Changing the network structure Leaving the past behind

Invited Paper Changing the network structure Leaving the past behind Ralf Herber T-Systems, Deutsche-Telekom-Allee 7, D 64295 Darmstadt, Germany ralf.herber@t-systems.com ABSTRACT The infrastructure of the existing network is determined by the old copper access technology. Not only the copper access itself, also the number of central offices and their geographical distribution are results of the copper network and its physical limitations. Today, in Germany, several thousand active locations cater for the delivery of plain old telephony services, as well as for the delivery of new fiber-based broadband services. Due to the fact that the attenuation of optical fibers is relatively low, new concepts for the design of the network structure become possible and are discussed in this paper. A reach of 40 km, for example, on an optical transport system is no problem. Longer possible link lengths can result in a reduced number of central offices, leading to reduced expenditures for the building, power supply and air conditioning. In the case of Germany a number of some hundred central offices is envisaged. However, a significant drawback of today s existing optical access technologies is the very limited number of customers on a single fiber. For instance, GPON (Gigabit Passive Optical Network) provides typically a 32-way split and a distance of 20 km. This paper discusses some new ideas to introduce higher splitting ratios and longer access lengths into the network. With WDM (Wavelength Division Multiplexing) and/or coherent optical receivers new options for a future proof access network are available. Keywords: Optical Access, copper infrastructure, reduction of central offices 1. INTRODUCTION Historically, the network development was driven by the installation of copper pairs from the central office (CO) to the buildings. The copper infrastructure together with the CO has imposed the configuration on today s network development based on optical fiber. However, the optical fiber provides higher flexibility due to the fact that attenuation is in the order of less than 1 db/km for practical installations including splice loss, connectors etc. Thus the overall network concept can be discussed in the light of new options: Lower attenuation, PON technology and wavelength division multiplexing are some of the topics which have to be considered when a new network optimization is analyzed. The trend is clear towards higher bandwidth per customer and towards a pure optical infrastructure which could cater for the future requirements such as regulatory issues and increased bandwidth demand. In the following the cost of putting fiber into the ground will be discussed together with the new options and the new requirements for for fiber-to-the-home. For the network operator the new planning features are given by the parameters of new optical transmission solutions in the access part of the network. 1.1 network The historical development of the network infrastructure was driven by the requirements for copper links for telephony. Depending on the distance between a customer and a central office, copper wires with different cross sections were used. Typical diameters are 0.4 mm or 0.6 mm. wires with different diameters are used solely or in a mixed fashion. Typically, the majority of copper wires is 0.4 mm. Interesting for the development of the infrastructure is the resulting link length as depicted in figure 1. Optical Metro Networks and Short-Haul Systems II, edited by Werner Weiershausen, Benjamin Dingel, Achyut Kumar Dutta, Atul K. Srivastava, Proc. of SPIE Vol. 7621, 762104 2010 SPIE CCC code: 0277-786X/10/$18 doi: 10.1117/12.847086 Proc. of SPIE Vol. 7621 762104-1

Figure 1. The distribution for the length of copper access links in the urban and rural region. In urban areas the average link length between customer and central office is approximately 2 km. [1] The network infrastructure which developed from the requirements for copper based telephony led to an infrastructure with a few thousand central offices catering for some 30 Mio. households. 1.2 Development in Germany after the reunification in 1989 In 1989 the Federal Republic of Germany and the German Democratic Republic were reunified. The communication system in the eastern part of Germany was relatively old and needed renovation. Gerd Tenzer, the technical head of the incumbent telecommunication company at that time, initiated the use of optical fiber wherever possible. Several optical access trials where initiated and fiber-to-the-home trials where realized. The problem was that the technology was not that advanced, so basically ISDN (Integrated Services Digital Network) and POTS (Plain Old Telephony Services) were realized over optical fiber. When in the late 1990s the ADSL (Asymmetric Digital Subscriber Line) technology evolved, the available bandwidth in areas with optical fiber in the access was very limited compared to those regions with old copper infrastructure and DSL technology. It took about 10 years before optical access technology became mature enough so that an installation of broadband access could be started. 1.3 GPON field trial in Dresden In Dresden nearly 100 % of the households are able to have VDSL (Very High Speed Digital Subscriber Line) with up to 50 Mbit/s.. Only in Dresden-Striesen and Dresden-Blasewitz, which are areas in the center of Dresden, no VDSL is available. Why? In 1994 an active transport system called HYTAS 94 was installed to provide ISDN and POTS. HYTAS 94 is a fiber-to-the-home system with an active splitting at the street cabinet. The idea was to utilize the spare fibers for up-to-date GPON systems. In 2008 GPON technology was started to be installed for more than 20000 households in more than 3000 buildings [3]. The systems are now running and more and more customers for triple play services are connected. The old narrowband HYTAS94 is still in operation. Proc. of SPIE Vol. 7621 762104-2

narrowband access system HYTAS 94 Cu fiber GPON extension Tel. ONT/ ONU OLD DIV splitter splitter Tel. ONT/ ONU OLD DIV PC DSL Router Settop ONU AGS2 VDSL2/ passive Box ADSL2+ coupler flat / home cellar street cabinet central office Figure 2: The existing narrowband HYTAS 94 system is upgraded by GPON with an in-house VDSL link [4]. OLD in HYTAS 94 is an active splitting point, DIV shows the switching center for telephony services. AGS2 is an aggregation switch. [Deutsche Telekom] The GPON system in Dresden is an off the shelf GPON system which was adapted mainly concerning the operational requirements of the network operator. On the optical layer no changes have been made. Only in the building a splitter has been introduced to separate old telephony services from new VDSL services. The GPON system provides the standard 2.5 Gbit/s downstream capacity, 1.25 Gbit/s upstream capacity and a 32-way splitting ratio. The broadband GPON extension is a single fiber solution. 2. PUSHING THE FIBER TOWARD THE CUSTOMER: VDSL It is appealing to think about fiber-to-the-home scenarios since it is the ultimate answer to all communication questions for fixed networks. However, the cost aspect has to be taken into account. The laying of a fiber optic cable in an urban area is some ten thousand Euros per km. Therefore the practical approach is to utilize fiber and copper in a mixed scenario: VDSL. In 50 cities in Germany VDSL access has been realized by pushing the fiber toward the customer. A fiber to the curb scenario was realized in order to provide up to 50 Mbit/s per customer. This project was started in 2006 for the soccer world championship which took place in Germany. A complete new network together with DSLAMs in the outdoor cabinets have been set up. Based on Microsoft MS-TV triple play services are provided. The basis of the network is L2/L3 Ethernet switches which are cascaded to form an aggregation network. The classical structure with aggregation (AGS1 and AGS2) and core network has been maintained. For TV services a multicast structure has been introduced into the network elements down to the AGS1. The last multicast replication point is currently within the AGS1. However, it is envisaged to have the next generation of DSLAM (Digital Subscriber Line Access Multiplexer) with a multicast replication point. This will allow the introduction of relatively large DSLAM with a single Gigabit-Ethernet uplink. For further traffic increase 10 Gbit/s uplink is prepared. Proc. of SPIE Vol. 7621 762104-3

Figure 3: The new concept for delivery of triple play services consists of a DSLAM for ADSL2+/VDSL and a dual stage aggregation. D-Switch and D-Server belong to the Microsoft TV concept and are introduced in order to reduce the zapping delay. STB = Set Top Box, BBRAS = Broadband Remote Access Server. For outdoor DSLAM large outdoor cabinets were installed, see figure 4. Figure 4: Outdoor cabinets have been installed to provide space for the outdoor DSLAM with VDSL line cards (left). In some cases the old smaller cabinets were destroyed and the new larger cabinet was used instead (middle) without affecting the service. In some dense urban areas it was found to be very difficult to install the larger cabinets (right). The price of the outdoor cabinet including the DSLAM for 100 customers is a few thousand Euros and thus very attractive compared to the cost of fiber digging. [2] Photos by Deutsche Telekom. 3. THE DEVELOPMENT OF THE NETWORK IN THE LIGHT OF OPTICAL TRANSMISSION TECHNOLOGY When the access technology was moving toward fiber-based technology, a discussion started as to whether it wouldn t be reasonable to utilize all the benefits from optical transmission in a new design of the network infrastructure. With optical fiber link lengths of more than 100 km without amplification can be bridged at 10 Gbit/s. The first analysis of the existing fiber graph showed that it is possible to increase the link length and to reduce the number of central offices passed. Proc. of SPIE Vol. 7621 762104-4

Figure 5: Changing the infrastructure. In the left viewgraph the existing infrastructure with a few thousand COs is shown. In the right viewgraph the alternative concept of reduction on only a few hundred central offices is depicted. [5] However, the cost issues have to be discussed: What are the benefits from reduction of central offices and what are potential drawbacks? 3.1 The cost of central offices, power and air conditioning versus the cost of transport Reducing the number of central offices (CO) is beneficial due to different reasons: Reducing the number of central offices leads to a reduction in areas of real estate which have to be maintained year over year. The more simply a central office can be operated, the lower the costs are. e.g. a CO with personnel requires an adequate infrastructure with sanitary rooms and recreation rooms. On the other hand central offices are used for co-location purposes. Therefore it might be difficult to give up a central office due to regulatory reasons. Power is a costly asset since it has usually additional features like guarantee for operation for some hours during a blackout. Batteries and aggregates have to be maintained. Air conditioning is costly. If the air conditioning systems can be concentrated in a larger central office the overall cost will be reduced. One larger air conditioning system is cheaper and easier to maintain than many smaller systems. In general, one larger CO is better and cheaper that many small COs. Synergy can be used if all components (switches, router, server etc.) are concentrated within a single location. Optimization will show whether it is better to run one big switch for instance rather that many small ones. On the other hand the cost of transport, longer link lengths, less dense traffic on a single fiber have to be considered: If the number of central offices is reduced, the link length increases. Increased link length leads to more attenuation thus to higher costs for advanced lasers and sophisticated receivers. Practically speaking, the cost for a long-reach SFP (Small Form-factor Pluggable) is higher that the cost of a short-reach SFP. The traffic on a given link can be less dense due to missing aggregation points. In other words: A single fiber (pair) carries the traffic of only a few customers. This effect depends on the access technology which has been chosen. There is a tradeoff between the cost for central offices and the cost for transmission. If fiber is available, transmission is cheap and the cost for central offices is high then definitely the reduction of number of central offices is beneficial. 3.2 Office consolidation in practical network development Office consolidation is not an easy task to perform. A lot of legacy technology is installed and is still under operation. Furthermore the demands of the customers have to be fulfilled with existing technology. In the same way that VDSL or ADSL technology is advantageous to provide fast internet access and/or triple play services it must be clear that these Proc. of SPIE Vol. 7621 762104-5

technologies require active components near the customers. The existing central offices will be used to produce these services. But it might be a reasonable strategy to withdraw all complicated, maintenance requiring components to some centralized central offices. This will reduce the load on smaller central offices in terms of power and air conditioning. 4. COMPARING DIFFERENT ACCESS TECHNOLOGIES In the chapters above the pros and cons for central office consolidation have been discussed. In this chapter we consider optical access technologies and their potential for supporting office consolidation. Ø2 km Home Building Curb some thousands of COs VDSL DSLAM Fiber = Residential Gateway ADSL2+ DSLAM Figure 6: The access network architecture today s example [6] Figure 6 shows a sketch of the existing infrastructure with a few thousand COs. FTTC is installed mainly outdoor for VDSL services up to 50 Mbit/s. ADSL2+ DSLAM can be found in the COs. Ø20 km Home Building Curb former CO VDSL DSLAM Fiber some hundreds of COs Redundancy optional Figure 7: The access network architecture tomorrow s option [6] Proc. of SPIE Vol. 7621 762104-6

The reduced number of COs can provide significant OPEX savings. On the other hand mean average distances of 20 km between the customer premises and a CO have to be overcome. One CO must support on average tens of thousands of customers and requires efficient fiber utilization between former and remaining COs. The lower the number of COs the longer the link length between customer and (new) CO. In a scenario where the number of COs is reduced to some hundreds the average distance is in the order of 20 km. On the other hand the utilization of fiber is a significant cost item for longer link lengths: The cost drops drastically the higher the splitting ratio of a fiber optic system can be chosen. When the splitting ratio is in the range of 512 or 1024 the cost of fiber plant cannot be reduced significantly. On the other hand, most operators do not want to have more than approximately 1000 customers on a single fiber without protection. 4.1 What is needed for future access networks? In the section above it was discussed that with the reduction in the number of central offices, longer link lengths and higher splitting ratios are favorable factors to build a cost effective network. The term splitting ratio in this context does not necessarily mean the splitting ratio in a PON system. Splitting ratio is more generally the accumulation of customer traffic on a single fiber (pair). If we have a look at standard GPON as shown in Figure 8 systems it is clearly visible that these systems are somewhat limited. The typical splitting ratio is 32 and the maximum distance is typically in the order of 20 km. GPON systems are under development and higher splitting ratios and longer link lengths have been demonstrated [7]. However, the demonstrated systems rely on optical amplifiers or active regenerators between the customer and the CO. Amplifiers and regenerators are active components and would require electrical power and maybe a temperature controlled environment. Small/ Medium Enterprise FTTBusOperator FTTBus unbundled spectrum Residential Broadband FTTH Operator FTTH ONT Splitter passive Filter passive Filter FTTB/ C Operator FTTB/ C DSLAM Figure 8: A comparison between the standard GPON systems and a future WDM-PON system with coherent detection. While a GPON system has a splitting ratio of 1:32 or 1:64, the WDM-PON system could provide a 1000-way split [6]. 4.2 The coherent option Coherent detection is a well known technique to create very sensitive receiving systems. However, coherent detection requires some amount of optical technology on the customer s side. With the advance of photonic integration the option for a sensitive receiver seems to be realistic for the next few years. Photonic integration has been shown and successfully commercialized by Infinera for 10 x 10 Gbit/s transmitters and receivers on a single chip, respectively. For a fiber-to-thehome system depicted in Figure 9, a local oscillator (tunable laser), a polarization diversity receiver and a modulator have to be realized in a cost-efficient way. The big advantage of a coherent system from a network operator point of view would be the fact that the infrastructure from GPON systems could be re-used. Proc. of SPIE Vol. 7621 762104-7

ONT Local Oscillator λ Control LO Data Data PDR 1 Gbps Polarization Diversity Control Logic Receiver selective Amplification at LO frequency Light 191-196 THz (1530 1565 nm) Tuning Range Data 1 Gbps Figure 9: Principle of coherent detection for access [8] 4.3 The options for future development If sophisticated systems with coherent detection and cost-effective integrated optical functionality are available, new and flexible options for unbundled access solutions could be realized in a very simple manner. By assigning wavelength band to individual operators and/or services a clear separation of services on a common access network would be possible. 5. SUMMARY The interest of network operators in network optimization and consolidation of central offices requires new access technologies. Longer link lengths and higher splitting ratio for future optical access networks are required. An option which comes into play, due to the advances in photonic integration, is coherent detection. If a cost-effective access solution can be realized a bandwidth of 1 Gbit/s per user is feasible. REFERENCES 1. Wartmann, H.. [Fernmelde-Linientechnik (I)], Schiele&Schoen, Berlin (1982). 2. Bögle, L. and Lormis, F.-J., Workshop IHK Potsdam 26.06.2009 The presentation can be found at: http://www.potsdam.ihk24.de/produktmarken/innovation_und_umwelt/anlagen/pdfs/dtag_timpe.pdf 3. Weckbrodt, H., Superschnell im Internet, Dresdner Neue Nachrichten, Nov. 13, (2008). 4. Ahlers, E., "Extrabreit - Internet rasant per Glasfaser," c t 3/2009, 80-82, (2009). 5. Greil, P., Deutsche Telekom, Detecon Networks Forum 2007, Dresden 6. Fricke, M., "Examining the Evolution of the Access Network Topology," IEEE GLOBECOM ACCESS '09 Business Forum, Session 104: Advanced Fiber Access Systems, Dec. 2, (2009). 7. Shea D. P. and Mitchell J. E., Long-Reach Optical Access Technologies, IEEE Network, vol. 21, no. 5, pp. 5-11, September, (2007). 8. Rohde, H., Smolorz, S., Gottwald, E. and Kloppe, K., "Next Generation Optical Access: 1 Gbit/s for Everyone" ECOC, Vienna, (2009). Proc. of SPIE Vol. 7621 762104-8