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A Self-Restorable WDM-PON with a Color-Free Optical Source Joon-Young Kim, Sil-Gu Mun, Hoon-Keun Lee, and Chang-Hee Lee, Senior Member, IEEE

Abstract— We propose and demonstrate a self-restorable wavelength division multiplexing-passive optical network (WDM-PON) with a color-free optical source. When the distribution and feeder fiber links are broken, we can successfully recover the network by using optical switches and/or Ethernet switches. If the fault monitors detect the failures, optical switches change the transmission path and the failures are recovered within 10 msec. In case we use only Ethernet switches, we also can protect the transmission path failure with help of link aggregation control protocol. Index Terms—Optical fiber communication, passive optical network, protection, self-restoration, availability.

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I. INTRODUCTION

he wavelength division multiplexing-passive optical network (WDM-PON) based on a colorless or color-free optical source is in the limelight as a way to realize a next generation access network [1]. It has many merits such as unlimited bandwidth, high security, high flexibility, no QoS issues, no requirement for burst-mode Rx and Tx, and no need for media access control (MAC). In spite of that, enormous data loss may occur when a transmission path is broken. In order to solve this problem, many protection methods have been investigated [2-7]. Some of them are able to protect either feeder fiber or distribution fiber [2-3]. There also exist protection schemes from failure of both kinds of fiber [4-7]. However, the protection schemes require some complex architectures [4-5], and devices with high insertion loss [5]. Also, the distribution fiber cannot be protected when feeder fiber has been recovered from the failure [4-7]. In this paper, we propose and demonstrate a self-restorable WDM-PON that the feeder and distribution fibers can be protected simultaneously. Colorless or color-free operation of transceivers at optical line terminal (OLT) and optical network unit (ONU) brings about simple and efficient protection architectures. We assigned two fibers for each ONU that is inevitable because fiber topology should have the route diversity for protection. The working path is switched to the protection path, when the fault monitor detects the failure in the working fiber. In this case, the assigned wavelength is shifted due to the characteristic of employed AWGs. It is also possible to protect with help of link aggregation control protocol in layer 2. The power penalty caused by these protection processes

Joon-Young Kim, Sil-Gu Mun, Hoon-Keun Lee, and Chang-Hee Lee are with the School of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 305-701, Korea (e-mail: [email protected]).

Fig. 1. The proposed self-restorable WDM-PON using 1 x 2 optical switches.

is negligible and these schemes can be adapted to various PON architectures with color-free sources. II. CONFIGURATION OF SELF-RESTORABLE WDM-PONS Fig. 1 shows schematic diagram of the proposed self-restorable WDM-PON. It comprises an OLT located in a central office (CO), a remote node (RN), and N ONUs. The OLT and RN are connected with two feeder fibers, one for working and the other for protection. On the same principle, the RN and each ONU are also connected with two distribution fibers. It should be noted that duplication of fibers (both feeder and distribution) requires route diversity for protection from fiber cable failure. The OLT is composed of two broadband light sources (BLS), two 1 x 2 optical switches connected to the feeder fibers, a fiber fault monitor, a 2 x 2N AWG, N transceivers, N 1 x 2 optical switches connected to transceivers, and N fiber fault monitors for the control of N switches. The two BLS operate at different wavelength bands separated by the free spectral range (FSR) of employed AWGs. The RN consists of a same 2 x 2N AWG as the OLT has. Each ONU has a transceiver (TRx), a 1 x 2 optical switch, and a fault monitor. A color-free optical source can be obtained at both OLT and ONU by using wavelength locked Fabry-Perot laser [8]. Under normal operation, signal is transmitted along a working feeder fiber and a working distribution fiber. In this condition, the output of BLS comes into the first input port of AWG and wavelengths of the spectrum-sliced output are {λ1, λ2, … , λ2N}. Then, assigned wavelengths to N ONUs for downstream and upstream are {λ1,D, λ3,D, … , λ2N-1,D} and { λ1,U, λ3,U, … , λ2N-1,U }, respectively, as shown in Fig. 2(a). If there is a fault in the working feeder fiber, however, the states of two 1

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Fig. 2. Wavelengths assignment under (a) normal state and (b) protection state.

x 2 optical switches associated with the BLS at OLT are changed, and then signal is transmitted along a protection feeder fiber and a working distribution fiber. In this condition, the output of BLS comes into the second input port of AWG and wavelengths of the spectrum-sliced output become {λ2, λ3, … , λ2N+1}. Then, the allocated wavelengths for downstream and upstream shift to { λ2,D, λ4,D, … , λ2N,D } and { λ2,U, λ4,U, … , λ2N,U }[6]. If the BLS come into first input ports of AWGs, the wavelengths of the first output ports are {λ1,D} and {λ1,U}. If the BLS come into second input ports of AWGs, the wavelengths of the first output ports are {λ2,D} and {λ2,U}, as shown in Fig. 2(b). In this case, fault monitors at ONUs can detect the failure occurring at the feeder fiber and may change to protection mode. For deterring this unwanted activation, the fault monitors should wait over the period of the feeder fiber restoration process. It may be noted that the activation of optical switches at the OLT for each transceiver can be avoided by using feeder fiber fault signal. On the other hand, when a distribution fiber is broken, the optical switches corresponding to the ONU and the transceiver of the OLT are reconfigured to the protection path. In this case, as mentioned above, the activation of optical switch at the ONU should wait for a certain time (the feeder fiber restoration time) at first. If there is not restoration of the optical signal after the waiting time, the fault monitors activates optical switch. Then, a communication path via the working feeder fiber and the protection distribution fiber is formed. For example, if the working distribution fiber of ONU-i is cut, the optical switches at ONU-i and at i-th transceiver at the OLT are reconfigured. As a result, the wavelengths for downstream and upstream shift from {λ2i-1,D, λ2i-1,U} to {λ2i,D, λ2i,U}. Finally, when both the feeder fiber and a distribution fiber are failed, configuration of the optical switches associated with the BLS and those corresponding to both the ONU and the transceiver of the OLT are changed. For example, if the working feeder fiber and the working distribution fiber for ONU-j are cut, the optical switches connected to the feeder fiber and the optical switches at ONU-j and j-th transceiver of the OLT are triggered. As a result, the wavelengths for downstream and upstream shift to { λ2j+1,D, λ2j+1,U }.

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Fig. 3. The proposed self-restorable WDM-PON using Ethernet switches.

Besides the protection in the physical layer, it is also possible to provide a protection function with help of an established protocol in a higher layer. The proposed architecture is shown in Fig. 3. At the ONU side, an optical switch and a transceiver was replaced with two transceivers. We applied the same modification at the OLT side. In addition, the Ethernet switch has two input ports to provide link aggregation control protocol (LACP) described in Ref. [9]. Thus in normal operation, each ONU enjoys two times bandwidth assigned to each channel with help of LACP [9]. This condition is maintained even under the failure of the feeder fiber. In this case, bandwidth of each ONU is not changed. However, the bandwidth is decreased by half for the ONU which experiences a failure in the working distribution fiber. This scheme can work, since it is a subset of Fig. 1 in terms of optical architecture, and the link aggregation in Ethernet switch is well established in most commercial switches [9]. The signal path only that has experienced a fault in the fiber is switched to the protection path according to the above protection scenarios, while the other paths maintain normal operation. In this way, impact of a fault in a particular path is localized only to the signal transmission associated with it. This is an important feature to have high availability of the network for high QoS. This feature can be compared with architectures where the impact of a fault influences to other channel through protection procedures [4-7]. III. EXPERIMENTAL RESULTS AND ANALYSIS A. Experimental results The proposed scheme illustrated in Fig. 1 is demonstrated with a 32-channel WDM-PON. The WDM-PON is based on wavelength locked F-P LDs [10]. Each channel was modulated at 1.25 Gbps with a 231-1 pseudo-random bit sequence (PRBS) data. Flattop type cyclic AWGs at the CO and RN have a 100 GHz (0.8 nm) channel spacing and a free spectral range (FSR) of 55 nm. The ONU can be accommodated up to 16, since we have 32-channel in one direction. The output power of wavelength locked F-P LD is about 1 dBm. And the insertion loss of 15 km SMF, two AWG, four optical switches, four WDM filter, and a circulator are 3 dB, 15

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Fig. 4. Measured switching time to restore (a) the feeder fiber failure, (b) the distribution fiber failure (the waiting time is not included.).

Fig. 6. The schematic architectures for calculation of the availability of (a) the WDM-PON, and self-restorable WDM-PON illustrated in (b) Fig. 1 and (c) Fig. 3.

ONU 4

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Fig. 5. Measured BER curves of upstream signals.

dB, 2 dB, 2 dB, and 1 dB, respectively. This results in the total insertion loss of 23 dB. Because the sensitivity of the photodiode at 1.25 Gbps is -26 dBm, we have the optical power margin of 4 dB. It may be noted that the insertion loss of the AWG is almost independent of the number of output ports. The measured protection characteristics are shown in Fig. 4(a) and (b). The fiber fault was simulated by using an optical switch connected to the working feeder fiber and working distribution fiber. The upper line shows the fault detection signal at the fault monitor, and the lower line means the electric signal converted by the receiver at the ONU. The break of fiber-link was restored within 10 msec. To investigate transmission characteristics, we also measured the bit-error-rate (BER) performance for upstream traffic in the normal state and the restoration state for ONU 4, 8, and 12 as shown in Fig. 5. In all cases, the power penalty was less than 1 dB without any sign of error floor. B. System availability We also quantified the performance of the proposed scheme by investigating system availability that is defined as the part of time, during which the system is running. The approximate equation of system availability A can be expressed as [11] U = MTTR/MTBF (1) (2) A = 1−U , where MTTR stands for mean-time-to-repair, MTBF stands for mean-time-between-failure. Reference models for calculation of system availability are shown in Fig. 6. The values for the MTBF and the MTTR of each element are given in Table 1. These values are based on the variables in the Ref. [12-13].

For a conventional WDM-PON not employing the proposed protection scheme, the overall unavailability of a channel is Uoverall = UOLT + UFF + UAWG + UDF + UONU. Because an OLT is composed of a transceiver, a WDM filter, an AWG, two BLS, and a bidirectional coupling device (CD) which consists of 2 circulators and 2 WDM filters, the unavailability of OLT is UOLT = UTRx + UWDM + UAWG + 2UBLS + UCD. Therefore, the overall unavailability and the overall availability for a single channel is 830.8×10-7 and 0.999917, respectively. However, in the proposed self-restorable WDM-PON in Fig. 1, the overall unavailability is reduced due to the parallel connection of fibers. It may be noted that the fiber is the most effective component in the calculation of the overall availability. We have the unavailability of the OLT as UOLT = UTRx + UWDM + UAWG + 3UOS + 2UBLS + UCD. Then, the overall unavailability is Uoverall = UOLT + UFF2 + UAWG + UDF2 + UOS + UONU = 162×10-7 and the overall availability is 0.9999838. The availability of the system is increased by factor of 5 by applying the proposed protection scheme shown in Fig. 1. Similarly, the availability of the system of Fig. 3 is calculated. Because the unavailability of the OLT is UOLT = (UTRx + UWDM)2 + UAWG + 2UOS + 2UBLS + UCD, the overall unavailability is Uoverall = UOLT + UFF2 + UAWG + UDF2 + UONU2 = 123.9×10-7 and the overall availability is 0.9999876. The availability of the system of Fig. 3 is improved by factor of 7 with protection. TABLE I. COLLECTED UNAVAILABILITY NUMBERS OF THE ELEMENTS FIT MTBF MTTR Elements (failures / Unavailability (years) (hours) 109hours) Tx 186 630 2 3.72×10-7 Rx 70 1630 2 1.4×10-7 WDM filter 498 229 2 9.96×10-7 Optical Swtich 200 570 2 4×10-7 AWG 200 570 2 4×10-7 Circulator 200 570 2 4×10-7 BLS 2000 57 2 40×10-7 Feeder Fiber (20km) 2280 50 24 547.2×10-7 Distribution Fiber 570 200 24 136.8×10-7 (5km)

C. Comparison of architectures The proposed protection scheme uses duplex fibers and this is inevitable for the route diversity. It may be noted that the

> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < protection is impossible with a signal route for one ONU. Excepting the fiber, we don’t use any duplication of resources including AWGs. When we duplicate TRx in second scheme (Fig. 3), we double the offered data rate for each ONU. In order to show the simplicity of the proposed architecture, we compared the proposed schemes to Ref. [4, 5, 7] as shown in Table 2. Compared with other schemes, the proposed schemes have following advantages. The proposed protection schemes use small number of AWGs and don’t use additional passive components such as power splitters and WDM filters which add insertion loss. We may use optical couplers to remove the active components in ONUs, but this makes extra 3 dB loss resulting in the degradation of the quality of signal. Because ONUs can provide the electric power, and the required power for monitoring and switching is very low, the employment of active components in ONUs may not be an issue. The proposed protection schemes can restore feeder and distribution fiber failure simultaneously. That is, even if the working feeder fiber and all working distribution fibers are cut at the same time, the services can be recovered in a short period. In addition, the restoration process of a distribution fiber does not have an effect on the signal traffic of other ONUs. In the structure of Fig. 2, the BLS power may be considered wasteful because the half of the BLS power goes to unconnected output ports of the AWG. However, as mentioned above, the route diversity is essential to protection. When someone uses one output port of AWG, a 3-dB coupler is needed. Then, it is required to increase BLS power by 3 dB to maintain the injection power. In this case, the optical transmission path has 3-dB more loss. TABLE II. THE NUMBER OF ELEMENTS IN SELF-RESTORABLE ARCHITECTURES

ONU Transceiver AWG size Feeder fiber (FF) Distribution fiber (DF) Interconnection fiber # of Wavelength Optical switch Splitter WDM filter Ethernet Switch Data rate per ONU in normal state of DF Insertion loss for protection Protection of DF with FF cut Isolation of other ONUs from a DF cut

Ref. [4] N 2N 2xNx3

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The protection scheme illustrated in Fig. 3 does not need any optical switch for distribution fiber protection though it requires Ethernet switches. It may be noted again, this scheme provides double data rate both in normal mode and protected mode of feeder fiber. In addition, our schemes employ similar number of feeder fibers and distribution fibers. To make the architecture in Ref. [7] working properly the OLT has to contain extra WDM filters, and optical switches. IV. DISCUSSION AND CONCLUSION We demonstrated the proposed self-restorable WDM-PON based on wavelength-locked FP-LDs at 1.25 Gbps for both the feeder fiber and distribution fiber faults as illustrated in Fig. 1. A color-free optical source was used for the proposed architectures in conjunction with 2 x 2N AWGs to serve N customers. The protection can be done only for the ONUs affected by the failure. Otherwise, there is no service interruption for the rest ONUs. The protection time was less than 10 msec. The measured power penalty was less than 1 dB. It is also possible to provide two times bandwidth, when we do protection in higher layer with an Ethernet switch as shown in Fig. 3. It is also noted that the proposed architecture can be applied to WDM-PON with any type of color-free optical sources. Finally, the overall availability was increased about 5 and 7 times by the proposed protection function. REFERENCES [1]

C. –H. Lee, Wayne V. Sorin, and B. Y. Kim, ”Fiber to the home using a PON infrastructure,” J. Lightw. Technol., vol.24, no.12, pp.4568-4583, Dec. 2006. [2] K. Lee, S. B. Lee, J. H. Lee, Y. –G. Han, S. –G. Mun, S. –M. Lee, and C. –H. Lee, “A self-restorable architecture for bidirectional wavelength-division-multiplexed passive optical network with colorless ONUs,” Opt. Express, Vol.15, No.8, pp.4863-4868, Apr. 2007. [3] Z. Wang, X. Sun, C. Lin, C. –K. Chan, and L. –K. Chen, “A Novel Centrally Controlled Protection Scheme for Traffic Restoration in WDM Passive Optical Networks,” IEEE Photon. Technol. Lett., vol. 17, no. 3, pp.717-719, Mar. 2005. [4] X. Sun, C. –K. Chan, and L. K. Chen, “A survivable WDM-PON Architecture with Centralized Alternate-Path Protection Switching for Traffic Restoration,” IEEE Photon., Technol., Lett., vol. 18, No. 4, pp.631-633, Feb. 2006. [5] Arshad Chowdhury, M. –F. Huang, H. –C. Chien, Georgios Ellinas, and G. –K. Chang, “A Self-Survivable WDM-PON Architecture with Centralized Wavelength Monitoring, Protection and Restoration for both Upstream and Downstream Links,” in Proc. Opt. Fiber Commun. Conf., San Diego, 2008, Optical Society of America, Paper JThA95. [6] K. Lee, S. –G. Mun, C. –H. Lee, and S. –B. Lee, “Reliable Wavelength-Division-Multiplexed Passive Optical Network Using Novel Protection Scheme,” IEEE Photon. Technol. Lett., vol. 20, no. 9, pp.679-681, May. 2008. [7] J. Chen, L. Wosinska, and S. He, “High utilization of wavelengths and simple interconnection between users in a protection scheme for passive optical networks,” IEEE Photon. Technol. Lett., vol. 20, no. 6, pp389-391, March 2008. [8] H. D. Kim, S. –G. Kang, and C. –H. Lee, “A low-cost WDM source with an ASE injected Fabry-Pérot semiconductor laser,” IEEE Photon. Technol. Lett., vol. 12, no. 8, pp.1067-1069, Aug. 2000. [9] IEEE Standard for Information Technology, IEEE Standard 802.3 ad-2000. [10] S. –G. Mun, J. –H. Moon, H. –K. Lee, J. –Y. Kim, and Chang-Hee Lee, “A WDM-PON with a 40 Gb/s (32 x 1.25 Gb/s) capacity based on wavelength-locked Fabry-Perot laser diodes,” Opt. Express, Vol. 16, No. 15, pp.11361-11368, 2008. [11] S. Verbrugge, D. Colle, P. Demeester, R. Huelsermann, and M. Jaeger, “General availability model for multilayer transport networks,” in

> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < Proceedings of 5th International Workshop on Design of Reliable Communication Networks (DRCN2005), pp. 85–92, 2005. [12] S. Verbrugge, D. Colle, M. Pickavet, P. Demeester, S. Pasqualini, A. Iselt, A. Kirstädter, R. Hülsermann, F. –J. Westphal, and M.Jäger, “Methodology and input availability parameters for calculating OpEx and CapEx costs for realistic network scenarios,” J. Opt. Netw.,Vol. 5, No. 6, pp.509-520, June 2006. [13] J. Chen, and L. Wosinska, “Analysis of protection schemes in PON compatible with smooth migration from TDM-PON to hybrid WDM/TDM-PON,” J. Opt. Netw., Vol. 6, No. 5, pp.514-526, May 2007.

Joon-Young Kim received the B.S. degree in information and communication engineering from Inha university, Incheon, Korea, in 2007. He is currently working in a M.S.-Ph.D. joint program at Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea. His research interest includes lightwave systems and optical access network based on WDM-PON.

Hoon-Keun Lee received the B.S. degree in electronics from Kyungpook National University, Daegu, Korea, in 2006. He is currently in a M.S.-Ph.D. joint program at Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea. His research interest includes lightwave systems and optical access network based on WDM-PON.

Sil-Gu Mun received the B.S. degree in electronics from Kyungpook National University, Daegu, Korea, in 2003 and the M.S. degree in electrical engineering from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2005. She is currently working toward the Ph.D. degree in optical communication systems at KAIST. Her research interest includes lightwave systems and high-speed optical access network.

Chang-Hee Lee (S’82–M’90-SM’08) was born in 1961. He received the M.S. and Ph.D. degrees from the Department of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejon, Korea, in 1983 and 1989, respectively. He became a Member (M) of IEEE in 1990, a Senior Member (SM) in 2008. He spent a year at Bellcore (Bell Communications Research) as a PostDoc. He worked with the Electronics and Telecommunications Research Institute, from 1989 to 1997, as a Senior Researcher. Since 1997, he has been a Professor with KAIST. His major interest is optical communications and networks. He has spent over 20 years as an Engineer in the area of optical communications, including semiconductor lasers. He was Technical Leader for 2.5- and 10-Gb/s optical-transmission-system development, including optical amplifiers at the Electronics and Telecommunications Research Institute. He is an author of 170 journal and conference papers. He is the holder of 23 U.S. patents and more than 50 additional patents are pending in the U.S.

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