Journal of Communication and Computer 9 (2012) 669-678 D DAVID PUBLISHING The Designing of NG-PON Networks Using the HPON Network Configuration Rastislav Róka Institute of Telecommunications, Slovak University of Technology, Ilkovičova 3, Bratislava 812 19, Slovakia Received: September 17, 2011 / Accepted: November 21, 2011 / Published: June 30, 2012. Abstract: NG-PON systems present optical access infrastructures to support various applications of many service providers. Therefore, some general requirements for NG-PON networks are characterized and specified. The HPON network presents a necessary phase of the future transition from TDM to WDM multiplexing techniques utilized in passive optical networks. Requirements for the HPON are therefore also introduced. A main part of the paper is dedicated to presentation of parameters for the HPON network configurator that allows preparing the full-value design of NG-PON networks from a viewpoint of various specific network parameters. Finally, a comparison of two basic HPON options based on simulation results is presented. Key words: Next-generation PON, the hybrid PON, the HPON network configurator. 1. Introduction With the emerging applications and the needs of ever increasing bandwidth, it is anticipated that the next-generation PON (NG-PON) with much higher bandwidth is a natural path forward to satisfy the demand and for network operators to further develop a valuable access network. Many network operators are motivated to leverage such NG-PON systems as a common access infrastructure to support broader market segments. The NG-PON must be able to protect the investment of legacy passive optical networks. There are several migration scenarios to meet disparate service provider s needs. Two likely introduction scenarios service-oriented and service-independent reflect recognition that differing service introduction strategies might affect requirements for the NG-PON specifications. NG-PON technologies can be divided into two Corresponding author: Rastislav Róka, Ph.D., associate professor, research fields: communication systems and networks, WDM and TDM technologies, DBA and DWA algorithms. E-mail: rastislav.roka@stuba.sk. categories. The NG-PON1 presents an evolutionary growth with supporting the coexistence with the GPON on the same ODN. The coexistence feature enables seamless upgrade of individual customers on live optical fibers without disrupting services of other customers. The NG-PON2 presents a revolutionary change with no requirement in terms of coexistence with the GPON on the same ODN [1-2]. There are several possible architectures that could meet the NG-PON requirements. The 10GPON system is referred to as the XG-PON, where the Roman numeral X signifies 10 Gbit/s transmission speed, and we can expect its various versions the XG-PON1, the XG-PON2, the extended reach and wavelength controlled XG-PON, the hybrid DWDM/XG-PON [2-4]. 2. Requirements for NG-PON Networks In present days, PON networks with the TDM multiplexing technique are creating in many countries, also including Slovakia [5]. In the near future, we can expect NG-PON technologies with different motivations for developing of HPON networks:
670 The Designing of NG-PON Networks Using the HPON Network Configuration (1) Creating the new PON network overcoming TDM-PON network possibilities with minimum financial costs. In this case, various optical resources are utilized, but it is not the full-value WDM-PON network, and the TDM approach is still utilized from various reasons. So created HPON network is not very expensive (in a comparison with the WDM-PON network) and provides a sufficient transmission capacity for customer needs in a long-time horizon. This variation can be included in the NG-PON1, where the WDM filter is installed to combine and separate G-PON and XG-PON1 signals into and out of the common ODN. The ODN, i.e. optical fibers and the RN, is not replaced or changed during the migration to the NG-PON1 [1]. (2) Preparing the transition from the TDM-PON to the WDM-PON network with minimum costs for rebuilding of the existing TDM-PON infrastructure. Such HPON network should satisfy the following features: a backward compatibility with the original TDM-PON architecture and a coexistence of TDM and WDM approaches; a maximum exploitation of the existing optical infrastructure, optical fibers and optical equipments; new bonus functions for the network protection and the fast traffic restoration in a case of failures. This variation can be included in the NG-PON2, where separate optical fibers and power splitters may be used. Also, a different device in place of the simple power splitter may be used [1]. In a case of the extended reach and wavelength controlled NG-PON version, the basic feature is a more controlled ONU wavelength. If we use a wavelength-controlled ONU, then the bandwidth of optical amplifiers can be reduced to about 0.5 nm and this reduction allows a sensitivity to be improved by many db. By this way, a wavelength-controlled ONU also opens possibility for WDM-based multiplexing upgrades in future. In a case of the hybrid DWDM/XG-PON, multiple NG-PONs are combined with using a DWDM MUX/DEMUX and a colorless technology for the ONUs. The key components for the system are a colorless transmitter (the seed light injected RSOA and a tunable LD) and WDM filter [2]. 2.1 Requirements for the HPON Network The HPON is a hybrid passive optical network in a way that utilizes on a physical layer both TDM and WDM multiplexing principles together. The HPON network utilizes similar or soft revised topologies as TDM-PON architectures. For the downstream and upstream transmission, TDM and WDM approaches are properly combined, i.e. it is possible to utilize the time-division or wavelength-division multiplexing of transmission channels in the common passive optical network [6-9]. Our HPON network model is based on principles of the evolutionary architecture from the TDM-PON network utilizing few WDM components (Fig. 1) [10-12]. A basic architecture of the optical distribution network (ODN) that distributes signals to users consists of the one-fiber topology and several topological links connected to the ONT through the remote node (RN). Logically, a connection of the point-to-point type is created between the optical line terminals (OLT) and the RN. The RN consists of either a passive optical power splitter (TDM-PON) or an arrayed waveguide grating (WDM-PON). As resources of optical radiations in the OLT, tunable lasers (TL) are utilized in order to decrease a number of necessary optical sources. A number of tunable lasers in the OLT is smaller than a number of transmission channels utilized in a network; therefore, various subscribers can dynamically share tunable lasers. Of course, there are also other possible HPON architectures that can be later also included in this HPON network model [1-4, 6-9]. The largest requirement for the NG-PON is its coexistence with an operational GPON on the same ODN. This presents a challenge due to multiplexing
The Designing of NG-PON Networks Using the HPON Network Configuration 671 ODN OLT receiver ONT 1 DWDM transmitters r. transmitter /receivers CWDM transmitters /receivers tr. receiver transmitter RN 1 N splitter receiver transmitter ONT 2 receiver ONT N transmitter Fig. 1 The general architecture of the HPON network. the new system with the old one. For this purpose, we can use WDM or TDM techniques. Each of these methods will have different requirements for the wavelength plan [2]: (1) The WDM in both directions The simplest scheme is created by using of the WDM to separate NG-PON1 signals from GPON signals. This system requires GPON ONUs to be equipped with a wavelength blocking filter, so that additional NG-PON1 wavelengths are ignored. If the existing OLT equipment is to be retained for a smooth migration, the WDM1 branching filter must be placed between the ODN and the GPON OLT. (2) The WDM downstream, the TDMA upstream In a case where the previous PON system uses the widest upstream spectrum band, the upstream channel spectrum of the NG-PON1 may need to be shared with the previous system using the TDMA channel sharing. In this scheme, the existing GPON OLT would be replaced with an OLT that supports both the GPON and NG-PON1 systems. The NG-PON1 OLT would be installed between the ODN and the existing GPON OLT and would perform two additional functions to amplify the upstream signal and to mimic a GPON ONU to the GPON OLT and thereby request and obtain upstream timeslots to allocate slots to NG-PON1 ONUs connected to it. (3) The TDM downstream, the TDMA upstream It is possible to construct signals that are sufficiently orthogonal, where the very same wavelength can be used to transmit both signals. In this approach, the bit-stacked signal is generated by two differential optical sources at the OLT. The relative optical modulation depth of signals is adjusted in the ratio of about 30%. This offers very simple upgrade opportunities for legacy GPONs by simply adding a second optical source. This upgrade can be accomplished by either replacing the GPON OLT by a new hybrid GPON/NG-PON OLT or by combining the optical data streams by employing an additional separate combiner box. 3. Parameters of the HPON Configuration 3.1 A Selection of the Environment for the HPON Configuration and Its Capabilities Our simulation model is created in the Matlab 7.0 and Visual C++ 6.0 [10-12]. In both programming environments, there exist possibilities for the
672 The Designing of NG-PON Networks Using the HPON Network Configuration graphical interface the GUI (Graphical User Interface) for the Matlab and the MFC (Microsoft Foundation Class) library for the C++. The simulation model has one main dialogue window (Fig. 2) for inserting input parameters of the deployed TDM-PON network and also for simulation results. Two additional dialogue windows serve the specific HPON configuration setup. Our simulation model is working in several steps: (1) Inserting the input parameters of the TDM-PON networks a number of TDM networks, a type of the network, a number of subscribers per one network, a distance between the ONT and the OLT. (2) Evaluating the input parameters a calculation of the total transmission capacity of the TDM network together with the average capacity per one subscriber, the total number of subscribers and the maximum attenuation of the TDM network; this step is terminating with the selection of a detailed design of the hybrid PON configuration. (3) Setting the input parameters for the hybrid PON configuration based on the stored TDM networks data and selecting one from two HPON types. (4) Application the input parameters and the specific parameters summary of the HPON network (the total capacity, the total number of subscribers, the average capacity per one subscriber, the maximum attenuation of the hybrid network between the OLT Fig. 2 The main window of the HPON configurator.
The Designing of NG-PON Networks Using the HPON Network Configuration 673 and the ONT, a number and type of used active and passive components) with presenting possibilities for future expanding of both HPON types. 3.2 Power Relationships For the XG-PON, there will be two loss budgets denoted Normal and Extended. The Normal loss budget is defined with a Class B+ loss budget plus an insertion loss from a WDM1 filter. The link loss will be approximately 28.5 db to 31 db at BER = 10-12. The Extended loss budget is defined with a Class C+ loss budget plus an insertion loss from a WDM1 filter [3]. In the HPON network, power relationships are depending on specific network characteristics and applied optical component parameters. We prefer real values of optical components utilized in real passive optical networks (Table 1). In addition to a distance between the OLT and particular ONU, the network architecture, a number and type of used splitters are included for calculating of the total network attenuation. Also, calculating of the total optical fibers attenuation for available wavelengths is included in simulations of specific network configurations. Also in this case, we prefer attenuation coefficients (Table 2) for common optical fibers according to ITU-T G.652 [13]. However, we can incorporate attenuation coefficients (Table 3) for new types of optical fibers according to ITU-T G.657 [14] and by this way evaluate their utilization in the HPON network. 3.3 An Availability of Wavelengths One of the issues with any DWDM scheme is provisioning of equipment wavelengths. For the OLT side, there is no issue a new OLT interface supporting all the channels in a single module. However, each ONU only supports a single wavelength in either direction. Wavelength selectable sources for the upstream transmitter might be a possibility. Alternatively, ONU interfaces may be colored and this may present operational problems. Table 1 Specifications of HPON optical components. Symbol Description Value CWDM the maximum fiber attenuation in the CWDM wavelength bands 0.35 db/km DWDM the maximum fiber attenuation in the DWDM wavelength bands 0.25 db/km L the distance between the OLT and the RN optional L RING the ring length optional l the distribution fiber length 2 km a AWG the AWG attenuation 5 db the 1:4 splitter attenuation 7.5 db the 1:8 splitter attenuation 11 db the 1:16 splitter attenuation 14.1 db a SPLIT1:N the 1:32 splitter attenuation 17.4 db the 1:64 splitter attenuation 21.0 db the 1:128 splitter attenuation 22.5 db the 1:256 splitter attenuation 26.4 db a TDM-RN the TDM node attenuation (including connectors) 1.5 db a WDM-RN the WDM node attenuation (including connectors) 1 db a ADD/DROP the attenuation of added/dropped wavelengths 1.2 db a CON the connector attenuation 0.2 db loss of a splice 0.15 db loss of the fiber span 0.25 db/km Table 2 ITU-T G.652 optical fibers attenuation. Class Wavelength Attenuation A B C D maximum at 1310 nm maximum at 1310 nm maximum at 1625 nm 0.5 db/km 0.35 db/km maximum from 1310 nm to 1625 nm 0.3 db/km maximum from 1310 nm to 1625 nm 0.3 db/km Table 3 ITU-T G.657 optical fibers attenuation. Class Wavelength Attenuation A B maximum from 1310 nm to 1625 nm maximum at 1383 nm ±3 nm maximum at 1310 nm maximum at 1625 nm 0.3 db/km 0.5 db/km 0.3 db/km The overall wavelength plan for the NG-PON is shown in Fig. 3. This plan pulls together the diverse
674 The Designing of NG-PON Networks Using the HPON Network Configuration XG-PON US ch.e GPON XG-PON US ch.d 1260 1280 1300 1320 1340 1360 XG-PON US ch.c ch.b XG-PON US ch.a 1500 1520 1540 1560 1580 1600 1620 (a) upstream GPON video overlay XG-PON DS Fig. 3 The NG-PON wavelength spectrum plan. 1480 1500 1520 1540 1560 1580 1600 (b) downstream set of requirements from PON deployments and the NG-PON system concept into a minimal set of wavelength assignment [3]: (1) The XG-PON1: The downstream wavelength band of 1575~1580 nm is used since it is the only wavelength band that is left in the system with the video overlay. For the upstream, five channel assignments can be discussed within the L-band (channel A), the C-band (channel B), the video-compatible C-band (channel C), the O-plus band (channel D) and the O-minus band (channel E). According to ITU-T G.987 series [15-17], parameters for the upstream wavelength allocation are determined to 1260~1280 nm. (2) The ER+WC XG-PON1: The 0.5 nm wide wavelength windows (the 200 GHz channel spacing) are specified. (3) The XG-PON2: The downstream band is the same as for the XG-PON1. The upstream band spans 1260~1280 nm that corresponds to the O-band placement and permits the use of directly modulated lasers without excessive dispersion penalty and the use of uncooled lasers. (4) The hybrid DWDM/XG-PON: The channel spacing can be selected at either 100 GHz or 50 GHz. It requires the C-band upstream allocation (channel B) because it is sensitive to the fiber loss, but this is in conflict with the video overlay. The downstream wavelengths are located from 1575 to 1582 nm. In the WDM/TDM HPON network, two sets of wavelengths can be utilized for CWDM and for DWDM systems. Because these bands are overlapped, the number of available wavelengths is depending on utilized bandwidths and on a density of the wavelength allocation (100 GHz or 50 GHz channel spacing). In our simulation program, a dependency between wavelengths is exactly scheduled [10-12]. In the SUCCES-HPON, a situation is different. In the first step of a transition to the HPON, a network topology is changing from various point-to-multipoint infrastructures to the one ring by means of the one optical fiber. Therefore, DWDM wavelengths are utilizing in the downstream direction (one wavelength per one TDM node). In the upstream direction, each
The Designing of NG-PON Networks Using the HPON Network Configuration 675 TDM node utilizes a different wavelength from the CWDM grid. Now, it is appropriate to clarify a relationship between a number of available CWDM and DWDM wavelengths in the network (Fig. 4). The less CWDM wavelengths are used, the more DWDM wavelengths can be utilized in a relevant spectrum. Simulation calculations are based on the following Eq. (1): 20 (18 C ) 125 D C (1) where λ C assigns the number of used TDM nodes, respectively CWDM wavelengths, λ D is a maximum number of available DWDM wavelengths for WDM subscribers, Δλ is the interval between DWDM channels (100 GHz). This relationship describes a maximum possible number of available DWDM wavelengths that can be utilized by WDM subscribers in a dependency on a number of utilized CWDM wavelengths. 4. Options of WDM/TDM HPON and SUCCESS-HPON Networks The WDM/TDM-PON network represents a hybrid network based on the combined WDM/TDM approach [6-8] (Fig. 5). The SUCCESS-HPON network introduces a sequential transition to the pure a number of available DWDM wavelengths a number of available CWDM wavelengths Fig. 4 The relationship between numbers of available wavelengths at the 50 GHz and 100 GHz channel allocation. WDM PON network in a compliance with the TDM and WDM technology coexistence [7-9] (Fig. 6). The number of WDM subscribers in the SUCCESS-HPON is limited by a number of available wavelengths (max. 150 without TDM nodes). A number of WDM subscribers can be increased by the wavelength and the AWG port sharing as in a case of the WDM/TDM-PON up to quadruple at the maintenance of acceptable attenuation values. In Table 4, output values from the simulation model for few selected configurations are presented. As we can see, until the 1:8 splitting ratio, the total attenuation Fig. 5 A window for the WDM/TDM-PON configuration.
676 The Designing of NG-PON Networks Using the HPON Network Configuration Fig. 6 A window for the SUCCESS-HPON configuration. Table 4 The WDM/TDM-PON and SUCCESS-HPON comparison. The TDM-PON networks with the 1:32 splitting ratio/subscribers The WDM/TDM-PON (AWG ports/splitting ratio/subscribers) 2/64 32/1:4/128 19.5 4/128 32/1:8/256 23.0 6/192 48/1:8/384 23.0 8/256 32/1:16/512 26.1 10/320 48/1:16/768 26.1 The TDM-PON networks with the 1:32 splitting ratio/subscribers The SUCCESS-HPON (TDM nodes/wdm nodes/subscribers) The TDM node attenuation (db) The TDM/WDM node attenuation [db] 2/64 3/1/128 30.4/15.3 4/128 6/2/256 33.0/18.0 6/192 9/3/384 35.7/20.7 8/256 11/4/480 37.6/22.6 10/320 12/4/511 38.3/23.3 in the WDM/TDM-PON is lower or equal to the original TDM-PON. In addition, the number of subscribers is much higher. In the SUCCESS-HPON, the attenuation for TDM nodes is higher at 4 nodes in the network. For a comparable number of subscribers, the attenuation in the SUCCESS-HPON is around 10 db above the value in the WDM/TDM-PON. In the first place, it is due to utilizing of CWDM wavelength with higher attenuation values. Due to a summary of simulation results and hybrid network comparison [10-12], we can see that both HPON network designs overcome actual TDM-PON network possibilities. Because the SUCCES-HPON has a high attenuation of TDM nodes, it is not possible to realize this network without modifications. For improvement of power relationships, it is necessary to utilize fewer nodes with CWDM wavelengths in the lower attenuation band, ADM multiplexors with the lower attenuation, the lower splitting ratio in TDM nodes. Another possibility is an exploitation of optical amplifiers, however, with the increasing of noise levels and other nonlinear effects. Globally, the SUCCESS HPON is outstanding from various viewpoints. By contrast, the WDM/TDM-PON is balanced and utilizes a more easily concept that is identical in the entire network. Average capacities per subscribers can be very high (in the order of Gbit/s). From a total standpoint, the WDM/TDM-PON is preferable to the SUCCESS-HPON for utilization in access networks. Even though financial costs of these network components are above actual TDM-PON components, costs per subscriber s capacity in the HPON are much below the TDM-PON. 5. Conclusions Possibilities for the practical development of our
The Designing of NG-PON Networks Using the HPON Network Configuration 677 HPON configuration in passive optical networks are very hopeful. The HPON configuration mitigates an understanding and a working with the design of hybrid passive optical networks that are engaged in a smooth transition process between TDM-PON and WDM-PON networks. Possibilities for extending of the HPON network configuration can be summarized as follows: A larger possibility for the selection and the variability of input parameters, e.g. entries for each TDM network and for each subscriber can be inserted particularly; An extension of selections for the CWDM and DWDM wavelength allocations, e.g. a possibility for the 25/50/100/200 GHz spacing can be included; A possibility for the interactive network model can be implemented; A capability for the adaptation some input parameters for correct network design and simulations; A specification of active components OLT and ONT; A specification of passive components, e.g. a splitter location, a split ratio, a split reduction. The created HPON simulation program represents a suitable tool for a comparison of various possible HPON network configurations from a viewpoint of the physical layer and moreover for evaluating of transitions between the existing PON to the expected NG-PON networks. The comparison is realized from various specific network aspects required for the complex data collection. The first aspect is the network robustness from a viewpoint of possible total transmission capacity and a number of subscribers. The second aspect is a number and type of active and passive optical components influential network financial costs. The third aspect is represented by the total network attenuation related to the maximum network reach. The fourth aspect is a possibility for further network extensions increasing of the transmission capacity, connection of other new subscribers and others. Acknowledgments This work is a part of research activities conducted at Department of Telecommunications, Faculty of Electrical Engineering and Information Technology, Slovak University of Technology Bratislava, within the scope of the project VEGA No. 1/0106/11 Analysis and proposal for advanced optical access networks in the NGN converged infrastructure utilizing fixed transmission media for supporting multimedia services. References [1] J. Kani, et al., Next-generation PON Part I: Technology roadmap and general requirements, IEEE Communications Magazine 47 (2009) 43-49. [2] F. Effenberger et al., Next-generation PON part II: Candidate systems for next-generation PON, IEEE Communications Magazine 47 (2009) 50-57. [3] F. Effenberger et al., Next-generation PON part III: System specification for XG-PON, IEEE Communications Magazine 47 (2009) 58-64. [4] K. Tanaka et al., IEEE 802.3av 10G-EPON standardization and its research and development status, Journal of Light Wave Technology 28 (2010) 651-661. [5] R. Róka, The evolution of optical access networks for the provisioning of multimedia services in the NGN converged networks, in: Design of forms in the marketing communication for support of implementation in new multimedia products in the praxis, EDIS ŽU Publishing house, Žilina (Slovakia), 2008, pp.138-143. [6] J.H. Lee, et al., First commercial deployment of a colorless gigabit WDM/TDM hybrid PON system using remote protocol terminator, Journal of Light Wave Technology 28 (2010) 344-351. [7] L.G. Kazovsky, et al., Next-generation optical access networks, Journal of Light Wave Technology 25 (2007) 3428-3442. [8] A. Banerjee, et al., WDM-PON technologies for broadband access A review, Journal of Optical Networking 4 (2005) 737-758. [9] F.T. An, et al., SUCCESS-HPON: A next-generation optical access architecture for smooth migration from TDM-PON to WDM-PON, IEEE Communications Magazine 43 (2005) 40-47. [10] R. Róka, The designing of passive optical networks using the HPON network configuration, International Journal of
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