Rapid growth of fibre-optic networks has resulted in an explosion of improved services to the residential consumer, with fibre to the home (FTTH or FTTx) passing more than 262 million homes globally.
As telecommunications companies replace old copper technologies with fibre, they are able to deliver more bandwidth, reliability, flexibility and security to end-users.
While many carriers with deployed twisted-pair copper networks utilise increasingly complex DSL technology to deliver broadband services, bandwidth demand will ultimately exceed the capacity which can be delivered over those networks.
With nearly unlimited bandwidth, FTTH fibre networks are becoming the go-to technology for next-generation communications worldwide, including distance learning, cloud computing, tele-medicine and more. Additionally, many governments have sponsored broadband plans to install and improve this critical fibre infrastructure in remote and rural areas.
FTTH passive optical networks (PON) are increasingly being deployed to provide optical fibre’s bandwidth advantages at a lower cost than point-to-point architecture affords. However, PONs present unique test challenges when installing and maintaining the FTTH network. This article provides an overview of FTTx PON architectures, identifies test challenges unique to them, and describes optical tests recommended to verify or troubleshoot FTTx PONs, including in-service (live) PONs.
FTTH network architectures
FTTH networks may be deployed using either point-to-point or point-to-multipoint network architecture. Point-to-point architectures include active Ethernet, and offer a dedicated fibre connection from the operator’s local exchange – either a central office (CO) or powered, environmentally controlled vault (EV) – to each subscriber’s premise. A point-to-point architecture also requires electro-optics for each subscriber at both CO or EV and the customer premise.
Gigabit-capable passive optical network (GPON) and Ethernet passive optical network (EPON) are two PON technologies commonly deployed in point-to-multipoint networks. In both, active electronics are located only at the end-points of the PON, reducing network cost and increasing reliability.
An optical line terminal (OLT) residing in the carrier’s CO communicates across the passive optical distribution network (ODN) to optical network terminals (ONTs) located at subscribers’ premises. In the ODN, a single feeder fibre connects the CO to a pedestal- or pole-mounted fibre distribution hub (FDH). The FDH may be located in a neighbourhood or the entry level of a multi-dwelling unit (MDU).
In single splitter PON architectures, a single 1x32, 1x64 or 1x128 splitter is installed at the FDH. A feeder fibre connects the central office to the splitter at the FDH. This splitter separates the downstream signal into 32, 64 or 128 copies. Each of the many splitter ports are then connected through a distribution fibre to a pedestal- or pole-mounted access point (AP) terminal. A drop fibre connects each AP terminal to an ONT installed at each subscriber’s premise. The ONT may be installed either inside or outside the customer premise.
In a distributed PON, two lower split ratio splitters are installed, one at the FDH followed by a second splitter at the multi-port access terminal. The effective split ratio in a distributed PON is the multiple of the distributed splitters (e.g. 1x4 followed by 1x8 results in an effective split ratio of 1x32). This configuration may be used in rural applications to save fibre, or in dense applications, such as MDUs or apartment buildings. For example, drop fibres from a secondary splitter located in the entry level of an MDU would connect to each subscriber’s apartment.
A PON’s trunk-and-branch architecture reduces the amount of fibre required. Additionally, the downstream signal is broadcast to all attached subscribers, so a single downstream transmitter serves multiple customers. In the upstream direction, subscribers are assigned unique time slots in which to transmit, with the signals from the subscribers combined at the passive splitter and sent to the CO over the single feeder fibre. Consequently, a single CO receiver also serves multiple customers.
Traffic on the network
Both GPON and EPON deliver voice, data and IP video signals downstream using 1490 nm wavelength. An optional video overlay may be transmitted downstream at 1550 nm. Voice and data is communicated upstream from the ONT on the same fibre using 1310 nm wavelength.
Downstream signals are optically divided at the splitter into multiple identical signals. The optical power of each signal is reduced by the split ratio plus some additional insertion loss. For example, the optical power level through a 1x32 splitter is reduced by approximately 16 dB, which is slightly more than a factor of 32. While each ONT sees the same downstream signal, downstream data is encrypted and addressed to specific ONTs, so each ONT can only access data addressed to it.
In the upstream direction, multiple subscribers may simultaneously wish to send data upstream to the CO. Since all ONTs use the same upstream wavelength, time division multiple access (TDMA) control is used to prevent multiple ONTs from sending signals at the same time. With TDMA, the OLT assigns each ONT unique time slots during which it can send data in the upstream direction, preventing multiple ONTs from transmitting simultaneously.
FTTH PON testing overview
Proper testing is a critical part of installing, activating and maintaining a PON. While most components are tested during the manufacturing process, they are tested again after splicing and installation of splitters and access terminals. Field testing is required to ensure no excess loss or reflectance has been introduced due to micro-bends in installed fibre, poor splices, macro-bends in splice closures or access terminals, or dirty, damaged or improperly seated connectors.
If not detected and corrected, excess loss or reflectance often results in poor network performance. Performance may initially seem acceptable, but over time, transmission errors may begin to increase long before the need for any maintenance activity would normally be expected. Tests commonly used to verify optical links include the following:
• Connector inspection.
• Insertion loss test.
• Optical return loss test.
• Optical time domain reflectometry (OTDR).
Connector inspection and cleaning during installation and maintenance are among the most effective methods for ensuring an optical network will deliver expected performance. Connector inspection is typically performed using an optical microscope.
To prevent accidental eye damage when inspecting fibres potentially carrying live traffic, a video microscope images the connector end-face and displays the magnified image on a handheld display. Dirt, debris, or damage are easily detected and images may be captured before and after cleaning, then compared for any variation. Connector contamination and damage are the most common causes of poor optical network performance, according to a recent study by NTT-Advanced Technology.
An insertion loss test measures the end-to-end loss of the installed link by injecting light with a known power level and wavelength at one end, and measuring the received power level output from the other end. The measured difference between the transmitted and received power levels indicates the optical loss through the network. Insertion loss is considered acceptable when the measured loss level is lower than the budgeted loss level.
An optical return loss test injects light with known wavelength and power level into one end and measures the power level returned to that same end. The difference between the injected power level and the measured return level is the return loss. Return loss is considered acceptable when it is higher than the budgeted return loss target. A low return loss value (below 35 dB) is often an indication of one or more sources of excess reflection in the network under test, typically due to dirty or damaged connectors or a fibre break.
Since optical network loss is wavelength-dependent, insertion and return loss testing is typically performed using wavelengths at or near those which will be used during network operation. In the case of FTTx PONs, downstream wavelengths of 1490 nm and 1550 nm may be used, while 1310 nm is used in the upstream direction. Consequently, insertion and return loss testing at 1310 nm, 1490 nm and 1550 nm may be required. In practice, testing is often performed only at 1310 nm and 1550 nm, reasonably expecting loss and return loss at 1490 nm to be between the levels measured at 1310 nm and 1550 nm.
If the loss and return loss measured at each wavelength are within the levels budgeted for the link, the optical network may be considered ready for activation. However, in many cases, the network operator requires the network to be more fully documented using an optical time domain reflectometer (OTDR).
An OTDR scans a fibre from one end to measure the length, loss and optical return loss of an optical network. It also locates and measures reflective and non-reflective events in the network due to splices, connectors, micro- or macro-bends, splitters or faults. Operating like a radar, an OTDR injects narrow pulses of light into the fibre-under-test. As each pulse travels down the fibre, imperfections in the fibre scatter some of the light, with some of this Rayleigh-scattered light being guided back up the fibre.
Optical pulses and backscatter experience some loss as they traverse a mated connector pair, mostly due to imperfect alignment between the two connectors. By measuring the difference between backscatter levels before and after the connection, the OTDR is able to measure the loss across each connection.
A Fresnel reflection is generated whenever the pulse encounters a mismatch in the index of refraction, usually at a mated connector. An air gap at a poorly mated connector or an open connector end will generate a strong reflection. This reflected energy is also guided back up the fibre. (Note: The ends of APC connectors are angled to ensure that light is reflected off the end-face at such an angle that it is not captured and guided back up the fibre.)
The OTDR measures the level of returned backscatter and reflections versus time. Since the speed of light through the optical fibre is known, the OTDR is able to convert time-of-flight into distance, creating a trace which plots changes in backscattered and reflected light levels versus fibre length. Losses due to connectors or macro-bends appear as abrupt changes in the backscatter signal level. Reflections due to connectors, air gaps and open ends appear as spikes in the OTDR trace.
End-to-end loss is measured by comparing backscatter level at the beginning of the fibre to backscatter level at the end of the fibre. Reflectance levels are determined by comparing the backscatter level just before a connector to its reflective spike level. Optical return loss can be computed by summing end-to-end backscatter and reflection levels and comparing them to the transmitted pulse power level.
For more accurate connector or end-to-end loss measurements, OTDR traces from opposite ends of a point-to-point network are often averaged using PC-based trace analysis software. Insertion and return loss tests each provide a single numeric value which can be compared to network-specific limits to determine if the optical plant is within acceptable performance values. However, when unacceptable or marginal values are found, insertion or return loss tests cannot locate the source of the problem. An OTDR also measures loss and return loss, but can additionally locate the sources of excess loss and reflections, reporting the distance to high loss or highly reflective events.
Which tests, when and where?
Optical testing is typically performed at various points in a network’s lifetime:
Installation verification testing occurs as the network is being constructed or after network installation is complete, but before the network is activated. This is usually when the most complete testing is performed, and may include insertion and return loss testing as well as OTDR testing. Pass/fail criteria may be applied to end-to-end length, loss and ORL results, as well as to individual event loss and reflectance measurements for splices, connectors and splitters. Formal reports may be generated, including all of the measurements, OTDR traces, pass/fail criteria and pass/fail results.
Maintenance troubleshooting is performed when service outages occur, and typically requires rapid response to restore service as quickly as possible. This is done through reroute restoration and/or fault location, repair and verification before restoring active service. Troubleshooting may also require non disruptive fibre identification to ensure in-service fibres are not disconnected. Maintenance personnel may require a visual fault locator (VFL) to precisely pinpoint the location of breaks or macro-bends in splice or access enclosures.
FTTx PON insertion loss tests
Insertion loss tests are primarily used to test FTTx PONs during installation. Insertion loss testing may be performed on individual fibre segments as they’re installed (e.g. test feeder fibre from CO to FDH, test distribution fibre from FDH to AP, or test drop fibre from AP to subscriber’s premise).
An end-to-end insertion loss test may also be performed on the FTTx PON after it is partially or fully installed (from CO through feeder fibre, splitter, distribution and drop fibres to the AP or customer’s premise). A stable optical light source and an optical power meter are required to measure insertion loss. Access to both ends of the fibre-under-test is required. Consequently, this is typically an out-of-service test.
To measure loss, received power at the far end of the fibre-under-test must be compared to transmit power injected into the fibre at the near end of the fibre-under-test. To simplify loss measurements, the power meter is initially connected to the source with a short jumper cable and the source power level is measured and stored as the 0 dB reference level for that wavelength. Since the source’s output power levels and the power meter’s detector response are different at each wavelength, the power meter must be referenced to the source at each test wavelength.
Once the source and power meter have been referenced at each of the test wavelengths, the source – with the reference jumper still attached – is connected to one end of the fibre-under-test; the power meter is connected to the other end. Received power level is measured and displayed. More conveniently, the power meter can compare the received power level to the stored reference, directly displaying optical loss in dB.
Simple power meters measure power at only one wavelength at a time. To make loss measurements at multiple wavelengths, the source must be configured for each test wavelength in turn. At the same time, the power meter operator must select the appropriate wavelength at the power meter so the correct detector calibration factor and reference level are applied. This is both time consuming and error prone, as it requires coordination between the source operator on one end and the power meter user at the other end of the fibre-under-test.
To reduce test time and eliminate this potential for errors, AFL’s FLX380 FlexTester includes Wave ID. A Wave ID source alternately transmits light at each wavelength. A Wave ID power meter automatically synchronises to the received wavelengths, eliminating the need for source and power meter to be manually switched between wavelengths.
FTTx PON out-of-service OTDR tests
OTDR testing is typically completed as the FTTx PON is being deployed. The feeder fibres connecting CO to FDH are typically the longest links in the PON, are usually the first fibres installed, and may include multiple splices. These may be tested as point-to-point links before the splitter is installed at the FDH.
If a splitter is spliced to the feeder fibre before testing, the loss through each of the splitter legs may be verified. However, this requires testing from each of the multiple splitter outputs, and requires a launch fibre (1000 μm recommended) to allow the splitter loss to be clearly seen and measured. Splitter losses through each of the legs cannot be easily verified by testing from the CO end of the feeder fibre.
Using an OTDR, distribution fibres are typically tested after installation and connection to the splitter. Once attached to the splitter, these fibres may only be tested from the downstream access point or subscriber premise (if the drop fibre is also installed and connected). High-resolution analysis of the distribution and drop fibres may be obtained using narrow pulses, but the OTDR may not be able to measure the splitter loss using narrow pulses. Wider pulses improve the OTDR’s dynamic range, enabling it to more accurately measure the loss of the attached splitter.
OTDR testing during FTTx PON installation testing is usually performed only at 1310 and 1550 nm. During operation, the FTTx PON always utilises 1490 nm in the downstream direction and 1310 nm in the upstream direction. It may additionally utilise 1550 nm as a second downstream wavelength.
Fibre loss is highest at 1310 nm and lowest at 1550 nm, while bending-induced loss is highest at 1550 nm. If end-to-end loss is within acceptable limits at both 1310 and 1550 nm, it is nearly certain to be acceptable at 1490 nm. If no excess losses or reflections are found at 1310 or 1550, none are likely to be found at 1490 nm. Even if the PON will initially be operated using only 1490 and 1310, testing at 1310 and 1550 nm can detect any micro- or macro-bends, and ensures the FTTx PON is capable of adding 1550 nm operation in the future.
Troubleshooting an in-service FTTx PON
Because of its point-to-multi-point architecture, it is possible for one or only a few subscribers to lose service while other subscribers on the same PON continue to receive service. There are several possible causes:
• Equipment or connection problem inside the customer’s premise.
• Failed ONT at the customer’s premise.
• Fault in the distribution or drop fibre from the splitter to the subscriber.
• Fault introduced at the splitter connection to the subscriber’s distribution or drop fibre (e.g. macro-bend introduced while adding another subscriber, or inadvertently disconnecting the distribution or drop fibre to an active subscriber).
If some, but not all, subscribers are affected in an FTTx PON built using distributed splitter architecture, it is possible that all of the affected customers are served from a single secondary splitter. In this case, likely causes include:
• Fault in the distribution fibre serving the secondary splitter.
• Fault in the secondary splitter itself.
In either case, a fault in the feeder fibre or a failure within the OLT is not likely, since the feeder fibre and OLT are also shared by subscribers who are still receiving service.
Troubleshooting normally requires a visit to the subscriber’s premise. A recommended troubleshooting process is described below.
Test procedure when troubleshooting an in-service (live) PON
1. Use an Active ONT Identifier to determine if the ONT at the subscriber’s premise is responding to downstream signals from the OLT. The Active ONT identifier clamps on to 900 nm buffered fibre, or 2 or 3 mm jacketed fibre, and senses and reports the presence or absence of the 1310 nm upstream response from an active ONT.
2. If an active ONT is detected, the fault is either an equipment or connection problem inside the customer’s premise (most likely) or the ONT itself (less likely). Optical tests at the ONT are unlikely to resolve the problem.
3. If an active ONT is not detected, the fault may either be a failed ONT or a fault in the splitter, distribution or drop fibre connecting the feeder fibre to the subscriber.
4. In this case, disconnect the drop fibre from the ONT, inspect and clean the optical connectors on the drop fibre and the ONT. If a damaged optical connector is found on the drop fibre, replace, clean and inspect the new connector before proceeding. If a damaged optical connector is found on the ONT, the ONT likely will have to be swapped out.
5. If connectors are clean and undamaged, check the downstream power level at the ONT using a PON power meter. Some OTDRs include a PON power meter integrated into their OTDR port, enabling immediate detection and measurement of downstream power levels at both 1490 and 1550 nm.
6. If the measured downstream power levels are acceptable, the problem is likely a failed ONT. Swap out the ONT, clean and reconnect the drop fibre, and verify the ONT synchronises to the upstream OLT.
7. If the measured downstream power level(s) are not acceptable, the problem is likely a fault in the distribution or drop fibre, or a fault introduced at the splitter in the FDH. In this case, connect a live PON OTDR to the drop fibre and initiate an upstream OTDR test using the out-of-band 1625 nm wavelength. To prevent disrupting service on the live PON, select an OTDR which prevents the user from initiating 1310, 1490, or 1550 nm OTDR tests when live traffic is present.
8. Some OTDRs also allow the operator to test only the customer fibre (distribution and drop) or to test through the splitter. Unless multiple customers are affected, the problem is most likely in the distribution and drop fibre, so testing only the distribution and drop fibre is a good bet.
9. Review the 1625 nm trace and event table to determine if there is a break or any excess losses or reflections in the distribution and drop fibres. If so, locate the problem location, repair the fault, then verify the fix by rescanning the fibre using the same 1625 nm test. To precisely pinpoint macrobends or breaks within a splice enclosure or access point, disconnect the OTDR and connect a visual fault locator (VFL, a visible red laser). Enable the VFL and look for the point where the fault causes red light to escape from the fibre.
10. If no excess losses or reflections were identified in the OTDR trace, rescan the FTTx PON from the same location at 1625 nm using the ‘Test through Splitter’ setup. This will provide a trace of the distribution and drop fibres with sufficient dynamic range to see through the splitter and measure the splitter loss. Since other probable causes have been eliminated, likely problems are a break or macrobend at the splitter, or the splitter has been disconnected from the distribution fibre. These will manifest themselves either as excess loss at the splitter, or as the fibre end being detected at the splitter. Repair the fault, then verify the fix by rescanning the fibre using the same setup.
11. Once fibre restoration is complete, verify the proper downstream power levels are available at the end of the drop fibre, clean and reconnect the drop fibre to the ONT, and verify the ONT synchronises to the upstream OLT.
Summary
Passive optical networks are being deployed worldwide to more cost-effectively deliver higher bandwidth broadband services to subscribers. FTTx PONs present unique installation verification and maintenance troubleshooting challenges. These challenges are effectively overcome when technicians understand FTTx PON architecture and are equipped with test tools designed to address the unique test requirements of FTTx PONs.
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