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Towards Greener Optical Access Networks K. Ennser, B. Devlin and S. Mangeni School of Engineering, Swansea University, SA2 8PP, Swansea, UK Tel: (+44) 1792 602450, e-mail:
[email protected] ABSTRACT Whereas the developments in passive optical networks are deemed to hold the promise for faster true broadband networks, there is still a window of opportunity to use semi-active optical networks or purely passive optical networks. In the meantime, a hybrid network would be an attractive idea as this ensures availability, guaranteed quality of services and efficient restoration of services in cases of outage. In this paper a solution is proposed that can be advanced into the possibility of providing green power for the active part of optical access networks as we draw closer to purely passive and hence even greener optical network. This discussion focuses on the requirements and sources of power for the active components of optical access networks. To ensure continuous reduction of the carbon footprint while advancing towards purely passive optical networks, we have emphasized the need for alternative, renewable and greener sources to supply the required power. Solar power would be used as the default power source while mains are used as backup or a standby source. This is because solar power can be harvested and stored with less carbon footprint. Keywords: high speed optical access network, passive optical network, green energy, solar power. 1. INTRODUCTION The increasing demand for quality, quantity and speed in the communications industry has been key drivers for the ongoing research, innovation and competition among services providers. It is generally believed now that the internet, on which many services depend, is poised to become truly pervasive providing services anywhere, anytime connectivity to people objects and things [1]. Optical access networks have been found to hold the promise of delivering services to meet most of the very demanding network resources. Optical fibre has a lot more advantages over other mediums of signal transmission. There is abundant unutilized bandwidth still available in a single fibre strand. In addition optical fibre is immune to the electromagnetic interference suffered by the copper cables, it suffers lower attenuation over large distance and is therefore able to give the required bit error rates (BER) in the order of 10-9 [2]. It is also lighter and much smaller in diameter so it can be less laborious to lay or install and once installed, the capacity it can provide is so huge compared to its counterpart media of signal transmission. Unlike in the initial phases of its use, where optical fibre was mainly used to replace copper cables in long haul inter-exchange networks, optical fibre has now dominated the Metropolitan (inter-office) networks. We are now at a stage where most research and discussions target bringing fibre into the last mile of the network. The problem associated with the increase in broadband is that there is obviously an increase in the amount of power needed to run the network. However with the world of today there is a large focus on reducing the amount of carbon emissions worldwide. The proposed solution is to run the optical network by way of solar power. The use of solar powered optical nodes in the metropolitan area will help combat the amount of emissions released. 2. BROADBAND GROWTH in UK The following section will highlight some of the key statistics associated with broadband in United Kingdom. These statistics have been taken from an up-to-date article from the British government group Ofcom [3]. The level of required broadband services is dependant on the number of users. At current about 70% of adults in the UK have access to broadband internet at home, which is approximately 43 million users. The majority of households consist of at least two adults, so therefore it is estimated that there are approximately 21.5 million homes that subscribe to broadband internet. However there is an ever increasing demand for broadband internet, there are approximately 17.5 million lines installed within the UK at present but this is expected to raise to approximately 24 million lines by the end of 2014. This is a growth of about 6.5 million lines or 37% on the current amount of lines. Figure 1 shows the British broadband predicted growth [4]. Overall the total broadband is mostly dominated by the consumer traffic over a period of seven years and the tendency is to continue to be the major contributor. It is easy to see that the business broadband isn’t predicted to increase that much from mid-2005 to end-2012 but does increase by about 1.5 million lines. It is also possible to see that the consumer broadband is predicted to increase by about 11.5 million lines from mid-2005 to end-2012 which is a 153% increase in consumer broadband.
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Total Number of Lines (million)
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Figure 1. UK Broadband predicted growth. Source: The Next Phase of Broadband UK [4]. Figure 2 depicts the UK broadband growth for different technologies [5]. It is predicted that FTTX (Fibre-to-theX where X is home, business, curb or cabinet) will start to increase the installation from 12/10 but will have a steep increase to about 7.5 million lines by 12/14. DSL is predicted to have increased the amount of lines to about 16 million lines from 6 million lines between 06/05 and 12/11; the Digital Subscriber Line (DSL) connections are then predicted to fall to about 12 million lines by 12/14. This is likely to be due to the FTTX line being installed. The impact of FTTX on the economy is likely to have a positive impact by increasing standalone internet businesses and by increasing sales via the internet for existing businesses. Although the relative delay of implementation of FTTX in UK, there is a considerable growth expected in the next years.
Figure 2. UK Predicted Broadband Growth. Source: Broadband Stakeholder Group [5]. 3. PASSIVE OPTICAL NETWORKS A passive optical network (PON) is a point-to-multipoint fibre to the building network architecture in which unpowered optical splitters that utilize Brewster's angle ideology are used to enable a single optical fibre strand to provide access to multiple customer premises, typically 32-128 in number [6]. A PON consists of an optical line terminal (OLT) at the service provider's central office (CO) and a number of optical network units or terminals (ONU/Ts) near to the end users. PON configurations reduce the amount of fibre and central office equipment required compared with point-to-point architectures [7]. ITU-T G.984 GPON has emerged as the fastest available TDM PON technology offering a highly efficient 2.5 Gbps transport featuring split ratios up to 128 subscribers resulting in significant network cost savings [8]. While PONs are the target to achieve the desired speed for truly broadband networks, it may be agreed that PON deployment is still underway hence the current need for hybrid networks that are capable of providing service while the system smoothly migrates to purely passive optical access networks. Among different broadband access technologies the PON is the most promising architecture to offer costeffective high speed services [9]. To focus the work, we evaluate the power consumption of a commercial PON
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[10]. The maximum power consumption of the OLT used to supply 256 subscribers with broadband internet is 200 W. This OLT can supply subscribers with broadband up to about 20 km away from the node. The power consumption per subscriber is calculated to be 0.78125 W. 4. SOLAR POWER The most suitable type of solar power generation to link with the broadband network is Photovoltaics because it can be used in a standalone manor and be linked to the mains. Photovoltaics produce electricity by harvesting the radiation energy emitted by the sun and convert this by the use of a circuit installed inside the module into electricity. In the UK we can harvest a maximum of 121 kWh per month of energy from a single 1 kW module during May and July, which works out to be about 4 kWh per day [11]. However in the winter months we can only produce 21 kWh per month from the same 1 kW module during December, which is roughly 0.7 kWh per day. A suitable Photovoltaic solar module could be a 215 W system which has dimensions of 1580 mm tall by 798 mm wide and a depth of 46 mm. To power the OLT the minimum amount of modules required would be six, this would then power the OLT continuously for the months May, June and July. However to power the OLT during December there would need to be 33 modules installed per OLT. Potentially there would be a reduction of approximately 4 million tonnes of CO2 emissions per year if the solar modules were installed over the whole broadband network. The solar modules would not solely power the OLTs in the winter months therefore requiring energy to be used from the mains national grid. The supplier would need to make an initial investment of approximately £1.41 million to implement the use of and install the solar modules just in the area of Swansea, UK. However there would be no costs after this apart from for any extra energy used from the mains to supplement the power obtained from the solar modules. 5. DISCUSSIONS AND CONCLUSIONS It is recommended that Photovoltaic modules, mounted on a passive tracker and linked to the mains electrical grid, are used to power the OLTs within the metropolitan area. They will not only reduce costs for the supplier, but also reduce the amount of CO2 emissions released into the environment. This would help the supplier to put across their stance as being green, which could encourage more customers to subscribe to their service. Also, linking the modules and OLT to the mains ensures that the subscribers never lose service due to technical faults (such as power cuts), as there would always be a source of energy available. The disadvantage of using the Photovoltaic modules is that the suppliers would need to make a large initial investment in the system before they would see the benefits. In summary, the use of solar power within the broadband network is theoretically feasible within the metropolitan area. However there is a large initial cost associated with the implementation of the system but there is also a very large reduction in the amount of CO2 released into the atmosphere which could bring with it cost benefits. This proposal could also be implemented all over the world especially in the countries where they receive higher and more constant solar radiation for example in countries such as Spain or Australia. This would massively decrease the amount of pollution released into the environment in particular metropolitan areas, and work in line with the carbon reduction agenda. ACKNOWLEDGEMENTS This work was carried out with the support of the BONE-project (“Building the Future Optical Network in Europe”), a Network of Excellence funded by European Commission through the 7th ICT-Framework Programme. REFERENCES [1] P. Hargrave. “The future of the internet”, Digital Communications knowledge transfer Networks (DCKTN) and Grid Computing workshop, The Royal Society of Edinburgh, 4th, June 2009. Available at www.dcktn.org.uk. [Accessed 12/04/2010]. [2] R. Ramaswami and K. Sivarajan “Transmission system engineering” in “Optical Networks” 2nd. Edition USA, Morgan Kaufman, 2002, pp. 284. [3] Ofcom. The Consumer Experience 2009. London: Ofcom, 2009. [4] Caio, Francesco. The Next Phase of Broadband UK: Action now for long term competitiveness. London : Department for Business, Enterprise & Regulatory Reform, 2008. URN 08/1228. [5] Broadband Stakeholder Group. UK Broadband Market Estimated Growth to 2014. Broadband Stakeholder Group. [Online]. [Cited: 13 March 2010.]. http://www.broadbanduk.org/content/view/267/55/. [6] M. Cover, Thomas; Joy A. Thomas. “Elements of Information Theory”. Wiley-Interscience (1991). [7] ITU-T “WDM PON blurs the boundary between metro and last mile”. Available at www.wikipedia.org/wikipassive_optical _network, GPON ITU-T 2003. [accessed 5th. August 2009].
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[8]
Dan Parsons “Innovations in cost reduction” Director of Marketing at BroadLight Inc. www.broadlight.com/docs/.../wp-innovations-in-pon-cost-reduction.pdf [accessed 12/04/2010]. [9] J. Baliga et al., “Energy consumption in access networks”, in Proc. OFC/NFOEC2008, USA, March 2008, paper OThT6. [10] Kikuchi, Kazuroh. Optical Communication & Device Technologies Edition. [Product Brochure] Tokyo : Mitsubushi Electric Advance, 2003. [11] JRC European Commision. Estimation of PV electricity generation for the chosen location. Fermi : JRC European Commision, 2008.
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