## 4 Network Layer: The Main Building Block of Fixed Networks
### F1G PONs
#### 4.1.8 Power Saving in PONs  
Since PON is the most promising access technology and is going to be widely deployed, it is worth taking a separate look at possible power-saving techniques, both in order to operate at a low cost and for sustainability reasons. According to Supplement 45 of the ITU-T G-series recommendations (GPON power conservation), three basic modes of operation have been adopted for ONUs/ONTs being in a power-saving state [38, 39]:  
* ONU/ONT Shedding--Receiver and transmitter operate with a minimum power (even zero) as long as they can maintain a fully operational optical link. (Estimated power savings: up to 60% from full power mode). * ONU/ONT Dozing--Transmitter operates at a minimum power (even zero), but the receiver remains continuously on. * ONU/ONT Sleeping (estimated power savings: up to 90% from full power mode): (i) Deep Sleep--Transmitter and receiver are switched off for the entire duration of the power save mode. (ii) Fast Sleep--Transmitter and receiver are switched OFF and ON repeatedly during the power save mode. In addition, three more techniques have been considered for power savings depending on the type of PON:Bit-interleaving--In the downstream direction, all users must process all data in order to select their own data, which are usually a small percentage. More precisely, after the clock and data recovery (CDR) stage (Figure 9), all data are aligned and processed (e.g., Forward Error Correction) at a full-rate, and then the own data are selected (to be further processed at a lower user rate). In the \"bit-interleaving\" PON, the CDR is accompanied by a decimator capable of dropping out all useless data, and thus the own data can be processed immediately after the CDR stage at the lower rate. In this way, the energy consumption at the ONU/ONT is substantially reduced. This method can be applied in addition to the sleep mode of operation [28]. * Adaptive Link Rate--In PONs such as GPON and 10G-EPON, where a transmission rate can be selected from a set of available rates (i.e., 1 Gb/s, 2.5 Gb/s, and 10 Gb/s), energy consumption can be reduced by selecting the lower possible rate, since the higher the transmission rate, the higher the energy consumption [38]. * Shutdown Wavelength(s)--The OLT plays a dominant role in PON, not only by scheduling the sending and receiving of data but mainly to synchronize the ONUs/ONTs. Thus, it is not a good practice to restrict the operations of OLT in order to reduce energy consumption. Nevertheless, in the case of NG-PON2, where multiple wavelengths are available, energy can be saved by utilizing a subset of wavelengths while shutting down the rest of the wavelengths [38]. Especially for the NG-PON2 or TWDM-PONs in general, several factors can be combined, and under certain circumstances, PON traffic can achieve energy savings [28]. Power saving is achieved at the expense of system complexity. In power-saving mode, ONU and ONT may lose their connectivity to the physical layer, especially in sleep mode. To hide it from the service level, the complexity of the system must be significantly increased. Software-based techniques employing effective mechanisms can leverage the ONU/ONT capabilities for energy saving, by combining a hybrid cyclic/deep sleep enabled at ONUs/ONTs, a batch-mode transmission at OLT and ONUs/ONTs and a DBA [40]. Figure 10 qualitatively depicts the ONU/ONT energy consumption according to the basic mode of operation, shown in the horizontal axis [38]. An example of the impact of ONU/ONT sleeping mode of operation is shown in Table 5[39].