ECS 259 Project Report Energy Efficiency in Optical Access Networks

Transcription

1 ECS 259 Project Report Energy Efficiency in Optical Access Networks Partha Bhaumik Department of Computer Science University of California Davis March 17,

2 Contents 1 Introduction 3 2 Key Roles of Green Technology for Access Network Systems 3 3 EEE enabled 10Gbps Copper PHYs or Fiber PHYs? 4 4 Energy Consumption of FTTB and FTTH Access Networks 5 5 Power efficiency of long reach PONs 6 6 Sleep Mode for Energy Saving PONs: Good or Bad? 7 7 Towards Green Broadband Access Networks 8 8 Conclusions 8 2

3 1 Introduction The efficient use of energy in communications is a growing concern for both industry and governments worldwide. The massive amount of communications devices that are used today, together with their expected growth, have led to the conclusion that significant energy can be saved by applying energy efficiency concepts in the design of communication systems. Access network is the last mile of the communication network that connects the telecom Central Office (CO) to the residential and business customers. With the proliferation of the Internet, customer demands for bandwidth-intensive services are rapidly increasing. Access network comprises a large part of the Internet. It is a major energy consumer in the Internet due to the presence of huge number of active elements. If we can reduce energy consumption in the access network, it will automatically provide a significant reduction of overall Internet power consumption. Access network power-consumption reduction not only has the potential of enormous cost savings, this will also lead us to develop environment-friendly technologies, thereby achieving the ultimate goal of green Internet. Energy efficiency in access networks has been an important topic of research for quite some time. Researchers have looked into energy efficient routing, sleep mode ONUs, elastic ONUs, green technologies for network management and operation, new-age architectures like wireless-optical hybrid access networks among others. In this short survey we look at some relevant topics like energy efficiency of EEE enabled copper PHYs, advantages and disadvantages of sleep mode ONUs, energy consumption of FTTH and FTTB access networks, power efficiency of long reach PONs and WOBANs. 2 Key Roles of Green Technology for Access Network Systems Somemura [5] has reviewed standardization activities at the International Telecommunication Union, Telecommunication Standardization Sector (ITU-T) related to ICTs and climate change, and has also outlined the discussions to date, has described recent progress, and introduces the future structure of such standardization activities in ITU-T. Impact of ICTs on environment: positive or negative? ICTs have both positive and negative impacts on the environment. In general, the negative impact is the environmental load caused by ICT devices and equipment themselves, such as energy and natural resource consumption and e-waste. On the positive impact side, ICT services can make lifestyle and business operations efficient. This efficiency of lifestyle and business activities indirectly reduces the environmental load. The author argues that in order to construct a sustainable society from an environmental viewpoint, it is important to minimize the negative impacts of 3

4 ICT ( Green of ICT ) and maximize the positive ones ( Green by ICT ). It is incumbent on the IT industry to standardize a methodology for objective and transparent evaluation of the environmental impacts of ICTs. Life cycle assessment (LCA) is one such methodology. Case study of Green of ICT and Green by ICT Somemura introduces case studies for energy saving by using the passive optical network (PON) system and fiber to the home (FTTH) as key roles of green technology for access network systems. As an example of Green of ICT, a major focus of ITUs work in recent years has been on next generation networks (NGNs), which are expected to reduce energy consumption by 40% compared to the current public switched telephone networks (PSTN). This energy saving is due to a significant decrease in the number of switching centers required and to more tolerant climatic range specifications for NGN switching locations. Furthermore, the introduction of Very high bit-rate Digital Subscriber Line (VDSL) 2 is expected to reduce energy consumption because it specifies three power modes (full, low-power, and sleep), whereas VDSL has only a single power mode (full power). As a case study of Green of ICT by utilizing a method for calculating environmental impact, a PON system has an advantage over a single star (SS) system in energy saving, because PON can decrease the environmental load (CO 2 emission) by sharing optical fibers among users. As a case study for Green by ICT, the effect of NTT Groups FTTH service, in reducing environmental impact has been quantitatively evaluated, taking the lifecycles of FTTH equipment and networks into consideration. By investigating the use of FTTH using statistical data, the researchers at NTT determined that there are 19 types of ICT services/actions using FTTH and conventional ones that can achieve the same objectives without using FTTH, such as sending e- mail instead of conventional mail, downloading movies instead of purchasing DVDs at a store, purchasing e-tickets instead of tickets from a store. FTTH has been found to reduce environmental impact by 70% compared with the use of conventional alternative means. 3 EEE enabled 10Gbps Copper PHYs or Fiber PHYs? The IEEE 802.3az Energy Efficient Ethernet (EEE) task force has given a standard that will greatly improve the energy efficiency of Ethernet PHYs. However, the new standard applies only to copper and backplane Ethernet PHYs and therefore optical fiber PHYs will not benefit from the adoption of EEE. Reviriego et al [4] compare the energy efficiency of EEE enabled 10Gbps Ethernet copper transceivers with fiber transceivers. The basic concept behind the EEE standard is pretty simple. It tries to reduce energy consumption in Ethernet devices by defining low power modes. 4

5 The idea is that a PHY that has no frames to transmit can be put into a low power mode and when new frames arrive, back into the active mode very quickly (in a few microseconds). This enables energy savings that are almost transparent to upper protocol layers. The estimated savings while in low power mode can be close to 90% and therefore for links that spend most of the time in low power mode the energy consumption is reduced drastically. Thus for lightly loaded links a copper PHY that implements EEE may be more energy efficient than an optical one. One problem with EEE enabled copper PHYs is that the transition times in and out of the idle mode are quite high. An important observation is that even for low loads the link may spend little time in low power mode as it may need to continuously move in and out of the low power mode as frames come and go. In addition, during those transitions the PHY consumes a significant amount of energy. To evaluate under which conditions an EEE enabled 10GBase-T copper PHY would be more efficient than a fiber PHY the authors did some performance simulations. They used a Poisson model for the frame arrivals. A relative measure was used, which is the times that the copper PHY power consumption in active mode can be larger than that of the optical PHY and still be more energy efficient because of the use of the low power mode. It was observed that the values tend to one for small load values and therefore the EEE copper PHY would only be more energy efficient if its power consumption in active mode is smaller than that of the fiber PHY. This is unlikely to occur in the near future as copper PHYs require much more complex processing than optical ones. From the results it can be concluded that optical PHYs will be more energy efficient even when the new Energy Efficient Ethernet standard is implemented on copper PHYs. This is an interesting result that ensures the advantage of optical Ethernet in terms of energy efficiency for the coming future. 4 Energy Consumption of FTTB and FTTH Access Networks Lange et al [2] have investigated the energy consumption of FTTB and FTTH access networks. An attractive option for providing optical access in FTTB and FTTH architectures is the use of passive optical networks (PON). As the distance bridged by a PON decreases, the number of subscribers served by a single PON at a constant bit rate can be increased. In the network model used by the authors, the OLT is assumed to be situated in the central office, in order to replace the existing copper-based or hybrid access systems by a FTTB or FTTH architecture with GPON. In the case of a FTTB architecture the data stream is distributed to n 1 buildings, where the ONU (optical network unit) terminates the optical transmission. The optical signal is converted to the electrical domain and the distance from 5

6 the building ONU to the m flats per building is bridged by copper-based VDSL transmission. The VDSL modem is the termination point in the subscribers home. In the FTTH case the data stream from the OLT is split to n 2 living units, where ONTs (optical network termination) terminate the optical connection. In each of the cases (FTTB and FTTH) the network section from the central office to the network termination in the living unit is considered, i.e. from the OLT to the modem (FTTB) or ONT (FTTH). An overall number of N living units is served with an average access bit rate of r. Numerical results obtained for an exemplary area show that power consumption of a FTTB network is higher than that of a FTTH network. The OLT power consumption in both cases stays the same, since the same number of customers is served at the same bit rate. However, the power consumption in the building and home differs. In the FTTB case there are building ONUs and modems as power consumers. By contrast, in the FTTH case the ONT in each subscribers home is fed directly by the OLT and there are no active intermediate components. The power consumption part which has to be provided by the network operator reduces drastically in the FTTH case (OLTs only) compared to the FTTB network (OLTs and ONUs). There is also a linear dependency between energy consumption and average access bit rate. The authors conclude that FTTB networks consume a considerable higher amount of power than FTTH networks, since there are more active devices. the high number of distributed elements (ONTs in the FTTH case as well as ONUs and modems in FTTB) drives the power consumption compared to the relatively low power consumption of the central network elements (OLTs). It is claimed that the optimization of the distributed network components is an attractive option for reducing the energy consumption in the access network. 5 Power efficiency of long reach PONs Lovrić et al [3] have made an attempt to estimate power consumption of several advanced PON technologies and architectures with and without reach extenders. In their study, they use three different access network architectures: point-topoint (P2P) Ethernet, PON and hybrid TDM/WDM PON, which can be optionally used with reach extenders (REs). For reach extenders (RE) they consider optical amplifiers (EDFA, PDFA, and SOA) and optical-electrical-optical (OEO) repeaters. They assume symmetrical trasmission data rates. For the calculation of power consumption they considered two cases. In case 1, they do not take into account the realistic limitations of aggregation switch at the central office (CO) but rather allow a theoretical maximum throughput of the access network. In case 2, they presume a realistic scenario and therefore limit the throughput at the CO. The assumption for the limitation included a 10-hop, 80-channel WDM metro ring providing 40 Gbps per wavelength channel and 1000 users per CO. From the results it was observed that for both case 1 and case 2, both P2P 6

7 and WDM scenarios consume the most power. Extended reach 10G-EPON is more efficient in terms of power consumption per user and bit than extended reach 1G-EPON. The hybrid TDM/WDM PON is the most promising solution among considered FTTH access networks. 6 Sleep Mode for Energy Saving PONs: Good or Bad? Wong et al [6] focus on some feasible implementations of sleep mode in passive optical networks (PONs). Advantages. In current TDM-PONs, OLT physically broadcasts downstream traffic to all ONUs that remain active even if they are not the destination of any data. There can be considerable potential energy savings if the ONUs stay active only in the receiving time slots (i.e., when data are destined to them) and switch to sleep mode in empty time slots. By exploiting the statistical multiplexing property of the downstream traffic, significant reduction in energy consumption can be achieved. To enable sleep mode in current TDM-PON, an ONU must wake up from the sleep mode and regain synchronization with the network. The paper describes the wake-up processes for ONU for both Gigabit PON (GPON) and EPON. The power consumption in active mode (i.e., when ONU is up) and sleep mode are compared for ONU with current receiver architecture. The authors also propose two novel ONU receiver architectures and study their respective power consumption behaviors. The proposed architectures allow ONUs to switch to sleep mode and quickly recover the OLT clock with minimal changes from current ONU design. The effects of sleep mode on the energy saving performance of the three ONU receiver architecture options are analytically computed and compared. Results show nearly 80% energy saving performance in light traffic and more than 50% energy saving under realistic TDM traffic. The current ONU architecture saves the most energy when the sleep time is much longer than the data transmission time. However, the novel architectures outperform the existing one when sleep time is less than 30ms. This is because the overhead time for the ONU with existing architecture becomes significant with respect to the sleep time. Disadvantages. The two main problems with implementing sleep mode ONUs are traffic loss and queuing delays as a result of switching an ONU from active mode to sleep mode. If an ONU is asleep when a downstream packet arrives, the OLT would either have to queue the packet or drop it. Similarly, when new packets arrive to an ONU during its sleep mode, ONU could queue the packets before it wakes up and regain synchronization with the OLT. This process may take more than one cycle time. To resolve these problems, the OLT must 7

8 adopt more advanced scheduling and protocol that would not, for example, allocate downstream packet or upstream bandwidth grants when an ONU switches to sleep mode. An example of such advanced scheduling and protocol is the energy-efficient multi-poll approach proposed for wireless local area networks. This approach has shown that effective sleep mode could be achieved with moderate buffering and enhanced scheduling of wake-up times. However, buffering increases packet delay. In TDM-PONs, the literature has also shown notable reduction of buffering delay when advanced traffic prediction models are used. Although minimizing buffering delay is quite important to mitigate the effects of the sleep mode technique on traffic, the system would still certainly work without this. For instance, a simple downstream TDMA scheme that buffers excess data could certainly be used as the sleep mode scheduler. There is also consideration to account for additional timing issues. In particular, the novel architectures suffer from the absence of accurate OLT clock synchronization during sleep mode. In conclusion, a suitable high performance OLT scheduler for energy saving PON would need to consider the impact of scheduling misses and the possibility of drifting in the wake-up time due to phase accumulation. 7 Towards Green Broadband Access Networks Chowdhury et al [1] develop energy-aware design techniques and routing protocol for green WOBAN (hybrid Wireless-Optical Broadband Access Networks). Although these techniques are developed for WOBAN, it is claimed that they will be proven to be more general and can be adapted to other access network technologies like wireless networks and different PON variants. The architecture of WOBAN, a mathematical model of energy-aware WOBAN design, and an energy-aware routing algorithm for WOBAN are presented. Through a series of rigorous formulations and numerical results, it is concluded that energy consumption of WOBAN can be reduced by efficient design and energy-aware routing. The impact of these energy-aware design decisions on the performance of the network is also examined. It appears that with suitable design decision parameters, comparable performance (of WOBAN) with the energy-aware mode can be achieved. The energy-saving in the optical part of WOBAN also does not increase the energy usage in the wireless part. 8 Conclusions The United Nations Intergovernmental Panel on Climate Change (IPCC) foresees a further rise of between 1.4 and 5.8 in average global temperatures by the end of the century. Climate change is a concern for everyone and requires efforts on the part of all sectors of society, including the information and communications technology (ICT) sector. Since energy consumption is more and more becoming an environmental, 8

9 social and political issue, for network operators it is a problem of economical concern. Power management approaches in networking research focus on three major directions increasing energy efficiency of equipments, energy-aware network design and energy-aware protocol design. Although ICTs contribute only an estimated 2.5% of total greenhouse gases, this is expected to grow as ICT usage expands globally, growing at a faster rate than the general economy. Access networks comprise a significant fraction of the total count of ICTs deployed worldwide and this number is expected to undergo a meteoric rise in the coming decade or so. Performance of access technologies is improving over time, reducing the cost per byte of traffic and making the broadband Internet affordable to more users. This refuels the tremendous growth of the Internet and scales up the size of broadband access networks. The observations brought forth by this survey demonstrate some interesting facts regarding energy efficient access network design. First, EEE enabled copper PHYs are still less energy efficient compared to optical fiber PHYs, hence future network deployments should focus on fiber PHYs rather than 802.3az. Another interesting result is that FTTH is much more energy efficient than FTTB due to much lesser number of active components. It is also showed that hybrid TDM/WDM PON is the most promising solution among considered FTTH access networks. As far as ONU design is considered, sleep mode ONUs can contribute around 50% savings in energy consumption; however they introduce new complexities like scheduling and synchronization. Hybrid architectures like WOBAN can also be very energy efficient as has been demonstrated. Access technologies such as xdsl, CM, Wireless, and Cellular networks do not live up to satisfying future broadband Internet demands, but hybrid architectures using one or more of these technologies in conjunction with PON can give rise to very energy efficient broadband access solutions in the first mile. This can be an exciting area for future research. 9

10 References [1] Pulak Chowdhury, Massimo Tornatore, Suman Sarkar, and Biswanath Mukherjee. Towards green broadband access networks. In Proceedings of the 28th IEEE conference on Global telecommunications, GLOBECOM 09, pages , Piscataway, NJ, USA, IEEE Press. [2] C. Lange and A. Gladisch. On the energy consumption of FTTH access networks. In Optical Fiber Communication - incudes post deadline papers, OFC Conference on, pages 1 3, [3] A. Lovric and S. Aleksic. Power efficiency of extended reach 10G-EPON and TDM/WDM PON. In Optical Fiber Communication (OFC), collocated National Fiber Optic Engineers Conference, 2010 Conference on (OFC/NFOEC), pages 1 3, [4] P. Reviriego, D. Larrabeiti, J.A. Maestro, J.A. Hernandez, P. Afshar, and L.G. Kazovsky. Energy efficiency in 10Gbps Ethernet transceivers: Copper versus fiber. In Optical Fiber Communication (OFC), collocated National Fiber Optic Engineers Conference, 2010 Conference on (OFC/NFOEC), pages 1 3, [5] Y. Somemura. Key roles of green technology for access network systems. In Optical Fiber Communication (OFC), collocated National Fiber Optic Engineers Conference, 2010 Conference on (OFC/NFOEC), pages 1 3, [6] Shing-Wa Wong, L. Valcarenghi, She-Hwa Yen, D.R. Campelo, S. Yamashita, and L. Kazovsky. Sleep Mode for Energy Saving PONs: Advantages and Drawbacks. In GLOBECOM Workshops, 2009 IEEE,