Microwave and Optical Technology Letters. Minhui Yan, Qing-Yang Xu 1, Chih-Hung Chen, Wei-Ping Huang, and Xiaobin Hong

Transcription

1 Page of Schemes of Optical Power Splitter Nodes for Direct ONU-ONU Intercommunication Minhui Yan, Qing-Yang Xu, Chih-Hung Chen, Wei-Ping Huang, and Xiaobin Hong Department of Electrical and Computer Engineering, McMaster University, 0 Main Street West, Hamilton, ON, Canada, LS K Department of Engineering Physics, McMaster University, 0 Main Street West, Hamilton, ON, Canada, LS L Abstract Two schemes of optical power splitter nodes to enable direct local intercommunications between optical network units (ONUs) in a passive optical network (PON) are proposed and analyzed. In comparison with the existing schemes, the new schemes dramatically increase the number of ONUs supported in a PON with the capability of direct local communication. Experimental demonstrations confirm that our proposed schemes can fit into GPON standard by meeting its requirement on the power penalty. Index Terms Passive optical network (PON), optical power splitter, local area network (LAN) emulation.

2 Page of I. INTRODUCTION The passive optical network (PON) architecture has been widely adopted as an optical access technology to realize fiber-to-the-home (FTTH) applications. The network is normally designed as a tree-like topology and communication between the optical network units (ONUs) connected to the same optical power splitting node is facilitated indirectly through the optical line terminal (OLT). In the current PON standards, e.g., gigabit PON (GPON) [], the recommendations are to use routers at the OLT side to route back the ONU-to-ONU packets. Such scheme doubles the required bandwidth on the fiber as the local communication packets have to be uploaded from the transmitting ONU to the OLT and then downloaded to the receiving ONUs. Furthermore, it suffers from the latency caused by the routing process at the OLT terminal as well as the propagation delay due to the long fiber length (up to 0 km). This delay is not desirable for latency-sensitive applications like online gaming or video conference. Therefore, more efficient schemes for the ONU-to-ONU communications are required to meet the surging demands for bandwidth hungry applications and the strict latency requirement within local communities. Several architectures have been proposed for local area network (LAN) emulation in the optical layer. They used additional distribution fiber to collect loop-back signals from other ONUs [], [], utilized additional fiber Brag grating (FBG) for local communications [], [], or looped back signals with short fiber cord with or without

3 Page of isolators at the ports toward the OLT []-[]. All these existing methods bear some disadvantages. The first scheme [], [] requires the deployment of additional fiber with the same length of the distribution fiber between the optical power splitter node and the ONU. This redeployment of another fiber to the existing PON network tremendously increases the costs to the internet service providers (ISPs). The ISPs deploying the PONs usually prefer to restrict the upgrades at the terminals (i.e., the OLT and the ONUs) or the optical splitter nodes. So we do not consider this scheme. The last two schemes lay the upgrades at the optical power splitter nodes and the terminals. However, they suffer from the significant loss due to the doubled light passage traveling through the power splitter. In [], multiple loop-back fiber cords are used to increase the optical signal power reaching the ONUs. However, this is only applicable to incoherent light emitter diodes (LED). For laser transmitters, such scheme suffers from severe interferences between coherent optical signals from different propagation paths. In this letter, we propose two designs of the optical power splitter nodes to solve the problem of the huge loss in the existing schemes. The proposed schemes for optical splitter nodes have much less attenuation in the optical signal of the local ONU-ONU direct communication, in comparison with the other schemes []-[]. Consequently, the new schemes can support more ONUs in the PON with local intercommunication capability.

4 Page of II. OPTICAL LAYER SCHEMES FOR THE LAN EMULATION Our proposed architectures for ONU-ONU interconnection are shown in Figure. The wavelength of 0 nm is used to transmit the signal from ONU to OLT and from ONU to ONU, and 0 nm wavelength is utilized to transmit the signal from OLT to ONUs. The wavelength allocation is consistent with the existing GPON standard []. In both Figure (a) and Figure (b), a circulator in the ONU node is used to isolate the transmitting and receiving paths utilizing the same wavelength at 0 nm []. The OLT structure remains the same as that defined in the GPON standard []. The differences between the two schemes are in the designs of power splitter nodes. In scheme A (Figure (a)), based on an N N star coupler (SC), where N is the number of ports on each side of the SC, a coupler and an isolator are added for each ONU at the power splitting node. Path for direct ONU-to-ONU communication is created through these two couplers between the transmitting and receiving ONUs, the SC, and the isolator close to the receiving ONU. The optical path between each ONU and the OLT is through the SC and the coupler close to the ONU. In scheme B (Figure (b)), a circulator is used for each ONU and OLT. Interconnections for OLT-to-ONU, ONU-to-ONU, and ONU-to-OLT are routed through these two circulators at the transmitting and receiving nodes, in addition to the N N SC. It should be noted that when an N N SC is used, both schemes can support N- ONUs. In Scheme A, one ONU port is not used because its corresponding port on the other side of the SC is taken by the OLT. On the other hand, in Scheme B, one ONU port is

5 Page of physically taken by the OLT. There are two operation modes of ONU in the network, namely the ONU-to-ONU and the ONU-to-OLT communications. The ONU-to-OLT communication is required when an ONU is to send information to the OLT. On the other hand, the ONU-to-ONU communication is to emulate the LAN operation. In both proposed schemes, the optical signal transmitted from one ONU can be re-directed to all other local ONUs as well as the OLT. Various media access control (MAC) protocols can be employed to enable each ONU or OLT to capture the optical packets intended for itself. Data encryption may also be implemented when security or privacy is of concern. Security issues like virus spreading or network attacking should be watched over. This feature can be implemented at the OLT side because the OLT can receive the packets in the communication between the ONUs and monitor the network usage among them. III. PERFORMANCE COMPARISON To assess the performance of the proposed schemes, we compared our proposed schemes against other existing ones []-[] using the parameters of a standard PON network. Here we assume that the commercially available transmitters and receivers are used, and their optical channel loss budget is consistent with class C, i.e., its attenuation is in the range of ~ 0 db []. This means that the attenuation of the optical links between the OLT and any ONU, or between any two ONUs, are in the

6 Page of range of ~ 0 db. To ease the analysis, we consider the PON network diagram illustrated in Figure. The distance between the farthest ONU and the power splitter node is denoted as the distribution radius of the ONUs in a distribution area. A larger distribution radius means a larger distribution area can be connected under one PON network. This is preferred because it leads to lower shared cost per user. When comparing the performance of the ONU-to-ONU communication between schemes, we consider the extreme case that both ONUs (i.e., ONU# and ONU# in Figure ) sit at the ONU distribution circle. Because of the channel loss requirement in class C, the optical link between these two ONUs should have an attenuation less than 0 db. Based on this criterion, we can calculate the distribution radius. To make a comparison with the existing schemes, Figure shows two existing schemes. The optical power splitter node shown in Figure (a) (denoted as Scheme C) uses an FBG to reflect the local optical signals [], [], and we assume a 0% reflectivity for the FBG at 0 nm. This ensures that the network can perform the ONU-ONU direct intercommunication, while not blocking the upstream communication from the ONUs to the OLT. The 0% reflectivity of the FBG introduces additional db loss for both ONU-to-ONU and ONU-to-OLT communication at the 0 nm wavelength. No loss is introduced at 0 nm channel by the FBG. The other existing scheme shown in Figure (b) (denoted as Scheme D) uses a short fiber cord with an isolator [], []. In both schemes C and D, the structures of the ONUs and the OLT are the same as those shown in Figure.

7 Page of In our calculation, the fiber has an attenuation of 0. db/km at 0 nm and 0. db/km at 0 nm [], respectively. The N N SC has an ideal insertion loss equal to 0 log 0 (N). For schemes A and B, N is the number of ONUs plus one. For schemes C and D, N is the same as the number of ONUs. The insertion loss of each coupler is db in the power splitter node of scheme A. The insertion losses for the circulators in all schemes are 0. db from port to port and. db from port to port, respectively. The loss of the isolator in scheme A is 0. db. Finally, we assume additional insertion loss of db to include all the losses in the wavelength division multiplexer (WDM) used in the OLT and ONUs, and in the connectors (or splicers) along the optical link. For the number of ONUs hosted in a PON, according to GPON standard [], it can be,, or. In our analysis, we also add an ONU number of to accommodate future extension. The calculated distribution radii, which guarantee that the attenuations between any two ONUs at 0 nm are within ~ 0 db, are listed in TABLE I. We can see that our proposed schemes (Schemes A and B) can support all the ONU numbers (i.e., ~ ) in the GPON standard, while other schemes (Schemes C and D) can only support ONUs with a relatively small distribution radius. On the other hand, our proposed Scheme B can even accommodate ONUs for the future PON applications.

8 Page of IV. EXPERIMENTAL DEMONSTRATION To demonstrate and assess the performance of the proposed schemes (Scheme A and Scheme B in Figure ), we carried out some transmission experiments based on the setups shown in Figure (b) and Figure (c) to realize Scheme A and Scheme B, respectively. For comparison purpose, we also conducted experiments using a conventional power splitting node shown in Figure (a) as the reference. In our experiments, a SC is used, and it is composed of four SCs, as shown in Figure (d). To demonstrate the performance of the proposed schemes, we measured the bit-error-rate (BER) as a function of the received optical power. Each experiment was conducted using the same pair of optical transmitter and receiver, both working at 0-nm-wavelength range, but placed at different locations (e.g., at ONU or OLT). Our goal is to demonstrate the feasibility of the proposed power splitting nodes which use only passive components. Our measurement setup is analogous to the full duplex operation using 0-nm/0-nm as the upstream/downstream wavelength, except that the losses on the fiber and passive components are different. To further simplify the experiment, the WDMs in the transceivers of the ONU and the OLT, as well as the circulators in the ONU transceivers shown in Figure, are not used in the experiments. Consequently, crosstalks between the different wavelength channels in the WDM and between up- and down-stream signals in the circulator within the ONU node are not shown, for either conventional PON or our proposed schemes.

9 Page of In the experiments, the transmitter (Tx) is a distributed feedback laser diode (DFB-LD). It is externally modulated and emits at wavelength of. nm with average output power of 0. dbm. The Tx output is ( ) pseudo-random bit sequence (PRBS) non-return-to-zero (NRZ) signal, driven by a. Gbps pulse pattern generator (PPG). The receiver (Rx) is an optical PIN receiver with sensitivity of. dbm at BER of 0 -. The fiber connected to the OLT is km of standard single mode fiber (SMF) with attenuation of 0. db/km. A fixed attenuation of. db is inserted between the power splitter node and ONU#. A short fiber cord is connected between the power splitter node and ONU#. The attenuation in this short fiber can be ignored. In the experiments, we used Anritsu MPB PPG and Anritsu MPA error detector to measure the bit-error-rate (BER) with respect to the received optical power. Figure shows the BER results for the three different configurations: ) Tx at OLT and Rx at ONU#, ) Tx at ONU# and Rx at OLT, and ) Tx at ONU# and Rx at ONU#. In each configuration, we compared the results of three schemes, namely ) conventional PON, ) Scheme A, and ) Scheme B against that of the back-to-back (B-to-B) connection. It is observed that in all three configurations, the receiver sensitivity at BER of 0 - is in the range of -. ~ - dbm. The corresponding sensitivity in the B-to-B case is -. dbm. This means that the power penalties relative to the B-to-B performance for all three configurations are about 0. db, well

10 Page 0 of below db. The power penalty includes the effects coming from reflection, dispersion or nonlinearity induced inter-symbol interference, and other degradations except the optical path attenuation []. The power penalty is required to be within db compared to the B-to-B situation to meet the GPON recommendation []. The above experiment demonstrates that our proposed schemes can meet such power penalty requirement. Our experiment used optical transmitter with wavelength of 0-nm range. For the 0 nm and 0 nm wavelengths used in a conventional PON, the power penalty is expected to be even less. For a G. fiber [], the dispersion is less in the wavelength of 0 nm or 0 nm than that in the 0 nm range. The reflection is usually flat over the three wavelengths of interest (i.e., 0 nm, 0 nm, and 0 nm). Based on the above analysis, we can foresee that the power penalty at 0 nm or 0 nm would be even smaller. V. CONCLUSION In this letter, we proposed two physical architectures for local customer intercommunication overlaid on the conventional PON network. We compared our proposed schemes with other existing schemes. Significantly improved capacity in terms of much more supported ONUs and wider distribution area is demonstrated. We also showed that only our proposed schemes can accommodate all the ONU numbers required in the GPON standard. Finally, the experiments demonstrated that our 0

11 Page of schemes meet the requirement of power penalty defined in GPON standard. ACKNOWLEDGEMENT This work was supported in part by National Sciences and Engineering Research Council (NSERC) of Canada, Ontario Photonics Consortium (OPC) through Ontario Research and Development Fund (ORDF), Canadian Foundation for Innovation (CFI), and Ontario Graduate Scholarship (OGS). Authors would like to thank Prof. M. J. Deen and Dr. O. Marinov of the Department of Electrical and Computer Engineering, McMaster University, Hamilton, ON, Canada for their assistance in measurement. REFERENCES. Gigabit-capable Passive Optical Networks (GPON), ITU-T Rec. G., 00.. E. Wong and C. J. Chae, CSMA/CD-based Ethernet passive optical network with optical internetworking capability among users, IEEE Photon Technol Lett (00), -.. N. Nadarajah, M. Attygalle, E. Wong, and A. Nirmalathas, Novel schemes for local area network emulation in passive optical networks with RF subcarrier multiplexed customer traffic, IEEE/OSA J Lightwave Technol (00), -.

12 Page of C. J. Chae, S. T. Lee, G. Y. Kim, and H. Park, A PON system suitable for internetworking optical network units using a fiber Bragg grating on the feeder fiber, IEEE Photon Technol Lett (), -.. S. R. Sherif, A. Hadjiantonis, G. Ellinas, C. Assi, and M. A. Ali, A novel decentralized Ethernet-based PON access architecture for provisioning differentiated QoS, IEEE/OSA J Lightwave Technol (00), -.. C. J. Chae, H. Park, and J. H. Eom, An ATM PON system overlaid with a -Mb/s optical star network for customer networking and fiber to the premises, IEEE Photon Technol Lett (00), -.. C. J. Chae, E. Wong, and R. S. Tucker, Optical CSMA/CD media access scheme for Ethernet over passive optical network, IEEE Photon Technol Lett (00), -.. Optical Access Networks to Support Services up to the ISDN Primary Rate or Equivalent Bit Rates, ITU-T Rec. G.,.. Characteristics of a Single-mode Optical Fibre Cable, ITU-T Rec. G.,.

13 Page of CAPTIONS Figure Proposed architectures for an optical power splitter node to enable direct ONU-ONU internetworking using (a) isolators and couplers (scheme A), and (b) circulators (scheme B). Figure Diagram to demonstrate the concept of an ONU distribution radius and its distribution circle (dashed line) in a PON network. Figure Existing schemes for optical power splitter nodes using (a) an FBG to reflect LAN data back [], [] (Scheme C), and (b) a short fiber cord with an isolator at the ports toward the OLT [], [] (Scheme D). Figure The setups for transmission experiments. (a) Config. : using the power splitting node in the conventional PON; (b) Config. : using the power splitter node described in Scheme A; (c) Config. : using the power splitter node described in Scheme B; and (d) Implementation of the SC by four SCs. Figure Measured BERs for different Tx/Rx pair configurations. (a) Tx at OLT and Rx at ONU#; (b) Tx at ONU# and Rx at OLT; and (c) Tx at ONU# and Rx at ONU#.

14 Page of TABLE I. CALCULATED ONU DISTRIBUTION RADII (KM) OF PROPOSED AND EXISTING PON ARCHITECTURES Note: / indicates the calculated radius is less than 0, which means the PON under this configuration and the ONU number is not physically feasible.

15 Page of Fig (a) x0mm ( x DPI)

16 Page of Fig (b) x0mm ( x DPI)

17 Page of Fig. x0mm ( x DPI)

18 Page of Fig (a) x0mm ( x DPI)

19 Page 0 of Fig (b) x0mm ( x DPI)

20 Page of Fig (a) x0mm ( x DPI)

21 Page of Fig (b) x0mm ( x DPI)

22 Page of Fig (c) x0mm ( x DPI)

23 Page of Fig (d) x0mm ( x DPI)

24 Page of Fig (a) 0x0mm (0 x 0 DPI)

25 Page of Fig (b) 0x0mm (0 x 0 DPI)

26 Page of Fig (c) 0x0mm (0 x 0 DPI)

27 Page of TABLE I x0mm ( x DPI)