Green In-Building Networks: The Future Convergence of Green, Optical and Wireless Technologies Leonid G. Kazovsky [1], Fellow, IEEE, Tolga Ayhan [1], Member, IEEE, Apurva S. Gowda [1], Member, IEEE, Ahmad R. Dhaini [1], Member, IEEE, Anthony Ng oma [2], Peter Vetter [3] [1] Stanford University, Photonics and Networking Research Laboratory, CA,USA [2] Corning Inc., USA [3] Alcatel-Lucent Bell Labs, NJ, USA ABSTRACT The global network energy consumption is increasing at an alarming rate due to proliferation of the Internet and its increasing bandwidth-intensive applications. In this paper, we propose an energy efficient Access/In-Building architecture that features energy efficient customer premises equipment (CPE) design in conjunction with an energy efficient access network protocol, and in-building optical/wireless integration using Radio-over-Fiber. We analyze the energy consumption of the proposed architecture and show that the use of these technologies could lead to up to 50% energy savings compared to the architectures that employ legacy technologies. Keywords: Energy efficiency, Networking, Optical-wireless networks, Radio-over-Fiber. 1. INTRODUCTION Communication networks today consume a small fraction of the total energy, about 1% 2% [1]. However, that fraction is expected to increase dramatically in the future due to the significant increase in the number of users and their demand for high data rates. Thus, energy-efficient or Green networking has become an important paradigm in the design and operation of modern communication networks. The Internet usage in North America increases at a sustained rate of 40% p.a. [1]. It is predicted that by year 2025, communications networks will consume almost two orders of magnitude more energy than today s networks, unless novel energy efficient networking technologies are employed. In-building networks in the US alone (Residential and Office buildings) consume approximately 7% (39.5 GWh excluding data centers) of the worldwide total Internet energy consumption [1]. In this paper, we propose a new green optical/wireless network architecture that achieves significant energy saving in both the access and in-building networks while offering high data rates to end-users. The new architecture features energy-efficient solutions in the Physical and Medium Access Control (MAC) layers. As illustrated in Fig. 1, Bit-Interleaving PON (BIPON) is employed in the access with Optical-Electrical-Optical (OEO) regeneration and downsampling at the Customer Premises Equipment (CPE)/ Main Distribution Frame (MDF). For the in-building network, we consider different optical/wireless architectures based on the Radioover-Fiber (RoF) technology. As we will see, the most energy-efficient in-building architecture depends on the cell size and coverage area. The remainder of the paper is organized as follows: Section 2 discusses the access part of the network and how the proposed solutions can increase energy-efficiency. In Section 3, we present and analyze the new green in-building architectures. In Section 4, we present our results, which highlight the advantages of the proposed architecture over legacy solutions. We also compare the energy efficiency of the considered in-building network variants. Figure 1. Illustration of the proposed green network architecture. 978-1-4799-0683-3/13/$31.00 2013 IEEE 1
2. ACCESS NETWORK AND ITS INTERFACE TO IN-BUILDING NETWORKS: Legacy TDM-PONs such as XGPON [2] are not optimized in terms of energy efficiency. In XGPON, every downstream frame is processed at a high network clock rate by all Optical Networking Terminals (ONTs), and the packets whose destination address do not match the ONT s are dropped, which is approximately 98% of the total downstream PON traffic. This leads to high power consumption at the ONT [2]. With BIPON, the payload of the frame is interleaved at the Optical Line Terminal (OLT) and broadcast in the downstream direction, and each ONT processes its designated payload bits at a low rate, and drops all other payload bits without processing them. This reduces the average power consumption for protocol processing in the ONT by more than an order of magnitude. With the consumption of other components like transceiver and home gateway processing, the total power consumption of the ONT can hence be reduced by about 67% in a 10 Gbps PON [2]. In the new architecture, with RoF employed in the in-building network, the OLT must insert the generated wireless data frame in the BIPON frame s payload, which then gets regenerated at the CPE/MDF. The OEO regeneration at the CPE/MDF is more energy efficient than the passive CPE/MDF. With OEO regeneration, the transceivers of the local network can be optimized for short reach and hence are energy efficient, unlike a passive CPE/MDF, in which the transceivers need to be designed for the long reach access network and cover the additional losses of the passive CPE/MDF. OEO regeneration allows for down-sampling the useful rate into the local network or switching the CPE/MDF into the sleep mode to save more energy. 3. OPTICAL/WIRELESS INTEGRATION - RADIO-OVER-FIBER: Current In-Building networks are comprised of wired and wireless networks. Today, each network works independently with its own protocol stack. However, the integration of these two networks (i.e., optical/wireless convergence) would realize a much more flexible architecture with higher end-user data rates. RoF technology allows for a seamless integration by eliminating wired transmission protocol processing [1]. RoF-based networks (but not links) can, with a proper design we propose, be more energy-efficient as RoF allows for centralized resources with simple remote antenna units (RAUs). Despite the fact that the RoF links are not more energy efficient than Baseband-over-Fiber (BoF) links, which limits the benefits of centralized architectures, simpler RAUs yield lower circuit energy consumption (i.e. the penalty of using smaller cells and increasing the number of RAUs will be lower [3]). Furthermore, moving from a Copper medium to an Optical one reduces energy consumption and allows for higher data rates [1]. Therefore in the proposed architecture, we employ an optical/wireless In-Building network based on the RoF technology. We analyzed the following five architectures, depicted in Fig. 2: (a) ARCH1, distributed processing using BoF: This is the legacy architecture (Fig. 2a). It employs wireless access points, each with its own processing circuitry, connected to the Main Distribution Frame (MDF) via 1Gbps Optical Ethernet links. (b) ARCH2, centralized and shared analog and digital processing using Analog RoF (ARoF): As shown in Fig. 2b, ARCH2 involves a DSP unit and one high bandwidth transceiver chain at the MDF shared by all the RAUs. We assume that the RAUs time-share the DSP unit and the analog transceiver chain and are sleep-mode-enabled. The RAUs consist of a band pass filter, a power amplifier and a low noise amplifier. The RAUs are connected to the MDF via ARoF links. (c) ARCH3, centralized and shared analog and digital processing using Digitized RoF (DRoF): This architecture differs from ARCH2 in the wired connection between the RAU and the MDF which is via DRoF links (Fig. 2c). The RAU and MDF s must include band-pass sampling Analog-to- Digital Converters (ADCs) and Digital-to-Analog Converters (DACs) to digitize the RF signal. (d) ARCH4, centralized and dedicated analog processing and centralized and shared digital processing using ARoF: In this architecture, all the RAUs share a DSP unit but have dedicated analog transceiver chains at the MDF (Fig. 2d). It employs ARoF links between the RAU and the MDF. (e) ARCH5, centralized and dedicated analog processing and centralized and shared digital processing DRoF: As illustrated in Fig. 2e, this architecture is similar to ARCH4 but employs DRoF links between then RAU and the MDF. In the architectures analyzed, a typical RoF consists of an RF analog transceiver chain that generates the RF signal. On the downlink, a high frequency signal is modulated onto an optical carrier, which is received at the RAU. After O/E conversion and band-pass filtering, this signal is transmitted over the air to the end-user. On the uplink, the received RF signal is modulated onto the optical carrier at the RAU. Thus the RAU only consists of OEO conversion circuitry, a band-pass filter and an antenna unit. In a DRoF link, the high frequency signal is digitized and the digital signal is modulated onto the optical carrier. Digitizing high frequency signals requires the use of band-pass sampling ADC s and DAC s [4]. A BoF link is assumed to be a conventional link with optical Ethernet connected to a wireless access point. 2
(a) (b) (c) (d) Figure 2. Green architectures: (a) ARCH1 (b) ARCH2 (c) ARCH3 (d) ARCH4 (e) ARCH5. 4. ANALYSIS RESULTS 4.1 Comparison of ARoF, DRoF and BoF links Based on end-to-end Signal-to-Noise Ratio (SNR) models for the optical link ([4], [5]) and the wireless link for both ARoF and DRoF links, we can calculate the transmission energy required to maintain a certain bit-errorrate. Assuming certain typical circuit energy consumption (listed below), we evaluate the total (transmission+circuit) energy consumption per bit of individual ARoF, DRoF and BoF links for BPSK (robust but low spectral efficiency) and 64QAM (spectrally efficient but low robustness). The investigated energy efficiency results depend on parameter values but the relative performance trends remain the same. Figures 3a and 3b compare the energy consumption per wireless PHY bit of an ARoF, DRoF and BoF on the uplink and downlink. The receiver on the downlink of the RoF links is the same as that of the BoF link (User s receiver). However, on the uplink, detection of the wireless signal occurs after the optical link. Hence, the SNR on the uplink and the downlink differs. This results in different power consumptions on the uplink and downlink. We assume a RF carrier frequency of 2.4 GHz with a bandwidth of 20 MHz. We also assume that the RF Analog Transceiver consumes 400 mw, the Ethernet Transceiver 1 W, Laser Driver 10 mw and the Transimpedance Amplifier 250 mw (based on surveys). On the uplink (Fig. 3a), we see that for DRoF at 64QAM, the energy consumption per bit increases rapidly at longer distances. This is due the higher required SNR for DRoF at 64QAM. On the downlink, DRoF has the highest energy consumption per bit as it has an extra ADC and DAC. ARoF has the highest circuit and transmission energy consumption per bit on the uplink due to the amplification required to maintain the received RF signal power above the noise floor of the optical link and RF transceiver chain. In DRoF, the signal gets degraded due to quantization and sampling, however, high amplification of the received RF signal is not needed as long as there is reliable communication on the digital optical link. However, in both cases, BoF is the most energy efficient. This is because the energy consumption per bit on the 1 Gbps Optical Ethernet link is much lower than that of the ARoF and DRoF link which operate at the wireless bit rate (24 Mbps for BPSK; 144 Mbps for 64QAM). (e) (a) (b) Figure 3. Link energy consumption (f c = 2.4 GHz BW = 20 MHz): (a) Uplink, (b) Downlink. 3
4.2 Green In-building Architecture vs. Legacy Networks Figure 4 compares the power consumption of the five In-building architectures described in Section 3 for a 100 m by 100 m floor plan (i.e., large In-building network). We assume that there is no frequency re-use and the total available network throughput is 360 Mbps. ARCH3, which employs DRoF links, is the most energy efficient architecture for cell sizes between 5 m 25 m, although DRoF links are not more energy efficient compared to BoF links (ARCH1). Thus, centralizing and sharing the processing units can help realise a significant amount of energy savings for areas which require a large number of antennas for complete coverage (above ~400 m 2 ). We also see that an optimum cell size exists for the architectures. At shorter distances the circuit energy consumption dominates owing to high number of antennas and at longer distances transmission energy dominates since strong power amplifiers are needed. For the parameters considered, the optimum cell size is between 15 20 m. Architectures employing ARoF links can be beneficial for small cell sizes, despite the high amplification required (ARCH2 at 10m). By using ARCH3, we can save up to 50% power compared to ARCH1. Figure 4. Total power consumption of green in-building architectures. Figure 5. Total power consumption of proposed green network vs. legacy solutions. Figure 5 compares the power consumption of the conventional In-building networks with the proposed Green In- Building network architecture. Our study is concerned with analyzing the power consumption of in-building networks. Therefore, we evaluate the proposed architecture without considering the OLT. We consider PON rate to be 10 Gbps. The average power consumption of the conventional in-building network is compared with that employing a CPE/MDF with downsampling and DRoF in both small In-building networks (~625 m 2 ) and large In-building networks (~10000 m 2 ). We assume that the processor and the Optical/Electrical (O/E) converter is sleep mode enabled and are idle 66.7% of the time. For the processor of conventional CPE/MDF, we make use of PowerPC 750FX [6]. The aggregate rate for small In-building network is 180 Mbps and for large In-building network 360 Mbps. We observe that using the green architecture, we can get power savings up to ~51% for small In-Building and 46% for large In-building networks. These results highlight the effectiveness of our proposed architecture. 5. CONCLUSIONS We proposed and analyzed an energy efficient Access/In-Building architecture that employs Bit-Interleaving PON in the access networks and Radio-over-Fiber architectures in large buildings. We analysed five In-Building architectures and compared their energy consumption. Analog Radio-over-Fiber (ARoF) distribution networks have reasonable energy efficiency for dense deployments even though point-to-point ARoF links are not energy efficient on their own. The best distribution network is ARCH3 which employs Digitized Radio-over-Fiber links with centralised and shared resources. Overall power savings that can be realised using the proposed green architecture are around 46% to 51% depending on the coverage area. The proposed architecture is a strong candidate for next-generation Green Access/In-Building Networks. ACKNOWLEDGEMENTS The authors would like to thank Alcatel-Lucent Bell Labs, NJ, USA and Corning Inc. USA for their support and guidance of this work. A. R. Dhaini is supported by the Natural Sciences and Engineering Council of Canada (NSERC), and by the National Science Foundation (NSF) via Grants CNS-0627085 and CNS-1111374. 4
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