IEEE Network • March/April 2012 28 0890-8044/12/$25.00 © 2012 IEEE iber-wireless (FiWi) access networks offer to combine the robustness and high capacity of optical access net- works with mobility, ubiquity and flexibility of the wireless access networks in the last mile of the Inter- net. A FiWi network consists of an optical back-end that employs a passive optical network (PON) technology, such as Ethernet PON (EPON) or wavelength-division multiplexing (WDM)-PON, and a wireless front-end technology, such as IEEE 802.11g, IEEE 802.16j, IEEE 802.16m, Third Genera- tion Partnership Project (3GPP) Long Term Evolution (LTE), or LTE-Advanced (LTE-A) [1]. Meanwhile, telecommunica- tion network architecture is evolving into a simpler infrastruc- ture where metro and access network capacities are consolidated in the long-reach passive optical network (LR- PON). LR-PON reduces operational expenditures by simplify- ing the telecom network infrastructure in addition to reducing the deployment cost per subscriber [2]. Furthermore, as a recent wireless access technology, LTE-A offers high data rates and supports large numbers of users. The advantages of LR-PON and LTE-A can be combined in the extended-reach FiWi by employing LR-PON in the optical back-end and LTE-A in the wireless front-end. On the other hand, extend- ed-reach FiWi has several challenges that need to be addressed. The optical back-end is subject to high propagation delay due to the extended reach, whereas the wireless front- end is challenged by meeting the targeted peak data rates up to 1 Gb/s (500 Mb/s) in downlink (uplink) using the existing Universal Mobile Telecommunications System (UMTS) [3]. Figure 1 illustrates the new telecommunication network infrastructure when metro-access convergence becomes widely adopted by LR-PON. As seen in the figure, convergence of fiber and wireless access networks further decreases the cost of running fiber to each user while adding the flexibility of wireless technologies in terms of mobility support. Hence, FiWi networks in long-reach access seem to be a break- through for the last mile of the Internet. Since mid-2000s, the dramatic increase of greenhouse gas (GHG) emissions on a global scale and the increasing contri- bution of information and communication technologies (ICT) to those GHG emissions have led to research toward energy- efficient design and management of telecommunication net- works. In the last few years, energy-efficient solutions for backbone networks have been proposed in several works, which consist of energy-saving architectures, energy-efficient design/planning, and energy-efficient protocols [4]. Access networks have not been the main focus in the energy-efficient ICT research since power consumption of a single access net- work component is significantly lower than the power con- sumption of a core network component. For instance, a single active IP core router port consumes around 1000 W, while a single optical network unit (ONU) in an EPON consumes around 5 W when all of its components (transmitter, receiver, serializer/deserializer) are on. However, since access networks form a big portion of the entire telecom network, employment of energy saving architectures and protocols in every single access network accumulatively decreases the overall energy consumption and consequently the GHG emissions of the telecom network. To the best of our knowledge, the research in [5] is one of the initial studies that consider putting the ONUs in the sleep mode when they are idle during uplink and downlink dynamic bandwidth allocation (DBA). This article provides an overview of the energy efficiency mechanisms for FiWi access networks. Furthermore, consider- ing energy-efficient DBA protocols for FiWi networks, it focus- es on a recently proposed scheme, EPON-LTE Deployment for a Green Access Network (ELEGANT) [6]. ELEGANT was ini- tially proposed for convergence of EPON and LTE-Advanced technologies in the conventional OLT-ONU reach of 20 km. Therefore, it introduces a delay penalty for long-reach access where ONU-optical line terminal (OLT) distance is extended to 100 km and beyond. In the long- reach access, energy-effi- ciency of the optical back-end of a FiWi network is shown to be F F Burak Kantarci and Hussein T. Mouftah, University of Ottawa Abstract Telecommunication networks call for novel energy-efficient design and management schemes as a result of the increasing contribution of the ICT sector to electricity consumption and greenhouse gas emissions. Access networks, being one of the significant contributors in the last mile, require power saving protocols and archi- tectures. As one of the emerging access network solutions, convergence of PONs and wireless access networks, also named as FiWi, offer to combine the robust- ness and high capacity of optical networks with the mobility and ubiquity of wire- less networks. In this article, we present an overview and a brief comparison of energy-efficient protocols and design approaches in FiWi networks. We further propose an energy-efficient bandwidth allocation mechanism in FiWi networks that adopts an optical burst switching (OBS)-like report generation mechanism in LR- EPON. Through simulations, we show that the proposed scheme leads to signifi- cant energy savings in the long-reach FiWi network while overcoming the delay penalty of the ONU-BS sleep modes. Energy Efficiency in the Extended-Reach Fiber-Wireless Access Networks