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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 23 — Nov. 5, 2012
  • pp: 25284–25291
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Energy-efficient optical line terminal for WDM-OFDM-PON based on two-dimensional subcarrier and layer allocation

Xiaofeng Hu, Pan Cao, Zhiming Zhuang, Liang Zhang, Qi Yang, and Yikai Su  »View Author Affiliations


Optics Express, Vol. 20, Issue 23, pp. 25284-25291 (2012)
http://dx.doi.org/10.1364/OE.20.025284


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Abstract

We propose and experimentally demonstrate a scheme to reduce the energy consumption of optical line terminal (OLT) in wavelength division multiplexing - orthogonal frequency division multiplexing - passive optical networks (WDM-OFDM-PONs). In our scheme, a wireless communication technique, termed layered modulation, is introduced to maximize the transmission capacity of OFDM modulation module in the OLT by multiplexing data from different ONU groups with signal-to-noise ratio (SNR) margins onto the same subcarriers. With adaptive and dynamic subcarrier and layer allocation, several ONU groups with low traffic demands can share one OFDM modulation module to deliver their data during non-peak hours of a day, thus greatly reducing the number of running devices and minimizing the energy consumption of the OLT. Numerical calculation shows that an energy efficiency improvement of 28.3% in the OLT can be achieved by using proposed scheme compared to the conventional WDM-OFDM-PON.

© 2012 OSA

1. Introduction

With rapid increase of global data traffic and massive deployment of new network devices and equipments, the energy consumption of network is growing fast and becoming a key environmental, social, and economic issue, receiving great attention in the past few years [1

1. R. S. Tucker, “Green optical communications – Part I: Energy limitations in transport,” IEEE J. Sel. Top. Quantum Electron. 17(2), 245–260 (2011). [CrossRef]

3

3. S. J. Yoo, “Energy efficiency in the future internet: the role of optical packet switching and optical-label switching,” IEEE J. Sel. Top. Quantum Electron. 17(2), 406–418 (2011). [CrossRef]

]. It was reported that access networks consume about 70% of overall network energy consumption owing to the large quantity of access nodes [4

4. C. Lange and A. Gladisch, “On the energy consumption of FTTH access networks,” in Proc. OFC2009, San Diego, CA, paper JThA79.

]. However, current access networks exhibit poor energy efficiency. A large portion of energy is wasted by idle devices [5

5. P. Chowdhury, M. Tornatore, S. Sarkar, and B. Mukherjee, “Building a green wireless-optical broad band access netwok (WOBAN),” J. Lightwave Technol. 28(16), 2219–2229 (2010). [CrossRef]

], as the access networks are engineered to satisfy the peak traffic-load requirement. In literatures, many methods have been proposed to reduce the energy consumption of time division multiplexing-passive optical network (TDM-PON) and wavelength division multiplexing (WDM) PON, such as sleep mode [6

6. L. Shi, S. S. Lee, and B. Mukherjee, “An SLA-based energy-efficient scheduling scheme for EPON with sleep-mode ONU,” in Proc. OFC2011, paper OThB4.

], adaptive line rate control [7

7. R. Kubo, J. Kani, H. Ujikawa, T. Sakamoto, Y. Fujimoto, N. Yoshimoto, and H. Hadama, “Study and demonstration of sleep and adaptive link rate control mechanisms for energy efficient 10G-EPON,” IEEE J. Opt. Commun. Netw. 2(9), 716–729 (2010). [CrossRef]

], and pilot-tone-based monitoring technique [8

8. K. H. Tse, W. Jia, and C. K. Chan, “A cost-effective pilot-tone-based monitoring technique for power saving in RSOA-based WDM-PON,” in Proc. OFC2011, paper OThB6.

].

Optical orthogonal frequency division multiplexing (OFDM) technique has recently been a promising technique in access networks due to its high spectral efficiency and robust dispersion tolerance [9

9. N. Cvijetic, “OFDM for next-generation optical access networks,” J. Lightwave Technol. 30(4), 384–398 (2012). [CrossRef]

]. WDM-OFDM-PON, combining the advantages of WDM and OFDM techniques, can provide higher data rate and more flexible bandwidth allocation for end users, which has been intensively investigated by many research groups [10

10. B. Liu, X. Xin, L. Zhang, J. Yu, Q. Zhang, and C. Yu, “A WDM-OFDM-PON architecture with centralized lightwave and PolSK-modulated multicast overlay,” Opt. Express 18(3), 2137–2143 (2010). [CrossRef] [PubMed]

12

12. D. Qian, T. Kwok, N. Cviject, J. Hu, and T. Wang, “41.25 Gb/s real-time OFDM receiver for variable rate WDM-OFDMA-PON transmission,” in Proc. OFC2010, paper PDPD9.

]. Nonetheless, OFDM modulation modules, consisting of high-speed digital signal processing (DSP) chips, digital-to-analog converters (DAC), and E/O modulators, are needed for the generation of optical OFDM signals in WDM-OFDM-PON. These components consume much more energy than the counterparts in conventional TDM-PON and WDM-PON, especially when the data rate increases up to 10 Gb/s. Moreover, each OFDM modulation module is fixed for one optical network unit (ONU) group in conventional WDM-OFDM-PON, which causes a rough granularity and wastes a large amount of bandwidth resource since the users do not fully utilize the network capacity all the time. Therefore, it is of great significance to design an energy-efficient WDM-OFDM-PON system. To date, however, there are few reports concentrating on improving the energy efficiency of WDM-OFDM-PON.

2. Operation principle

Layered modulation, also termed hierarchical modulation, is one of the wireless communication technologies for multiplexing and modulating multiple data streams into one single symbol stream. Although layered modulation can be implemented in many constellations, we limit our discussions to hierarchical QPSK/16QAM format in this paper for simplicity. Figure 1(a)
Fig. 1 (a) QPSK/16QAM layered modulation. (b) Two-dimensional subcarrier and layer allocation (LS: low-SNR; HS: high-SNR). (c) Schematic diagram of the proposed energy-efficient WDM-OFDM-PON (LR: long-reach; HSR: high-split-ratio).
describes the principle of QPSK/16QAM layered modulation. Two separate and independent data streams, data1 and data2, are mapped onto Layer1 and Layer2 with QPSK constellations, respectively. The two QPSK symbols are then superimposed together to constitute a 16QAM symbol. The mapping of the information bits is indicated in the hierarchical QPSK/16QAM constellation in Fig. 1(a), where the first two bits represent the data in the Layer1 and the two remaining bits represent the data in the Layer2 [15

15. H. Jiang and P. Wilford, “A hierarchical modulation for upgrading digital broadcast systems,” IEEE Trans. Broadcast 51(2), 223–229 (2005). [CrossRef]

]. The minimum distances between adjacent points in two QPSK constellations are denoted by d1 and d2, respectively. Since d1 is larger than d2, data1 shows better bit error rate (BER) performance than data2 at the same receiving condition when hierarchical QPSK/16QAM signal is transmitted [16

16. M. Hossain, P. Vitthaladevuni, M. S. Alouini, V. Bhargava, and A. Goldsmith, “Adaptive hierarchical modulation for simultaneous voice and multiclass data transmission over fading channels,” IEEE Trans. Vehicular Technol. 55(4), 1181–1194 (2006). [CrossRef]

]. Here, we define SNR1, SNR2, SNRQPSK, and SNR16QAM as the required minimum SNRs to achieve error-free transmission for data1 and data2 in hierarchical QPSK/16QAM signal, QPSK signal and 16QAM signal, respectively. In addition, we also define a hierarchical parameter α as the ratio of d1 to d2 (α = d1/d2). With the increase of α, SNR1 is decreased at the cost of the increase of SNR2. Based on the analysis in Ref [15

15. H. Jiang and P. Wilford, “A hierarchical modulation for upgrading digital broadcast systems,” IEEE Trans. Broadcast 51(2), 223–229 (2005). [CrossRef]

,16

16. M. Hossain, P. Vitthaladevuni, M. S. Alouini, V. Bhargava, and A. Goldsmith, “Adaptive hierarchical modulation for simultaneous voice and multiclass data transmission over fading channels,” IEEE Trans. Vehicular Technol. 55(4), 1181–1194 (2006). [CrossRef]

], an inequality, SNR2SNR16QAMSNR1SNRQPSK, can be obtained.

The basic principle of our proposed two-dimensional (2-D) subcarrier and layer allocation is depicted in Fig. 1(b). Owing to diverse transmission and reception conditions, the received SNRs of ONU groups are different. For instance, the SNRs of long-reach ONU Groupi and high-split-ratio ONU Groupj (SNRONUi and SNRONUj) in Fig. 1(c) are relatively low, while the SNR of general ONU Grouph (SNRONUh) is high. Normally, the modulation format in the subcarriers allocated to the ONU depends on the SNR of that ONU, which is referred to bit loading technique [14

14. Q. Yang, W. Shieh, and Y. Ma, “Bit and power loading for coherent optical OFDM,” IEEE Photon. Technol. Lett. 20(15), 1305–1307 (2008). [CrossRef]

]. We assume that the SNRs of three ONU groups in Fig. 1(c) meet the inequalitySNRONUhSNR16QAMSNRONUi,jSNRQPSK. Thus, by employing subcarrier allocation and bit loading technique, k subcarriers with QPSK constellation are assigned to ONU Groupi and Groupj to ensure error-free transmission, while m-k subcarriers with 16QAM format are assigned to ONU Grouph. However, if the ONUs still have some remaining SNR margins satisfying the inequality:
SNRONUhSNR2SNR16QAMSNRONUi,jSNR1SNRQPSK
(1)
a hierarchical QPSK/16QAM modulation format can be employed in the k QPSK-modulated subcarriers. As shown in Fig. 1(b), data of ONU Grouph originally carried by the last m-k subcarriers is moved onto Layer2 of the hierarchical QPSK/16QAM constellation in the first k subcarriers. Meantime, data of ONU Groupi and Groupj are mapped onto Layer1 of the first k subcarriers. This allocation scheme is referred to 2D subcarrier and layer allocation in our paper. By controlling the hierarchical parameter α, one can adaptively adjust SNR1 and SNR2 to meet the above inequality, achieving error-free operation. Thus, m-k subcarriers originally assigned to ONU Grouph can be re-allocated to other ONU groups, greatly increasing the transmission capacity of OFDM modulation module. It is worth noting that the hierarchical modulation formats and hierarchical parameter can be properly adjusted according to the practical access network conditions.

It is well known that the traffic load of access networks fluctuates during the course of a day, mainly determined by the behavior patterns of the customers. For conventional WDM-OFDM-PON, all the OFDM modulation modules have to be running all the time. However, in our proposal, by using dynamic reconfiguration of optical switch and 2-D subcarrier and layer allocation in the OLT, all ONU groups can share the least number of OFDM modulation modules to deliver data according to network traffic demands, thus powering off other OFDM modulation modules and therefore reducing the energy consumption of the OLT of WDM-OFDM-PON.

3. Experimental setup and results

In Fig. 2, we also mark the optical signal powers at the output port of the OLT and input ports of three ONUs. The output optical power of downstream OFDM signal in each wavelength is about 5 dBm in the OLT. On the other hand, it is easy to understand that the receiver sensitivity in ONU Group2 is worse than that in ONU Group1,3 due to the longer fiber transmission. The minimum received power for the conventional downstream 16QAM-OFDM signal to achieve a BER of 2 × 10−3 is approximately −18.3 dBm for ONU Group2 as shown in Fig. 3(b)
Fig. 3 BER curves for hierarchical QPSK/16QAM signals with α = 2 and 3 after (a) 12.5-km transmission, (b) 75-km transmission (FEC threshold: 2 × 10−3).
. With the use of forward error correction (FEC) module, error-free transmission can be achieved. Thus, the power budget of our system is ~23.3 dB considering the worst receiver sensitivity of −18.3 dBm for conventional 16QAM-OFDM signal.

Figure 3 shows the BER curves for downstream hierarchical QPSK/16QAM signals after transmission of 12.5-km and 75-km SSMF, which are obtained by offline Matlab processing. In a PON system, received powers can be used to evaluate the network performance instead of SNRs. The cross points A-D and A'-D' in Fig. 3 denote the required minimum received powers (PA-PD, PA'-PD') to achieve a BER of 2 × 10−3 for data on two layers in hierarchical QPSK/16QAM signal with α = 2 or 3. Here, we assume that error-free transmission can be realized at these points taking into account the use of FEC module. It is clearly observed that PC and PD are higher than PA and PB, demonstrating that the BER performance of data on Layer1 is better than that on Layer2 when QPSK/16QAM layered modulation is implemented. In our experiment, the received powers at the ONUs of ONU Group1 are −14 dBm and those of long-reach ONU Group2 and high-split-ratio ONU Group3 are −20.5 dBm and −20 dBm, respectively. As shown in Fig. 3(a), PA and PB are lower than −20 dBm, while PC and PD are lower than −14 dBm but higher than −20 dBm. It indicates that the data from high-split-ratio ONU Group3 should be mapped onto Layer1 of QPSK/16QAM symbol, while the data from ONU Group1 can be modulated on both layers. For long-reach ONU Group2, the data should be loaded onto Layer1 of hierarchical QPSK/16QAM symbol with α = 3 since only PA' is lower than −20.5 dBm as shown in Fig. 3(b). Thus, only QPSK format can be employed in the subcarriers allocated to ONU Group2 and Group3 if one uses bit loading technique. In order to maximize the transmission capacity of OFDM modulation module, the hierarchical QPSK/16QAM modulation format with α = 3 is used in our experiment with Layer1 delivering the data from ONU Group2 and Group3, and Layer2 transmitting data for ONU Group1, respectively. Based on the above analysis, error-free transmissions are achieved for all data from three ONU groups.

4. Numerical analysis for energy efficiency

Based on the data provided in Ref [19

19. B. Skubic, E. Betou, T. Ayhan, and S. Dahlfort, “Energy-efficient next-generation optical access networks,” IEEE Commun. Mag. 50(1), 122–127 (2012). [CrossRef]

,20

20. B. Sedighi, K. Lee, R. Tucker, H. Chow, and P. Vetter, “Energy-efficiency in future 40-Gb/s fiber access networks,” in Proc. OFC2012, paper JTh2A.59.

], we estimate that the power consumptions of OFDM modulation module, DFB laser, optical switch, EDFA, and upstream receiver are 8 W, 1 W, 5 W, 6 W, and 2 W, respectively. Thus, the total power consumption of the OLT is 395 W in the conventional 32-wavelength WDM-OFDM-PON. By using 2D subcarrier and layer allocation, a power saving of 43.7% in the OFDM modulation modules can be achieved, corresponding to 111.9-W power reduction. Therefore, our proposed scheme improves the energy efficiency of the OLT by ~28.3%.

5. Conclusion

We have proposed and demonstrated a scheme to realize energy-efficient OLT for WDM-OFDM-PON by sharing OFDM modulation modules with 2-D subcarrier and layer allocation. A proof-of-concept experiment utilizing one OFDM modulation module to serve three ONU groups is performed, verifying the feasibility of our proposal. Numerical analysis results show that up to 28.3% energy saving in the OLT can be achieved by using the proposed scheme.

Acknowledgments

This work was supported in part by NSFC (61077052/61125504), MoE (20110073110012), and Science and Technology Commission of Shanghai Municipality (11530700400).

References and links

1.

R. S. Tucker, “Green optical communications – Part I: Energy limitations in transport,” IEEE J. Sel. Top. Quantum Electron. 17(2), 245–260 (2011). [CrossRef]

2.

R. S. Tucker, “Green optical communications – Part II: Energy limitations in networks,” IEEE J. Sel. Top. Quantum Electron. 17(2), 261–274 (2011). [CrossRef]

3.

S. J. Yoo, “Energy efficiency in the future internet: the role of optical packet switching and optical-label switching,” IEEE J. Sel. Top. Quantum Electron. 17(2), 406–418 (2011). [CrossRef]

4.

C. Lange and A. Gladisch, “On the energy consumption of FTTH access networks,” in Proc. OFC2009, San Diego, CA, paper JThA79.

5.

P. Chowdhury, M. Tornatore, S. Sarkar, and B. Mukherjee, “Building a green wireless-optical broad band access netwok (WOBAN),” J. Lightwave Technol. 28(16), 2219–2229 (2010). [CrossRef]

6.

L. Shi, S. S. Lee, and B. Mukherjee, “An SLA-based energy-efficient scheduling scheme for EPON with sleep-mode ONU,” in Proc. OFC2011, paper OThB4.

7.

R. Kubo, J. Kani, H. Ujikawa, T. Sakamoto, Y. Fujimoto, N. Yoshimoto, and H. Hadama, “Study and demonstration of sleep and adaptive link rate control mechanisms for energy efficient 10G-EPON,” IEEE J. Opt. Commun. Netw. 2(9), 716–729 (2010). [CrossRef]

8.

K. H. Tse, W. Jia, and C. K. Chan, “A cost-effective pilot-tone-based monitoring technique for power saving in RSOA-based WDM-PON,” in Proc. OFC2011, paper OThB6.

9.

N. Cvijetic, “OFDM for next-generation optical access networks,” J. Lightwave Technol. 30(4), 384–398 (2012). [CrossRef]

10.

B. Liu, X. Xin, L. Zhang, J. Yu, Q. Zhang, and C. Yu, “A WDM-OFDM-PON architecture with centralized lightwave and PolSK-modulated multicast overlay,” Opt. Express 18(3), 2137–2143 (2010). [CrossRef] [PubMed]

11.

M. F. Huang, J. Yu, D. Qian, N. Cvijetic, and G. K. Chang, “Lightwave centralized WDM-OFDM-PON network employing cost-effective directly modulated laser,” in Proc. OFC2009, San Diego, CA, paper OMV5.

12.

D. Qian, T. Kwok, N. Cviject, J. Hu, and T. Wang, “41.25 Gb/s real-time OFDM receiver for variable rate WDM-OFDMA-PON transmission,” in Proc. OFC2010, paper PDPD9.

13.

X. Hu, L. Zhang, P. Cao, K. Wang, and Y. Su, “Energy-efficient WDM-OFDM-PON employing shared OFDM modulation modules in optical line terminal,” Opt. Express 20(7), 8071–8077 (2012). [CrossRef] [PubMed]

14.

Q. Yang, W. Shieh, and Y. Ma, “Bit and power loading for coherent optical OFDM,” IEEE Photon. Technol. Lett. 20(15), 1305–1307 (2008). [CrossRef]

15.

H. Jiang and P. Wilford, “A hierarchical modulation for upgrading digital broadcast systems,” IEEE Trans. Broadcast 51(2), 223–229 (2005). [CrossRef]

16.

M. Hossain, P. Vitthaladevuni, M. S. Alouini, V. Bhargava, and A. Goldsmith, “Adaptive hierarchical modulation for simultaneous voice and multiclass data transmission over fading channels,” IEEE Trans. Vehicular Technol. 55(4), 1181–1194 (2006). [CrossRef]

17.

B. Schmidt, A. Lowery, and J. Armstrong, “Experimental demonstrations of electronic dispersion compensation for long-haul transmission using direct-detection,” J. Lightwave Technol. 26(1), 196–203 (2008). [CrossRef]

18.

Sandvine, “Global Internet Phenomena Spotlight – North America, Fixed Access, Spring 2011” (Sandvine, 2011). http://www.sandvine.com/news/global_broadband_trends.asp.

19.

B. Skubic, E. Betou, T. Ayhan, and S. Dahlfort, “Energy-efficient next-generation optical access networks,” IEEE Commun. Mag. 50(1), 122–127 (2012). [CrossRef]

20.

B. Sedighi, K. Lee, R. Tucker, H. Chow, and P. Vetter, “Energy-efficiency in future 40-Gb/s fiber access networks,” in Proc. OFC2012, paper JTh2A.59.

OCIS Codes
(060.2330) Fiber optics and optical communications : Fiber optics communications
(060.4250) Fiber optics and optical communications : Networks

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: July 24, 2012
Revised Manuscript: September 5, 2012
Manuscript Accepted: September 16, 2012
Published: October 22, 2012

Citation
Xiaofeng Hu, Pan Cao, Zhiming Zhuang, Liang Zhang, Qi Yang, and Yikai Su, "Energy-efficient optical line terminal for WDM-OFDM-PON based on two-dimensional subcarrier and layer allocation," Opt. Express 20, 25284-25291 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-23-25284


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References

  1. R. S. Tucker, “Green optical communications – Part I: Energy limitations in transport,” IEEE J. Sel. Top. Quantum Electron.17(2), 245–260 (2011). [CrossRef]
  2. R. S. Tucker, “Green optical communications – Part II: Energy limitations in networks,” IEEE J. Sel. Top. Quantum Electron.17(2), 261–274 (2011). [CrossRef]
  3. S. J. Yoo, “Energy efficiency in the future internet: the role of optical packet switching and optical-label switching,” IEEE J. Sel. Top. Quantum Electron.17(2), 406–418 (2011). [CrossRef]
  4. C. Lange and A. Gladisch, “On the energy consumption of FTTH access networks,” in Proc. OFC2009, San Diego, CA, paper JThA79.
  5. P. Chowdhury, M. Tornatore, S. Sarkar, and B. Mukherjee, “Building a green wireless-optical broad band access netwok (WOBAN),” J. Lightwave Technol.28(16), 2219–2229 (2010). [CrossRef]
  6. L. Shi, S. S. Lee, and B. Mukherjee, “An SLA-based energy-efficient scheduling scheme for EPON with sleep-mode ONU,” in Proc. OFC2011, paper OThB4.
  7. R. Kubo, J. Kani, H. Ujikawa, T. Sakamoto, Y. Fujimoto, N. Yoshimoto, and H. Hadama, “Study and demonstration of sleep and adaptive link rate control mechanisms for energy efficient 10G-EPON,” IEEE J. Opt. Commun. Netw.2(9), 716–729 (2010). [CrossRef]
  8. K. H. Tse, W. Jia, and C. K. Chan, “A cost-effective pilot-tone-based monitoring technique for power saving in RSOA-based WDM-PON,” in Proc. OFC2011, paper OThB6.
  9. N. Cvijetic, “OFDM for next-generation optical access networks,” J. Lightwave Technol.30(4), 384–398 (2012). [CrossRef]
  10. B. Liu, X. Xin, L. Zhang, J. Yu, Q. Zhang, and C. Yu, “A WDM-OFDM-PON architecture with centralized lightwave and PolSK-modulated multicast overlay,” Opt. Express18(3), 2137–2143 (2010). [CrossRef] [PubMed]
  11. M. F. Huang, J. Yu, D. Qian, N. Cvijetic, and G. K. Chang, “Lightwave centralized WDM-OFDM-PON network employing cost-effective directly modulated laser,” in Proc. OFC2009, San Diego, CA, paper OMV5.
  12. D. Qian, T. Kwok, N. Cviject, J. Hu, and T. Wang, “41.25 Gb/s real-time OFDM receiver for variable rate WDM-OFDMA-PON transmission,” in Proc. OFC2010, paper PDPD9.
  13. X. Hu, L. Zhang, P. Cao, K. Wang, and Y. Su, “Energy-efficient WDM-OFDM-PON employing shared OFDM modulation modules in optical line terminal,” Opt. Express20(7), 8071–8077 (2012). [CrossRef] [PubMed]
  14. Q. Yang, W. Shieh, and Y. Ma, “Bit and power loading for coherent optical OFDM,” IEEE Photon. Technol. Lett.20(15), 1305–1307 (2008). [CrossRef]
  15. H. Jiang and P. Wilford, “A hierarchical modulation for upgrading digital broadcast systems,” IEEE Trans. Broadcast51(2), 223–229 (2005). [CrossRef]
  16. M. Hossain, P. Vitthaladevuni, M. S. Alouini, V. Bhargava, and A. Goldsmith, “Adaptive hierarchical modulation for simultaneous voice and multiclass data transmission over fading channels,” IEEE Trans. Vehicular Technol.55(4), 1181–1194 (2006). [CrossRef]
  17. B. Schmidt, A. Lowery, and J. Armstrong, “Experimental demonstrations of electronic dispersion compensation for long-haul transmission using direct-detection,” J. Lightwave Technol.26(1), 196–203 (2008). [CrossRef]
  18. Sandvine, “Global Internet Phenomena Spotlight – North America, Fixed Access, Spring 2011” (Sandvine, 2011). http://www.sandvine.com/news/global_broadband_trends.asp .
  19. B. Skubic, E. Betou, T. Ayhan, and S. Dahlfort, “Energy-efficient next-generation optical access networks,” IEEE Commun. Mag.50(1), 122–127 (2012). [CrossRef]
  20. B. Sedighi, K. Lee, R. Tucker, H. Chow, and P. Vetter, “Energy-efficiency in future 40-Gb/s fiber access networks,” in Proc. OFC2012, paper JTh2A.59.

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