# Advances in Improving Energy Efficiency of Fiber-Wireless Access Networks: A Comprehensive Overview
## 3 Energy Efficiency Analyses of Radio-and-Fiber Networks
### Power-Saving Techniques in the Optical Domain of FiWi Networks

Table 1: _Cont._In ONU dozing mode, the ONU transmitter is powered off for a considerable amount of time, while the ONU receiver maintains an active state at all times. This corresponds to even lower instantaneous power consumption of the ONU dozing mode when compared with the power consumption of the power-shedding mode (Figure 3). The ONU sleeping modes are, according to [24], further divided into two subgroups: deep sleep and fast sleep (also known as cyclic sleep) (Figure 3). The ONU sleeping modes are characterized by powering off both the ONU receiver and the transmitter for a considerable amount of time. When using ONU sleeping mechanisms, it is necessary to make a trade-off between energy savings and network performance. Due to powering off the receiver and the transmitter during the entire PS mode period, during the ONU deep sleep mode maximum energy conservation can be achieved at the cost of significant performance degradation. On the other hand, during the ONU fast/cyclic sleep mode, the transmitter and the receiver alternate between on and off periods, forming sleep cycles, which results in lower energy savings when compared with the energy savings of deep sleep mode (Figure 3). Another PS method in PONs is the implementation of the adaptive link rate (ALR) concept. It is based on energy reduction that can be achieved by adapting different transmission rates between optical devices [51]. In optical access networks where multiple data transmission rates are available (e.g., GPON and G-EPON), reducing transmission rates in low-traffic periods can contribute to the reduction of the energy consumption of the optical unit and this reduction improves network EE. Moreover, these PS techniques can be combined with ONU sleep modes for even greater energy savings. The authors in ref. [25] proposed a hybrid scheme that combines the ONU sleep mode with ALR mechanisms in order to improve the EE of 10G-EPON systems (Table 1). Such a hybrid PS scheme activates the ONU sleep mode in the absence of network traffic, while in the presence of network traffic, the ALR function for adapting the downlink data rates is activated based on the levels of traffic load. It was shown in ref. [25] that such a hybrid PS approach can contribute to significant energy consumption reductions in the optical part of the network. In addition to the aforementioned PS modes, a newer PS scheme called watchful sleep mode (WSM) has emerged and is also included in all major PON standards [52; 53; 54]. The WSM acts as a unified solution that combines cyclic sleep and doze PS modes. It eliminates the need for mode negotiation between optical units and maximizes the ONU's EE. It is expected that WSM will be used as the only PS mode for PONs in future (Table 1) [26]. In ref. [27], the authors conducted a performance comparison between PS fast/cyclic sleep modes and a WSM for a GPON ONU and concluded that both approaches offer similar energy savings. However, the WSM performed better regarding state transition latency (Table 1). The simulation results in ref. [28] showed a decrease in downstream and upstream transmission delays with a significant energy savings when WSM was implemented in combination with dynamic bandwidth allocation (DBA) in a 10-gigabit-capable passive optical network (XG-PON) (Table 1). In addition, the authors in ref. [29] recently proposed a new energy-efficient scheme called adaptive watchful sleep mode (AWSM). The AWSM increases the energy savings of the standard WSM scheme by minimizing the