A Primer on ROADM Architectures

Transcription

1 TECHNOLOGY WHITE PAPER A Primer on ROADM Architectures As bandwidth consumption continues to explode in a challenging economic environment, service providers and enterprises need to maximize their network s efficiency deliver more with less. Effective use of Reconfigurable Optical Add Drop Multiplexers (ROADMs) is key to this strategy. Author: Jim Theodoras, ADVA Optical Networking ADVA Optical Networking All rights reserved. Increasing the flexibility, scalability and remote configurability of a network lowers Operational Expenses (OPEX). Using ROADMs, a bandwidth provider can quickly turn up new services, alter networks as needed, protect his revenue stream and reduce truck rolls through remote management. However, the ROADM landscape is large and diverse, and all the available technologies and architectures can at times be confusing and potentially prevent operators from maximizing their networks potential. This white paper explores ROADM technologies and architectures and the various ways they are used in networks to increase efficiency. Background The advent of Wavelength Division Multiplexing (WDM) has been one of the key impetuses for the bandwidth revolution of the past decade. WDM technologies have brought more than an order of magnitude of increase in the amount of bandwidth that can be transported over a fiber, and this is where WDM thrived first and most, over point-to-point transport networks. However, while transport networks may be thought of as the roads of the network, the intersections are where the intelligence lies. Early WDM switches were electronic, and color conversion was simply appended to their input and output. These became known as OEO switches, as a WDM data stream was converted from Optical, to Electrical and back to Optical when traversing any node on a WDM network. However, full OEO conversion is power-hungry and can have difficulty scaling as data rates and channel counts grow. Effective use of ROADMs is key to network efficiency. Eventually, photonic switches of various optical technologies were used to build optical crossconnects (OXCs). The primary advantage of OXCs is that they are data rate transparent and do not need to be upgraded every time data rates jump to a new generation. Early OXCs did have an Achilles heel, though. While they worked great for grey traffic, when colored WDM traffic was used, wavelength blocking occurred. That is to say, after a path was switched, the signal was statistically possible to be blocked by a subsequent WDM filter or combiner [Figure 1]. The problem of wavelength-blocking led the optical industry to invest a substantial amount of time and money into finding a solution.

2 Figure 1: Wavelength-blocking in an OXC. If a transparent optical crossconnect is used after a channel de-multiplexer to switch WDM wavelengths, those channels are blocked after the switch by the following re-multiplexer. ROADM Types One of the primary functions needed at a node in a WDM-based network is add/ drop, which is the ability to drop an appropriate color from a WDM aggregate of wavelengths (rainbow of colors on a single fiber), and then re-add the incoming traffic of the same color back to the optical aggregate again. The simplest way of achieving an add/drop is with a pair of 3-port WDM dielectric filters, which selectively add or drop a wavelength to its add/drop port [Figure 2]. Such a filter can only add/drop a pre-designated color, as the wavelength affected depends upon the physical structure of the filter itself, namely the spacings of consecutively stacked dielectric layers. Dielectric filters are simple to use, yet relatively inexpensive and are still popular today for fixed network installations. Figure 2: Dielectric filter-based OADM. The colors that can be add/drop are fixed. Add/drop ports are pre-assigned fixed colors. However, today s networks are much more dynamic than in the past, and such networks demand greater flexibility. In order to provide the necessary flexibility, Reconfigurable Optical Add Drop Multiplexers (ROADMs) were developed. ROADMs perform exactly the function the name implies, and early versions were complex assemblies of hundreds of discrete optical switches, dielectric filters and sometimes even variable optical attenuators. These rats nests of fiber were eventually helped by Arrayed WaveGuide (AWG) technology supplanting the dielectric filter sets. In AWG s, the multiplexing and demultiplexing of channels in a WDM aggregate of colors is done in waveguides integrated onto a single substrate, so fewer fiber connections are needed. As technology advanced, so 2

3 did the level of integration, with some component vendors eventually integrating all the key functions required into a single substrate using Planar Lightwave Circuit (PLC) technology. Figure 3: Demux-switch-mux ROADM. Building blocks are interconnected to form a ROADM. Any combination of colors may be added or dropped. Add/drop ports are pre-assigned a fixed color. These ROADM architectures, regardless of the technology employed to realize them, are referred to simply as demux-switch-mux, and PLCs are merely one of many ways of implementing them [Figure 3]. Even with advances in PLC technology pushing the level of optical integration to new extremes, the sheer number of functions required to implement this ROADM architecture has burdened these products. The search for a more suitable ROADM technology continued, and this research led to an alternative approach, the Wavelength Selective Switch (WSS). WSSs are not a unique technology, but rather a new class of optical technologies that may be arranged in new ways to form a ROADM with different characteristics. Just as demux-switch-mux ROADM architectures may be formed with a variety of technologies, similarly WSS elements may be formed with Micro Electro Mechanical technologies (MEMs), Liquid Crystals (LC), micro-mirrors, gratings, etc. Figure 4: Broadcast-and-select ROADM with WSS used as blocker. As colors are added, the WSS blocks them as needed. Any combination of colors may be added or dropped. Add/drop ports are still pre-assigned a fixed color. WSS elements began their life humbly, used as wavelength blockers. The concept in itself seemed enigmatic, as the industry had invested so much money to avoid wavelength blocking in transparent optical switches, and the first WSS 3

4 solution actually intentionally blocked wavelengths [Figure 4]. The concept was simple: Use color-agnostic power splitters and combiners to add and drop aggregated traffic at a network node, and use the WSS based blocker to prevent any dropped channels from continuing through the network node and colliding with added channels. WSSs can switch any color in an aggregate to an add/drop port. At first, WSS based ROADMs were simply dropped into architectures optimized for fixed dielectric filters, and wavelength tunability was used only for spares reduction, as a single WSS could be stocked in place of filters. This configuration is sometimes referred to as broadcast-andselect, as all the colors are spread out and then the appropriate color(s) selected, thus solving the problem of wavelength blocking, as well as enabling remote add/drop node channel assignments to be made dynamically [Figure 5]. Dynamic ROADMs, like those WSS-based, ushered in a new era in WDM network management. In a WDM network, all the links between nodes within a network are in a delicate balance, with dozens of parameters such as individual channel power tuned to give optimal performance. Whenever a color is added, dropped, or switched, this balance must still be maintained in the traffic not being added or dropped, since this bypass traffic is not being re-generated at the node. This pass through traffic is sometimes referred to as the express path. Figure 5: Broadcast-and-select ROADM with WSS used as a selector. Any combination of colors may be added or dropped. Add/drop ports are still pre-assigned a fixed color. WSSs can also channel balance. Before the advent of ROADMs with power balancing capabilities, a network designer had two options: 1) Limit the number of nodes and link distances to that which corresponds to the maximum power variation (both in time and across the spectrum) that can reasonably be expected, or 2) Maximize the number of nodes and link distances, and rely on manual power balancing with attenuator pads at all points in the network, a very intensive process to say the least. WSSs have the important ability to vary the optical attenuation on a per channel basis. With WSS-based dynamic ROADMs overall network channel balance can be automatically maintained as various colors are added and dropped in a dynamic fashion. 4

5 -less is More Colorless ROADMs Meanwhile, while ROADM technology and architectures were advancing, so were transmitter technologies. As WDM laser transmitters transitioned from fixed, to narrowband tunable, to full C-band tunable technology, a new issue was eventually encountered. While a tunable transmitter coupled with re-configurable add/drop should provide full channel assignment flexibility, a new impediment was encountered: physical port blocking. When a transmitter is tuned to a new color channel, and the ROADM is reconfigured to match, the signal may now come out a different physical port at the network site, which would still require manual configuration and intervention. For example, if the dropped color at a node in a network is connected to a router, a physical cable connects the ROADM to the router, port to port. If the drop traffic is re-assigned a new color, the data will come out a different physical ROADM port and the router will no longer be connected to the ROADM [Figure 6]. Figure 6: Port blocking. If traffic is switched from λ1 to λ2 on network, it moves to a physical drop port not connected to the equipment port. The only way to prevent a port blocking situation in this scenario is to either 1) connect all ROADM ports to all router ports, which wastes expensive router ports, or 2) pre-plan network color and port assignments, which defeats the advantage of having used a ROADM for flexibility in the first place. Colorless ROADMs prevent port blocking. It did not take long for creative network designers to envision better implementations. The aforementioned issue of port blocking was resolved with the advent of colorless ROADMs. In these newer architectures, twice as many WSSs are used, and color selective multiplexers and de-multiplexers have been replaced with non-selective power combiners and splitters [Figure 7]. The advantage of this approach is colorless operation without port blocking. This advantage comes at a cost, though, as twice as many WSSs are required. This disadvantage has led to a variety of hybrid colorless architectures that compromise between the number of colorless add/drop channels and number of WSSs required. 5

6 Figure 7: WSS-based colorless ROADM. Any combination of colors may be added or dropped. Add/drop ports are no longer pre-assigned a fixed color. In other words, any color may be added or dropped at any port. Directionless ROADMs In directionless architectures, color-agnostic power splitters are used to broadcast the entire WDM aggregate (rainbow of colors) to all directions at the add/drop node in the network, where directions refers to North, South, East and West. WSSs then handle the rest, selecting which colors go where [Figure 8]. The powerful advantage of directionless ROADMs is that any port at the add/ drop node can be assigned any color and network direction, seamlessly, automatically and without disturbing the balance of the express network. All this flexibility complicates matters a bit, though, since once a wavelength is assigned a network direction it becomes unavailable to other directions. Figure 8: WSS-based directionless colorless ROADM. Any color may be added or dropped at any port in any direction as long as that color port is available on each WSS. 6

7 Another way of thinking about directionless ROADMs is that any signal may be sent in any direction, but there are pre-assigned colors for each direction, as shown in Figure 9. Figure 9: In directionless ROADMs, any signal may be sent in any network direction, but colors are pre-assigned to each direction. Directionless, Colorless ROADMs Since having colors pre-assigned to network directions can limit network flexibility, it didn t take long for ROADMs that are both directionless and colorless to arrive. In these designs, any color may be sent in any network direction, as shown in Figure 10. However, once a color is assigned to a network direction, it then becomes unavailable to all other network directions. This additional shortcoming can also impact overall network efficiency, as colors sit around unused. When two networks request the same resource at the same time, and only one may be served at a time, then we say the resource is in contention. In this case, if the same color is needed by two different directions, and only one direction gets to use the color, then that color is in contention. Figure 10: In colorless, directionless ROADMs, any color may be sent in any network direction, but only used once. 7

8 Colorless, Directionless, Contentionless (CDC) ROADMs As the name suggests, colorless, directionless and contentionless (CDC) ROADMs do not have contention issues, thus solving the efficiency problems introduced by wavelengths sitting around unused due to configuration rules. CDC ROADMs offer truly unrestricted wavelength (color) connectivity with no strings attached. Figure 11: In colorless, directionless and contentionless (CDC) ROADMs, any color may be routed to any and all network directions. Gridless ROADMs At their debut, CDC ROADMs were thought of as the epitome of color switching. After all, any color can be switched to any port or direction at any time. Nonetheless, over time shortcomings have been gradually uncovered. The key limitation in CDC ROADMs is in the very thing they are switching the colors. WDM is the transport of multiple data streams in parallel on the same fiber optic cable by assigning them different colors. Ever since the arrival of WDM technology, the colors have been pre-assigned by a standard grid of wavelengths with evenly spaced frequency intervals, such as 200GHz or 100GHz. There are two disadvantages to running a fixed grid, and both relate to future migration. First, a grid is usually chosen for a network up front and then rarely changed. Moving one step in granularity, say from 100GHz to 50GHz spacing, using a novel optical device called an interleaver is possible, especially if the equipment has been predesigned to make the move painless. However, if interleavers were not Figure 12: Interleavers may be used to migrate to a denser channel spacing. 8

9 part of the initial network planning, or moving more than one level of granularity is required, changing grids can be very difficult if not impossible. Modulating a signal makes the carrier wider. To understand the second limitation of running a fixed grid, one must first step back and take a look at basic communication theory. All basic communication relies on encoding information onto a carrier that transports it from point A to B. When the carrier is modulated with the desired information, the characteristics of the carrier change. In this case, the carrier is a color of light from a laser, and the information is a 0s and 1s bit stream of data. When the light is modulated with the data, the frequency of the light spreads out, and the color becomes less pure. The purity of the color of light is measured by spectral width, and the spreading of the color due to modulation is known as the modulation bandwidth. Figure 13 shows the spectrum of a Non- Return-to-Zero (NRZ) modulated 10Gbit/s signal. Figure 13: Spectrum of 10Gbit/s NRZ versus 50GHz ITU channel When WDM first arrived on the scene, data rates were so much smaller than the bandwidth available on the wavelength grid that the modulation width of the signal being carried was considered to be negligible and completely ignored. For example, the modulation bandwidth of a 622Mbit/s signal consumes less than 1% of a 200GHz channel band. As data rates have continued to climb, and channel spacing fell to 50GHz, modulation bandwidths are threatening to exceed channel bandwidths, forcing a move to modulations with higher spectral efficiencies. Today, a transport network may be a mix of many different data rates and modulation techniques. Trying to force fit these signals into a predetermined wavelength grid creates inefficiencies due to incomplete matching. As data rates migrate from 100Gbit/s to 400Gbit/s and even 1Tbit/s, the sheer variety of signal rates and spectral widths necessitates a more flexible approach to wavelength allocation within a fiber. Enter gridless ROADMs. In a gridless ROADM, rather than discrete color channels being switched, sections of the optical spectrum may be sliced out and switched. The starting and ending points of the spectrum being switched may be tuned so that the amount of spectrum being re-routed can be as narrow or as wide as needed. There are many advantages to a gridless approach. When channels are evenly spaced a gap is left between them. This gap is known as the guard band, and as much as 25% of a fiber optic cable s total bandwidth carrying capacity can be lost to guard bands. With a gridless ROADM, these guard bands 9

10 Figure 14: As data rates climb, NRZ modulation width exceeds ITU channel width can be squeezed down to the minimum required, rescuing stranded bandwidth. Running gridless can also help future proof a network, as ROADMs today can be set to standard spaced grids, yet be reconfigured as wide or narrow as needed to support future signal protocols. Figure 15: In a gridless ROADM, channels may be sized to each signal s width. Perhaps the most exciting potential of gridless ROADMs is support of Software Defined Optics (SDO), where the modulation type of a signal is varied depending upon the reach needed. The bit rate may be kept fixed, say at 400Gbit/s, but the symbol rate is increased or decreased to tradeoff information rate and physical reach. While in this scenario the bit rate is a constant, since the modulation type is changing, so does the spectral width. In order to efficiently support SDO, a gridless approach is needed to optical channel management, otherwise the required guard bands would negate any efficiency improvements. 10

11 Now, it turns out gridless ROADMs do in fact have a grid, for example 12.5GHz, though this is more a concession to software design than physical limitations of the optical design. It turns out managing wavelengths to several decimal points can get a bit unwieldy. Choosing to run a gridless ROADM at a finer granularity grid offers most of the benefits of running gridless, with fewer resulting headaches. ADVA Optical Networking s Client-Selectable Interface Client-selectable ROADMs are ultimate in flexibility. There really is no right and wrong ROADM architecture, as the optimal configuration depends upon the customer requirements, network design, and individual add/drop node characteristics. This is why ADVA Optical Networking does not force fit a ROADM architecture upon customers. Not only does the ADVA Fiber Service Platform (FSP) have a wide range of ROADMs to choose from, it s unique client-selectable interface architecture allows customers to individually tailor each ROADM to each add/drop node in the network, using the same common building blocks. In addition, by offering both low (2) and high (8) port WSS variants, customers are not forced into paying for the extra ports that may not be needed at a network add/drop node. Figure 16: Client-selectable building blocks Client-selectable ROADMs consist of a power combiner or splitter and a WSS packaged together, with all of the fiber access ports brought to the faceplate. The combiners and splitters are colorless, while the WSS is available in two sizes and channel spacings. Since all the ports are accessible to the customer, the ROADMs can be cross-cabled to perform any needed function. For example, the 8 CH WSS with colorless combiner ROADM may simply be used by itself to make a 40 channel colorless terminal with 8 channel add/drop capability. Or, it can be combined with a 2 CH WSS colorless splitter ROADM to form a two-degree 40 channel colorless ROADM. Or, alternatively, it can be combined with the 8 CH WSS colorless splitter ROADM to form a multi-degree 40 channel CDC ROADM with 8 channels of add/drop in as many as seven network directions [Figure 9]. 11

12 Figure 17: Client-selectable building blocks configured as CDC ROADM Network Considerations In order to appreciate the benefits of these latest and greatest ROADMs, one has to look beyond the simple add/drop scenarios at a single network node, and instead look at overall network architecture. One hurdle in using the latest ROADMs may be encountered when connecting them to customer premise equipment (CPE). While a ROADM may be able to switch any color to any port and direction, it still needs to know what color has been connected to each of its ports. The traditional way of connecting CPE to transport gear is client-to-client. In this case, the color is created in the transport gear transponder, which is in communication with the ROADM, so the transponder and ROADM can sync up on colors. This works great, but some customers prefer to avoid the additional power and cost of the client optics and directly connect CPE to ROADMs. In this second scenario, the color is generated in the CPE, which is not in the same management domain as the transport equipment, so no communication can occur between the transponder and ROADM. These colors are called alien wavelengths because the ROADM has no idea the color, power, or other characteristics of the signal that is being input. While able to support either, ADVA Optical Networking has developed a third option that offers all the benefits of both of the aforementioned approaches, without any of the shortcomings. With intelligent wavelengths, an Optojack transceiver is installed in the CPE. The cost and power of redundant client connections are avoided, yet the Optojack transceiver resides in the management domain of the transport equipment, and is in full communication with the transport gear. Using an out of band communication channel, the ROADM is able to read the color of the attached signal and tune itself appropriately. After a ROADM switches, span design must still be met. A second hurdle in exploiting ROADMs to their greatest potential benefit is network span constraints. While a CDC ROADM can slice and dice wavelengths at an add/drop node any way you like it, what good is it, if after the switch the signal does not make it to the next node? While configuration rules may allow a color switch, network span design rules may not. This is why ADVA Optical Networking closely matches ROADMs with other synergistic technologies to form a holistic Agile Core Transport solution. Span loss due to attenuation 12

13 Figure 18: Three ways of connecting CPE to transport in glass fiber is combated with both hybrid EDFA/RAMAN optical amplifiers and coherent detection based receivers. The latter uses digital filtering techniques to null out both chromatic and polarization mode dispersion. These two disruptive technologies combine to provide enough span budget for gridless CDC ROADMs to have the freedom to switch any color to any network span. Gridless CDC ROADMs, coherent receivers, and hybrid amplifers together form a holistic Agile Core Transport solution that enable any range of colors to be switched out any port, in any network direction, over any span distance. Figure 19: Agile Core Transport 13

14 Coherent Detection Based ROADM Architectures The aforementioned coherent detection based receivers provide additional benefits beyond increased span distance. Coherent detection relies on a stronger laser source at the receive end of link being mixed with the weaker incoming received laser signal to make it bigger. For 100Gbit/s coherent receivers, intradyne detection is used, which allows the use of a smaller local oscillator that does not have to be phase and/or frequency locked to the incoming signal. Figure 20: Coherent intradyne receiver If the color of the local oscillator is varied, rather than fixed, then a wavelength selective receiver is realized. Having a tunable receiver allows the simplification of some leading ROADM architectures. For example, in a colorless ROADM, one of the WSS s may be replaced with a simple power splitter. Power splitters are much smaller, cheaper, and have lower signal loss than WSS s. However, in practice the number of channels that can be simultaneously sent to a coherent selective receiver is limited due to signal-to-noise considerations. Rather than hitting the receiver with all 96 channels at once, typically a subset of channels is selected by a drop side WSS, and then the coherent receiver selects one of the colors in the subset. Figure 21: Coherent detection based colorless ROADM 14

15 Similarly, coherent detection based wavelength selective receivers may be used to simplify the CDC ROADM shown in Figure 17. Note how the WSS that is used to prevent port blocking at the drop site can be replaced with a simple power splitter. In this example, the drop side WSS first selects any 8 channels from the 40 channel WDM aggregate, and then each coherent receiver selects one of these 8 channels. Figure 22: CDC ROADM updated to coherent In both examples, only coherent receivers will be able to make sense of the dropped signal, so this type of simplified ROADM architecture can only be used to drop coherent signals. Both direct detection and coherent based ROADMs can easily co-exist on the same network, so with modern hybrid networks, both are likely to be used wherever most appropriate. Control Plane ROADMs are typically all interconnected and controlled by messages from a network manager sitting somewhere in the network. This is generically known as the control plane. In ADVA Optical Networking solutions, a simple yet sophisticated GMPLS-based control plane is used to create plug-andplay configuration of end-to-end connections. Channel assignments, port connectivity, as well as any necessary channel-to-channel power balancing are all done in a fully automated fashion. For example, the simple task of lighting up an additional wavelength in a WDM transport network can involve hundreds of individual commands. Tunable transmitters, attenuators, amplifiers, ROADMs, etc. all need to be reconfigured on both working and protect paths. With ADVA Optical Networking s Low Touch Provisioning (LTP), all of these commands are automatically issued and ripple through the network to where they are needed. Network installations are quick and painless, and later re-configurations are a snap. As a network grows, add/drop channels may be added whenever and wherever needed. OPEX is greatly reduced. Networks are more efficient when routers switch ROADMs. In general, all transport layer network managers are good at bringing up and tearing down wavelength services. And, for basic service management this is sufficient. However, if higher network layer performance is to be optimized, the transport layer lacks the information necessary to make higher level decisions. If one wants to optimize router-to-router interconnect, then the routers need the ability to gather information from the transport layer and then switch it as needed. To put it more succinctly, networks are more efficient when routers switch ROADMs. 15

16 ADVA Optical Networking is a leader in control plane integration and has developed an External Network-to-Network Interface (ENNI) interface that allows communication between the packet routing and transport layers of the network. When connected to this ENNI, routers have full visibility into available wavelengths, path constraints and transport network topology. Routers may directly setup and tear down Line Switched Paths (LSPs) through their own operating system. New services may be provisioned in seconds rather than weeks. End-to-end service protection and restoration may be done entirely at the routing layer of the network, greatly reducing the amount of redundant protection paths and overprovisioned links needed on a service. Figure 23: ENNI between routing and transport domains Summary A new paradigm in network efficiency has been achieved. WDM technology increased fiber link capacity, but also created the need for colored add/drops at intermediate network nodes. Early Optical Add/Drop Multiplexers (OADMs) met this need for optical bypass. The dynamic nature of both traffic and networks led to the advent of Reconfigurable OADMs. These ROADMs come in various configurations, based upon differing technologies. The optimal type depends upon the individual network and node configuration. ADVA Optical Networking s client-selectable technology allows a customer to tailor the ROADM to each application. When the ADVA Fiber Service Platform (FSP) clientselectable building blocks are configured as gridless CDC ROADMs, any range of colors of traffic at any network port may be routed in any direction. When the flexibility of gridless CDC ROADMs is coupled with the additional gain of hybrid EDFA/RAMAN amplifiers and coherent detection receivers, a truly Agile Core Transport network is achieved. Agile Core Transport allows remote provisioning of color and direction, thus enabling on demand applications, while lowering the overall number of truck-rolls in the process, all of which greatly reduces OPEX for quicker and easier service bring up and protection. When Agile Core Transport is coupled to routers through an ENNI interface, routers may directly initiate and switch LSPs across a WDM transport network, thus achieving a new paradigm in network efficiency. 16

17 About ADVA Optical Networking ADVA Optical Networking is a global provider of intelligent telecommunications infrastructure solutions. With software-automated Optical+Ethernet transmission technology, the Company builds the foundation for high-speed, next-generation networks. The Company s FSP product family adds scalability and intelligence to customers networks while removing complexity and cost. Thanks to reliable performance for more than 15 years, the Company has become a trusted partner for more than 250 carriers and 10,000 enterprises across the globe. The ADVA FSP 3000 ADVA Optical Networking s scalable optical transport solution is a modular WDM system specifically designed to maximize the bandwidth and service flexibility of access, metro and core networks. The unique optical layer design supports WDM-PON, CWDM and DWDM technology, including 100Gbit/s line speeds with colorless, directionless and contentionless ROADMs. RAYcontrol, our integrated, industry-leading multi-layer GMPLS control plane, guarantees operational simplicity, even in complex meshed-network topologies. Thanks to OTN, Ethernet and low-latency aggregation, the FSP 3000 represents a highly versatile and cost-effective solution for packet optical transport. ADVA Optical Networking North America, Inc Peachtree Industrial Blvd. Norcross, Georgia USA For more information visit us at ADVA Optical Networking SE Campus Martinsried Fraunhoferstrasse 9 a Martinsried / Munich Germany ADVA Optical Networking Singapore Pte. Ltd. 25 International Business Park # German Centre Singapore Version 08 /