Superchannels A to the rescue! S THE NEED for ever- increasing amounts of DWDM transmission capacity shows no sign of waning, the optical transport industry is moving toward a new type of DWDM technology the superchannel. A superchannel is a set of DWDM wavelengths generated from the same optical line card, brought into service in one operational cycle, and whose capacity can be combined into a higher-data-rate aggregate channel. It s the DWDM industry s answer to the question, What comes next after 100 Gbps? Scaling optical-fiber capacity By GEOFF BENNETT Superchannels will save carriers from the dilemma of how to flexibly scale capacity, particularly as requirements exceed 100 Gbps. The capacity and service f lexibility of optical fiber is remarkable, but still governed by strict rules of physics and engineering practicality. Although written in 2006, Emmanuel Desurvire s paper still gives an excellent overview of those limits, while a more recent paper by Adel Saleh and Jane Simmons points out that increases in the spectral efficiency of optical transport systems u ltimately provides the biggest bang for the buck in terms of capacity scaling to meet growing internet demand.1,2 But what neither of these papers covers is that, despite 40% compound growth in demand over the past five years (equivalent to a factor of five increase), service providers are not able to hire an army of extra network engineers. In fact, in most cases headcount will be frozen. So it s clear that the optical transport networks of the future must be capable of turning up much larger amounts of DWDM capacity for a given operational effort without sacrificing optical GEOFF BENNETT is the director of solutions and technology for Infinera. He has more than 20 years experience in the data communications industry, including IP routing with Proteon and Wellfleet, ATM and MPLS with FORE Systems, and optical transmission and switching with Marconi as distinguished engineer in the CTO Office. Reprinted with revisions to format, from the March/April 2012 edition of LIGHTWAVE Copyright 2012 by PennWell Corporation
O/S Processing virtualization layer Multi-core CPU reach or total fiber capacity. Today that capacity unit in long-haul networks is 100 Gbps a data rate enabled by a series of advances in optical transmission, namely: High-order phase modulation (typically -multiplexed quadrature phase-shift keying, or PM-QPSK). Coherent detection using a very stable local oscillator laser. Advanced digital signal processing in the receiver to compensate for fiber impairments. High-gain forward error correction (FEC), including soft-decision FEC that can offer more than 11 db of gain for a typical span. Services Let s refer to the combination of these four items as coherent technology, which offers a quantum leap Bandwidth virtualization layer Multi-carrier superchannel in terms of optical performance compared to non-coherent systems. While there will likely be incremental improvements in future coherent technology, these advances alone are unlikely to keep up with bandwidth demands. It s interesting to note that computer manufacturers are facing a similar problem. You may be aware that CPU clock speeds appeared to stop getting faster about five years ago. Yet the famous Moore s law remains valid in that the number of transistors on a chip is still increasing. CPU and GPU (graphics-processing-unit) manufacturers are using FIGURE 1. Virtualized parallel processing in the CPU and GPU world (left) and virtualized multi-carrier superchannel in the DWDM transport world (right). those additional transistors to build multiple cores, rather than running individual cores at faster data rates. But the chips they produce appear as a single unit of processing capacity to the operating system. Likewise, a DWDM superchannel consisting of multiple wavelengths appears as a single unit of operational capacity to the network engineer. This analogy is shown in Figure 1. Implementing superchannels So what s the best way to implement coherent superchannels? Let s assume that a service provider 1 laser 4 modulators 320-Gbaud electronics ~ 11-nm silicon ~10 years C-band 1 Tbps PM-QPSK Option A Option B Option C 2 lasers 8 modulators 160-Gbaud electronics ~16-nm silicon ~7 years needs to turn up a terabit of optical capacity in a single operational cycle. Today that would mean installing ten 100G transponders an approach that actually takes more than 10X the effort of a single transponder because each time a transponder is added it affects the existing wavelengths in the fiber. Since this approach offers no value for operational scaling, we will not consider it further. Instead, Figure 2 shows three engineering options A, B, and C that we will consider. All three 10 lasers 40 modulators 32-Gbaud electronics Photonic ICs ~2 years FIGURE 2. Comparison of spectral efficiency and electroniccomponent performance for single-carrier, dualcarrier superchannel, and 10-carrier superchannel implementations.
examples will use PM-QPSK as experiments before this, of course). the modulation technique: So let s take that to the next step Option A is a single-carrier with Option C, a superchannel with (i.e., one wavelength) transponder operating at 1 Tbps. 10 subcarriers, which divides the That s effectively a 100G transponder where the A series of incremental field electronics run 10X faster. trials culminated in 1 Tbit of Unfortunately, electronics superchannel capacity transmitted (particularly the analogto-digital converter and fiber link. over a production DWDM DSP chips) that run at the 320-Gbaud rate required will not be available for another decade, according to electronics performance by 10 certain industry roadmaps. also and 32-Gbaud electronics is Option B is a superchannel implementation consisting of two 10 subcarriers imply 10 optical actually available today. However, 500-Gbps subcarriers, which circuits, and coherent technology are electronically combined in the already requires a rather large transponder card to appear as a number of high-quality and therefore expensive optical components 1-Tbps superchannel. The advantage is that the performance of even for a single optical circuit. the electronics is halved to 160 In fact, a 10-carrier 1-Tbps superchannel line card would involve Gbaud. Unfortunately, we still have to wait about seven years around 600 optical functions in total before chips with this performance for the transmitter and receiver level are available for products circuit quite impractical if built (they may be available for hero using discrete optical chips. Fortunately, DWDM systems based on large-scale (i.e., multicarrier) photonic integrated circuits (PICs) have been commercially available since 2004. These PICs predate the more recent move toward coherent technologies, and many skeptics in the DWDM industry had initially expressed their doubts that such an advanced level of optical performance could be delivered in a commercial PIC. During the course of 2010 and 2011, however, a series of incremental field trials was completed culminating in a terabit of superchannel capacity being transmitted over a production DWDM fiber link between San Jose and San Diego on the TeliaSonera International Carrier network last November. The TeliaSonera trial used twin pre-production 500G coherent superchannel line cards, thanks to large-scale PIC technology. Turning up this 1 Tbps of capacity took two operational cycles, one for each 500 Gbps of capacity. This implementation compares much more favorably to the multiple rack implementation that s typical for a discrete-component superchannel demonstration requiring 10 line cards of 100 Gbps of capacity. High-order modulation Those of you with a cable, wireless, or xdsl technology background may already be familiar with higherorder phase modulation. Figure 3 shows the basic principle. Binary phase-shift keying (BPSK) uses two phase states per modulation symbol, which encodes 1 bit in that symbol. By adding multiplexing, PM-BPSK encodes 2 bits per symbol. We can add phase states to each symbol to encode additional bits, enabling us to transmit higher data rates with much better spectral efficiency. PM-BPSK will deliver 4 Tbps in the C-band, while PM-16QAM increases that to about 16 Tbps. But higher-order modulation comes at a price. Because optical fiber is a non-linear medium, each modulation symbol can only be transmitted at a certain power level before non-linear effects are triggered.
While PM-BPSK superchannels could from about 750 GHz (PM-BPSK) flexibility for Optical Transport depending on the balance of well be used for transpacific subma- to about 200 GHz (PM-16QAM). Network (OTN) transport contai- capacity and reach needed by the rine links, the reach of a PM-16QAM But all of these superchannels ners. The current OTN hierarchy network designer, it s necessary to superchannel may be limited. can be accommodated efficiently defines ODU0 (1.25G), ODU1 (2.5G), define an adaptable OTN contai- using a multiple of 12.5 GHz. ODU2 (10G), ODU3 (40G), ODU4 ner that can be sized accordingly. Going gridless In the short term, however, (100G), and ODUflex (n x 1.25G). At last December s ITU Study In explaining Figure 2, I had said service providers will need a super- ODUflex was ITU-T s response Group 15 meeting, an OTUadapt that 1 Tbps of capacity will require channel that can be deployed on for more flexible, lower-data-rate proposal gained widespread about the same amount of fiber an existing grid-based DWDM line containers. Since superchannels support from vendors, component spectrum regardless of how many system. So the first generation of may vary in their total capacity, companies, and service provi- subcarriers make up the super- commercial superchannel products ders. This flexibility would help channel. That s not true if the channels are forced apart to comply with a fixed grid ITU-T will use split spectrum superchannels, a term coined by the IETF. Split spectrum superchannels could BPSK 1 bit per symbol per to solve the nagging problem that OTN containers are often out of sync with next generation Ethernet G.694.1 spacing. This recommendation defines several grid spacings, including 25 and 50 GHz. If we potentially be designed to operate on 25- or 50-GHz G694.1 grid line systems and will provide a seamless QPSK 2 bits per services. Gigabit Ethernet (GbE), 10GbE, and 40GbE all had different but significant issues in their assume a 10-carrier superchannel of 100G per subcarrier, using PM-QPSK, then the carrier width is migration from a grid-based to gridless architecture. Meanwhile, they ll also offer the required opera- 8QAM 3 bits per OTN mapping. OTUadapt will avoid these issues in the future especially since the data rate for about 37 GHz. That s too wide for a 25-GHz grid, yet using a 50-GHz grid will waste about 25% of tional scaling benefits and only sacrifice about 20 25% of the maximum ideal fiber spectrum 16QAM 4 bits per Ethernet services beyond 100GbE has not yet been defined (note the IEEE standard is expected the available fiber spectrum. (assuming PM-QPSK modulation). in the 2016-17 timeframe). For this reason ITU-T has updated G.694.1 to include a flex grid option based on a 12.5-GHz granularity. The spectral width for a 1-Tbps gridless superchannel varies OTN flexibility An interesting technical challenge that results from superchannel architectures is the need for more FIGURE 3. Adding more bits to a symbol increases spectral efficiency, but the total power per symbol (before non-linear threshold is reached) is shown by the thick black circle. Flexible capacity DWDM superchannels potentially offer an ideal solution to the twin problems of increasing optical
transport capacity beyond 100 electronics, allowing this techno- for an engineering design. of Lightwave Technology, Vol. Gbps and providing the flexibi- logy to be delivered much more 24, No. 12, December 2006. lity to maximize the combination quickly than other options. The References 2. A. Saleh, J. Simmons, Technology of optical capacity and reach. By key to a multi-carrier superchan- 1. E. Desurvire, Capacity Demand and Architecture to Enable implementing a superchannel nel is the use of large scale PICs to and Technology Challenges the Explosive Growth of the with many optical carriers, we can reduce optical-circuit complexity for Lightwave Systems in the Internet, IEEE Communications reduce the requirement for exotic and offer the maximum flexibility Next Two Decades, Journal Magazine, January 2011.