Embedding optical time domain reflectometry (eOTDR) into a small form factor pluggable (SFP) transceiver makes continuous fiber monitoring more cost effective. With today’s growing dependence on broadband services, the need for cost-effective fiber monitoring is escalating. OTDR technology has been in use for several years, primarily to troubleshoot transport networks. This form of fiber testing can be especially valuable when there is a total loss of signal on the network. However, because OTDR has typically been provided through external test equipment, it is not well suited for large-scale residential fiber networks. External solutions that offer continuous monitoring are costly and complex, usually on demand, and can result in service interruptions. To provide a better fit with the needs of passive optical networks (PONs), Bell Labs technology innovations have enabled a whole new category of OTDR testing equipment. This pioneering hardware has OTDR functionality embedded in a small form factor pluggable (SFP) transceiver, which plugs into an optical port in the access node. The OTDR capabilities fit into the SFP chiefly because testing is integrated with data transmissions. In other words, fiber monitoring operates at exactly the same wavelength as user data — keeping it in-band, rather than using a dedicated proxy out-band wavelength. This embedded approach to OTDR – eOTDR - balances the performance of fiber network troubleshooting with its costs. So network operators have a simpler, more effective way to gain the benefits of continuous monitoring and proactive troubleshooting.
Finding fiber problems
Fiber is resistant to environmental factors, such as humidity and noise. But a range of faults can still occur, including accidental cuts during civil works. These problems can happen nearly anywhere on an optical network. Finding the location of a fault can be challenging for several reasons. Most fiber is buried underground, so a visual inspection is difficult. Particularly in passive optical networks, a single PON often covers a wide area, using a point-to-multipoint topology. This topology often uses two levels of splitters, which divide a single fiber coming from the central office (CO) into 32 or more branches running to end users. Localizing a fiber problem on one of the multiple branches beyond a splitter is a key technical challenge of fiber troubleshooting.
How OTDR works
OTDR testing works by sending a series of optical pulses from one end of the fiber and extracting the reflected signal from the same end. The returned signal is then measured and analyzed to provide a view into the network and to detect problems. OTDR findings can show the signal attenuation characteristics of the fiber, splices, connectors and splitters. They also offer a view of highly reflective network components, such as SC/PC connectors, splitters and most types of fiber cuts.
External OTDR solutions
OTDR capabilities can play a key role in maintaining service quality. Currently, two types of external test equipment are available — and both options present operating challenges.
- Remote monitoring — With this approach, equipment placed in the CO can provide proactive troubleshooting for all lines in a network. These solutions can continuously monitor long distances, as well as high split ratios. However, very few network operators are using remote monitoring, because it is costly, operationally challenging and not best fitted for typical PON networks.
- On-demand troubleshooting — This portable OTDR equipment can be deployed as needed, after a problem becomes apparent. That makes it a less-costly way to locate fiber faults. But the on-demand option can result in more service interruptions for subscribers.
Embedded OTDR breakthrough
Bell Labs technology innovations have enabled new OTDR testing equipment that balances measurement performance, cost and speed to offer a better fit with PON needs. As shown in Figure 1, this hardware has OTDR functionality embedded in a SFP transceiver, which plugs into the Alcatel-Lucent Intelligent Services Access Manager (ISAM) and links to Motive Network Analyzer - Fiber.
With OTDR capabilities in the access node, the embedded solution can provide continuous proactive monitoring over large portions of the network, which is shared by a large number of users. But no additional equipment is required, and there are no service interruptions during testing. As a result, operators can gain the benefits of continuous monitoring with far less complexity and cost.
The most revolutionary concept
To allow OTDR capabilities to be embedded within an SFP, innovators at Bell Labs found a way to integrate OTDR testing methods with the data transmission system. That is, they chose to execute fiber monitoring at exactly the same wavelength as user data — keeping it in-band, rather than using a dedicated proxy out-band wavelength, as shown in Figure 2.
How does this work? While conventional OTDR provides reflections from a single pulse, eOTDR uses a binary bit pattern modulated on top of the data stream. To understand how it produces meaningful information, imagine “listening” to reflections from a set of sine waves added on top of normal PON downstream data traffic. Multiple such waves can be Fourier-transformed to produce the same reflection information available from conventional pulse-based OTDR. This unique concept is important for the following reasons:
- Different wavelengths in an optical signal behave in different ways. A proxy wavelength has a different sensitivity than the data wavelength, resulting in OTDR events detected on a proxy wavelength, which are not necessarily impacting traffic.
- If a dedicated proxy wavelength was used for OTDR, subscribers’ Optical Network Terminals (ONTs) would need a filter to block that signal — which would increase complexity and cost.
- A proxy wavelength could also require additional hardware in the SFP, again increasing costs as well as the size of the SFP.
- Passive optical networks are evolving to next-generation technologies, such as TWDM-PON, which introduce more wavelengths. This evolution can make it more difficult to find an available wavelength for testing, if a dedicated proxy wavelength is required. In addition to that, the proxy wavelength would have to be routed around wavelength selective channel filters placed into the network. But with in-band embedded OTDR, the issue is eliminated.
The right OTDR for PONs
OTDR technology has been in use for years. But solutions have used external test equipment that is not a good fit for mass-volume residential fiber networks. But now, groundbreaking developments at Bell Labs have enabled OTDR capabilities to be embedded in the network. This new hardware has been designed to balance the performance and costs of fiber network troubleshooting. So network operators have a simpler, more affordable way to gain the benefits of continuous monitoring. To contact the authors or request additional information, please send an email to firstname.lastname@example.org. Editor's note: This article contains results from a project funded by the German Bundesministerium für Bildung und Forschung under the grant Nr.01BP0701. The responsibility for the content lies solely with the author.