Bridging the Gap in Optical Network Testing by Bill Heselden

Transcription

1 Bridging the Gap in Optical Network Testing by Bill Heselden TABLE OF CONTENTS: 1.0 Meeting Demand Installation Testing Commissioning the MON 05 The evolution of service is driving a desire for seamless access between the end-user and the ultra high-bandwidth optical backbone. ABSTRACT To meet the ever growing demand for more bandwidth and services, Metro Optical Networks (MONs) are being upgraded for seamless connection between the end-user and the ultra high-bandwidth optical backbone. To make this transition, the MON's performance requires a combination of increased data rates, Optical-Add Drop Multiplexers (OADM), and Dense Wave Division Multiplexing (DWDM). Each of these techniques present unique challenges to installers to effectively test this evolving network and its new characteristics. This paper provides an overview of proper testing techniques and equipment needed to effectively install, commission and maintain these evolving networks. Technical Paper

2 1.0 Meeting Demand Even in the midst of the current economic situation, the world continues to demand more bandwidth. The Internet continues to permeate every aspect of our day-to-day lives and we are standing at the threshold of advanced, bandwidth intensive services such as video-on-demand, High Definition TeleVision (HDTV) and increased video conferencing. This evolution of services is driving a desire for seamless access between the end-user and the Ultra high-bandwidth optical backbone. Historically, this access was provided via a fragmented network adopted, and adapted, from the traditional voice networks. Saying that these networks only allow modest data transfer rates is an understatement. To satiate the demand for increased bandwidth requires that the traditional access networks be upgraded. Traditional Metro Optical Networks (MONs) were originally designed to handle low-speed voice applications. When deployed, the explosive growth in data traffic was not foreseen and consequently, these original networks were not designed with high-speed (10GB/sec and above) transmission rates in mind. To upgrade the MONs performance requires a combination of increased data rates, Optical-Add Drop Multiplexers (OADM), and Dense Wave Division Multiplexing (DWDM). Each of these techniques present unique challenges to installers to effectively test this evolving network and its new characteristics. Before the challenges of such a network can be fully appreciated, several aspects of the network must be understood. First, the physical architecture of the MON and the type of fiber spans associated with the network must be evaluated. The typical MON loop is between 40 and 120 km with the optical interface nodes randomly located throughout the loop. Node spacing depends on the placement of the network interface between the Local Access Networks and the Long-haul providers. Second, the basic characteristics of DWDM must be understood. In a DWDM network, a series of closely spaced wavelengths (channels), typically in either the C- band ( nm) or possibly a combination of the C- and L- bands ( nm), are applied simultaneously. This can dramatically increase the overall bandwidth per fiber by up to 160 times. Bridging the Gap in Optical Network Testing Page 2 of 5

3 Finally, these advanced networks require different test and measurement equipment and techniques to effectively install and commission them. They typically require the use of optical test equipment such as an OTDR (Optical Time Domain Reflectometer), Loss Test Set (Power Meter and Light Source) with Optical Return Loss (ORL) Meter, Chromatic Dispersion (CD) and Polarization Mode Dispersion (PMD) measurement instruments, and an OSA (Optical Spectrum Analyzer) Installation Testing During the installation phase the primary objectives of the installer is to measure and verify the bi-directional loss and reflectance within the MON loop and to characterize the relationship of the nodes within the network architecture. This is achieved through the use of the OTDR and LTS. When testing MON loops, bi-directional measurements are imperative. This may be facilitated by the architecture of some historical MON loops, as they traditionally start and end at the same physical location. This will become less common with the insurgence of newer, more desirable Mesh topologies. The testing can also become more complicated when the loop is designed with an add/drop functionality, where some fibers drop out at intermediate nodes, and others enter at that node. This produces a series of segments that are not optically connected. Bi-directional OTDR testing is a necessity for accurate splice loss measurements. Due to mismatched fiber types and core size variations, splices or connectors may appear as a gain in power in one direction and a loss in the opposite direction, neither of which is accurate. Taking the average of the losses in both directions will result in the true loss of the event. With the OTDR being the primary piece of test equipment, it is critical to have proper functionality to address these technical issues. The following guide should be used for proper OTDR selection: Modular Platform - Provides the maximum flexibility of test modules to be used on the same platform, thus providing greater compatibility of test results and the reduction of equipment cost. Multiple Wavelength OTDR Modules - Optical modules with 2, 3 or 4 wavelengths must be available to properly test and document a MON. Specialized Testing Modes - Designed for single setup, repetitive bi-directional testing of high-count fiber cables. This will ensure consistency, eliminate operator-induced errors and reduce the cost of repetitive measurements during installation and commissioning. CMA4000 OTDR Metro Mode or MON Optimization - Allows the OTDR to test with high dynamic range, ensuring complete span evaluation while maintaining high resolution to see all critical network nodes and characteristics. Data Emulation Software - A software package must be available that will allow viewing, printing and bi-directional analysis of all test data acquired. Report generation capabilities are also required for documentation and proper system commissioning Bridging the Gap in Optical Network Testing Page 3 of 5

4 Even though the OTDR will provide total end-to-end attenuation, the most accurate method to measure loss is the use of a Loss Test Set (Power Meter and Light Source). The Light Source injects a known signal level into a fiber and the Power Meter measures the amount of light at the opposite end of the fiber, directly measuring the loss of the entire span. The LTS measurements should also be bi-directional to allow the inclusion of both end connectors and to help detect swapped fibers mid-span. The following guide should be used for proper selection of LTS with ORL equipment: Optical Power Meter - Calibrated at 1310, 1550, and 1625 nm with auto-wavelength detection, auto-switching, data storage, and referencing capability. Sensitivity is not usually an issue, however it should be greater than - 45 dbm. Stabilized Light Source - Including sources transmitting at 1310, 1550 and 1625 nm with a minimum -10 dbm output and longterm stability for accurate results. ORL Meter - ORL functionality is required to accurately document the returned signal level from the entire fiber link. Note: ORL is defined as the ratio of reflected power to incident power Loss Test Set The following table provides general guidelines for each of the fiber's cable and connector loss characteristics: Fiber Cable and Connector Loss Characteristics Characteristic Splice loss (bi-directionally averaged) Mechanical connector loss (bi-directionally averaged) Mechanical connector reflectance Fiber attenuation Typical Specification < 0.15 db < 0.50 db < -45 db (super polish) < -50 db (ultra polish) < -55 db (angled polish) < nm < nm Today's MON generally operate at 2.5 Gb/s and consequently the testing of CD and PMD are not currently required. However, so as not to get caught in the same predicament we face today, many providers are designing MONs with future upgrades in mind. These upgrades will entail more DWDM channels and higher data rates, and as a result, CD and PMD measurements, which were once thought to be only long-haul tests, are now becoming increasingly important in shorter, metro networks. To guarantee optimal system performance as the data rates increase to 10 Gb/s and above, CD and PMD measurements will become necessary to properly install and commission MONs. Bridging the Gap in Optical Network Testing Page 4 of 5

5 In summary, the following table lists some standard measurements that are conducted and documented during the installation phase: Standard Measurements During Installation Installation Measurement OTDR Loss Test Set w/orl CD and PMD End-to-end attenuation (bi-directional) Span/Segment length Location and loss of splices and connectors (bi-directional) Location of add/drops Total ORL (Optical Return Loss) Connector Reflectance Dispersion >10Gb/s NetTest Center Green, Building 4 6 Rhoads Drive Utica, NY USA Toll Free: Tel: Fax: info@nettest.com web: Commissioning the MON Once the fiber installation is complete, final MON commissioning is essential to verify network performance. This is accomplished by conducting many of the same measurements that were performed during the installation phase, plus the analysis of the DWDM system characteristics. Due to the economics involved, and to meet the need for bandwidth expansion, network designers have migrated to DWDM technology. In doing so, the Optical Spectrum Analyzer (OSA) becomes an essential tool for documenting and verifying the performance of the individual channels (wavelengths) in the DWDM system. In order to obtain optimal system performance, each channel is measured for center wavelength, power, optical signal to noise ratio (OSNR) and the overall system gain tilt. Failure to meet these system parameters can adversely affect the data within the system by causing channel loss or increased Bit Error Rate (BER). In addition to spectral range, the OSA module should have superior resolution, to measure tight channel spacing, and provide accuracy over the entire spectrum and temperature range. Furthermore, with the add/drop architecture of the MON, some fibers or wavelengths drop out at certain nodes, while others enter. Therefore, it's paramount to identify and document what fibers or channels are present. By doing so, the actual MON performance may be compared to the designed standards of network architecture to verify that the system meets end-of-line specifications. As a final step, before the final turn-up and commissioning of the network, a comprehensive test of SONET functionality should be performed and documented, if applicable. With the proper test equipment and an understanding of the tests required during the installation and commissioning phases, MONS may be efficiently and effectively deployed. In doing so, the installer opens the floodgates enabling high-speed traffic between the end-user and the Ultra high bandwidth optical networks, thus bridging the gap for seamless data transmission. NetTest Sales Offices Australia Italy Brazil Mexico Canada Singapore China Spain Denmark Sweden France UK Germany USA NetTest is a leading worldwide provider of testing, monitoring and management systems across both the optical and network layers of communications networks. NetTest provides network operators, network equipment manufacturers, component manufacturers and enterprise service providers with the network testing solutions they need. ISO 9000 certified. Bridging the Gap in Optical Testing - LTR 2002 NetTest Inc. All Rights Reserved. Specifications subject to change without notice.