C
+ L band wavelength division
multiplexing access network with
distributed-controlled protection
architecture
Chien Hung Yeh
Chi Wai Chow
C
+ L band wavelength division multiplexing
access network with distributed-controlled
protection architecture
Chien Hung Yeh
Industrial Technology Research Institute (ITRI) Information and Communications Research
Laboratories
Chutung, Hsinchu 31040, Taiwan E-mail: [email protected]
Chi Wai Chow
National Chiao Tung University
Department of Photonics and Institute of Electro-Optical Engineering
Hsinchu 30010, Taiwan
Abstract. In this work, we propose and experimentally demonstrate a novel distributed-controlled protection architecture for automatic and fast network restoration in wavelength division multiplexing-passive optical network (WDM-PON). The proposed scheme can support both C and L bands. Besides, duplication of network equipments, such as optical networking unit (ONU) or optical line terminal, is not required. In this distributed-controlled system, each ONU can always keep track of the network status. Hence, this can facilitate the network manage by removing the work loads from the central office. Besides, the proposed scheme can tolerate simultaneous fiber cuts in the feeder and distributed fibers.
C
2011 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.3658759] Subject terms: distributed controlled; self-protection; WDM-PON.
Paper 110750RRR received Jul. 1, 2011; revised manuscript received Sep. 22, 2011; accepted for publication Oct. 17, 2011; published online Nov. 18, 2011.
1 Introduction
The wavelength division multiplexing-passive optical net-work (WDM-PON) is a promising candidate to deliver high capacity broadband services to business and residential subscribers.1 As end-users are demanding faster and higher quality broadband services, reliable WDM-PON are highly desirable in the near future. However, when a fiber cut oc-curs in the distributed fiber [fiber between the remote node (RN) and the optical network unit (ONU)] or in the feeder fiber [fiber between the central office (CO) and the RN], the ONU will become unreachable from the CO, leading to data loss. Therefore fault management is one of the important is-sues to improve the network reliability. Recently, several net-work protection architectures have been proposed.2–7 They usually require duplicated network resources, such as ONU and optical line terminal (OLT) for the network protection. References8and9require active electrical control and man-agement at the RN and at the OLT in the CO, respectively, for optical path protection against fiber fault. Reference10
requires wavelength tunable laser source at each ONU. The tunable laser can simultaneously provide an upstream signal and optical path restoration using wavelength switching.
In this work, we propose and experimentally demonstrate a novel distributed-controlled protection architecture for the WDM-PONs. Our proposed scheme can also support both C-and L-bC-ands. It can also tolerate simultaneous fiber cuts in the feeder and distributed fibers. In the proposed scheme, each ONU can keep track of the network status, and active control in the RN and OLT is not required when compared with Refs.8and9. Hence, this can facilitate the network manage by removing the work loads from the OLT. Besides, tunable laser sources for optical path restoration are not required when compared with Ref.10.
The paper is organized as follows: in Sec.2, the network protection architecture is proposed. The operation principle for protection in feeder or distribution fibers is discussed. In
0091-3286/2011/$25.00C2011 SPIE
Sec.3, the experimental results showing the performance or the protection scheme are provided. A comprehensive com-parison of our proposed scheme with other recently reported schemes in the literatures are also presented. Finally, a con-clusion is given in Sec.4.
2 Architecture
To improve the bandwidth utilization of the optical fiber, both C- and L-bands will be used in the proposed WDM-PON, as shown in Fig.1. Figure2shows the network architecture of the protection scheme. In the CO, there are two line terminals (LTs) for the C-band (LTC) and L-band (LTL), respectively. In each LT, the three-port optical circulator (OC) is used to separate the upstream and downstream signals. The down-stream signal from the LT is divided into two paths by a 1×2 optical coupler (CP) and transmitted into the normal fiber (path 1) and protection fiber (paths 1). The RN con-sists of two 1×2 C-/L-band WDM couplers (WC) and two 1×N arrayed waveguide gratings (AWGs). The WC inside the RN can combine the C- and L-band optical downstream signals sent from LTC and LTL, respectively. The common port of the WC is connected to the AWG. Although the upper AWG is used to connect the ONUs in C-band while the lower AWG is used to connect the ONUs in L-band, the two AWGs can operate in C+ L band with the same spectral periodicity property.
Two ONUs (ONUCi and ONULi, i= 1, 2, 3, . . . , N) are assigned to act as a group, one operates in C-band and the other operates in L-band. The proposed wavelength assign-ments for the downstream and upstream channels are shown in Fig. 3. The downstream wavelengths (Cdi, Ldi) and the upstream wavelengths (Cui, Lui) in the i’th ONU group (for i = 1, 2, 3, . . . , N) are assigned with the wavelength separa-tion by one free spectral range of the AWG, as illustrated in Fig.3. This means that one port of the AWG can support the four wavelengths simultaneously due to the spectral period-icity property.
Yeh and Chow: C+ L band wavelength division multiplexing access network...
Fig. 1 Schematic of the proposed C+ L band WDM-PON.
Inside each ONU, a C-band optical coupler (CCP) or L-band optical coupler (LCP), a 1×1 optical switch (OS), and a C+ L band 2×2 CP are used in front of the transmitter and the receiver (Rx), as shown in the inset of Fig.2. The CCP in the C-band ONU can remove the L-band downstream signal Ldi, while the LCP in the L-band ONU can remove the C-band downstream signal Cdi.
In the normal state (there is no fiber cut), the two OSs in each ONU pair are “OFF.” The OS is controlled by medium access control (MAC) layer. The downstream and upstream traffics are transmitting in fiber paths 1 and 2 while the signals in fiber paths 1and 2are blocked by the OS in each ONU in the normal state.
When a fiber cut occurs on the distributed fiber between the RN and ONUC1 as shown in Fig.4, the data traffic be-tween the CO and ONUC1 will be disconnected. Since the
Fig. 2 The proposed distributed-controlled protection WDM-PON ar-chitecture.
Fig. 3 The wavelength assignment plan for the upstream and down-stream signals.
Rx of the ONUC1 cannot receive the downstream signal, then the OS will be switched “ON” by the MAC. Hence the ONUC1reconnect both the downstream and upstream signals through the partner ONUL1via the protecting fiber on path 1, as shown in the inset of Fig.4. Without the loss of gen-erality, the same automatic re-routing mechanism can also be applied to the ONUL1while the fiber cut is located in the distributed fiber connecting the L-band ONUs.
When a fiber cut occurs in the feeder fiber path 1 between the RN and CO, as shown in Fig.5. All C-band ONUCs can-not detect the downstream signals from CO. Thus, the OS in each ONUCwill switch “ON” for signal reconnection via the corresponding ONULs through the protection fiber on path 1. Without the loss of generality, the same automatic re-routing mechanism can also be applied when the fiber cut occurs in feeder fiber path 2 for the network restoration. It is also worth mentioning that the proposed scheme can tolerate simultane-ous fiber cuts in both the feeder and distributed fibers. When fiber cuts occur in both the feeder and distributed fibers, all the C-band ONUCs cannot detect the downstream signals from CO. Thus, the OS in each ONUCs will switch “ON” for signal reconnection via the corresponding ONULs through the protection fiber on path 1.
Fig. 4 A fiber cut occurs at the distributed fiber between the RN and the ONUC1, and the network restoration path.
Fig. 5 A fiber cut occurs at the feeder fiber between the CO and the RN, and the network restoration path.
3 Experiments and Discussion
To investigate the performance of the proposed WDM-PON, we selected four wavelengths to emulate the downstream and upstream wavelengths for ONUC1 and ONUL1. In this experiment, the wavelengths of Cd1, Cu1, Ld1, and Lu1 are 1546.0, 1548.0, 1562.0, and 1564.0 nm. The total length of the feeder and distributed fibers is 20 km. The fiber con-necting the ONU pair is 2 km. All the fibers are standard single mode fiber (SSMF). Both the downstream and up-stream signals are modulated at 10 Gb/s nonreturn-to-zero format with 231to 1 pseudorandom binary sequence data via a LiNbO3 Mach–Zehnder modulator. Optical preamplified Rx using erbium-doped fiber amplifier (EDFA) was used for the bit error rate (BER) analysis. Figures6(a)and6(b)show the BER performances of downstream and upstream signals between the CO and the ONUC1 back-to-back without fiber transmission, in the normal state [20 km single mode fiber (SMF) transmission] and the protection state (22 km SMF transmission). Figure6(a)shows the measured power penal-ties between CO and ONUC1are less than 0.5 dB at a BER of 10− 9without and with fault cut. Figure6(b)also shows the measured power penalties are about 0.8 dB at a BER of 10− 9. The larger penalty of Fig. 6(b)is due to the amplification limitation of the EDFA used in the experiment. Moreover, the insets of Fig.6(a) are the measured corresponding eye diagrams of downstream and upstream signals at the BER of 10− 9with and without protecting operation after 20 and 22 km fiber transmissions, respectively. These measured eyes are wide opening and clear.
Besides, the automatic switching time was also measured, and the switching time of the OS is about 10 ms as shown in Fig. 7. By considering the insertion losses of the OS (∼0.5 dB), an AWG (∼5 dB), two CPs (∼6 dB), a WC (<1 dB), an OC (<0.5 dB), and the 22 km SSMF (0.2 dB/km), the total loss budget is about 18 dB.
In the proposed scheme, each ONU can always keep track of the network status and control its own OS for network restoration. Hence this is a distributed-controlled system.
Fig. 6 BER performances of downstream and upstream traffic (a) between CO and ONUC1(C-band) and (b) between CO and ONUL1
(L-band) with and without fault protection, respectively. The insets of (a) are measured corresponding eye diagrams for downstream and upstream signals with and without protection, respectively.
This can facilitate the network manage by removing the work loads from the CO. Besides, the proposed scheme can tolerate simultaneous fiber cuts in the feeder and dis-tributed fibers. In the experiment, we only demonstrated four wavelengths. In practice, the proposed scheme can support 80 WDM wavelengths using both C+ L bands (40 wave-lengths in the C-band and 40 wavewave-lengths in the L-band). The number of supported wavelengths can be further increased if dense WDM (DWDM) is used.
We have experimentally compared our proposed scheme with other recently reported schemes in the literatures as shown in Table 1. The yellow shaded boxes are the ad-vantages provided by the scheme. The experimental results showed that our proposed scheme has a higher power penalty between working and protection states (∼0.8 dB), while other schemes in Refs.9and10and in Ref.8have about 0.5 dB and negligible power penalty, respectively. Our proposed scheme also has a longer switching time between the two states (10 ms), while other schemes8–10 have the switching time from 1 to 8 ms. The switching time can be improved by
Yeh and Chow: C+ L band wavelength division multiplexing access network...
Table 1 Comparison of our proposed scheme with other recently reported schemes.
Protection of both feeder and distribution fibers Active control between CO and ONU Power penalty between working and protection states Switching time (ms) Duplication of ONU and OLT for protection
Requirement of tunable laser in ONU
Operation
Bandwidth Additional cost Proposal by
KAIST, et al. (Ref.8)
Yes Yes negligible 1 No No C-band Optical switches
and RN control unit Proposal by CUHK (Ref.9) No No ∼0.5 dB 5 No No C-band Mach-Zehnder modulator with 40 GHz clock source at the CO Proposal by Georgia Tech. et al. (Ref.10)
Yes No ∼0.5 dB 8 No Yes C-band High speed
tunable laser at each ONU Our proposed scheme Yes No ∼0.8 dB 10 No No C-plus L-bands Optical switches, couplers and fiber
paths
using a better optical switch. Our scheme can protect both feeder and distribution fibers, and it does not require active control between the CO and ONU. We can also see that the additional cost of our proposed scheme is due to the optical switches, fiber couplers, and fiber paths connecting adja-cent ONUs. We think that this additional cost is worthy for the protection. We also believe that the cost could be lower for the scheme requiring a Mach–Zehnder modulator and 40 GHz clock source,9 and the scheme using high speed tunable laser at each ONU.10 Moreover, other protection schemes8–10 could only be employed in C-band for WDM operation. For example, 40 wavelength channels can be used in the C-band (1530 to 1560 nm) for WDM transmis-sion while the channel spacing is 100 GHz (0.8 nm). Here, our proposed PON architecture can also support both C- to L-bands (1530 to 1600 nm) for providing twice the capacity when compared with Refs.8–10.
4 Conclusion
We proposed and experimentally demonstrated a novel distributed-controlled protection architecture for automatic and fast traffic restoration against fiber cut in WDM-PONs. Duplication of network equipments, such as ONU or OLT, is not required in the proposed scheme. Power penalties of <1 dB at BER of 10− 9was measured in the proposed tection scheme. The switching time between normal and pro-tection states was about 10 ms. In this distributed-controlled system, each ONU can always keep track of the network sta-tus, and control its own OS for network restoration. Hence this can facilitate the network manage by removing the work loads from the CO. Besides, the proposed scheme can tol-erate simultaneous fiber cuts in the feeder and distributed fibers.
References
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Chien Hung Yeh received his PhD degree from the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan, in 2004. His research interests are optical fiber communication, fiber laser, optical fiber amplifier, optical sensor, terahertz application, long-reach PON, and ROF-PON access. He is a lead researcher with the Information and Communications Research Laboratories, Industrial Technology Research Institute (ITRI), Taiwan, for leading the Advanced Project of Optical Access Network.
Chi Wai Chow received the BEng (First-Class Hons) and PhD de-grees both from the Department of Electronic Engineering, the Chi-nese University of Hong Kong in 2001 and 2004, respectively. After graduation, he was appointed as a postdoctoral fellow at the CUHK, working on hybrid integration of photonic components and silicon waveguides. Between 2005 and 2007, he was a postdoctoral re-search scientist, working mainly on two European union projects: PIE-MAN (Photonic Integrated Extended Metro and Access Network) and TRIUMPH (Transparent Ring Interconnection Using Multi-wavelength Photonic switches) in the Tyndall National Institute and Department of Physics, University College Cork in Ireland. In 2007, he joined the Department of Photonics, National Chiao Tung University in Tai-wan. He is an associate professor with the Department of Photonics, National Chiao Tung University, Taiwan.
