT). The most promising solution is to reuse the wavelength that is assigned to each user for downstream and upstream transmission. A component of the transmitted signal from the OLT is modulated by each user in the inverse return-to-zero format, while the upstream signal is modulated/amplified by SOAs (semiconductor optical amplifiers) in the return-to-zero format. This solution does not require the installation of optical transmitters for each user; thus, the cost is significantly reduced. An alternative solution is to use low-cost tunable lasers as transmitters (no need for expensive thermoelectric coolers and wave-lockers). In the uplink, tuning is achieved by utilizing the common OLT wave-locker. Nevertheless, tunable receivers are needed at each ONU. Moreover, a self-seeded scheme could be used for the colorless operation, with the severe drawback of limiting the reach of the WDM-PON. Due to the use of different wavelengths per ONU/ONT, a WDM-PON offers substantially better security than a TDM-PON. The NG-PON2 resulted from the combination of TDM and WDM technologies and is characterized as a TWDM-PON [12, 28, 33]. The NG-PON2 (Figure 8) is based on a splitter, while in order for an ONU/ONT to select a wavelength, it must use a wavelength filter. _DS/US Rates and Wavelengths:_ Four to eight TWDM wavelength (channel) pairs are introduced. The data rates of 10 Gb/s and 2.5 Gb/s are symmetrically offered per wavelength, while 10 Gb/s and 2.5 Gb/s are offered per wavelength in the downlink and uplink, respectively. Thus, for eight wavelengths, the maximum rate is 80 Gb/s. The DS wavelengths are in the L-band, with a range of 1596-1603 nm, while the US wavelengths are in the C-band, with a range of 1524-1544 nm (wide band), 1528-1540 nm (reduced band) or 1532-1540 nm (narrow band). _Splitting ratio and Maximum reach:_ A split ratio of at least 1:256 is assumed, and a minimum ONU-OLT distance of at least 40 km is considered without reach extenders. The high splitting ratio (even through multistage splitting) and the support of a high number of users are a must in order to justify the high system cost. On the other hand, higher splitting ratios and the number of users substantially increase the required power budget between OLT and ONU/ONT, while lasers of higher transmitting power and receivers of higher sensitivity are needed.