GATE: Greening At The Edge

Dramatic data traffic growth, especially wireless data, is driving a significant surge in energy consumption inthe last mile access of the telecommunications infrastructure . The growing energy consumption not only escalates the operator s’ operational expenditures (OPEX) but also leads to a signific ant rise of carbon footprints. Therefore, enhancing the energy efficiency of broadband access networks is becoming a necess ity to bolster social, environmental, and economic sustainabi lity. This article provides an overview on the design and optimiza tion of energy efficient broadband access networks, analyzes the energy efficient design of passive optical networks, discus ses the enabling technologies for next generation broadband wirel ess access networks, and elicits the emerging technologies for enhancing the energy efficiency of the last mile access of the networ k infrastructure. I. BROADBAND ACCESSNETWORKS Access networks are the last mile in the Internet access. As Internet traffic surges, broadband access networks should b e upgraded to provide high network capacity. Currently, broa dband access can be offered over the following communication media: digital subscriber line, hybrid fiber coaxial cable, broadband over powerline, wireless, and optical fiber. Owin g to the immense capacity of optical fiber, optical access netw ork will be extensively deployed for future broadband access. Furthermore, the ubiquity of mobile and wireless devices warrants wireless access networks an indispensable option for broadband access. Optical (Fig. 1(a)) and wireless (Fig. 1( b)) access will be the predominant choice of the last mile access . GATE (Greening At The Edges) refers to the transformation of the access portion of communications infrastructure int o an energy efficient version, i.e., an environmentally frien dly version. Thus, the GATE’s objective is to green both wirelin and wireless access networks which currently consume a significant amount of energy of communications infrastruct ure owing to the large quantity of access nodes. A. Optical Access Networks Optical access networks feed the metro and core networks by gathering data from end subscribers. In general, there ar e three major types of optical access networking technologie s [1]: • Point-to-Point Fiberconnects fiber directly between the central office and the optical network terminals (ONTs) located at the subscribers’ homes. Since the fiber is dedicated to individual subscribers, the optical power experiences small loss, and the transmission power budget allows a distance of up to 10 km between the central office and the subscribers’ homes. Point-to-Point fiber access networks (Fig. 2(a)) has a simple network architecture and does not require expensive optical components. • Active Ethernet Network (AEN) adopts a point to multiple points architecture as shown in Fig. 2(b). It utilizes electrical equipment, e.g., switches and routers, to distribute signals, thus enabling network operators to completely control their infrastructures and provision quali ty of service at different levels. The electrical equipment aggregates fiber delivered directly to subscribers and dramatically reduce the number of fibers terminated in the central office. AEN can dedicate each subscriber with a high data rate link, e.g., 1 Gb/s. • Passive Optical Network (PON) has a similar architecture as AEN. PON (Fig. 2(c)) uses a passive optical splitter to aggregate the signal. In the downstream, the splitter divides the light signal sending from the central office and then broadcasts it to all optical network units (ONUs). In the upstream, the optical splitter aggregates the light signals coming from ONUs and transmit the aggregated signal to the optical line terminal (OLT) over fiber. The network is referred to as passive optical network because there is no optical repeaters or any active devices in the network. As compared to AEN, the optical splitter adopted in PON requires zero power consumption and maintenance. As a result, the cost of deploying PON is significantly reduced. PON is also an enabling option for radio-over-fiber for integrated optical and wireless acces s [2]. B. Broadband Wireless Access Networks Broadband wireless access networks provide wireless data communications at a comparable data rates to that of wirelin e access networks for subscribers. In general, wireless acce ss networks can be classified into three categories according t o their coverage areas: wireless local area networks (WLANs) , wireless metropolitan area networks (WMANs), and wireless wide area networks (WWANs). WLANs provide wireless data communications in an area with a cell radius of up to hundreds of meters. WMAN covers a wider area that can be as large as an entire city while WWAN provides wireless data services to an area covering multiple cities. The most popular WLAN technology is WiFi which adopts IEEE 802.11 standards. The service radius of WiFi is usually from 50 to 100 meters. WiFi initially provides an aggregated throughput of 11 Mbps, which has been significantly enhanced by recent IEEE standards, e.g., IEEE 802.11ac. WiFi network s are usually self-deployed at home and office environment. Recently, carriers such as Comcast and AT&T also deploy outdoor WiFi access points as hot spots to offloading traffic from mobile cellular networks [3]. Mobile cellular networks are one of the major WWAN technologies. The network capacity of cellular networks ha been significantly enhanced via the network evolution from GSM to LTE-Advanced, which can provide a peak data rate as high as 1 GB/s. The capacity of mobile cellular networks will continue to improve by adopting advanced technologies such as massive Multiple-Input-Multiple-Output (MIMO), coord inated multi-point (CoMP), small cell networks, and device t o device communications. In addition, mobile cellular netwo rks and WiFi networks are gradually integrated with each other to provision high capacity and low latency wireless access networks. II. GREEN OPTICAL ACCESSNETWORKS Owing to the passive nature, PON consumes a small amount of energy per transmission bit. However, considering world wide deployment of PON, it is worthwhile to further reduce