When troubleshooting point-to-point fiber optics links (or PON links) with an OTDR, field technicians often run into active links, in which light is still present in the fiber. This may be a showstopper because a regular 1310nm/1550nm OTDR may not work or (in the case of PON) its test pulses could introduce disturbances (rogue signals) to the network and affect other subscribers. This becomes even more important with bi-directional (BiDi) Ethernet links, since their single fiber strand carries signals in both directions. In traditional duplex ethernet transmission, technicians could at least shoot OTDR pulses towards the receiver, since there should not be any light sources in that dedicated strand, however that is no longer applicable to BiDi.
Bi-Directional (BiDi) Transceivers enable full-duplex transmission over a single optical fiber using a simplex LC connector, with combined transmit (TX) and receive (RX) functions, unlike conventional transceivers with duplex LC for separate TX and RX ports.
Using conventional OTDRs, with 1310/1550 nm pulses, only works for dark fibers, otherwise it would interfere with BiDi or active PON services and generate rogue signals, which could adversely affect live network performance and disrupt service.
A "rogue ONT" refers to an unauthorized or malfunctioning Optical Network Terminal (ONT) that transmits data outside of its assigned time slot, disrupting network performance and potentially causing issues to other subscribers.
To address this roadblock, field technicians could contact the NOC, ask them to turn off the laser for the link under test, hope they are not busy running other tasks, wait for confirmation, to then start testing on that direction. However, to test towards the customer, technicians may have to drive to the customer’s premises and disconnect the transmitter before they can start testing. All this can significantly increase the mean time to repair/resolution (MTTR), decrease the workflow efficiency and increase the operational cost (OPEX).
PROPOSED SOLUTION: (1) Use filtered OTDRs so their sensors don’t get affected by the light present in the fiber. (2) Use test signals with wavelengths that are less intrusive to live services (e.g., common BiDi transceivers use different combinations of 1270, 1310, 1330, 1490 and 1550 nm). VeEX’s FX150+ OTDR, with the Macrobend Buster™ option, has a narrow FWDM filter to only receive the test pulses’ reflections and uses safer in-band 1350nm as well as out-of-band 1650nm pulses for testing, allowing field technicians to perform the test regardless of light being present on the fiber, improving workflow efficiency, reducing truck rolls and overall cost. These two carefully selected wavelengths are compatible with common BiDi versions, GPON, EPON, 10G-EPON and XGS-PON.
Given that common BiDi wavelengths include 1270/1330, 1310/1490, 1310/1550, and 1490/1550 nm, any incoming signal within these ranges will be effectively blocked if present in the fiber link under test. The use of 1350 and 1650 nm test pulses, and their respective filters, guarantee reliable measurements even if the CO or CPE sides are still "hot". When in doubt of the exact wavelength for the signal present in the link/fiber, we recommend using the 1650 nm out-of-band pulses only.
On the other hand, if a common 1310/1550 nm OTDR is used in the presence of a live BiDi signal, it will display a "Signal Present!" warning message and needs to be disconnected from the link. In that case, depending on the direction being troubleshot, field technicians must either call the NOC or go to the customer's premises to turn the signal off, before continuing with the link diagnostic procedures. Either way, it takes time before the technician can resume troubleshooting the link, hindering the mean time to repair (MTTR), efficiency, productivity and increasing operational cost.
Warning: With regular OTDRs, if you ignore the live traffic detection warning message and the signal level is extremely hot, you could damage the OTDR's APD sensor! Also, even if the OTDR can manage to show a trace, there is a good chance that the results will be non-linear due to inability to correctly detect the noise floor.
Caution: The company you work for may not allow the use of OTDRs in live links, as suggested in this article, so you should double check first.
The following video presents a more visual description of the problem and proposed solution.
About the Macrobend Buster Option
Macrobend Buster™ is VeEX's trademark name that identifies a type of OTDRs that use a macro-bend detection technique for dark and live fiber optics networks, including P2P and PON. It helps experienced and inexperienced OTDR users identify macro-bends easily. Besides the obvious application implied by its name, their carefully selected wavelengths enable these OTDRS to be used in multiple live optical distribution network (ODN) test scenarios.
The Macrobend Buster functionality extends beyond mere macro-bend detection; it also safeguards the OTDR sensor from saturation due to signals originating from the Central Office (CO). The Macrobend Buster OTDR's sensor employs FWDM filters to ensure its sensitivity is restricted to the supported wavelengths. The 1350 and 1650 nm wavelengths were carefully selected to avoid conflicts with common services.
Macro-bends are non-reflective events caused by bending fiber optics strands beyond their critical curvature value. Exceeding their critical angle for total internal reflection, forces light to escape. The effects of macro-bends are wavelength dependent, affecting longer wavelengths the most. The maximum bend radius and total loss also depend on the type of fiber being used.
To effectively detect macro-bends, the OTDR analyzes the fiber link using two wavelengths to pinpoint event inconsistencies between two traces, which manifest as characteristic macro-bend signatures. The Macrobend Buster test uses a specialized 1650 nm out-of-band signal, frequently employed in live monitoring of optical distribution networks (ODN), as well as a carefully chosen 1350 nm wavelength to avoid disruptions to active PON services.
Related Test Solutions
- FX150+ - PON-optimized Mini OTDR for FTTx and Metro Fiber Networks
- What is the Macrobend Buster™ OTDR? (article)
- Get a copy of the OTDR Basics poster