Troubleshooting PON Networks Effectively with
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1. OPTICAL COMMUNICATIONS Troubleshooting PON Networks Effectively with Carrier Grade Ethernet and WDM PON Rafael Sanchez Jos Alberto Hernandez and David Larrabeiti Universidad Carlos Ill de Madrid ABSTRACT WDM PONSs have recently emerged to pro vide dedicated and separated point to point wavelengths to individual optical network units In addition the recently standardized Ethernet OAM capabilities under the IEEE 802 1ag stan dard and ITU T Recommendation Y 1731 together with state of the art optical time domain reflectometry provide new link layer and physical tools for the effective troubleshooting of WDM PON This article proposes an integrated troubleshooting box ITB for the effective com bination of both physical and link layer informa tion into an effective and efficient set of management procedures for WDM PONs We show its applicability in a number of realistic troubleshooting scenarios including failure situ ations involving either the feeder fiber one of its branches or even Ethernet links after the ONU INTRODUCTION Passive optical networks PONs have been pro posed and standardized to open up the band width capacity of access networks At present network operators have begun to deploy time division multiplexing TDM based PONs in high density urban areas while wavelength divi sion multiplexing WDM PONs are still in the stage of research and standardization Concerning TDM PONSs curre2. Yes Delay jitter Y 1731 on faulty channels Figure 7 The troubleshooting algorithm running on the ITB latter case the algorithm must combine Ether net OAM measurements with the OTDR to identify and isolate the failure Results of those tests are sent to the ITB using SNMP OAM measurements from OLT or SFTP OTDR traces and received by the operator This infor mation is of key importance for the operator to properly diagnose the failure As shown in Fig 7 the troubleshooting algo rithm starts with an alarm received from the IEEE Communications Magazine February 2014 S11 SSF Essentially the Ethernet OAM makes it possible to quickly identify either network failures or performance degradation while the OTDR can further investigate the exact failure location at the physical level with a very fine resolution Network Operator Ae i SNMP S amp S ia ea gt aN Cll i oo i i SFTP aaa i e ne t OTDR Hien ate PT i ee Figure 8 Use cases 1 2 and 3 OLT There are many types of alarms and events some of them are more important than others For instance an alarm related to OLT misconfig uration should be ignored by the ITB since it is not related to network failures whereas an alarm associated with signal loss on a specific wave length is particularly important In this case the following set o
3. Wavelength 1535 8 nm 4km fiber Lengttt Cursor Position Sensitivity 71 dB 2 5 km a NY bes Sensitivity 71 dB b Figure 4 OTDR snapshots for Test 1 a OTDR snapshot channel 4 b OTDR snapshot channel 5 4km Fiber Length 2 5km Fiber vi Sensitivity 71 dB Cursor Position Sensitivity 42 dB 18 5 km Wavelength 1545 3 nm ONT refiection d s Figure 5 OTDR snapshots for Test 2 a OTDR snapshot channel 16 b zoomed in OTDR snapshot channel 16 instance every 10 ms this value can be config ured by the network manager If three consecu tive CCMs are not received a failure is assumed to have occurred At this point an alarm is reported to the network management plane Fault notification All devices supporting ITU T Y 1731 can be configured to report alarm indication signals AISs to the network manage ment plane upon failure suspicion either after three lost CCMs or any other misbehaving event At this point the network manager should verify and isolate the failure as explained next Fault verification in charge of verifying that an actual failure has occurred Under failure sus picion the network manager can configure the device to send a loopback message LBM to a specific destination which would answer with a loopback reply LBR Obviously in the case of an actual failure no reply would arrive back at the source The key difference betwee
4. 6 5 0 5 Connector amp g5 AWG reflection 7 va absorption J 0 4 0 3 0 3 02 0 2 7 0 fi fi fi 1 1 1 0 fi ji fi fi fi fi E b DMH A PD DPD Tunable OTDR C band a Feeder fiber 4km 4km 4km 4 km Branch fiber oT gy a ine card b Figure 3 Test setups a AWG pass through b reach and termination test feeder and branch fibers can be inspected even with the large insertion loss introduced by the AWG of 5 5 dB at most Any AWG ports with out a fiber can easily be identified from the OTDR response Test 2 Reach and Termination Tests Test 2 takes one step further by increasing the trunk fiber length for up to 16 km and terminating port 16 instead of port 4 of the AWG with an unpowered ONT Fig 3 In this setup the branch is 2 5 km long rather than 4 km as before Again all connections were performed with SC APC connectors The OTDR was then tuned to channel 16 i e wavelength 1545 3 nm show ing the snapshots of Fig 5 The first snapshot shows the entire 18 5 km fiber length on a 20 km window view The OTDR sensitivity is set to the maximum value 71 dB but even so the very large attenuation observed hides any details about the power drop at the AWG or the banch fiber section The OTDR automatically switches to Raylei
5. makes it possible to quickly identify either network failures or performance degrada tion while the OTDR can further investigate the exact failure location at the physical level at very fine resolution This article proposes an algorithm to bring 12 IEEE Communications Magazine February 2014 together these two historically separated worlds Ethernet OAM and physical measurements into an integrated and effective troubleshooting tool to ease management of WDM PON networks This algorithm is capable of diagnosing different failure situations in a WDM PON setup includ ing failures in the feeder fiber one of its branch es or even after the ONT One of the main drawbacks of the proposed solution is related to the cost of the tunable OTDR and its associated filters required for in service operations Nevertheless it is worth noticing that OTDR equipment is shared among a number of OLT line cards each serving up to 32 ONTs in current deployments but may reach 128 ONTs 7 and beyond in the near future Hence the total cost of the integrated solution would be shared among N x 128 ONTs where N refers to the number of OLT line cards per chassis at present ranging between 8 and 16 Concerning future work the recently pro posed software defined networking SDN paradigm may be very well suited for a real implementation of the ITB 8 SDN is a new paradigm where the control plane in particular forwarding decisions a
6. x axis In the figure we observe the attenuation due to Rayleigh scat tering AWG absorption connector reflections and a fiber break reflection LABORATORY FIELD TRIALS Previous studies from Park et al 3 and Kaiser et al 4 have demonstrated the use of a tunable OTDR for in service monitoring of fiber faults in an experimental unstandardized WDM PON In their experimental setup they used a colorless WDM PON based wavelength locked Fabry Perot lasers with Broadband Light Sources BLS 5 on the C and S bands The authors used a wavelength locked Fabry Perot laser tuned by an L band BLS to emulate the tunable OTDR signal Our laboratory setup is very similar to those of 3 4 but uses a standardized WDM PON ITU T G 698 3 compliant 6 and standard frequency grids rather than experimental WDM PON technology The OTDR equipment used in our experiment is also commercially available Two different test scenarios were set up for the experiments Fig 3 The first test was aimed at demonstrating basic AWG pass through fea tures of the OTDR whereas the second one was focused on exploring the whole fiber path across the WDM PON Test 1 AWG Pass Through Tests In Test 1 two 4 km fiber spools were assembled to build an 8 km trunk fiber at the output of the OLT and further connected to the common port of the AWG using SC APC connectors Fig 3 In port 4 of the AWG another 4 km fiber spool was connected but not termi
7. DR capabilities to actually locate the failure and isolate it from the rest of the network Such manual operational procedures cause high OPEX and it would be desirable to make them automatic Indeed the IEEE and ITU T have standard ized a number of operations administration and maintenance OAM procedures for Ethernet networks under IEEE 802 1ag 1 and ITU T Y 1731 2 These mechanisms include the gen eration of loopback messages measurements of packet delay or loss and others at the Ethernet layer which in conjuction with the raw physical alarms provided by most vendor equipment and the OTDR measurements can provide a means of automatically troubleshooting WDM PON networks This article explores this idea of integrating troubleshooting information from multiple inde pendent sources equipment alarms OTDR traces and Ethernet OAM features and further proposes an integrated troubleshooting box ITB for effective and proactive i e without user intervention management of failures in WDM PONS Thanks to this box the network manager will be provided with accurate real time information about the PON status including the detection isolation and verification of failures upon their occurrence Fig 1 The remainder of this article is organized as follows We describe the troubleshooting capa bilities of OTDRs at the optical layer We review the Ethernet OAM mechanisms described in IEEE 802 lag and ITU T Y 1731 at the li
8. as for device management In this light future work will try to implement OF Config as part of the ITB An interest ing research direction may also be to use these protocols instead of CLI to configure the OLT and the OTDR IEEE Communications Magazine February 2014 13
9. e in real time services since these require strict service level agreements SLAs To conclude Ethernet offers a comprehen sive set of OAM tools with enhanced trou bleshooting capabilities when combined with optical tests The next section introduces the integrated troubleshooting box ITB which combines both approaches and further shows its applicability with a number of realistic use cases THE INTEGRATED TROUBLESHOOTING BOX The ITB is a software module that brings togeth er optical and link layer troubleshooting Figure 6 overviews the architecture of the ITB interop erating with the OLT and a tunable OTDR and their interfaces As shown both OLT and OTDR support command line interfaces CLIs for third party provisioning by the ITB although other typical interfaces such as NETCONF could be supported In addition the OLT exports alarms through Simple Network Management Protocol SNMP while the OTDR uses Secured FTP STFP to send its traces to the ITB In a real scenario the OTDR should be prop erly connected to the WDM PON for in service measurements that is the OTDR signal must not be affected by users traffic carried in other wavelengths The following set of requirements are necessary for such in service tests e Permanent low loss optical tap to be insert ed into each line card for connecting the OTDR point A in Fig 6 A single tunable OTDR to be coupled to all line cards with an optical switch po
10. erot Laser Diodes Injecting a Low Noise bls IEEE Photonics Tech Letter vol 8 2006 pp 1167 69 6 ITU T Rec G 698 3 Multichannel Seeded DWDM Appli cations with Single Channel Optical Interfaces 2012 7 D Seyringer Design and Simulation of 128 Channel 10 GHz AWG for Ultra Dense Wavelength Division Mul tiplexing 2012 14th Int l Conf Transparent Optical Networks 2012 pp 1 4 8 Open Networking Foundation Software Defined Net working The New Norm for Networks white paper 2012 9 ONF OpenFlow Management and Configuration Pro tocol OF Config 1 1 1 white paper 2013 BIOGRAPHIES RAFAEL SANCHEZ rsfuente it uc3m es holds an M Sc 2008 from University Carlos III of Madrid and his telecom munications engineer degree 1996 from the Polytechnic University of Valencia Since 1996 he has been involved in multiple networking projects in areas like optical networks SDH DWDM IPTV digital transmission fiber access FTTH WDM PON and IP carrier Ethernet in companies like Lucent Technologies Nortel and LG Nortel Currently he works for Google in the enterprise division on projects related to cloud computing and is pursuing a Ph D degree in telematic engineering at the University Carlos IIl Jose ALBERTO HERNANDEZ jahgutie it uc3m es completed his five year degree in telecommunications engineering at Universidad Carlos Ill of Madrid in 2002 and his Ph D degree in computer sc
11. f alarms should be considered by the ITB to initiate the troubleshooting procedure of Fig 7 link down AIS of 802 1ag remote defect indication RDI or three missing CCMs on any wavelength In addition those events resulting from exceeding performance thresholds such as bit error rate delay or jitter indications should have been configured in advance by the network operator according to a specific SLA in order to be treated by the ITB The next section further explores the opera tion of the ITB in detail with a generic WDM PON topology where two ONTs are connected in an Ethernet ring beyond the PON tree Fig 8 This configuration allows end to end Ether net OAM tests across multiple ONTs USE CASE 1 A FAILURE IN THE FEEDER FIBER This first case failure 1 in Fig 8 considers a severe fiber problem in the feeder namely fiber break or strong bending In this case the ITB is expected to receive several alarms involving all channels or most of them The ITB infers from the multiple alarms that the problem affects the feeder fiber so the next action is to find the exact failure location using the OTDR as noted from Fig 7 No Ethernet OAM measurement is needed since the failure is likely related to a physical issue USE CASE 2 SINGLE FAILURE IN A FIBER BRANCH In this case failure 2 in Fig 8 the ITB would receive a single alarm coming from a faulty channel At this point the ITB needs to decide whether or not this
12. failure is after the ONT For this reason the ITB must next launch LBM LTM measurements on the faulty channel In this case no reply is received from the ONT so the ITB understands that the failure is affecting a fiber branch of the PON The final step is to launch the OTDR to identify the exact failure location inside the fiber branch In addition the ITU T Y 1731 performance measurements jitter delay are encouraged if real time services traverse this particular fiber branch USE CASE N3 SINGLE FAILURE AFTER THE ONT In this case we consider a failure after the ONT failure 3 in Fig 8 The ITB behaves similarly as in case 2 except that the ONT would reply to the LBM LTM measurements hence diagnosing a problem after the ONT Furthermore thanks to the end to end nature of LTMs the network operator is capable of isolating the exact failing link since LBMs do not provide this informa tion Clearly the OTDR does not need to be launched since it cannot traverse active ele ments This troubleshooting use case finishes with an OAM report submitted to the network operator detailing the actual link failure SUMMARY DISCUSSION AND FUTURE WORK This article has shown the benefits of combining the recently standardized OAM features of carri er grade Ethernet IEEE 802 lag and ITU T Y 1731 together with current state of the art OTDR equipment for effective troubleshooting of WDM PON networks Essentially the Ether net OAM
13. gh mode for this view In order to better see the details at the end of the fiber the second snapshot of Fig 5 pro vides a 50 m window view at the very end of the fiber i e at 18 5 km Sensitivity is now reduced to 42 dB and the OTDR has automatically switched to the Fresnel mode for this zoomed in view of the last 50 m The reflection produced by the ONT is now clearly evident Both window size sensitivity and window position can be manually adjusted along the entire fiber length to identify and locate any fiber anomaly includ ing fiber breaks dirty connectors and so on CARRIER GRADE ETHERNET OAM In WDM PONs the point to point wavelengths between the OLT and the ONTs can but not necessarily carry Ethernet frames In this case the WDM PON can leverage from the Ethernet carrier grade capabilities which can show multi ple advantages for troubleshooting The OAM features of Ethernet specified in IEEE 802 lag and ITU T Y 1731 can be split into two main areas fault management and performance moni toring FAULT MANAGEMENT Fault management is in charge of detecting and isolating failures and reporting them to the net work operator To this end it provides the fol lowing functionality Fault detection supported through the use of continuity check messages CCMs CCMs are periodically issued between two endpoints for IEEE Communications Magazine February 2014 s9 Cursor Position 8 km
14. ience at Loughborough University Leichester United Kingdom in 2005 From 2005 to 2009 he was a postdoctoral researcher and teaching assistant at Universidad Aut noma de Madrid where he participated in a number of both national and European research projects concerning the modeling and performance evaluation of communication networks particularly optical burst switch ing technology In 2009 he moved to Universidad Carlos III de Madrid where he became an associate professor He has published more than 60 articles in both journals and conference in proceedings His research interests include the areas at which mathematical modeling and computer networks overlap He is the co author of the book Proba bilistic Modes for Computer Networks Tools and Solved Problems DAVID LARRABEITI dlarra it uc3m es is a professor of switching and networking architectures at Universidad Car los III of Madrid Since 1990 he has participated in EU funded research projects related to next generation networks and protocols In 2009 2010 he was a visiting researcher at Stanford University under Spanish mobility grant PR2009 0221 He is UC3M s principal investigator at the BONE network of excellence on optical networking His current research interests include the design of hybrid elec tro optical packet switches and multipoint optical commu nications E The ONF has pro posed a new proto col called OF Config 9 that defines a number of XML schem
15. int B This way the OTDR may take measure ments in all line cards but not simultane ously e A low pass filter LPF between the OTDR and the optical switch point C that iso lates the OTDR from stray light e Disable the L Band laser on the OLT line card associated with the channel under inspection The OTDR must be able to be tuned on the L Band downstream band for fiber testing The software module at the ITB runs the fol lowing algorithm Fig 7 Upon reception of one or many alarms the OLT forwards these events to the ITB via SNMP With this information the ITB s first task is to determine whether or not the problem comes from the PON s feeder fiber or one of its branches In the former case the next action is to launch the OTDR measurement to effectively locate the failure position In the boced i Integrated CLI troubleshooting ae i box i OLT Other line card line cards Alarm received link down AIS RDI 3xCCM Failure on feeder or branch fiber s Failure on one or more branches LTM LTB packet loss on faulty channels Failure on feeder fiber Real time service Launch OTDR measurement Yes a No Alarm after ONT send combined Send OAM port to operator report to OTDR OAM Operator
16. n fault detection and verification is that in the former the CCMs are periodically sent whereas the LBMs have to be manually launched by the operator Fault isolation achieved through the use of linktrace messages LTMs and linktrace reply LTR messages also provided by the manage ment plane The network manager may config ure a device to initiate an LTM toward an end node In this case each intermediate device along the source destination path must reply with an LTR back to the source This allows the network operator to detect the exact faulty link In a nutshell the LBM LBRs are like ICMP pings while the LTM LTRs act as traceroutes at the Ethernet layer PERFORMANCE MONITORING The ITU T Y 1731 standard complements the fault management procedures defined in IEEE 802 lag with extra performance monitoring fea tures Essentially the network manager may S10 IEEE Communications Magazine February 2014 decide to use the ETH LM and ETH DM fields inside the CCM frame to collect information regarding loss measurements ETH LM or delay and delay variation information ETH DM These two counters allow the network management plane to trigger alarms to the net work operator when certain thresholds are exceeded These counters can be used to estimate use ful metrics for the network operator such as frame loss ratio FLR frame delay FD and frame delay variation FDV This information is particularly valuabl
17. nated on any ONT In fact this branch fiber was terminated on another SC APC connector No fiber was connected to any of the other 31 ports of the AWG for the following reason Essentially the OTDR equipment is very sensi tive to external light sources Hence if other active ONTs at different wavelengths are con nected in the laboratory setup the OTDR would receive the power from all of them hence mask ing the signal of interest on channel 4 This issue is typically solved by using appropriate filtering at the input of the OTDR but this device was not available at the time of writing For this rea son we decided not to connect any ONT to the other AWG ports The tuning accuracy of the OTDR below 0 1 nm allows for the selection of individual user wavelengths over the full C band range where channel spacing is approximately 0 8 nm The OTDR was then tuned to the 1535 8 nm and 1536 6 nm wavelengths channels 4 and 5 of the the LG Ericsson EA1100 wavelength If the fiber break occurs in a branch AWG The two responses are displayed in Fig model the OTDR must be tuned to that particular 4 We conclude from the figures that both the s8 IEEE Communications Magazine February 2014 Attenuation of fiber T Att nuation of fiber 1 Railegh scattering J 1 Railegh scattering 0 9 0 9 Splice 71 Fiber break 7 7 0 8 absorption 0 8 reflection 5 0 7 S 0 7 gt 06 40 0
18. nd OTDR 1490 1310 nm or a different one out of band OTDR typ ically at 1625 nm In the former hardware changes are required in both the optical line ter minal OLT and optical network terminals ONTs to prevent the OTDR signal from affect ing the traffic of non faulty users In the latter case hardware changes are only required in the ONTs basically to make them capable of reflect ing the OTDR wavelength In either case signif icant hardware changes are required However in WDM PONSs the OTDR can be tuned on each user wavelength in band OTDR with minimal hardware changes only those involving the coupling of the OTDR equipment itself as shown in Fig 1 which poses a clear ben efit over TDM PON troubleshooting Fiber breaks may occur in either the feeder section of the PON or a user s branch In the first case all users will experience service disrup tion so the OTDR should detect the same prob lem at exactly the same location in every 1 The WDM PON used is channel in order to detect the exact location of the break Thanks to its WDM nature the fail ure can be diagnosed without affecting other users of the WDM PON Figure 2 shows two OTDR trace examples The first trace gives an example of the expected measurement displayed by the OTDR under normal operation whereas the second one exhibits the expected displayed figure under a fiber break The y axis depicts the signal strength vs distance shown in the
19. nd learning is decoupled from the data plane The research community has nade great progress toward the standardiza tion of a unified management plane For exam ple the Open Networking Foundation ONF has proposed a new protocol called OF Config 9 that defines a number of XML schemas for device management In this light future work will try to implement OF Config as part of the ITB An interesting research direction may also be to use these protocols instead of CLI to con figure the OLT and the OTDR ACKNOWLEDGMENTS The authors would like to acknowledge the sup port of the CRAMnet project funded by the Spanish government under grant no TEC2012 38362 C03 01 to the development of this work Also the authors would like to thank Mr Russ Jones from Ericsson LG for his valuable sup port especially concerning the laboratory trial setup of an earlier section REFERENCES 1 IEEE 802 1ag Connectivity Fault Management http www ieee802 org 1 pages 802 1ag html 2007 2 ITU T Rec Y 1731 OAM Functions and Mechanisms for Ethernet Based Networks 2007 3 C H Lee J Park and J Baik Fault Detection Tech nique in WDM PON 2007 4 G Kaiser Status Monitoring Concept for a WDM PON Int l Congress on Ultra Modern Telecommun and Control Systems and Wksps 2010 5 J S Baik K M Choi and C H Lee Color Free Opera tion of Dense WDM PON based on the Wavelength Locked Fabry P
20. nk layer We propose the above mentioned ITB device which will integrate both physical and link layer functionalities and automatize the IEEE Communications Magazine February 2014 0163 6804 14 25 00 2014 IEEE S7 gt Network operator Equipment alarms events OAM 802 1ag Y 1731 PON networks automatic troubleshooting S S OTDR traces Figure 1 Integrated troubleshooting box ITB architecture process of detection verification and isolation of the failure Finally we conclude this article with a summary and discussion of its main con tributions along with future work worth investi gation THOUBLESHOOTING WDM PON NETWORKS AT THE OPTICAL LAYER OTDR BACKGROUND OTDR equipment allows fiber breaks to be detected and located with a very fine resolution on the order of millimeters Essentially the OTDR equipment launches a very narrowband pulse into the fiber and a response is then received back at the OTDR when any air glass interface in the cable is detected Typical exam ples of air glass interfaces are due to fiber con nectors or fiber breaks The exact location of a fiber break can be inferred from the measured amplitude and delay of the response OTDR equipment can be applied to PONs for the detection of fiber breaks in either the feeder or a branch In TDM PONs the OTDR pulse can be tuned on either the same up down stream wavelength in ba
21. nt standards such as the Gigabit PON International Telecommunication Union Telecommunication Standards Sector ITU T G 984 the Ethernet PON IEEE 802 802 3ah and their recent enhancements XG PON1 ITU T G 987 and 10G EPON IEEE 802 3av use a 1 x N passive splitter combiner to divide the optical signal to all users in the downstream direction and aggre gate the users data in the upstream direction TDM access sharing is required in the upstream direction to avoid collisions between users data On the other hand for PONs based on WDM the power splitter combiner is replaced by a wavelength selective filter usually an array waveguide grating AWG thus allowing a dedi cated wavelength with symmetric bandwidth between each user and the central office Despite their differences both types of PONs share a main drawback related to the high oper ational expenditures OPEX derived from their manually troubleshooting procedures as follows Typically most vendor equipment offers proac tive alarms related to physical and link layer aspects such as link down frame loss or power level events These alarms are often followed by a set of manual measurements launched by the network manager to detect and locate the fail ure usually comprising fiber breaks or dirty con nectors In this light the network manager must devote some time to manually connecting exter nal measurement equipment with optical time domain reflectometry OT
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