Proposal of Stackable ROADM for Wavelength Transparent IP-over-CWDM Networks

Transcription

1 Proceedings of APC8 copyright 28 IEICE 8 SB 83 Proposal of Stackable ROADM for Wavelength Transparent IP-over-CWDM Networks MD. Nooruzzaman, Yuichi Harada, Osanori Koyama, and Yutaka Katsuyama Graduate School of Engineering, Osaka Prefecture University, Gakuen-cho 1-1, Naka-ku, Sakai, Osaka, Japan {noor, harada, koyama, katsu}@eis.osakafu-u.ac.jp Abstract Stackable ROADMs (s) have been proposed for use in regional IP-over-CWDM networks. The can be constructed by connecting modules with different wavelengths required in the node. The experimental results clarified that the could mux and demux the wavelengths successfully, and gave no limit to the passing-through wavelengths, making the network be wavelength transparent. Contrary to the, the existing fixed ROADMs were not wavelength transparent. I. INTRODUCTION Recently, optical networks have been introduced commonly to satisfy the rapid increase of traffic demands. In addition to the high capacity characteristics of optical fibers, WDM (Wavelength-Division Multiplexing) technologies provide more bandwidth per fiber, and are used in core backbone networks, metropolitan area networks (MANs), and regional area networks. The regional area networks include local networks to connect government departments, research organizations, and local communities, enterprise networks, university campus networks, and so on. In MANs and the regional area networks, wavelength routing capability in WDM networks is also investigated as well as the effective use of the high capacity of fibers. The design procedures for the wavelength-routed networks have been investigated and published. The design issues include how many wavelengths are required to satisfy the traffic demands between nodes with low blocking probability [1]-[2], and to minimize the required node system scale [3]. The need for high bandwidth in today s IP-based Internet accelerated the investigation on IP-over-WDM networks. In an IP-over-WDM network, the nodes employ wavelength routing devices and layer-3 switches (L3SWs) for IP routing. Another design issue is the reconfigurability of optical connections, called lightpaths [4], in WDM networks to groom traffic effectively and for connection provisioning []. The lightpaths can be reconfigured dynamically or statically, depending on the wavelength-routing device. For the reconfigurable purpose, wavelength mux/demux devices with the reconfigurable function were proposed: optical add/drop multiplexers (OADM) with optical switches [4] or with wavelength-tunable devices [6]. The networks for regional access applications can be classified as ring and star topologies [7]. Now the double star topology with optical power splitters is used widely as PON (Passive Optical Network) for FTTH (Fiber-To-The- Home) applications, and the ring topology is dominant [1]-[], [7] for the MAN and WAN applications. In such MAN and WAN applications, lots of business data are transmitted as well as voice data for telephony service. To transmit such business data, cell relay and frame relay services are used in public networks. Recently more IP and Ethernet transmissions are used, because many services are integrated and provided over IP, including business data storage/processing by web servers and VoIP (Voice-over-IP). Moreover, all communication services are going to shift to IP networks as the next generation network. The technical background and the trend motivated us to investigate regional IP-over-WDM networks, used mainly for business as intranet, service provider networks, local government networks, campus networks, and so on. The major traffic in the networks is supposed to be data transmission for computer communication service and voice data for telephony service. The technologies for the regional optical IP networks should be scalable in such a way to be applicable to wide range from a small LAN (Local Area Network) inside buildings to such a wide area network as installed to cover regional areas. The network scale should be variable by the design, depending on the applications. Optical LAN technologies are well developed: Gigabit Ethernet optical transceivers are standardized and CWDM (Coarse WDM) technologies provide multiplexed wavelength transmission in an easy and low cost way, because no temperature control is necessary in CWDM optical transceivers. This means that the optical IP networks can be scalable by adjusting the number of wavelengths and/or optical transceivers. Thus, we investigated scalable optical IP networks for regional area application [8-9]. The issues for such optical IP networks were addressed as follows: The networks should be managed and controlled effectively by a few network administrators, when the traffic change occurs. For the control purpose, suitable technologies are necessary for the reconfiguration of the lightpaths. We developed an effective design and control system [1] for such regional networks. A reconfigurable OADM (ROADM) was also developed [11] for such IP networks, and can be controlled by the developed system. However, the ROADMs had some problems: no reconfigurable wavelengths could be added to the ROADM, if additional wavelengths were required in the network. Therefore, the network scalability was limited by the ROADMs. In this paper, we propose stackable ROADMs (s) to give more flexibility to mange lightpaths in IP-over- CWDM networks. The proposed structure is described in details as well as the design considerations. The experimental results are also described in this paper to show the performance.

2 Proceedings of APC8 copyright 28 IEICE 8 SB 83 1 To Node 1 II. To Other Network SM Fibers ROADM CCW CW Figure 1 PROPOSAL OF STACKABLE ROADM Figure 1 shows an IP-over-CWDM network considered in this paper. Each node represents a user building, and consists of a ROADM, optical transceivers (OTRs) with different wavelengths, and a layer-3 switch (L3SW). The nodes are connected with single-mode (SM) fibers to form an optical ring, as adopted in many WAN and MAN applications. The optical ring is constructed by unidirectional single fibers or bi-directional dual fibers as in Fig.1, depending on the applications. The ROADM has wavelength-dependent couplers, splitters, and 2x2 optical switches (SWs). This is a common ROADM structure [4], and we developed this type of ROADMs for our experiments. In this network, the traffic to/from other networks is transmitted through Node 1, and some lightpaths are designed to connect nodes so as to satisfy the traffic demands between nodes. For example, wavelength is added and dropped in Node to make a lightpath connecting Nodes 1 and. To reconfigure the lightpath, the path of wavelength can be changed so as to pass through the ROADM by the SWs in Node. This change makes the lightpath be reconfigured to connect Node 1 and, say Node 4, when wavelength is added and dropped by SWs in Node 4. Thus, the lightpaths can be reconfigured by the ROADMs in the network. The ROADM could reconfigure lightpaths successfully in CWDM networks. Figure 2 shows the structure of the ROADM used in our experiment. The ROADM could be used for uni-directional transmission, or bi-directional transmission as shown in Fig.1, if a pair of the ROADMs was used in a node [1]. The ROADMs were controlled by receiving control packets via the network interface. The ROADM had some problems. One of the problems is the limited wavelengths in each ROADM: i.e. the 4 wavelengths - can be reconfigured by the ROADM, but no other wavelengths can be reconfigured, if one more wavelength, e.g. is required in the node. The coupler/splitter must be replaced. Therefore, we call this type of ROADM fixed ROADM (F-ROADM). The second problem is that the coupler/splitter should 2 Multiplexed 3 Lightpaths SW Layer-3 Switch Lightpath 4 CCW CW To Node 4 Node Optical Transceiver IP-over-CWDM network considered in this paper mux/demux more wavelengths than the 4 wavelengths - of the ROADM, if other wavelengths are used in the network: e.g. the coupler/splitter should mux/demux 8 wavelengths, if wavelengths - λ 8 are used in other ROADMs. The F-ROADM structure is shown in Fig.3 for such 8-λ transmissions. The 4 wavelengths - can be reconfigured in this F-ROADM, and wavelengths - λ 8 pass through this ROADM. To distinguish the type of the wavelengths, we call the 8 wavelengths - λ 8 working wavelengths (W-λs) in the network, and call such 4 wavelengths as - reconfigurable wavelengths (R-λs) of the ROADM. And we call such 4 wavelengths as - λ 8 passing-through wavelengths (PT-λs) of the ROADM. It is possible to have some wavelengths of the PT-λs to be R-λs in the ROADM, if the ROADM has reserve SWs, and the fibers connecting the coupler and splitter are reconnected so as to connect coupler/splitter and the SWs. Two SWs are reserved in Fig.3, and therefore, 2 of the PT-λs, e.g. and λ 6 can be R-λs in this F-ROADM. We call such wavelengths as and λ 6 reserve wavelengths (RV-λs). As a result, the W-λs are - λ 8 in the network, where the F-ROADM shown in Fig.3 is used, but no other wavelength, e.g. λ 9 can be used in the network. Thus, the F-ROADMs restrict the wavelength transparency of the network, giving the limitation to the network scalability. The R-λs of the F-ROADM are - λ 6, including 2 RV-λs, but no other wavelengths can be reconfigured by this F- ROADM. Thus, the F-ROADMs restrict the lightpath reconfigurability in the node. To remove all the problems of the F-ROADMs, we propose a stackable ROADM (), shown in To Network To L3SW To Network To L3SW Fixed ROADM Control Signal Network Interface Figure 2 Fixed ROADM Figure 3 Control Signal Network Interface SW1 F-ROADM SW1 SW SW6 F-ROADM (R-λs=4 and W-λs=8) λ 6

3 Proceedings of APC8 copyright 28 IEICE 8 SB 83 Fig.4. The can be constructed by connecting stackable modules with different wavelengths. The stackable module consists of an OADM connected by a 2x2 optical SW. The SW has 2 states and, and works in the same way as in the F-ROADM. When muxed lightwaves are input to a common input (CI) port of the S- ROADM, only passes the OADM1 in the first module, and other wavelengths are reflected. When SW1 state is, the passed is dropped, and received by a -OTR. The lightwave from the -OTR is added by SW1, muxed by OADM1, sent to the following modules, reflected by the modules, and sent to the network. When SW1 state is in the first nodule, is sent back to the network: i.e. passes through this. Thus, one module has the reconfiguration function for the specified wavelength. The reflected lightwaves other than at the first module are input to the second module, and is added/dropped (A/D) or passes through (PT) the module, depending on the state of. Thus, each wavelength can be set to be A/D or PT at the module independently, and gave no influence on the other wavelengths. Therefore, can be constructed in a stackable way by connecting the modules with as many different wavelengths as required. Table 1 summarizes the properties, comparing S- ROADM and F-ROADM. The has wavelength transparent (λ transparent) both for W-λs and PT-λs, whereas the W-λs and PT-λs for F-ROADM are limited within the W-λs. The R-λs of the are not limited, and are equal to the λs of connected modules. III. PERFORMANCE OF STACKABLE ROADM The loss properties of and F-ROADM were investigate and compared. Firstly, loss estimation equations were derived as follows. Three kinds of loss affect the transmission between nodes: i.e. the add, drop and PT losses denoted by L ad, L dp and L PT, respectively. The losses depend on the position of the module in the S- ROADM. When a module is located in the n-th position from the CI port, as shown in Fig.4, the losses are given by L ad (n) = (N-n)L r + L p + L SW (1-1) L dp (n) = (n-1)l r + L p + L SW (1-2) L PT (N) = NL r, (1-3) where L r and L p are the reflection and passing losses at/through the OADM, respectively, and N is the total module number connected in the. The loss of the SW is denoted by L SW, assuming that the losses are the same value for the and states. In the equations the losses are assumed to be independent of wavelength. The losses of the F-ROADM can be estimated by the similar equations, because the coupler and the splitter have optical bandpass filters which can mux or demux one wavelength at each filter in the similar way as the connected OADM in the. Therefore, the estimation equations for the F-ROADM are given by L ad (n) = (W-n)L r + L p + L SW (2-1) L dp (n) = (n-1)l r + L p + L SW (2-2) L PT (W) = (W-1)L r + 2L p (2-3) Here, W denotes the W-λ number in the network, and primes denote the values of the coupler/splitter. The values of L r and L p were measured for 4 sets of the OADMs used in the, and the values were obtained by averaging the 4 values at each λ of the 8 wavelengths from 147 nm to 161 nm. The wavelength dependence of the averaged values is found to be small. Therefore, L r =.41dB, and L p =.38dB were obtained by averaging all the values. The loss L SW =.684dB was obtained by averaging measured losses of SWs. In the same way, L r =.22dB, and L p =.38dB were obtained for the 3 sets of coupler and splitter in the F-ROADM. Figure shows optical loss properties of the S- and F- ROADMs, when the numbers of R-λs and W-λs are 4 and 8, respectively. The 4 R-λs are the standard values from =147nm to =13nm, and the number m in Fig. denotes λ m. The was constructed by connecting the 4 modules as shown in Fig.4, and the add and drop losses were measured. The results are plotted in Table 1 Comparison between 2 types of ROADMs Wavelength F-ROADM W-λ λ Transparent Limited within W-λs No Limitations Limited within R-λs R-λ R λs = wavelengths of Possible to add RVλs, Connected Modules if Reconnected PT-λ λ Transparent Limited within W-λs RV-λ Not Necessary Limited within W-λs 3 Optical n = 1 n = 2 n = 3 n = CI CO Stackable Module OADM1 SW1 Drop λ1 OADM2 λ2 OA DM3 λ3 OA DM4 λ4 Figure 4 Add Eq.(1-2) Eq.(2-1) Eq.(2-2) 1 Eq.(1-1). Measured F-ROADM Number of Wavelengths m Figure Optical loss comparison between and F-ROADM To L3SW λ1 λ2 λ3 λ4 Network Interface

4 Proceedings of APC8 copyright 28 IEICE 8 SB 83 Fig.. The calculated losses L ad (m) and L dp (m) by Eqs.(1-1) and (1-2) are also shown in Fig.. In the same way, the F-ROADM was constructed as shown in Fig.3, and the add and drop losses were measured. The measured values are plotted in Fig., together with the calculated lines by Eqs.(2-1) and (2-2). It is found that the measured and calculated values are consistent with each other for S- and F-ROADMs, respectively, and the maximum losses are comparable between S- and F-ROADMs. The add losses of F-ROADM are higher than those of, whereas the drop losses of F-ROADM are lower than those of. The coupler/splitter of the F- ROADM must mux/demux 8 wavelengths, because the number of W-λs is required to be 8. Therefore, W=8 in Eq.(2-1) gives higher add loss. On the other hand, the angle of inclination for loss lines is steeper than that of F-ROADM loss lines, due to the large reflection loss L r of OADM in. This is because the reflection loss L r of contains the reflection loss at the OADM and the connection loss between modules. The connection loss by connectors in the is higher than the loss connecting adjacent bandpass filters in the waveguide structure of the coupler/splitter for the F-ROADM. Therefore, the maximum losses are comparable between S- and F- ROADMs in the present case, but the maximum loss of S- ROADM shall be higher than that of F-ROADM, if numbers of R-λs and W-λs are 8. As the next experiment, 2 sets of the were constructed by connecting 4 modules with wavelengths from =149 to =1nm, respectively. The states of the 4 SWs were set to be, and the optical loss properties were evaluated. Firstly, wavelength dependence of the optical loss was measured for the first adding path, i.e. between the add port for and the common output (CO) port of the. The result is shown in Fig.6. The other losses for adding paths were also measured, and the results are also shown in Fig.6. It is found that the 4 low-loss windows to add 4 wavelengths were obtained, showing that the add losses decrease slightly with increasing the wavelength number, as expected by Eq.(1-1). Fig.6 also shows that the isolations of around 3 db were obtained by the modules for other wavelengths than the add wavelength. Four OTRs with wavelengths - were connected to the add ports, respectively, and the wavelength dependence of the output optical power was measured. The result is shown in Fig.7. It is seen clearly that the 4 wavelengths could be multiplexed by the S- ROADM. The optical powers increase with increasing the wavelength number, consistent with the loss decreasing property of the. To examine the demultiplexing properties of the S- ROADM, the drop loss was measured for the path from the common input port CI to the drop port (DP) of the module in another, having the SW of the module set to be sate. The measured loss is shown in Fig.8. The low-loss window for 1nm is obtained. The multiplexed lightwaves by the first were input to the second. The wavelength dependence of the output power from the drop port was measured. The result is shown in Fig.9, clarifying that could be demultiplexed successfully by the. The passing-through (PT) properties of the were evaluated. Figure 1 shows the measured loss Optical Power (dbm) Optical Power (dbm) Figure 7 Measured multiplexing properties of Figure 9 Input Port= Add for Figure 6 Input Port=CI Output Port=CO Measured add losses of Output Port =CO Input Port=CI Output Port= DP for Wavele ngth (nm) Figure 8 Measured drop Loss of Measured demultiplexing property by between CI and CO ports of the first, when all the SW states were set to be. It is found that the losses at the 4 R-λs were around 3 db, showing that the 4 wavelengths were dropped. It is also found that smaller PT loss ranges were obtained at wavelengths below =

5 Proceedings of APC8 copyright 28 IEICE 8 SB nm, and over λ 6 =17nm. The PT losses were almost constant, being about db. The values agree with the estimated value L PT (4)= 4L r =1.8dB, given by Eq.(1-3). Thus, the gives no restrictions to the transmissions of the lightwaves with other wavelengths than the 4 R-λs of the, showing that PT transmissions are λ transparent. The same PT properties were measured for the F- ROADM with 8 W-λs from to λ 8. The 4 SWs and the coupler/splitter were connected such that 4 wavelengths from =149 to =1nm were R-λs, and other 4 wavelengths were PT-λs. The PT losses were measured for the F-ROADM with the 4 SWs set to be state. The measured losses are shown in Fig.11. It is found that the losses at wavelengths from to were around 22 db, showing that the 4 wavelengths were dropped. And the PT losses at the 4 PT-λs were as small as about 2.3dB, which agrees with a value of L PT (8)=2.17dB calculated by Eq.(2-3). Fig.11 also shows that the PT losses are higher than 2dB at smaller wavelengths than. This property of the F-ROADM restricts the transmissions of other wavelengths than the W-λs: i.e. the F-ROADM is not λ transparent at other wavelengths than W-λs. IV. CONCLUSION A structure has been proposed for use in regional IP-over-CWDM networks, campus networks, large-scale LANs and so on. The wavelength types were classified as working, reconfigurable, passing-through and reserve wavelengths so as to determine what wavelength ranges are λ transparent and what wavelengths can be used for reconfiguration between particular nodes, when applying ROADMs to such networks. The s and F-ROADMs were manufactured for the case that the Input Port: CI Output Port: CO 4 SWs: State Figure 1 Passing-through loss of Input Port: CI, Output Port: CO 4 SWs: State Figure 11 Passing-through loss of F-ROADM numbers of W-λs and R-λs are 8 and 4, respectively, and the performance was evaluated. The equations to estimate the add, drop and passing-through losses were derived and the 3 types of losses were measured for the S- and F- ROADMs. The measured and calculated results are consistent with each other, showing that the maximum losses were identical for the 2 types of ROADMs. It has been clarified that the could mux the 4 R-λs, and demux one of the 4 wavelengths successfully. It has been shown that the losses at the other wavelengths than the R-λs were as small as dB for the, clarifying that the provides λ transparent networks. Contrary to the, the losses at other wavelengths than the W-λs were as large as over 2 db for the F-ROADM, showing that the network by F- ROADMs is not λ transparent. As a result, the S- ROADMs provide more flexible CWDM networks in terms of scalability and reconfigurability. ACKNOWLEDGMENT The authors express their thanks to the students in Multimedia Network Research Group of Osaka Prefecture University for their assistance in the experiments. 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