Effect of link margin and frequency granularity on the performance of a flexgrid optical network Abhijit Mitra,1,2,* Andrew Lord,1 Subrat Kar,2 and Paul Wright1 1
British Telecom Laboratories, Polaris House.BT Adastral Park, Martlesham Heath, Ipswich Suffolk IP5 3RE, UK 2 Department of Electrical Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India * [email protected]
Abstract: We show how dynamically adjustable modulation formats can be used to reduce link margins in flexgrid networks, reverting to lower order QAM due to reduced OSNR, if ageing occurs. Spectral savings amount to as much as 63% gain in capacity across a network using 64QAM with a fine frequency granularity of 6.25 GHz, with variable baud rate transponder. Further, a fixed baud rate, demand multiplexed transponder with adaptive modulation has been suggested. These transponders provide twice as much network capacity as compared to widely used fixed baud rate transponders operating at fixed grid of 50 GHz. ©2013 Optical Society of America OCIS codes: (060.1155) All-optical networks; (060.1660) Coherent communications; (060.4080) Modulation.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9.
M. Jinno, B. Kozicki, H. Takara, A. Watanabe, Y. Sone, T. Tanaka, and A. Hirano, “Distance-adaptive spectrum resource allocation in spectrum-sliced elastic optical path network [Topics in Optical Communications],” IEEE Commun. Mag. 48(8), 138–145 (2010). A. Mitra, S. Kar, and A. Lord, “Effect of link margin on spectrum saving and advantages of flexgrid optical networking,” in Proceedings of National Conference on Communications (NCC) (Indian Institute of Technology Delhi, New Delhi, 2013), pp.1–5. J. Auge, “Can we use flexible transponders to reduce margins?” in Proceedings of Optical Fiber Communication Conference/National Fiber Optic Engineers Conference(OFC/NFOEC) (OFC/NFOEC, 2013), OTu2A.1. O. Gerstel, M. Jinno, A. Lord, and S. J. B. Yoo, “Elastic optical networking: a new dawn for the optical layer?” IEEE Commun. Mag. 50(2), s12–s20 (2012). E. Lach and W. Idler, “Modulation formats for 100G and beyond,” Opt. Fiber Technol. 17(5), 377–386 (2011). A. Mitra, A. Lord, S. Kar, and P. Wright, “Effect of link margin and frequency granularity on the performance of a flexgrid optical network,” in Proceedings of European Conference and Exhibition on Optical Communication (ECOC, 2013),We.2.E.3. G. Bosco, V. Curri, A. Carena, P. Poggiolini, and F. Forghieri, “On the performance of Nyquist-WDM terabit superchannels based on PM-BPSK, PM-QPSK, PM-8QAM and PM-16QAM subcarriers,” J. Lightwave Technol. 29(1), 53–61 (2011). D. Yoon, K. Cho, and J. Lee, “Bit error probability of M-ary quadrature amplitude modulation,” in Proceedings of Vehicular Technology Conference, 2000 (IEEE-VTS Fall VTC, 2000), pp. 2422–2427. R. Essiambre, J. Kramer, P. J. Winzer, G. J. Foschini, and B. Goebel, “Capacity limits of optical fiber networks,” J. Lightwave Technol. 28(4), 662–701 (2010).
1. Introduction The network traffic continues to grow exponentially due to high data rate multimedia and internet services. These services use high bandwidth and put the onus on the network operators to efficiently use the network resources [1]. However, with the current 50 GHz ITU fixed grid this becomes difficult due to its lack of flexibility. A network that is flexible, adaptive and can adjust to the required traffic can help resolve these issues. The combination of a flexible wavelength grid and components such as WSS and coherent transceivers has led to the new paradigm of flexgrid optical networks. There is currently a great deal of consideration being given to how to capitalize on this flexibility.
#199165 - $15.00 USD (C) 2014 OSA
Received 8 Oct 2013; revised 15 Nov 2013; accepted 15 Nov 2013; published 20 Dec 2013 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000041 | OPTICS EXPRESS 41
One area, unexplored until now, is the potential to use adaptable modulation formats to meet the requirements of the optical path, without additional margin. In this concept, the modulation format occupying the minimum spectrum is used, calculated on a path by path basis [2]. Each modulation format (ranging from BPSK, QPSK through to 64QAM) has a different associated OSNR requirement for good error rate performance. So for a channel with high link OSNR, a matching high order M-ary modulation format can be assigned to transmit more bits per symbol and thereby save on the spectrum resources [2]. Unallocated margins, design margins and system margins mentioned in [3] constitute the overall Link Margin (LM) and arise from ageing and uniformity effects: both operators and equipment vendors have good reasons for including margins in traditional optical network planning and these are typically several dB. In general, the LM is subtracted from the estimated OSNR as a precaution to guarantee that the link remains operational in the event of degradation. This gives the Link Operational OSNR which forms the basis of traditional system design – the system should continue to run error free even if all the margins are eroded. If the link degrades further than this then the link will fail – so margins are set relatively high. Dynamic modulation format control can be used to operate the system with reduced margin, but with the scope to adjust to lower order QAM under any equipment or link degradation [4]. This can lead to reduced spectrum utilization and hence more traffic on day one, together with a targeted response to ageing. In this paper, we explore the magnitude of the benefits of reduced margins by doing simulations over a BT network for 100 Gb/s demands using PM-BPSK, PM-QPSK, PM16QAM and PM-64QAM with variable baud rate transponders. We show the impact of reducing LM by 4 dB and 6 dB using dynamic modulation format operation. Our approach measures the amount of random traffic supported by the network at a given Cumulative Blocking Probability (CBP). We compare the results for different Frequency Granularities (FG) of 50 GHz, 25 GHz, 12.5 GHz and 6.25 GHz. Further, we introduce a demand multiplexed, fixed baud rate transponder and compare its performance with traditional fixed baud rate transponders for a given CBP over 50 GHz, across all the LMs.
Fig. 1. EDFA Placement in km for an optical link.
2. Estimation of link OSNR For a pair of source and destination nodes, we first find the shortest path on our network. As an example suppose we have a link shown in Fig. 1. Intermediate nodes 2, 3 and 4 are ROADMs. Each ROADM is modeled having a span loss of 22 dB which is immediately compensated by an EDFA. The line EDFAs are symmetrically placed with a maximum EDFA inter span spacing of 60 km. The resulting noise model [2] determines the estimated link OSNR, assuming an EDFA noise figure of 4.5 dB. In the example shown, an estimated link OSNR of 25.4 dB is calculated. Comparing this with OSNR thresholds for the key modulation formats given in Table 1, derived from [5] with pre-FEC BER of 4*10−3, we choose to operate on PM-64QAM and allocate a corresponding bandwidth of 16 GHz. This would then use either a single 50 GHz, a single 25 GHz, two 12.5 GHz or three 6.25 GHz flexgrid channels to accommodate this demand.
#199165 - $15.00 USD (C) 2014 OSA
Received 8 Oct 2013; revised 15 Nov 2013; accepted 15 Nov 2013; published 20 Dec 2013 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000041 | OPTICS EXPRESS 42
Table 1. OSNR Threshold for 100 Gb/s Mod Format PM-BPSK PM-QPSK PM-16QAM PM-64QAM
Data rate (Gb/s) 112 112 112 112
Baud Rate (Gs/s) 56 28 14 10
OSNR (dB)
BW (GHz)
British Telecom Laboratories, Polaris House.BT Adastral Park, Martlesham Heath, Ipswich Suffolk IP5 3RE, UK 2 Department of Electrical Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India * [email protected]
Abstract: We show how dynamically adjustable modulation formats can be used to reduce link margins in flexgrid networks, reverting to lower order QAM due to reduced OSNR, if ageing occurs. Spectral savings amount to as much as 63% gain in capacity across a network using 64QAM with a fine frequency granularity of 6.25 GHz, with variable baud rate transponder. Further, a fixed baud rate, demand multiplexed transponder with adaptive modulation has been suggested. These transponders provide twice as much network capacity as compared to widely used fixed baud rate transponders operating at fixed grid of 50 GHz. ©2013 Optical Society of America OCIS codes: (060.1155) All-optical networks; (060.1660) Coherent communications; (060.4080) Modulation.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9.
M. Jinno, B. Kozicki, H. Takara, A. Watanabe, Y. Sone, T. Tanaka, and A. Hirano, “Distance-adaptive spectrum resource allocation in spectrum-sliced elastic optical path network [Topics in Optical Communications],” IEEE Commun. Mag. 48(8), 138–145 (2010). A. Mitra, S. Kar, and A. Lord, “Effect of link margin on spectrum saving and advantages of flexgrid optical networking,” in Proceedings of National Conference on Communications (NCC) (Indian Institute of Technology Delhi, New Delhi, 2013), pp.1–5. J. Auge, “Can we use flexible transponders to reduce margins?” in Proceedings of Optical Fiber Communication Conference/National Fiber Optic Engineers Conference(OFC/NFOEC) (OFC/NFOEC, 2013), OTu2A.1. O. Gerstel, M. Jinno, A. Lord, and S. J. B. Yoo, “Elastic optical networking: a new dawn for the optical layer?” IEEE Commun. Mag. 50(2), s12–s20 (2012). E. Lach and W. Idler, “Modulation formats for 100G and beyond,” Opt. Fiber Technol. 17(5), 377–386 (2011). A. Mitra, A. Lord, S. Kar, and P. Wright, “Effect of link margin and frequency granularity on the performance of a flexgrid optical network,” in Proceedings of European Conference and Exhibition on Optical Communication (ECOC, 2013),We.2.E.3. G. Bosco, V. Curri, A. Carena, P. Poggiolini, and F. Forghieri, “On the performance of Nyquist-WDM terabit superchannels based on PM-BPSK, PM-QPSK, PM-8QAM and PM-16QAM subcarriers,” J. Lightwave Technol. 29(1), 53–61 (2011). D. Yoon, K. Cho, and J. Lee, “Bit error probability of M-ary quadrature amplitude modulation,” in Proceedings of Vehicular Technology Conference, 2000 (IEEE-VTS Fall VTC, 2000), pp. 2422–2427. R. Essiambre, J. Kramer, P. J. Winzer, G. J. Foschini, and B. Goebel, “Capacity limits of optical fiber networks,” J. Lightwave Technol. 28(4), 662–701 (2010).
1. Introduction The network traffic continues to grow exponentially due to high data rate multimedia and internet services. These services use high bandwidth and put the onus on the network operators to efficiently use the network resources [1]. However, with the current 50 GHz ITU fixed grid this becomes difficult due to its lack of flexibility. A network that is flexible, adaptive and can adjust to the required traffic can help resolve these issues. The combination of a flexible wavelength grid and components such as WSS and coherent transceivers has led to the new paradigm of flexgrid optical networks. There is currently a great deal of consideration being given to how to capitalize on this flexibility.
#199165 - $15.00 USD (C) 2014 OSA
Received 8 Oct 2013; revised 15 Nov 2013; accepted 15 Nov 2013; published 20 Dec 2013 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000041 | OPTICS EXPRESS 41
One area, unexplored until now, is the potential to use adaptable modulation formats to meet the requirements of the optical path, without additional margin. In this concept, the modulation format occupying the minimum spectrum is used, calculated on a path by path basis [2]. Each modulation format (ranging from BPSK, QPSK through to 64QAM) has a different associated OSNR requirement for good error rate performance. So for a channel with high link OSNR, a matching high order M-ary modulation format can be assigned to transmit more bits per symbol and thereby save on the spectrum resources [2]. Unallocated margins, design margins and system margins mentioned in [3] constitute the overall Link Margin (LM) and arise from ageing and uniformity effects: both operators and equipment vendors have good reasons for including margins in traditional optical network planning and these are typically several dB. In general, the LM is subtracted from the estimated OSNR as a precaution to guarantee that the link remains operational in the event of degradation. This gives the Link Operational OSNR which forms the basis of traditional system design – the system should continue to run error free even if all the margins are eroded. If the link degrades further than this then the link will fail – so margins are set relatively high. Dynamic modulation format control can be used to operate the system with reduced margin, but with the scope to adjust to lower order QAM under any equipment or link degradation [4]. This can lead to reduced spectrum utilization and hence more traffic on day one, together with a targeted response to ageing. In this paper, we explore the magnitude of the benefits of reduced margins by doing simulations over a BT network for 100 Gb/s demands using PM-BPSK, PM-QPSK, PM16QAM and PM-64QAM with variable baud rate transponders. We show the impact of reducing LM by 4 dB and 6 dB using dynamic modulation format operation. Our approach measures the amount of random traffic supported by the network at a given Cumulative Blocking Probability (CBP). We compare the results for different Frequency Granularities (FG) of 50 GHz, 25 GHz, 12.5 GHz and 6.25 GHz. Further, we introduce a demand multiplexed, fixed baud rate transponder and compare its performance with traditional fixed baud rate transponders for a given CBP over 50 GHz, across all the LMs.
Fig. 1. EDFA Placement in km for an optical link.
2. Estimation of link OSNR For a pair of source and destination nodes, we first find the shortest path on our network. As an example suppose we have a link shown in Fig. 1. Intermediate nodes 2, 3 and 4 are ROADMs. Each ROADM is modeled having a span loss of 22 dB which is immediately compensated by an EDFA. The line EDFAs are symmetrically placed with a maximum EDFA inter span spacing of 60 km. The resulting noise model [2] determines the estimated link OSNR, assuming an EDFA noise figure of 4.5 dB. In the example shown, an estimated link OSNR of 25.4 dB is calculated. Comparing this with OSNR thresholds for the key modulation formats given in Table 1, derived from [5] with pre-FEC BER of 4*10−3, we choose to operate on PM-64QAM and allocate a corresponding bandwidth of 16 GHz. This would then use either a single 50 GHz, a single 25 GHz, two 12.5 GHz or three 6.25 GHz flexgrid channels to accommodate this demand.
#199165 - $15.00 USD (C) 2014 OSA
Received 8 Oct 2013; revised 15 Nov 2013; accepted 15 Nov 2013; published 20 Dec 2013 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000041 | OPTICS EXPRESS 42
Table 1. OSNR Threshold for 100 Gb/s Mod Format PM-BPSK PM-QPSK PM-16QAM PM-64QAM
Data rate (Gb/s) 112 112 112 112
Baud Rate (Gs/s) 56 28 14 10
OSNR (dB)
BW (GHz)
