Multicasting based optical inverse multiplexing in elastic optical network Bingli Guo, Yingying Xu, Paikun Zhu, Yucheng Zhong, Yuanxiang Chen, Juhao Li,* Zhangyuan Chen, and Yongqi He State Key Laboratory of Advanced Optical Communication Systems and Networks, Peking University, Beijing, 100871, China * [email protected]
Abstract: Optical multicasting based inverse multiplexing (IM) is introduced in spectrum allocation of elastic optical network to resolve the spectrum fragmentation problem, where superchannels could be split and fit into several discrete spectrum blocks in the intermediate node. We experimentally demonstrate it with a 1-to-7 optical superchannel multicasting module and selecting/coupling components. Also, simulation results show that, comparing with several emerging spectrum defragmentation solutions (e.g., spectrum conversion, split spectrum), IM could reduce blocking performance significantly but without adding too much system complexity as split spectrum. On the other hand, service fairness for traffic with different granularity of these schemes is investigated for the first time and it shows that IM performs better than spectrum conversion and almost as well as split spectrum, especially for smaller size traffic under light traffic intensity. ©2010 Optical Society of America OCIS codes: (060.4250) Networks; (060.4251) Networks, assignment and routing algorithms.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
M. Jinno, H. Takara, B. Kozicki, Y. Tsukishima, Y. Sone, and S. Matsuoka, “Spectrum-efficient and scalable elastic optical path network: architecture, benefits, and enabling technologies,” IEEE Commun. Mag. 47(11), 66–73 (2009). H. Takahashi, K. Takeshima, I. Morita, and H. Tanaka, “400-Gbit/s optical OFDM transmission over 80 km in 50-GHz frequency grid,” in Proc. ECOC 2010, Tu.3.C.1. Z. Chen, Y. Chen, J. Li, P. Zhu, Y. Xu, B. Guo, and Y. He, “Demonstration of elastic optical networks baseD on OFDM/SCFDM subbands,” in Proc. ACP 2013, AF1H.3. http://www.finisar.com/products/optical-instrumentation/WaveShaper-4000S Y. Wang, X. Cao, and Y. Pan, “A study of the routing and spectrum allocation in spectrum-sliced Elastic Optical Path networks,” in Proc. INFOCOM 2011, pp. 1503–1511. Y. Yin, K. Wen, D. J. Geisler, R. Liu, and S. J. Yoo, “Dynamic on-demand defragmentation in flexible bandwidth elastic optical networks,” Opt. Express 20(2), 1798–1804 (2012). F. Cugini, F. Paolucci, G. Meloni, G. Berrettini, M. Secondini, F. Fresi, N. Sambo, L. Poti, and P. Castoldi, “Push-pull defragmentation without traffic disruption in flexible grid optical networks,” J. Lightwave Technol. 31(1), 125–133 (2013). T. Takagi, H. Hasegawa, K. Sato, Y. Sone, A. Hirano, and M. Jinno, “Disruption minimized spectrum defragmentation in elastic optical path networks that adopt distance adaptive modulation,” in Proc. ECOC 2011, Mo.2.K.3. M. Xia, R. Proietti, S. Dahlfort, and S. J. Yoo, “Split spectrum: a multi-channel approach to elastic optical networking,” Opt. Express 20(28), 29143–29148 (2012). Z. Zhu, W. Lu, L. Zhang, and N. Ansari, “Dynamic service provisioning in elastic optical networks with hybrid single-/multi-path routing,” J. Lightwave Technol. 31(1), 15–22 (2013). J. Duncanson, “Inverse multiplexing,” Communications Magazine 32(4), 34–41 (1994). O. Gerstel, M. Jinno, A. Lord, and S. J. B. Yoo, “Elastic optical networking: a new dawn for the optical layer?” Communications Magazine 50(2), s12–s20 (2012). R. Muñoz, R. Vilalta, R. Casellas, R. Martínez, S. Frigerio, and A. Lometti, “Design and experimental evaluation of dynamic inverse-multiplexing provisioning in GMPLS-controlled flexi-grid DWDM networks with sliceable OTN BVTs,” in Proc. ECOC 2013, 2013, p.5.7. Y. Xu, J. Li, P. Zhu, B. Guo, Y. Chen, Y. Zhong, Y. Wang, Y. He, and Z. Chen, “Demonstration of all-optical inverse multiplexing in elastic optical networks,” in Proc. OFC/NFOEC 2014, Th1E.6.
#209726 - $15.00 USD (C) 2014 OSA
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15133
15. Y. Chen, J. Li, P. Zhu, Y. Xu, B. Guo, F. Zhang, Y. He, and Z. Chen, “Experimental demonstration of 1.2 Tb/s optical PDM SCFDM superchannel multicasting by HNLF,” in Proc. ACP 2013, AW4E.4. 16. Y. Chen, J. Li, P. Zhu, B. Guo, L. Zhu, Y. He, and Z. Chen, “Experimental demonstration of 400 Gb/s optical PDM-OFDM superchannel multicasting by multiple-pump FWM in HNLF,” Opt. Express 21(8), 9915–9922 (2013). 17. P. Zhu, J. Li, Y. Chen, Y. Xu, N. Zhang, B. Guo, Z. Chen, and Y. He, “Demonstration of 1-to-13 PDM-8QAM SCFDM superchannel multicasting in HNLF,” in Proc. OFC/NFOEC 2014, Th1D.2. 18. B. Guo, J. Li, Y. Wang, S. Huang, Z. Chen, and Y. He, “Collision-aware routing and spectrum assignmenT in GMPLS-enabled flexible-bandwidth optical network,” J. Opt. Commun. Netw. 5(6), 658–666 (2013).
1. Introduction Optical networks represent an important physical layer infrastructure to support the explosive growth of data traffic. And, a rigorous fact, that traffic increases almost 50% per year while capital and operational expenditures constrain the future infrastructure upgrades, is pressuring service providers to construct even more cost-efficient and scalable networking infrastructure. Recent research advancements propose Optical Orthogonal Frequency-Division Multiplexing (O-OFDM) as a promising paradigm (referred as Elastic Optical Network, EON) [1] for the future networks to cope with the high-capacity requirements, which migrates from the fixed WDM single line rate (SLR) system to systems with flexible spectrum allocation capability. Enabling technologies, including bandwidth variable transponder, bandwidth variable wavelength selective switching (BV-WSSs) and coherent detection, have been experimentally demonstrated [2–4]. However, EON also poses new challenges to the resource assignment in the network, known as the routing and spectrum allocation (RSA) problem [5]. In RSA, not only the traditional wavelength continuity constraints as in routing and wavelength assignment should be taken into account, but also other new constraints including the spectrum contiguity which requires subbands allocated for requests to be contiguous in the spectrum domain. Spectrum contiguity constraint is derived from the limitation of multi-carrier generation technique (e.g., frequency shifting) in transmitter and consideration that consecutive OFDM channels (as subbands of superchannel) could save the spectrum guard band required in optical switching. Also, the selection of signal modulation level is considered to adapt to the physical layer impairment. With these constraints, in network with dynamic traffic arrival/release, the spectrum would inevitably be split into discrete fragment which are neither contiguous on the spectrum axis nor continuous in the fiber link direction, making themselves hard to be utilized by future connection requests (especially for those with multi-hop and/or large bandwidth) [6]. This would decrease the probability of finding sufficient contiguous spectrum for future requests and potentially increase blocking. Authors in [6–10] respectively proposed different approaches to improve the spectrum utilization efficiency, such as spectrum conversion (SC) [6, 7], rerouting [8], split spectrum (SS) [9] and multipath routing [10], which could alleviate the impact of spectrum fragmentation (SF) problem in some extent, but bear their own flaws. Specifically, spectrum conversion requires either a contiguous spectrum block equal to the bandwidth of superchannel to be converted [6] or a large free contiguous spectrum segment in [7] to implement a hitless spectrum moving. Furthermore, multiple conversion processes may be needed when the fragments locate more discretely, which impedes its efficiency. Traffic rerouting [8] would cause traffic interruption and multipath routing introduces different delay between signals in carriers of same superchannel which would need complex data buffering/reassembly in the destination node. Also, in split spectrum [9], a huge traffic demand is carried by multiple inconsecutive channels to fit them into small free spectrum fragments, yet at the cost of complex multi-carrier generation in transmitter (e.g., costly optical comb generation approach to get discrete optical carriers) and consuming excessive guard band spectrum which may deteriorate the spectrum utilization efficiency. On the other hand, this paper addresses a technique to improve spectrum utilization in EON, referred as inverse multiplexing, which has already been widely used in Asynchronous Transfer Mode (ATM), Optical Transport Network (OTN) architecture [11]. Generally, IM is the aggregation of multiple independent channels across a network to create a single higher#209726 - $15.00 USD (C) 2014 OSA
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15134
rate information channel. For example, 400 Gb/s signal could be transmitted as 4 × 100 Gb/s, not necessarily adjacent subbands. Recently, the IM is also proposed in EON to improve the spectrum utilization efficiency [12], and OTN based IM in flex-grid network is also reported in [13], combining sliceable bandwidth-variable transponders (BVTs) and OTN signals. In this paper, IM is implemented based on superchannel multicasting we implemented in [14–17], where Highly Non-Linear Fiber (HNLF) is used to generates 3, 7 and 13 copies of 1.2Tb/s, 400Gb/s and 240 Gb/s polarization-division-multiplexed (PDM) OFDM/ (singlecarrier frequency-division-multiplexing) SCFDM superchannel in spectral domain respectively. With these copies generated by optical multicasting, one superchannel could be split into multiple small subchannels through properly filtering each subband of the superchannel from one of its copies (or itself) respectively. And, the superchannel inverse multiplexing will be implemented in intermediate node in accordance with the spectrum availability in the downstream links. Compared with the aforementioned spectrum conversion approach, the proposed IM scheme is supposed to utilize spectrum resources more efficiently since it relieves the subbands contiguity constraints in some extent. Also, the proposed IM is implemented in optical domain without introducing too much control complexity. This paper is organized as follows. First, the principle of optical inverse multiplexing and node architecture are introduced in Sec. II and experimentally demonstrated in Sec. III. Sec. IV evaluates its performance through comparing with various existing solutions. Finally, this paper is concluded in Sec. V. 2. Principle of optical inverse multiplexing 2.1 Optical multicasting based inverse multiplexing
Original
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(c) Fig. 1. (a) EON scenario with and without inverse multiplexing at node (C). (b) Principle of optical inverse multiplexing module (IMM). (c) Node architecture with inverse multiplexing degree of M.
Figure 1(a) shows an EON scenario where two optical paths (A-C-D and B-C-D) share same link CD and a superchannel from BC would spectrally overlap in CD with channels from AC.
#209726 - $15.00 USD (C) 2014 OSA
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15135
This would happen if the corresponding spectrum segment is concurrently reserved by more than one optical path (e.g., backward blocking in GMPLS) or under some reason that the spectrum availability of whole path is not fully considered before the superchannel is sent out. Such spectrum overlap/collision would induce traffic disruption. However, the collision can be avoided if IM is applied at node C. As shown in Fig. 1(a), with IM, the superchannel could be separate into several parts to fit into the available subbands, e.g., {(f2, f3), (f1)} or {(f1), (f2), (f3)}. Specifically, the proposed all-optical inverse multiplexing can be realized by a superchannel optical multicasting module and bandwidth-variable wavelength selective switch (BV-WSS), as shown in Fig. 1(b). Accordingly, if IM is properly applied in intermediate nodes of the optical path, it is helpful to reduce the traffic rejection due to no available contiguous spectrum segment. Figure 1(c) also presents one of the possible node designs with IM degree of M which implies that at most M superchannels could be inversely multiplexed in each input fiber. The superchannels which are supposed to take IM processing are filtered out separately by the first stage 1 × (M + 1) BV-WSS and sent into different IMMs. Then, the channels after IM processing and those not employing IMM are combined together as the input of second stage 1 × N BV-WSS where superchannels are delivered to their desired output fiber. It is worth noting that, this design poses a constraint that the superchannels after IM should still be able to fit into the original spectrum subbands. Of course, if an (M + 1) × N BV-WSS is available to replace the coupler and second stage BV-WSS, this constraint could be properly resolved. Figure 1(c) also gives an example of inverse multiplexing applied at node C in Fig. 1(a), which enables the channel in link BC together with those in link AC to be delivered to node D. 2.2 The proposed IM approach with four-wave mixing based optical multicasting In this paper, we employ HNLF to implement four-wave mixing (FWM) based optical multicasting. For multicasting with K co-polarized pumps, new copies of superchannel can be generated with the following frequency relationships [16]:
ωc = ωs + ω p − ω p (i = 1, 2 K , i, j
i
j
j = 1, 2 K , j ≠ i )
(1)
where ωc , ωs , ω p and ω p j represent the center frequencies of the new copies, the original i
superchannel and 2 of the K pumps respectively. In other words, every 2 pumps among K pumps will interact with the original signal and produce 2 copies of the original signal. Accordingly, to inverse multiplex a superchannel and let it fit into the fragmented spectrum, first, the position of superchannel and its copies need to be specified considering the availability of the downstream fragmented spectrum. Then, the number of pumps and their frequencies are defined with Eq. (1) to generate the required superchannel multicasting. Also, a guard bandwidth is required to avoid the impact of FWM idlers generated by the interaction of pumps to the superchannel copies [16]. The BV-WSS is used to select those desired subbands to generate multiple sub-channels, which are transmitted separately. On the other hand, considering the penalty suffering from optical multicasting, it is worth noting the superchannel occupying wider spectrum would have less potential to implement inverse multiplexing which is the result of fact that, the wider the superchannel is, the fewer valid copies it could have.
#209726 - $15.00 USD (C) 2014 OSA
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15136
3. Experimental demonstration of optical inverse multiplexing 1 symbol delay
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Fig. 2. Experimental setup of optical inverse multiplexing (insets: optical spectra).
In this section, we present a proof-of-principle experimental demonstration of optical inverse multiplexing. The setup is depicted in the top of Fig. 2. Two transmitters (Tx1 and Tx2) are used to generate optical OFDM superchannels in different links. A superchannel A centered at 1557.30 nm and including 3 subbands is generated by Tx1 (Fig. 2-inset a). Superchannel B, C, D and E consisting of 3, 6, 9 and 3 subbands respectively are generated by Tx2 (Fig. 2inset b). The width of each subband is 10 GHz and there are three free spectrum fragments between each pair of adjacent superchannels from Tx2 (i.e., between BC, CD and DE) with 10 GHz width for each. Apparently, A would be overlapped with D unless the proposed inverse multiplexing scheme is performed here. The multicasting block used here is similar with [16]. Three co-polarized pumps with frequency spacing of 60 GHz and 120 GHz are coupled with A and sent to the HNLF with a length of 1 km and a zero-dispersion wavelength of 1567 nm (Fig. 2-inset c). After multicasting, three copies of A are generated symmetrically at both sides (six in total), as given by Eq. (1). Figure 2-inset d shows the spectra after multicasting, including the three pump lasers, A and its six copies, and the FWM idlers. We employ Finisar Waveshaper 4000S to implement bandwidth variable WSS. The WSS is configured to select the desired subbands centered at 1556.32 nm, 1556.88 nm and 1557.68 nm (same with the center wavelength of three fragments), which are the second subband from the second copy (from left to right), the third one from the third copy and the first one from the forth copy, to construct three separated sub-channels. (i.e., A2, A3 and A1 in Fig. 2-inset e). These three sub-channels form the new superchannel A. After inverse multiplexing, A2, A3 and A1 are combined with B, C, D and E without blocking (Fig. 2-inset f). The detailed setups of the two optical OFDM transmitters are depicted as follows. For Tx1, three lasers with the frequency spacing of 10 GHz are coupled by cascaded polarization
#209726 - $15.00 USD (C) 2014 OSA
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15137
maintaining couplers (PMCs). A Tektronix arbitrary waveform generator (AWG7122B) operating at 10 GS/s is used to generate the baseband OFDM signals. The in-phase (I) and quadrature (Q) parts of the signals are directly converted to 3 optical carriers by an optical IQ modulator. The PDM is emulated with a polarization controller (PC), a polarization beam splitter (PBS), a tunable optical delay line and a polarization beam combiner (PBC). The delay of the optical delay line is exactly one OFDM symbol. Thus a QPSK PDM-OFDM superchannel with 30 GHz spectral bandwidth and 120 Gb/s data rate is generated. For Tx2, seven lasers with frequency spacing of 40 GHz, 30 GHz, 40 GHz, 30 GHz, 30 GHz and 40 GHz pass an intensity modulator driven by 10 GHz radio frequency (RF) signal, to generate the optical comb. An Agilent AWG (M9505A) operating at 10 GS/s, an IQ modulator and a PDM emulator are used to generate 4 QPSK modulated PDM-OFDM superchannels with bandwidths of 30 GHz, 60 GHz, 90 GHz and 30 GHz, respectively. We adopt the same scheme with [16] for coherent detection.
Fig. 3. Q-factor performances of all subbands.
Figure 3 shows the Q-factor performances of the inverse multiplexing process. The backto-back (BTB) Q-factors of all subbands in Superchannel A, B, C, D and E are measured as reference. Compared with the BTB case of A, the sub-channels (A2, A3 and A1) suffer a Qfactor penalty of 2 dB introduced by FWM. After being coupled with other superchannels, the Q-factors decrease 1.6 dB further due to the crosstalk from the residual signals of neighboring subbands, as shown in Fig. 2-inset b. The Q-factors of all subbands in B, C, D and E after coupling are also measured. We can observe a Q-factor penalty of 0.3~0.5 dB for the subbands adjacent to A2, A3 and A1, while other subbands suffer slight penalties. This can be explained by the imperfect filtering characteristics of the Waveshaper, as shown in Fig. 2inset e. After coupling, the Q-factors of all subbands are better than the forward error correction (FEC) limit, which validates the proposed inverse multiplexing scheme. It is worth nothing that due to the degradation of Q factor, IM process should be only utilized for only once in actual network scenarios. Also, the allowed highest modulation level to be used for a given lightpath should fully consider not just the general loss/impairments of transmission and switching, also the optical processing impairments like inverse multiplexing. 4. Spectrum allocation algorithm considering IM capability in nodes In this section, a spectrum allocation (SA) algorithm is proposed based on the node architecture with build-in IM capability. With the discussion above, first, we conclude the constraints for the spectrum allocation approach as following: (1) the generation of new superchannel copies follows Eq. (1); (2) superchannel with different spectrum width would have different IM potential because the wider the superchannel is, the fewer valid copies it could have; (3) a superchannel could employ IMM only once in the whole optical path considering the penalty suffered from optical multicasting and filtering. Also, it is worth noting that the inverse multiplexed superchannels should be carefully selected to avoid
#209726 - $15.00 USD (C) 2014 OSA
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15138
introducing more fragments. So, following these constraints, the proposed SA algorithm is elaborated in Fig. 4. Request arrive (s,d) with a given path p={li}
collect the spectrum utilization info. of li Find continuous available spectrum segment for p Select the available continuous spectrum segment {sx, sy} that could reach the farthest node n
Success?
Success? find optimal IM solution with Eq.(2) and (3) from available spectrum of path n to d Success?
Blocked
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(a) 12345
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C (b) Fig. 4. (a) Flow Chart of IM based Spectrum Allocation Algorithm. (b) An example of IM spectrum allocation
As shown in Fig. 4(a), if a contiguous spectrum segment could not be found to satisfy the requirement of a request, the spectrum segment which could reach the farthest node (e.g., n) would be selected. Then, the IM would be performed in node n, which would not only consider the spectrum availability from n to destination node d, also the IM potential of this superchannel. Furthermore, as shown in Fig. 4(b), for some given superchannel copies, there may be more than one IM options, e.g., scenario A, B and C. So, Eq. (2) and (3) is employed to evaluate the potential impact of IM on the spectrum utilization and they are defined based on the following observations: (1) it is harmful to fill in the middle of the free spectrum block, and (2) it is efficient to fully occupy the free spectrum block or quite small part of that, not half of it. #209726 - $15.00 USD (C) 2014 OSA
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15139
max[ Bi ]
(2)
I
(
)
2 i i k bx − bx Bi = α b' b − 0.5 2 i i
(
i
i
if block bi is split into two or more blocks (b1 , bk )
)
(3)
' i
if IM changes the size of block bi to b
where α is a coefficient and I is the set of contiguous free spectrum block which part of the superchannel may fill in, e.g., b1, b2 and b3 in the Fig. 4(b). Bi is the utilization benefit of spectrum block bi achieved from superchannel IM and its definition is presented in Eq. (3). Accordingly, an IM solution could be selected. Otherwise, the traffic is blocked. 5. Numerical results and discussion In this section, the service blocking performance and service fairness [18] (of different granularity traffic) of the proposed scheme are evaluated by OPNET with the typical 14-node NSFNET topology and mixed granularity traffic. We assume that every link carries 300 FSs in both directions. 106 requests are generated for each simulation and requests are distributed equally among all the node pairs. Each request may require 10 Gb/s, 20 Gb/s, 30 Gb/s, 40 Gb/s, 80 Gb/s, 100 Gb/s or 400 Gb/s respectively, and the same number of request for each granularity is generated. The symbol rate of each subband is 10 G-baud and signal format is decided by its transmission distance. For simplicity, the highest available signal format for one request is employed according to the hops of its route, as shown in Table 1. So, in this simulation, for a given request, we employ fixed shortest path which is helpful to improve the spectrum utilization efficiency. The inter-arrival time and holding time of the lightpath requests are exponentially distributed with an average of 1/l and 1/m seconds, respectively. The load offered to the network is expressed in Erlangs (Erl.) as the ratio l/m. In addition, a guard band of 1 subband is employed between the adjacent optical channels, so that the switching node is able to add/drop any of the channels. The IM degree M is 15. Table 1. Required FSs for Different Granularity Traffic with Different Transmission Length Required subbands/allowed maximum copies Hops Signal Format =1or =2 16QAM =3 or =4 QPSK = or > 5 BPSK
10
20
1/13 1/13 1/13
1/13 1/13 2/13
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400
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10/7 20/3 40/3
1/13 2/13 4/13
22/13 4/13 8/7
3/13 5/13 10/7
4.1 Bandwidth blocking performance In this paper, bandwidth blocking probability (BBP) (ratio of bandwidth utilized by accepted requests to the total requested bandwidth [18]) is employed to measure the blocking performance of network with multi-granularity traffic. Figure 5(a) presents the BBP of IM in terms of α, which implies that α in the range of (6.5, 7.5) could keep the blocking probability in a lower level. Also, the variation of α would impact the blocking performance more severely under heavy traffic load than that under lower load, e.g., BBP varying from 21% to 28% under 100 Erl. while from 13.7% to 16% under 60 Erl. This is the result of fact that a proper value of α could balance the selection of two type of IM spectrum fitting strategy. At the same time, performance of different spectrum allocation schemes (e.g., first-fit, IM, SC, SS) is compared in Fig. 5(b). It shows that, comparing with SC and first-fit, IM has lower BBP under various traffic loads, especially when traffic intensity varying from 60 Erl. to 85 Erl. Besides, it is worth noting that BBP of IM is quite close with that of SS (the largest gap is less than 3.5% under 80 Erl.), but without complex packet store-reordering strategy in the
#209726 - $15.00 USD (C) 2014 OSA
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15140
receiving side. This is due to the fact that IM works like an optical partial split spectrum strategy. 0.30 100 Erl. 90 Erl. 70 Erl. 60 Erl.
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4.2 Service fairness of different granularity traffic In this section, the service fairness of traffic with different granularity is discussed, which investigates the blocking performance of different granularity traffic. Service fairness ratio (SFR) defined in Eq. (4) and Eq. (5) [18] is employed (which is based on granularity based normalized blocking probability, G-NBP). G -NBPx = Px
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x
SFR y / x = G − NBPy / G − NBPx
(5)
where Px is the blocking probability of traffic with size x. As shown in Table 2, generally, SFRy/x increase with traffic load and the size of traffic y (for a fixed x), which is due to the fact that traffic with larger size is more likely to be blocked under heavy traffic. Specifically, comparing with SC and FF, IM serves the traffic with different granularities more fairly (the lower the value of SFR is, the more fair the service is), which is because that, in IM, the consecutive constraint in spectrum allocation is relaxed in some extent. Also, SFR of larger size traffic could achieve more improvement with IM than that of smaller one, e.g. under 100 Erl., SFR30/10 reduces from 3.6/4.1 (SC/FF) to 2.1 while SFR100/10 from 5.5/6.6 (SC/FF) to 3.1. On the other hand, IM could perform as well as SS, especially for traffic with smaller size under light traffic load. Table 2. SFR in terms of Traffic Intensity SFR Traffic Intensity 60 80 100 120
#209726 - $15.00 USD (C) 2014 OSA
SFR30/10
SFR80/10
SFR100/10
IM
FF
SC
SS
IM
FF
SC
SS
IM
FF
SC
SS
1.6 1.9 2.1 2.5
3.1 3.7 4.1 4.6
2.6 3.1 3.6 3.9
1.5 1.7 1.8 2.0
1.8 2.2 2.5 2.9
3.8 4.0 4.7 4.9
2.9 3.4 3.8 4.2
1.6 1.9 2.2 2.4
2.4 2.9 3.1 3.4
5.6 6.0 6.6 7.1
4.6 4.9 5.5 6.1
2.1 2.3 2.6 3.0
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15141
5. Conclusion A superchannel multicasting based optical inverse multiplexing is proposed to resolve the spectrum fragmentation problem in EON. IM could improve the spectrum utilization efficiency through separating the superchannel to fit into the discrete spectrum fragments, thus reducing the traffic blocking probability. At the same time, with IM, service fairness of different granularity traffic is improved significantly. To enable IM functionality in optical switching node, tunable laser sources are required in multicast module to generate pump signals as required. However, the number of laser source required for each superchannel would not been huge, e.g., 13 copies need 4 tunable laser sources. So, the amount of laser sources needed in a node depends on the requirements of concurrent converting superchannels. Acknowledgments Part of this work is published in OFC 2014. This work was supported by National Basic Research Program of China (973 Program, No. 2010CB328201 and 2010CB328202), the National Natural Science Foundation of China (NSFC, No. 61377072, No. 61275071, No. 61205058).
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Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15142
Abstract: Optical multicasting based inverse multiplexing (IM) is introduced in spectrum allocation of elastic optical network to resolve the spectrum fragmentation problem, where superchannels could be split and fit into several discrete spectrum blocks in the intermediate node. We experimentally demonstrate it with a 1-to-7 optical superchannel multicasting module and selecting/coupling components. Also, simulation results show that, comparing with several emerging spectrum defragmentation solutions (e.g., spectrum conversion, split spectrum), IM could reduce blocking performance significantly but without adding too much system complexity as split spectrum. On the other hand, service fairness for traffic with different granularity of these schemes is investigated for the first time and it shows that IM performs better than spectrum conversion and almost as well as split spectrum, especially for smaller size traffic under light traffic intensity. ©2010 Optical Society of America OCIS codes: (060.4250) Networks; (060.4251) Networks, assignment and routing algorithms.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
M. Jinno, H. Takara, B. Kozicki, Y. Tsukishima, Y. Sone, and S. Matsuoka, “Spectrum-efficient and scalable elastic optical path network: architecture, benefits, and enabling technologies,” IEEE Commun. Mag. 47(11), 66–73 (2009). H. Takahashi, K. Takeshima, I. Morita, and H. Tanaka, “400-Gbit/s optical OFDM transmission over 80 km in 50-GHz frequency grid,” in Proc. ECOC 2010, Tu.3.C.1. Z. Chen, Y. Chen, J. Li, P. Zhu, Y. Xu, B. Guo, and Y. He, “Demonstration of elastic optical networks baseD on OFDM/SCFDM subbands,” in Proc. ACP 2013, AF1H.3. http://www.finisar.com/products/optical-instrumentation/WaveShaper-4000S Y. Wang, X. Cao, and Y. Pan, “A study of the routing and spectrum allocation in spectrum-sliced Elastic Optical Path networks,” in Proc. INFOCOM 2011, pp. 1503–1511. Y. Yin, K. Wen, D. J. Geisler, R. Liu, and S. J. Yoo, “Dynamic on-demand defragmentation in flexible bandwidth elastic optical networks,” Opt. Express 20(2), 1798–1804 (2012). F. Cugini, F. Paolucci, G. Meloni, G. Berrettini, M. Secondini, F. Fresi, N. Sambo, L. Poti, and P. Castoldi, “Push-pull defragmentation without traffic disruption in flexible grid optical networks,” J. Lightwave Technol. 31(1), 125–133 (2013). T. Takagi, H. Hasegawa, K. Sato, Y. Sone, A. Hirano, and M. Jinno, “Disruption minimized spectrum defragmentation in elastic optical path networks that adopt distance adaptive modulation,” in Proc. ECOC 2011, Mo.2.K.3. M. Xia, R. Proietti, S. Dahlfort, and S. J. Yoo, “Split spectrum: a multi-channel approach to elastic optical networking,” Opt. Express 20(28), 29143–29148 (2012). Z. Zhu, W. Lu, L. Zhang, and N. Ansari, “Dynamic service provisioning in elastic optical networks with hybrid single-/multi-path routing,” J. Lightwave Technol. 31(1), 15–22 (2013). J. Duncanson, “Inverse multiplexing,” Communications Magazine 32(4), 34–41 (1994). O. Gerstel, M. Jinno, A. Lord, and S. J. B. Yoo, “Elastic optical networking: a new dawn for the optical layer?” Communications Magazine 50(2), s12–s20 (2012). R. Muñoz, R. Vilalta, R. Casellas, R. Martínez, S. Frigerio, and A. Lometti, “Design and experimental evaluation of dynamic inverse-multiplexing provisioning in GMPLS-controlled flexi-grid DWDM networks with sliceable OTN BVTs,” in Proc. ECOC 2013, 2013, p.5.7. Y. Xu, J. Li, P. Zhu, B. Guo, Y. Chen, Y. Zhong, Y. Wang, Y. He, and Z. Chen, “Demonstration of all-optical inverse multiplexing in elastic optical networks,” in Proc. OFC/NFOEC 2014, Th1E.6.
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Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15133
15. Y. Chen, J. Li, P. Zhu, Y. Xu, B. Guo, F. Zhang, Y. He, and Z. Chen, “Experimental demonstration of 1.2 Tb/s optical PDM SCFDM superchannel multicasting by HNLF,” in Proc. ACP 2013, AW4E.4. 16. Y. Chen, J. Li, P. Zhu, B. Guo, L. Zhu, Y. He, and Z. Chen, “Experimental demonstration of 400 Gb/s optical PDM-OFDM superchannel multicasting by multiple-pump FWM in HNLF,” Opt. Express 21(8), 9915–9922 (2013). 17. P. Zhu, J. Li, Y. Chen, Y. Xu, N. Zhang, B. Guo, Z. Chen, and Y. He, “Demonstration of 1-to-13 PDM-8QAM SCFDM superchannel multicasting in HNLF,” in Proc. OFC/NFOEC 2014, Th1D.2. 18. B. Guo, J. Li, Y. Wang, S. Huang, Z. Chen, and Y. He, “Collision-aware routing and spectrum assignmenT in GMPLS-enabled flexible-bandwidth optical network,” J. Opt. Commun. Netw. 5(6), 658–666 (2013).
1. Introduction Optical networks represent an important physical layer infrastructure to support the explosive growth of data traffic. And, a rigorous fact, that traffic increases almost 50% per year while capital and operational expenditures constrain the future infrastructure upgrades, is pressuring service providers to construct even more cost-efficient and scalable networking infrastructure. Recent research advancements propose Optical Orthogonal Frequency-Division Multiplexing (O-OFDM) as a promising paradigm (referred as Elastic Optical Network, EON) [1] for the future networks to cope with the high-capacity requirements, which migrates from the fixed WDM single line rate (SLR) system to systems with flexible spectrum allocation capability. Enabling technologies, including bandwidth variable transponder, bandwidth variable wavelength selective switching (BV-WSSs) and coherent detection, have been experimentally demonstrated [2–4]. However, EON also poses new challenges to the resource assignment in the network, known as the routing and spectrum allocation (RSA) problem [5]. In RSA, not only the traditional wavelength continuity constraints as in routing and wavelength assignment should be taken into account, but also other new constraints including the spectrum contiguity which requires subbands allocated for requests to be contiguous in the spectrum domain. Spectrum contiguity constraint is derived from the limitation of multi-carrier generation technique (e.g., frequency shifting) in transmitter and consideration that consecutive OFDM channels (as subbands of superchannel) could save the spectrum guard band required in optical switching. Also, the selection of signal modulation level is considered to adapt to the physical layer impairment. With these constraints, in network with dynamic traffic arrival/release, the spectrum would inevitably be split into discrete fragment which are neither contiguous on the spectrum axis nor continuous in the fiber link direction, making themselves hard to be utilized by future connection requests (especially for those with multi-hop and/or large bandwidth) [6]. This would decrease the probability of finding sufficient contiguous spectrum for future requests and potentially increase blocking. Authors in [6–10] respectively proposed different approaches to improve the spectrum utilization efficiency, such as spectrum conversion (SC) [6, 7], rerouting [8], split spectrum (SS) [9] and multipath routing [10], which could alleviate the impact of spectrum fragmentation (SF) problem in some extent, but bear their own flaws. Specifically, spectrum conversion requires either a contiguous spectrum block equal to the bandwidth of superchannel to be converted [6] or a large free contiguous spectrum segment in [7] to implement a hitless spectrum moving. Furthermore, multiple conversion processes may be needed when the fragments locate more discretely, which impedes its efficiency. Traffic rerouting [8] would cause traffic interruption and multipath routing introduces different delay between signals in carriers of same superchannel which would need complex data buffering/reassembly in the destination node. Also, in split spectrum [9], a huge traffic demand is carried by multiple inconsecutive channels to fit them into small free spectrum fragments, yet at the cost of complex multi-carrier generation in transmitter (e.g., costly optical comb generation approach to get discrete optical carriers) and consuming excessive guard band spectrum which may deteriorate the spectrum utilization efficiency. On the other hand, this paper addresses a technique to improve spectrum utilization in EON, referred as inverse multiplexing, which has already been widely used in Asynchronous Transfer Mode (ATM), Optical Transport Network (OTN) architecture [11]. Generally, IM is the aggregation of multiple independent channels across a network to create a single higher#209726 - $15.00 USD (C) 2014 OSA
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15134
rate information channel. For example, 400 Gb/s signal could be transmitted as 4 × 100 Gb/s, not necessarily adjacent subbands. Recently, the IM is also proposed in EON to improve the spectrum utilization efficiency [12], and OTN based IM in flex-grid network is also reported in [13], combining sliceable bandwidth-variable transponders (BVTs) and OTN signals. In this paper, IM is implemented based on superchannel multicasting we implemented in [14–17], where Highly Non-Linear Fiber (HNLF) is used to generates 3, 7 and 13 copies of 1.2Tb/s, 400Gb/s and 240 Gb/s polarization-division-multiplexed (PDM) OFDM/ (singlecarrier frequency-division-multiplexing) SCFDM superchannel in spectral domain respectively. With these copies generated by optical multicasting, one superchannel could be split into multiple small subchannels through properly filtering each subband of the superchannel from one of its copies (or itself) respectively. And, the superchannel inverse multiplexing will be implemented in intermediate node in accordance with the spectrum availability in the downstream links. Compared with the aforementioned spectrum conversion approach, the proposed IM scheme is supposed to utilize spectrum resources more efficiently since it relieves the subbands contiguity constraints in some extent. Also, the proposed IM is implemented in optical domain without introducing too much control complexity. This paper is organized as follows. First, the principle of optical inverse multiplexing and node architecture are introduced in Sec. II and experimentally demonstrated in Sec. III. Sec. IV evaluates its performance through comparing with various existing solutions. Finally, this paper is concluded in Sec. V. 2. Principle of optical inverse multiplexing 2.1 Optical multicasting based inverse multiplexing
Original
D
B
Input
BVWSS
1 2
IMM IMM
M
IMM
IMM
BVWSS
…
…
M
IMM IMM
Output
replaced by an (M+1)×N BV-WSS
…
1 2
…
…
…
N
…
…
BVWSS
…
IMM
…
M
Copies (b)
…
…
2
IMM IMM
…
BVWSS
1 2
…
Subchannels
BVWSS
1
BVWSS
inverse multiplexing (a)
Fragmented spectrum … … BV-WSS
C
Multicasting
could not fit in without inverse multiplexing
A
(c) Fig. 1. (a) EON scenario with and without inverse multiplexing at node (C). (b) Principle of optical inverse multiplexing module (IMM). (c) Node architecture with inverse multiplexing degree of M.
Figure 1(a) shows an EON scenario where two optical paths (A-C-D and B-C-D) share same link CD and a superchannel from BC would spectrally overlap in CD with channels from AC.
#209726 - $15.00 USD (C) 2014 OSA
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15135
This would happen if the corresponding spectrum segment is concurrently reserved by more than one optical path (e.g., backward blocking in GMPLS) or under some reason that the spectrum availability of whole path is not fully considered before the superchannel is sent out. Such spectrum overlap/collision would induce traffic disruption. However, the collision can be avoided if IM is applied at node C. As shown in Fig. 1(a), with IM, the superchannel could be separate into several parts to fit into the available subbands, e.g., {(f2, f3), (f1)} or {(f1), (f2), (f3)}. Specifically, the proposed all-optical inverse multiplexing can be realized by a superchannel optical multicasting module and bandwidth-variable wavelength selective switch (BV-WSS), as shown in Fig. 1(b). Accordingly, if IM is properly applied in intermediate nodes of the optical path, it is helpful to reduce the traffic rejection due to no available contiguous spectrum segment. Figure 1(c) also presents one of the possible node designs with IM degree of M which implies that at most M superchannels could be inversely multiplexed in each input fiber. The superchannels which are supposed to take IM processing are filtered out separately by the first stage 1 × (M + 1) BV-WSS and sent into different IMMs. Then, the channels after IM processing and those not employing IMM are combined together as the input of second stage 1 × N BV-WSS where superchannels are delivered to their desired output fiber. It is worth noting that, this design poses a constraint that the superchannels after IM should still be able to fit into the original spectrum subbands. Of course, if an (M + 1) × N BV-WSS is available to replace the coupler and second stage BV-WSS, this constraint could be properly resolved. Figure 1(c) also gives an example of inverse multiplexing applied at node C in Fig. 1(a), which enables the channel in link BC together with those in link AC to be delivered to node D. 2.2 The proposed IM approach with four-wave mixing based optical multicasting In this paper, we employ HNLF to implement four-wave mixing (FWM) based optical multicasting. For multicasting with K co-polarized pumps, new copies of superchannel can be generated with the following frequency relationships [16]:
ωc = ωs + ω p − ω p (i = 1, 2 K , i, j
i
j
j = 1, 2 K , j ≠ i )
(1)
where ωc , ωs , ω p and ω p j represent the center frequencies of the new copies, the original i
superchannel and 2 of the K pumps respectively. In other words, every 2 pumps among K pumps will interact with the original signal and produce 2 copies of the original signal. Accordingly, to inverse multiplex a superchannel and let it fit into the fragmented spectrum, first, the position of superchannel and its copies need to be specified considering the availability of the downstream fragmented spectrum. Then, the number of pumps and their frequencies are defined with Eq. (1) to generate the required superchannel multicasting. Also, a guard bandwidth is required to avoid the impact of FWM idlers generated by the interaction of pumps to the superchannel copies [16]. The BV-WSS is used to select those desired subbands to generate multiple sub-channels, which are transmitted separately. On the other hand, considering the penalty suffering from optical multicasting, it is worth noting the superchannel occupying wider spectrum would have less potential to implement inverse multiplexing which is the result of fact that, the wider the superchannel is, the fewer valid copies it could have.
#209726 - $15.00 USD (C) 2014 OSA
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15136
3. Experimental demonstration of optical inverse multiplexing 1 symbol delay
AWG 1 EDFA
LD
IQ
a PC
Modulator 1
PBS
HNLF
A
PBC
EDFA
Tx1
LD
Pump 1 30 GHz
10 GHz LD
EDFA
PMCs
EDFA
LD
IQ
Intensity Modulator
Pump 3
···
B C
D E
b
PBC
B
C
-20 -30 -40
0
10 GHz 10
c
A
Pumps
-20 -30 -40
1557.6
1558.8
1560.0
0
-10
-20
d
A2 A3 A1
-30
10 GHz
10 GHz
10 GHz
-50
-40 -50
1555.6 1556.0 1556.4 1556.8 1557.2 1557.6 1558.0 1558.4
Wavelength (nm) (span=3.2 nm)
A1
-40
1555.6 1556.0 1556.4 1556.8 1557.2 1557.6 1558.0 1558.4
Wavelength (nm) (span=3.2 nm) 0
A
-20
-30
A3
-30
10
Power (dBm)
E
Power (dBm)
D
A2
-20
1561.2
10
C
e
Wavelength (nm) (span=6.6 nm)
Wavelength (nm) (span=3.2 nm)
-10
0 -10
-50 1556.4
10
Power (dBm)
10 GHz
-10
1555.6 1556.0 1556.4 1556.8 1557.2 1557.6 1558.0 1558.4
B
CO-OFDM Receiver
f
E
-50
-50
-40
D
Power (dBm)
Power (dBm)
Power (dBm)
A
-10
0
Coupler
Multicasting
10 GHz
b
EDFA
EDFA
10
a
e
WSS
Tx2
10 0
d
PC
Modulator 2
PBS
LD
Pump 2
1 symbol delay
AWG 2
c Coupler
PMCs
PMCs
LD
1556.4
1557.6
1558.8
1560.0
1561.2
Wavelength (nm) (span=6.6 nm)
f
A2
B
C
A3
D
A1
E
-10 -20 -30 -40 -50 1555.6 1556.0 1556.4 1556.8 1557.2 1557.6 1558.0 1558.4
Wavelength (nm) (span=3.2 nm)
Fig. 2. Experimental setup of optical inverse multiplexing (insets: optical spectra).
In this section, we present a proof-of-principle experimental demonstration of optical inverse multiplexing. The setup is depicted in the top of Fig. 2. Two transmitters (Tx1 and Tx2) are used to generate optical OFDM superchannels in different links. A superchannel A centered at 1557.30 nm and including 3 subbands is generated by Tx1 (Fig. 2-inset a). Superchannel B, C, D and E consisting of 3, 6, 9 and 3 subbands respectively are generated by Tx2 (Fig. 2inset b). The width of each subband is 10 GHz and there are three free spectrum fragments between each pair of adjacent superchannels from Tx2 (i.e., between BC, CD and DE) with 10 GHz width for each. Apparently, A would be overlapped with D unless the proposed inverse multiplexing scheme is performed here. The multicasting block used here is similar with [16]. Three co-polarized pumps with frequency spacing of 60 GHz and 120 GHz are coupled with A and sent to the HNLF with a length of 1 km and a zero-dispersion wavelength of 1567 nm (Fig. 2-inset c). After multicasting, three copies of A are generated symmetrically at both sides (six in total), as given by Eq. (1). Figure 2-inset d shows the spectra after multicasting, including the three pump lasers, A and its six copies, and the FWM idlers. We employ Finisar Waveshaper 4000S to implement bandwidth variable WSS. The WSS is configured to select the desired subbands centered at 1556.32 nm, 1556.88 nm and 1557.68 nm (same with the center wavelength of three fragments), which are the second subband from the second copy (from left to right), the third one from the third copy and the first one from the forth copy, to construct three separated sub-channels. (i.e., A2, A3 and A1 in Fig. 2-inset e). These three sub-channels form the new superchannel A. After inverse multiplexing, A2, A3 and A1 are combined with B, C, D and E without blocking (Fig. 2-inset f). The detailed setups of the two optical OFDM transmitters are depicted as follows. For Tx1, three lasers with the frequency spacing of 10 GHz are coupled by cascaded polarization
#209726 - $15.00 USD (C) 2014 OSA
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15137
maintaining couplers (PMCs). A Tektronix arbitrary waveform generator (AWG7122B) operating at 10 GS/s is used to generate the baseband OFDM signals. The in-phase (I) and quadrature (Q) parts of the signals are directly converted to 3 optical carriers by an optical IQ modulator. The PDM is emulated with a polarization controller (PC), a polarization beam splitter (PBS), a tunable optical delay line and a polarization beam combiner (PBC). The delay of the optical delay line is exactly one OFDM symbol. Thus a QPSK PDM-OFDM superchannel with 30 GHz spectral bandwidth and 120 Gb/s data rate is generated. For Tx2, seven lasers with frequency spacing of 40 GHz, 30 GHz, 40 GHz, 30 GHz, 30 GHz and 40 GHz pass an intensity modulator driven by 10 GHz radio frequency (RF) signal, to generate the optical comb. An Agilent AWG (M9505A) operating at 10 GS/s, an IQ modulator and a PDM emulator are used to generate 4 QPSK modulated PDM-OFDM superchannels with bandwidths of 30 GHz, 60 GHz, 90 GHz and 30 GHz, respectively. We adopt the same scheme with [16] for coherent detection.
Fig. 3. Q-factor performances of all subbands.
Figure 3 shows the Q-factor performances of the inverse multiplexing process. The backto-back (BTB) Q-factors of all subbands in Superchannel A, B, C, D and E are measured as reference. Compared with the BTB case of A, the sub-channels (A2, A3 and A1) suffer a Qfactor penalty of 2 dB introduced by FWM. After being coupled with other superchannels, the Q-factors decrease 1.6 dB further due to the crosstalk from the residual signals of neighboring subbands, as shown in Fig. 2-inset b. The Q-factors of all subbands in B, C, D and E after coupling are also measured. We can observe a Q-factor penalty of 0.3~0.5 dB for the subbands adjacent to A2, A3 and A1, while other subbands suffer slight penalties. This can be explained by the imperfect filtering characteristics of the Waveshaper, as shown in Fig. 2inset e. After coupling, the Q-factors of all subbands are better than the forward error correction (FEC) limit, which validates the proposed inverse multiplexing scheme. It is worth nothing that due to the degradation of Q factor, IM process should be only utilized for only once in actual network scenarios. Also, the allowed highest modulation level to be used for a given lightpath should fully consider not just the general loss/impairments of transmission and switching, also the optical processing impairments like inverse multiplexing. 4. Spectrum allocation algorithm considering IM capability in nodes In this section, a spectrum allocation (SA) algorithm is proposed based on the node architecture with build-in IM capability. With the discussion above, first, we conclude the constraints for the spectrum allocation approach as following: (1) the generation of new superchannel copies follows Eq. (1); (2) superchannel with different spectrum width would have different IM potential because the wider the superchannel is, the fewer valid copies it could have; (3) a superchannel could employ IMM only once in the whole optical path considering the penalty suffered from optical multicasting and filtering. Also, it is worth noting that the inverse multiplexed superchannels should be carefully selected to avoid
#209726 - $15.00 USD (C) 2014 OSA
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15138
introducing more fragments. So, following these constraints, the proposed SA algorithm is elaborated in Fig. 4. Request arrive (s,d) with a given path p={li}
collect the spectrum utilization info. of li Find continuous available spectrum segment for p Select the available continuous spectrum segment {sx, sy} that could reach the farthest node n
Success?
Success? find optimal IM solution with Eq.(2) and (3) from available spectrum of path n to d Success?
Blocked
Success
(a) 12345
1'2' 3' 4' 5'
b1 4'
12345
1" 2"3"4" 5"
b2
b3
12
3" 5"
12
3"4" 5"
A B 2' 3' 4'
1
5"
C (b) Fig. 4. (a) Flow Chart of IM based Spectrum Allocation Algorithm. (b) An example of IM spectrum allocation
As shown in Fig. 4(a), if a contiguous spectrum segment could not be found to satisfy the requirement of a request, the spectrum segment which could reach the farthest node (e.g., n) would be selected. Then, the IM would be performed in node n, which would not only consider the spectrum availability from n to destination node d, also the IM potential of this superchannel. Furthermore, as shown in Fig. 4(b), for some given superchannel copies, there may be more than one IM options, e.g., scenario A, B and C. So, Eq. (2) and (3) is employed to evaluate the potential impact of IM on the spectrum utilization and they are defined based on the following observations: (1) it is harmful to fill in the middle of the free spectrum block, and (2) it is efficient to fully occupy the free spectrum block or quite small part of that, not half of it. #209726 - $15.00 USD (C) 2014 OSA
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15139
max[ Bi ]
(2)
I
(
)
2 i i k bx − bx Bi = α b' b − 0.5 2 i i
(
i
i
if block bi is split into two or more blocks (b1 , bk )
)
(3)
' i
if IM changes the size of block bi to b
where α is a coefficient and I is the set of contiguous free spectrum block which part of the superchannel may fill in, e.g., b1, b2 and b3 in the Fig. 4(b). Bi is the utilization benefit of spectrum block bi achieved from superchannel IM and its definition is presented in Eq. (3). Accordingly, an IM solution could be selected. Otherwise, the traffic is blocked. 5. Numerical results and discussion In this section, the service blocking performance and service fairness [18] (of different granularity traffic) of the proposed scheme are evaluated by OPNET with the typical 14-node NSFNET topology and mixed granularity traffic. We assume that every link carries 300 FSs in both directions. 106 requests are generated for each simulation and requests are distributed equally among all the node pairs. Each request may require 10 Gb/s, 20 Gb/s, 30 Gb/s, 40 Gb/s, 80 Gb/s, 100 Gb/s or 400 Gb/s respectively, and the same number of request for each granularity is generated. The symbol rate of each subband is 10 G-baud and signal format is decided by its transmission distance. For simplicity, the highest available signal format for one request is employed according to the hops of its route, as shown in Table 1. So, in this simulation, for a given request, we employ fixed shortest path which is helpful to improve the spectrum utilization efficiency. The inter-arrival time and holding time of the lightpath requests are exponentially distributed with an average of 1/l and 1/m seconds, respectively. The load offered to the network is expressed in Erlangs (Erl.) as the ratio l/m. In addition, a guard band of 1 subband is employed between the adjacent optical channels, so that the switching node is able to add/drop any of the channels. The IM degree M is 15. Table 1. Required FSs for Different Granularity Traffic with Different Transmission Length Required subbands/allowed maximum copies Hops Signal Format =1or =2 16QAM =3 or =4 QPSK = or > 5 BPSK
10
20
1/13 1/13 1/13
1/13 1/13 2/13
Traffic Request (Gb/s) 30 40 80 100
400
1/13 2/13 3/13
10/7 20/3 40/3
1/13 2/13 4/13
22/13 4/13 8/7
3/13 5/13 10/7
4.1 Bandwidth blocking performance In this paper, bandwidth blocking probability (BBP) (ratio of bandwidth utilized by accepted requests to the total requested bandwidth [18]) is employed to measure the blocking performance of network with multi-granularity traffic. Figure 5(a) presents the BBP of IM in terms of α, which implies that α in the range of (6.5, 7.5) could keep the blocking probability in a lower level. Also, the variation of α would impact the blocking performance more severely under heavy traffic load than that under lower load, e.g., BBP varying from 21% to 28% under 100 Erl. while from 13.7% to 16% under 60 Erl. This is the result of fact that a proper value of α could balance the selection of two type of IM spectrum fitting strategy. At the same time, performance of different spectrum allocation schemes (e.g., first-fit, IM, SC, SS) is compared in Fig. 5(b). It shows that, comparing with SC and first-fit, IM has lower BBP under various traffic loads, especially when traffic intensity varying from 60 Erl. to 85 Erl. Besides, it is worth noting that BBP of IM is quite close with that of SS (the largest gap is less than 3.5% under 80 Erl.), but without complex packet store-reordering strategy in the
#209726 - $15.00 USD (C) 2014 OSA
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15140
receiving side. This is due to the fact that IM works like an optical partial split spectrum strategy. 0.30 100 Erl. 90 Erl. 70 Erl. 60 Erl.
0.28
Split Spectrum Inverse Multiplexing Spectrum Conversion First-fit
0.25
0.24
BBP
BBP
0.20
0.20
0.15 0.10
0.16
0.05 0.12
4
5
6
α
7
8
9
40
60
80
Traffic Intensity (b)
(a)
100
Fig. 5. (a) Impact of α on BBP; (b) Blocking performance of different spectrum allocation algorithm.
4.2 Service fairness of different granularity traffic In this section, the service fairness of traffic with different granularity is discussed, which investigates the blocking performance of different granularity traffic. Service fairness ratio (SFR) defined in Eq. (4) and Eq. (5) [18] is employed (which is based on granularity based normalized blocking probability, G-NBP). G -NBPx = Px
(4)
x
SFR y / x = G − NBPy / G − NBPx
(5)
where Px is the blocking probability of traffic with size x. As shown in Table 2, generally, SFRy/x increase with traffic load and the size of traffic y (for a fixed x), which is due to the fact that traffic with larger size is more likely to be blocked under heavy traffic. Specifically, comparing with SC and FF, IM serves the traffic with different granularities more fairly (the lower the value of SFR is, the more fair the service is), which is because that, in IM, the consecutive constraint in spectrum allocation is relaxed in some extent. Also, SFR of larger size traffic could achieve more improvement with IM than that of smaller one, e.g. under 100 Erl., SFR30/10 reduces from 3.6/4.1 (SC/FF) to 2.1 while SFR100/10 from 5.5/6.6 (SC/FF) to 3.1. On the other hand, IM could perform as well as SS, especially for traffic with smaller size under light traffic load. Table 2. SFR in terms of Traffic Intensity SFR Traffic Intensity 60 80 100 120
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SFR30/10
SFR80/10
SFR100/10
IM
FF
SC
SS
IM
FF
SC
SS
IM
FF
SC
SS
1.6 1.9 2.1 2.5
3.1 3.7 4.1 4.6
2.6 3.1 3.6 3.9
1.5 1.7 1.8 2.0
1.8 2.2 2.5 2.9
3.8 4.0 4.7 4.9
2.9 3.4 3.8 4.2
1.6 1.9 2.2 2.4
2.4 2.9 3.1 3.4
5.6 6.0 6.6 7.1
4.6 4.9 5.5 6.1
2.1 2.3 2.6 3.0
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15141
5. Conclusion A superchannel multicasting based optical inverse multiplexing is proposed to resolve the spectrum fragmentation problem in EON. IM could improve the spectrum utilization efficiency through separating the superchannel to fit into the discrete spectrum fragments, thus reducing the traffic blocking probability. At the same time, with IM, service fairness of different granularity traffic is improved significantly. To enable IM functionality in optical switching node, tunable laser sources are required in multicast module to generate pump signals as required. However, the number of laser source required for each superchannel would not been huge, e.g., 13 copies need 4 tunable laser sources. So, the amount of laser sources needed in a node depends on the requirements of concurrent converting superchannels. Acknowledgments Part of this work is published in OFC 2014. This work was supported by National Basic Research Program of China (973 Program, No. 2010CB328201 and 2010CB328202), the National Natural Science Foundation of China (NSFC, No. 61377072, No. 61275071, No. 61205058).
#209726 - $15.00 USD (C) 2014 OSA
Received 21 Apr 2014; revised 9 Jun 2014; accepted 10 Jun 2014; published 12 Jun 2014 16 June 2014 | Vol. 22, No. 12 | DOI:10.1364/OE.22.015133 | OPTICS EXPRESS 15142
