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A Survey of Recent Developments on Flexible/Elastic Optical Networking I. Tomkos, E. Palkopoulou, M. Angelou Athens Information Technology (AIT) Center, Peania, Greece e-mail:
[email protected] ABSTRACT There is a growing awareness that the utilized bandwidth of deployed optical fiber is rapidly approaching its maximum limit. Given the possibility for such capacity crunch, the research community has focused on seeking solutions that make the most out of the scarce network resources (such as the fiber bandwidth) and allow accommodating the ever-increasing traffic demands. In such context, new spectrum efficient optical networking techniques have been introduced as a way to offer efficient utilization of the available optical resources. "Flexible", "elastic", "tunable", "gridless" or "adaptive" are few examples of the terms used in literature to describe solutions that migrate from the fixed WDM single line rate systems to systems that provide support for the most efficient bandwidth utilization. In this paper, we review the recent developments on the research topic of flexible/elastic networking and we highlight the future research challenges. Keywords: flexible optical networking, optical OFDM, Nyquist WDM, network planning. 1. INTRODUCTION As bandwidth-hungry applications are becoming widely adopted, the network traffic demand is continuously growing. However, this increase in the transported bits is not reflected in the revenues of the network operators – since the average revenue per user (ARPU) remains fairly flat. As a result, there is an increasing pressure to drive down the average cost per transported bit. Traditionally, the pressure to drive down network costs was transferred to the network equipment vendors – leading to requirements for cost efficient equipment and network architectures. Recently, the assumption that the spectrum requirements pose no restriction has been challenged. There is a growing awareness that the physical capacity of the optical fiber is rapidly approaching its maximum limit [1]. This growth has been enabled by important technological advancements, including, among others, the introduction of WDM, erbium-doped-fiber amplifiers and dispersion compensation. The current trends of traffic growth and system capacity increase will result in system capacity falling behind traffic by a factor of 10 over the same time period. Scaling interface rates and system capacity to meet these challenges will be difficult and will require breakthroughs on a similar scale to the introduction of large-scale wavelength division multiplexing. This growth in traffic coupled with observed trends in commercial practice will result in a requirement for interfaces to the core network to migrate from the current 10 Gb/s and 40 Gb/s to 100Gb/s in the next few years and to 1 Tb/s by 2020. Even assuming the future availability of 100 GSample/s analogue to digital converters (ADCs), sending bit rates as high as 1Tb/s over a single carrier would require the use of extremely large constellations, such as PM-1024QAM [2]. A recently proposed alternative solution is that of assembling a “superchannel” using several densely packed subchannels, as shown in Fig. 1. With the capacity crunch looming, the concept of flexible optical networking has been introduced as a way to offer efficient utilization of the available optical resources – while simultaneously accommodating extremely high data-rates [3]. “Flexible”, “elastic”, “tunable”, “gridless” or “adaptive” are few examples of the terms used in literature to describe solutions that migrate from the fixed WDM single line rate (SLR) systems to systems with improved and heterogeneous transmission characteristics. Prior to the introduction of flexible optical networking, the mixed line-rate (MLR) solution, where channels with different rates – such as 40-Gb/s and 100-Gb/s – can co-exist has been studied to upgrade the capacity of the existing systems. However, such solutions provide limited flexibility and cannot scale to the envisioned capacities of future systems. The term “flexibility” in optical networks refers to the ability of the network to dynamically adjust its resources (wavelength channels, bandwidth, transmission format, data rate, etc.) in an optimum and elastic way according to the continuous varying traffic conditions (traffic churn) and demands, while taking into consideration the Quality of Transmission (QoT) requests of the both the pre-established and newly assigned connections. Recent advances in optical orthogonal frequency division multiplexing (OFDM), coherent transmission, and Nyquist WDM (N-WDM) have set the stage for envisioning fully flexible optical networks, which are capable of dynamically adapting to the requirements of each connection. The required bit-rate and the transparent reach can be dynamically tuned – for example via the flexible allocation of spectrum and the selection of the appropriate modulation format. Thus, scarce resources – such as spectrum – can be more efficiently utilized. In order to fully realize the vision of flexible optical networking it is necessary to migrate
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Figure 1. A “superchannel” is composed by several densely packed subchannels. Coherent optical orthogonal frequency division multiplexing (Co-OFDM) and Nyquist WDM (N-WDM) techniques can lead to great spectral efficiency. Additional spectrum savings can be achieved by migrating from the rigid ITU-T frequency grid to a flexible configuration. from the rigid ITU-T frequency grid to a flexible configuration. The “Flexi-Grid” enables the occupation of spectrum slots with finer granularity, while the “Grid-less” case enables full flexibility – as shown in Fig. 1. In Fig. 2 a categorization of different research areas with respect to flexible optical networking is presented. The following areas are identified: performance (examined on the connection level and on the network level), cost (consisting of capital and operational expenditures), energy efficiency, and control plane requirements. The main parameters of interest concerning the performance on the network level are the bit-rate, the transparent reach, and the spectral efficiency. On the network level, it is important to examine the overall utilized spectrum, the blocking probability, the utilized interfaces, and the availability. In this paper, we review the recent developments on the research topic of flexible networking and we highlight the future research challenges. First, the technologies providing the basis for flexible optical networks (i.e., OFDM and N-WDM) are presented in Section 1. This corresponds to examining the performance on the connection level. We then proceed to describe how flexible optical networks can be optimized, as new challenges arise compared to traditional WDM network planning. Different optimization objectives can be defined on the network level, which may additionally include cost and energy targets. Control plane aspects are discussed in Section 3. Finally, we conclude this paper presenting the research challenges that flexible networking poses. 2. TECHNOLOGIES FOR FLEXIBLE OPTICAL NETWORKS New technologies are required to be developed in order to achieve high capacity and tuneable transport. As spectrum is becoming a scarce resource, it is especially important to achieve these targets at the highest possible spectral efficiency. A first step towards this direction is the deployment of multi-level modulation formats using polarization multiplexing, which achieve high spectral efficiency [1]. A recently proposed solution is based on combining multiple tightly spaced channels, which can assume such multi-level modulation formats. These multiple tightly spaced channels form superchannels offering tunable bit-rates in the terabit per second range. The channels forming the superchannels are referred to as subcarriers in the following. The bit rate of such systems is dependent on the subcarriers modulation format, the symbol rate, the FEC, and on the number of subcarriers. Thus, additional degrees of freedom are offered, which support tunability in terms of the bit rate and reach. There are two main approaches that enable the subcarriers to be closely spaced: Coherent-OFDM (Co-OFDM) [4] and N-WDM [2]. The ultimate spectral efficiency is the same for both methods. In the following we describe these methods in more detail.
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Figure 2.Categorization of different research areas with respect to flexible optical networking. Recent technological advances – for example in optical detection technologies- have enabled the adoption of OFDM in optical systems. OFDM can be used for the creation of subcarriers with nearly rectangular spectra, which can then be assembled into superchannels. In this case (i.e., when OFDM is viewed as a subcarrier modulation format), each subcarrier is formed by a multitude of “electrical” subcarriers. In [5] the generation, transmission, and reception of a 1.21 Tb/s OFDM signal reaching 400 km was demonstrated. Recently, in [6] a 400 Gb/s multi-flow, multi-rate, and multi-reach optical transmitter was demonstrated – while in [7] the transmission of a 1.12 Tbit/s 32 QAM OFDM superchannel was presented. For the transport of high bit rates, modulation formats up to 256-QAM have been examined. An alternative is for OFDM to be used at the optical level to multiplex conventionally-modulated subcarriers. This corresponds to the Co-OFDM case. In Co-OFDM, the subcarriers are heavily overlapping in the spectral domain. However, due to the orthogonality of the subcarriers, they can ideally be detected with no mutual crosstalk. In principle, both OFDM and Co-OFDM can achieve the maximum theoretical spectral efficiency. In N-WDM the subcarriers are spectrally shaped so that they occupy a bandwidth close or equal to the Nyquist limit for inter-symbol-interference-free and cross-talk-free transmission. In this case the subcarrier spacing can then be ideally as small as the symbol rate. In N-WDM there are two alternatives to achieve the quasi-rectangular subcarrier spectral shaping. The first one is via band-limiting the signal from each transmitter through an optical filter (“optical N-WDM”). The second alternative is via driving the electro-optical modulator with suitable electrical signals in order for the optical modulated signal to take the desired spectral shape (“digital N-WDM”). In this case DACs are required to generate the electrical driving signals. Experimental results have been presented for optical and digital N-WDM. Ultra-long-haul transmission of DP-QPSK N-WDM has been demonstrated at 100 Gb/s at transoceanic distances [8-9], with channel spacing equal to the symbol rate. Recently, 100 Gb/s DP-16QAM with a channel spacing value of 1.05 times the symbolrate was achieved having a reach of 3,700 km using digital N-WDM [10]. 3. NETWORK OPTIMIZATION FOR FLEXIBLE OPTICAL NETWORKS The emergence of flexible optical networking as a method to increase resource efficiency and provide advanced functionalities poses significant challenges on the networking level. In Fig. 3 a categorization of approaches for network optimization of flexible optical networks is presented based on the design scope, the application scope, and the selected methodology. Novel resource allocation algorithms are required to be developed – as the conventional routing and wavelength assignment (RWA) algorithms of traditional WDM networks can no longer be applied. Instead of assigning a certain wavelength to each connection, a number of contiguous subcarrier slots are now to be assigned. Additionally, the continuity of these subcarrier slots should be guaranteed in a similar manner as wavelength continuity constraints are imposed. This leads to the development of routing and spectrum allocation algorithms (RSA) ([11], [12], [13]). Moreover, as additional degrees of freedom are allowed by flexible optical networks – new relevant constraints are required to be considered. For example as the modulation level can be
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selected on a connection basis, constraints tying it to the required bit-rate of the traffic demand as well as to the achieved transparent reach are necessary. To this end routing modulation level and spectrum allocation algorithms (RMLSA) have been recently proposed ([14], [15]). Physical layer impairments can be considered in the planning procedure. Note that in this case the maximum transmission distance is commonly used metric. Additionally, restrictions can be imposed on the manner in which connections are allowed to be re-routed. Rerouting may be desired for example in order to avoid blocking of new demands. The discussed algorithms can address the offline network planning phase ([11], [12], [14]), or they can be applied to dynamically provision connection requests ([13], [14]). As connections are dynamically established and released, the issue of bandwidth fragmentation arises – leading to increased blocking probabilities. Thus, the development of spectrum defragmentation algorithms is required. Another categorization involves the method used to conduct the planning. Mathematical optimization methods, such as integer linear programming (ILP) and mixed integer linear programming (MILP), can be applied. Heuristic approaches can also be applied – especially if the computational complexity is a restricting factor. In this case optimality can be sacrificed in order to reduce the computation time. This leads to heuristic methods being the method of choice for dynamic planning. Note that the discussed network planning approaches can be applied with different optimization objectives. While most studies concentrate on achieved spectrum savings, the ultimate selection criterion for the deployment of new technologies is cost. In [16] one of the first attempts to quantify the cost savings of flexible optical networking was presented. Recently, energy efficiency has gained increasing importance due to environmental awareness and the pressure to drive down the operational costs ([17], [18]). However, the energy efficiency of flexible optical networks has only recently begun to be investigated [19].
Figure 3. A categorization of approaches for network optimization of flexible optical networks. 4. CONTROL PLANE FOR FLEXIBLE OPTICAL NETWORKS As flexible networks offer new capabilities, it is necessary to design and develop an advanced control plane solution fully supporting these emerging features. In [20] adoption scenarios from current rigid grid optical networks are presented. Possible extensions of OTN and ASON/GMPLS standards are discussed. In [21] the requirements for GMPLS control of flexible grids are studied. Additionally, the definition of GMPLS lambda labels is presented [22]. The extensions of OSPF-TE functionalities in order to support the flexible grid are discussed in [23-25]. In the following we describe recent works considering control plane aspects for flexible optical networks. In [26] the first demonstration of an impairment-aware flexible-bandwidth network test-bed with a real-time adaptive control plane is presented. The modulation format and the spectrum position can be adjusted, leading to greater efficiencies in terms of network resources. In [27] an effective lightpath provisioning scheme is proposed for path computation element (PCE) based optical networks assuming a flexible grid. It is shown that 12.5 GHz
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spacing is satisfactory to guarantee efficient network resource utilization for the expected modulation formats at 400 Gb/s. In [28] an experimental demonstration shows the capability of the PCE to trigger dynamic rerouting with bit-rate or modulation format adaptation. Dynamic frequency slot assignment and format adaptation from DP-16QAM to DP-QPSK at 100 Gb/s, and bit-rate adaptation at DP-16QAM from 200 Gb/s to 100 Gb/s is demonstrated. 5. FUTURE RESEARCH CHALLENGES FOR FLEXIBLE OPTICAL NETWORKS A key open research challenge associated with flexible optical network planning is the development of advanced static and dynamic algorithms for routing and resource allocation in flexible networks. While initial results indicate that there is significant potential in such technologies, holistic studies are yet to be explored. In particular, it is important to examine the design of dynamic algorithms, taking into account failure probabilities and limitations on the amount of information available about the network status. Finally, spectrum defragmentation processes along with spectrum allocation policies, which proactively combat spectrum fragmentation, need to be addressed. Additionally, the development of protocol extensions, which can provide the basis for flexible optical networks to be efficiently controlled and managed, is required. The collection of relevant information from the physical layer, as well as the integration of this information in the dynamic planning algorithms is important. In addition to the efficient use of network resources, it is important to consider the computational efficiency of the algorithms as well as the signaling requirements. This issue becomes more critical as stringent real-time requirements are imposed during the network operation. REFERENCES [1] René-Jean Essiambre et al., "Capacity limits of optical fiber networks," J. Lightwave Technol., vol. 28, 662-701 (2010). [2] G. Bosco, V. Curri, A. Carena, P. Poggiolini, F. Forghieri, “On the performance of Nyquist-WDM terabit superchannels based on PM-BPSK, PM-QPSK, PM-8QAM or PM-16QAM subcarriers,” J. Lightwave Technol., vol. 29, no. 1, Jan 1, 2011, pp. 53-61. [3] M. Jinno et al., "Spectrum-efficient and scalable elastic optical path network: Architecture, benefits, and enabling technologies," IEEE Communications Magazine, 47, 66-73 (2009). [4] B. Zhu, X. Liu, S. Chandrasekhar, D. W. Peckham, and R. Lingle, Jr., “Ultra-long-haul transmission of 1.2-Tb/s multicarrier no-guard interval CO-OFDM superchannel using ultra-large-area fiber,” IEEE Photon. Technol. Lett., vol. 22, pp. 826–828, Jun. 2010. [5] R. Dischler and F. Buchali, “Transmission of 1.2 Tb/s continuous waveband PDM-OFDM-FDM signal with spectral efficiency of 3.3 bit/s/Hz over 400 km of SSMF,” in Proc. OFC 2009, Mar. 22–26, 2009, Paper PDPC2. [6] H. Takara, T. Goh, K. Shibahara, K. Yonenaga, S. Kawai, and M. Jinno, "Experimental demonstration of 400 Gb/s multi-flow, multi-rate, multi-reach optical transmitter for efficient elastic spectral routing," in Proc. ECOC 2011, paper Tu.5.A.4. [7] X. Liu, S. Chandrasekhar, X. Chen, P. Winzer, Y. Pan, B. Zhu, T. Taunay, M. Fishteyn, M. Yan, J. M. Fini, E. Monberg, and F. Dimarcello, "1.12-Tb/s 32-QAM-OFDM superchannel with 8.6-b/s/Hz intrachannel spectral efficiency and space-division multiplexing with 60-b/s/Hz aggregate spectral efficiency," in Proc. ECOC 2011, paper Th.13.B.1 [8] J. Cai, et al., “20 Tbit/s transmission over 6,860 km with sub Nyquist channel spacing”, J. Lightwave Technology, accepted for publication, available in pre-print on IEEE Xplore. [9] E. Torrengo, et al. “Transoceanic PM-QPSK terabit superchannel transmission experiments at baud-rate subcarrier spacing,” in Proc. of ECOC 2010, paper We.7.C.2 [10] R. Cigliutti et al, “Ultra-long-haul transmission of 16x112 Gb/s spectrally-engineered DAC-generated Nyquist-WDM PM-16QAM channels with 1.05x(symbol-rate) frequency spacing,” in Proc. OFC 2012. [11] Y. Wang, X. Cao, and Y. Pan, “A study of the routing and spectrum allocation in spectrum-sliced elastic optical path networks,” in Proc. IEEE International Conference on Computer Communications (INFOCOM), 2011. [12] M. Klinkowski and K. Walkowiak, “Routing and spectrum assignment in spectrum sliced elastic optical path network”, IEEE Communications Letters, vol. 15, no. 8, pp. 884-886, 2011. [13] X. Wan, L. Wang, N. Hua, H. Zhang, and X. Zheng, “Dynamic routing and spectrum assignment in flexible optical path networks,” in Proc. OFC 2011. [14] K. Christodoulopoulos, I. Tomkos, M. Varvarigos, "Elastic bandwidth allocation in flexible OFDM-based optical networks", J. Lightwave Technol., 2011. [15] K. Christodoulopoulos, I. Tomkos, M. Varvarigos, “Dynamic bandwidth allocation in flexible OFDMbased networks”, in Proc. OSA/IEEE OFC/NFOEC 2011.
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[16] K. Christodoulopoulos, M. Angelou, D. Klonidis, P. Zakynthinos, M. Varvarigos, I. Tomkos, “Value analysis methodology for flexible optical networks”, in Proc. ECOC’2011. [17] J. C. Cardona Restrepo, C. Gruber, and C. Mas Machuca, “Energy profile aware routing,” in Proc. First International Workshop on Green Communications at IEEE International Conference on Communications, Dresden, Germany, June 2009. [18] G. Shen and R. S. Tucker, “Energy-minimized design for IP over WDM networks,” Journal of Optical Communications and Networking, 1(1):176{186, 2009. [19] M. Angelou, K. Christodoulopoulos, D. Klonidis, A. Klekamp, F. Buchali, E. Varvarigos, and I. Tomkos, “Spectrum, cost and energy efficiency in fixed-grid and flex-grid networks,” in Proc. OFC/NFOEC 2012. [20] M. Jinno et al., "Elastic and adaptive optical networks: Possible adoption scenarios and future standardization aspects", IEEE Communications Magazine, vol. 49, no.10, pp.164-172, Oct. 2011. [21] F. Zhang et al., “Requirements for GMPLS control of flexible grids”, IETF Internet Draft, Oct. 2011. draftzhang-ccamp-flexible-grid-requirements-01.txt [22] D. King et al., “Generalized labels for the flexi-grid in lambda-switch-capable (LSC) label switching routers”, IETF Internet Draft, October 2011. draft-farrkingel-ccamp-flexigrid-lambda-label-01.txt [23] F. Zhang et al., “GMPLS OSPF-TE extensions in support of flexible-grid in DWDM networks”, IETF Internet Draft, October 2011. draft-zhang-ccamp-flexible-grid-ospf-ext-00.txt [24] F. Zhang, O.G. de Dios, D. Ceccarelli, “RSVP-TE signaling extensions in support of flexible grid”, IETF Internet Draft, October 2011. draft-zhang-ccamp-flexible-grid-rsvp-te-ext-00.txt [25] A. Dhillon, I. Hussain, R. Rao, “OSPF-TE extension to support GMPLS for flex grid”, IETF Internet Draft, October 2011. draft-dhillon-ccamp-super-channel-ospfte-ext-01.txt [26] D.J. Geisler et al., "The first testbed demonstration of a flexible bandwidth network with a real-time adaptive control plane", in Proc. 37th European Conference and Exhibition on Optical Communication (ECOC), pp.1-3, 18-22 Sept. 2011. [27] N. Sambo, et al. , "Lightpath provisioning in wavelength switched optical networks with flexible grid", in Proc. Optical Communication (ECOC), 37th European Conference and Exhibition on Optical Communication (ECOC), pp.1-3, 18-22 Sept. 2011. [28] F. Cugini et al., "Demonstration of flexible optical network based on path computation element," J. Lightwave Technol., 2012.
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