How to Troubleshoot a Faulty PON With an OTDR

It is easy to trouble shoot the failure which occurs on a point-to-point FTTx network by using an optical time domain reflectometer (OTDR) test. However, troubleshooting a faulty point-to-multipoint network (i.e., PON network) differs significantly and are more complex than a point to point network. This post will introduce the potential faults which may occur in a PON, and explain how to troubleshoot them with an OTDR.

Brief Introduction of PON
A PON (passive optical network) is a telecommunications network that uses point-to-multipoint fiber to the premises in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises. A basic PON (see Figure 1) consists of an optical line terminal (OLT) at the service provider’s central office and a number of optical network termination (ONT) or optical network units (ONUs) near end users. Sometimes, a second splitter can be connected in cascade to the first splitter to dispatch services to buildings or residential areas (see Figure 2). The International Telecommunications Union (ITU-T) and Institute of Electrical and Electronic Engineers (IEEE) have created several standards for optical access systems based on PON architecture (G.982, G.983 or G.984 for ITU and 802.3ah or 802.3av for IEEE).

Figure 1. simple PON topology

Figure 1. Simple PON Topology

Figure 2.Cascaded PON topology

Figure 2. Cascaded PON Topology

Due to its architecture, operators can easily determine which subscribers are affected, and can also identify possible fault elements such as how many customers are affected and whether the PON is cascaded by using the network monitoring system at the Network Operation Center (NOC).

Possible Scenarios & Potential Faults of a PON
In general, we divide the faulty case of a PON as three scenarios. One case is that only one customer is affected. And the other case is occured in the cascaded PON and all affected customers are connected to the same splitter. The last case is all customers dependant on the same OLT are affected whether the PON is cascaded or not. In the first case, there are three probable potential faults. Fault may appear in the distribution fiber between the cutsomer and the closest splitter, or in the ONT equipment, or even appear in the customer’s home wiring. See Figure 3 (a) & (b).

Figure 3. PON case1.1

Figure 3 (a). PON Case 1—Possible Faults When Only One Subscriber is Affected

Figure 3. PON case1.2

Figure 3 (b). PON Case 1—Possible Faults When Only One Subscriber is Affected

When all customers connected to the same splitter cannot receive service, but others connected to the same OLT can, namely the second case, the cause may be that there is a fault at the last splitter or in the fiber link between the cascaded splitters. See Figure 4.

Figure 4 PON Case 2

Figure 4. PON Case 2—Cascaded PON with Affected Subscribers Connected to Last Splitter

To the thrid case described above, if all customers are affected, the fault may occur in the splitter closest to the OLT, or in the feeder fiber/cable of the network, or directly in the OLT equipment, as the Figure 5 shown.

Figure 5 PON Case 3

Figure 5. PON Case 3—All Subscribers are Affected (All Connected to the First Splitter)

In addition, we should know that if connectors are available at the splitters, terminals, or drops, isolating part of the faulty network will become easier. Inspecting connectors and taking OTDR measurements using 1310/1550 nm wavelengths are often performed on network sections that are out of service.

Why Use The Specific In-service Portable OTDR Device?
In order to troubleshoot PON networks in service, two dedicated tools are available — PON power meter and In-service 1625 or 1650 nm OTDR. As we know, a PON power meter is normally employed to verify that the signal is transmitted correctly to and from the ONT. A PON meter measures the power levels of all the signals and can then discriminate whether the issue comes from the customer’s ONT or from the network. However, you might be very confused with that why use In-service OTDR. The use of a classical OTDR with 1310 or 1550 nm test wavelengths would interfere with the traffic signals and disturb the traffic. At the same time, the traffic signals could also disturb the receiver of the OTDR, making it difficult to interpret OTDR traces. Due to mutual disturbances, classical OTDRs cannot be used, and specific in-service OTDRs are required.

The in-service OTDR was designed specifically for testing live fiber networks. This dedicated device uses an out-of band wavelength (test wavelength far away from traffic wavelength) to enable OTDR testing without disturbing either the network transmitters or the receivers. In the case of a PON network, WDM is no longer needed, except for monitoring purposes (using a remote fiber test system). The PON network is a point-to-multipoint configuration and the troubleshooting test is performed directly from an accessible element (ONT or splitter). The operator can disconnect the element because service is already off downstream toward the customer. First, the in-service OTDR must not disturb the other customers while shooting the OTDR test wavelength upstream toward the OLT, which is most likely the case, as OLTs reject signals above 1625 nm, based on ITU-T recommendations. Second, the traffic signals that the OTDR receives will be rejected to obtain accurate OTDR traces. The specific long-pass filter used to protect the OTDR diode can be added either via a jumper between the OTDR and the network or built into the OTDR.

Most equipment providers enable the use of the 1625 nm wavelength for safe testing. Some countries, such as Japan, are nevertheless pushing the 1650 nm wavelength as reflected in the ITU-T L.41 recommendation, which provides maintenance wavelengths on fiber-carrying signals. The 1650 nm wavelength is preferred based on the design of the filters and also because it is further away from the traffic signals (current and future PON technologies).

Case of PON Troubleshooting with OTDR
In order to make the whole troubleshooting or testing work smothly, it is essential to select the right OTDR tool, the correct pulse width, and the best location to start troubleshooting. OTDR configuration should be set according to the equipment being qualified and the distance to cover.

In response to the possible scenarios and potential faults of a PON described above, here are some solutions with OTDR to be introduced in the following. To avoid complexity, this document only analyzes the cases where connectors are only available at the ONT/OLTs.

Solution 1: Troubleshooting of the Distribution Fiber
Simple PON—Only one subscriber affected. Consider that no connectors are available at the splitter.(see Figure 6)

Case Test Location OTDR Direction What must be Seen Comment Pulse Width to be use Specific OTDR
Case 1 one customer down Customer’s home Disconnect the ONT Upstream Distribution fiber up to the closest splitter Testing through the splitter is not required, as the issue is only on the disrtibution fiber side. Short pluse 3 to 30 ns In-service OTDR

Figure 6-solution1

Figure 4. OTDR is Shot Upstream and Trace only Matters up to the Splitter

Solution 2: Troubleshooting of the Distribution Fiber and the Fiber between the Two Splitters in case of a Cascaded Network
All customers linked to the second splitter are down. Let’s consider the case where no connectors are available at the splitter.(See Figure 7.)

Case Test Location OTDR Direction What must be Seen Comment Pulse Width to be use Specific OTDR
Case 2 all the customers are down after the second splitter Customer’s home Disconnect the ONT Upstream Distribution fiber up to the two splitters Testing through the closest splitter is required. Medium pluse 100 to 300 ns In-service OTDR – short dead zone

This case requires viewing the signal after the splitter. The OTDR used must be optimized for this application and have the shortest possible dead zone as the splitter typically provides 7 to 10 dB loss.

Figure 7-solution 2

Figure 5. OTDR is Shot Upstream and Trace should Display the Traffic through the Last Splitter up to the First One

Solution 3: Troubleshooting of the Feeder
Information received at the NOC shows that all customers are down. As the problem likely comes from the feeder side, the most common way to test the faulty network is to shoot an OTDR downstream from the OLT.(See Figure 8.)

Case Test Location OTDR Direction What must be Seen Comment Pulse Width to be use Specific OTDR
Case 3 all customers are down OLT Downstream Feeder Testing through the splitter is unnecessary. Short pluse 3 to 30 ns Unnecessary

Figure 8-solution 3

Figure 6. OTDR is Shot Downstream and Trace should Display the Traffic Down to the First Splitter

Note: OTDR testing directly from the OLT is certainly the preferred choice when a faulty feeder is suspected (Solution 3), but this method is not recommended in the other cases.

Other PON Test Tools
Except the OTDR, there are some test tools used in PON troubleshooting in different phases, such as PON power meter, loss test set, IP testers (voice, data, video) and coaxial testers etc. Fiberstore can offer these test tools for you with high quality and competitive price. Such as ODTR, we can offer a variety of types of ODTRs in different brands, e.g. JDSU, YOKOGAWA. In addition, more PON related products can be found in Fiberstore. For more information, please visit our website or contact us over sales@fiberstore.com.

Do You Still Worry About The Cost of Fiber Optic Transceivers?

transceiver moduleTo many users, there is an inevitable issue that the cost of fiber optic transceivers will keep adding up over time. This is why the demands of 3rd party compatible fiber optic transceivers have emerged in the market. Actually, 3rd party compatible fiber optic transceivers are the direct solution for a tight budgets. However, some issues mayoccur when using 3rd party compatible fiber optic transceiver that drive users to give it up. The worry of the cost of fiber optic transceivers still exists. This paper is going to talk about the fiber transceiver industry and discuss something you should know about the 3rd party compatible fiber optic transceivers.

Fiber Optic Transceiver Industry
When you buy transceivers for your switch, you are told to buy them from your network equipment manufacturer in order to keep your system running properly and safely. However, the switch vendor doesn’t actually manufacture these transceivers. In fact, the fiber interface transceiver manufacturers will supply a variant of their standard transceiver to the switch vendor for resale. The switch vendor will perform testing of that transceiver against their switch, create a compatibility matrix and SKU for that transceiver and start selling the transceiver. They mark up the price of the transceivers to cover their costs (to test/procure/stock etc..) and make a profit. This is why the “brand” transceiver modules are more expensive.

However, as long as the transceiver complies with the required IEEE and MSA standards all it would take is a quick compatibility test and for the vendor could publish a list of all supported transceivers. Thus, 3rd party compatible transceivers are not hard to be realized. In order to corner the market, the switch vendor will request that the transceiver vendor flash the transceivers EEPROM with a vendor specific identifier. The switch operating system will use the I2C bus to query the transceiver EEPROM data, and verify that the transceiver has the correct identifier. If the identifier doesn’t match, then the OS will not power up the laser. The idea is that the switch vendor doesn’t want you to put anything into your router which hasn’t been approved by them. This is why many users will face error warning when using the 3rd transceivers.

How To Solve? – “My 3rd party transceiver does not work on my switch”
So, how to solve this issue and successfully use 3rd party transceivers on your switch? First, you should know the hidden commands of your switch. I believe some of my blog fans may know it as I have explain it some weeks ago in another papers. Yes, the “service unsupported-transceiver” command. Certainly, it is take Cisco for example, but it is easy to find the equivalent commands in other brand switches along the way. (For more details can visit this paper link.)

3rd Party Transceivers vs “Brand” Transceivers
User who have experience of buying 3rd party transceivers and “brand” transceivers may know that the the major difference is cost. So, how much difference? Assuming you get an identical transceiver from Cisco and Fiberstore, the list price for an SR SFP+ transceiver from Cisco is $1,495 USD, while Fiberstore’s one just listed at $ 18.00 USD. This difference is incredible, but it is the truth. The truth is that you won’t have to sacrifice any quality or reliability with all of the savings you receive. In contrast, you get everything you’ve come to expect from the 3rd party transceivers at up to 90% off list price. As high-density merchant-silicon based switches become mainstream, the per-port cost of the switch is dropping dramatically. The transceiver costs now become a very large part of the total system cost and, for a 48-port switch the transceiver costs could easily exceed the base cost of the switch. 3rd party transceivers help users to save more on their cost of transceivers, so why not do it?

Of course, 3rd party transceivers are good option for your transceivers solutions. However, at least so far, the market is not fully normalized. Though the prices of 3rd party transceivers are very attractive, but the good and bad are intermingled. If you plan to buy the 3rd party transceivers for your switch, you had better to choose a vendor with high reputation. I recommend Fiberstore for you. Why? You may know the answer after you try.

Article Source: http://www.fiber-optic-transceiver-module.com/do-you-still-worry-about-the-cost-of-fiber-optic-transceivers.html

Passive Optical Components — Optical Circulator

Optical Circulators are microoptic devices and can be made with any number of ports but 3 and 4 port versions are most common. Also, it is common to build an asymmetric version where the last port does not circulate around to the first. While this saves some cost this is not the most important reason for doing it. If we make sure the last port does not circulate around to the first, we can use the device in systems where we do not need (or want) this feature. For example, if the input to the first port is directly connected to a laser we certainly don’t want spurious signals to be returned back into it.

One of the great attractions of optical circulators is the relatively low level of loss. Typical devices give a port-to-port loss of between 0.5 dB and 1.5 dB. Optical circulators are very versatile devices and may be used in many applications. For example, a bidirectional link consisting of two fiber strands (one for each direction) is multiplexed onto a single strand of fiber. This might be done to save the cost of fiber. Of course if you did something like this you would need to take particular care to minimise reflections on the link.

Operating Principle

By itself there is no single, simple principle behind the optical circulator. Optical circulators are made of an assembly of optical components. There are many different designs but the key principle is like that of the optical isolator. The basic function of a circulator is illustrated in the figure below. Light entering at any particular port travels around the circulator and exits at the next port. Light entering at port 1 leaves at port 2, entering at port 2 leaves at port 3 and so on. The device is symmetric in operation around a circle.

4-Port Optical Circulator

Light travelling in one direction through a Faraday rotator has its polarisation rotated in one particular direction. Light entering the Faraday rotator from the opposite direction has its phase rotated in the opposite direction (relative to the direction of propagation of the light). Another way of looking at this is to say that light is always rotated in the same direction in relation to the rotator regardless of its direction of travel. This is complicated by the presence of unpredictable polarisation. We could filter the unwanted polarisation out but we would lose (on average) half our light in doing that—and often a lot more. So we separate the incident “ray” into two orthogonally polarised rays and treat each polarisation separately. The two halves of the ray are then re-combined before being output to the destination port.

Here is a figure showing a basic 3-port optical circulator. Its components function as following:

  • Polarising Beam Splitter Cube: This device separates the input ray into two orthogonally polarised rays.
  • Birefringent “Walk-off” Block: This is just a block of birefringent material cut at 45° to the optic axis. A ray incident at a normal to the air-crystal interface is split into two rays of orthogonal polarisation. The ordinary ray is not refracted and passes through unaffected. The extraordinary ray is refracted at an angle to the normal.
  • Faraday Rotator and Phase Plate: This combination passes light in one direction completely unchanged! (In the figure this is the right-to-left direction.) In the opposite direction polarisation of incoming light is rotated by 90°. In the left-to-right direction the Faraday rotator imparts a phase rotation of 45° (clockwise) and the phase plate rotates the light another 45° (also clockwise). Thus we get a net 90° clockwise rotation. In the right-to-left direction the phase plate rotates the light in the same direction (in relation to the direction of the ray of light) as before, that is, anti-clockwise at 45°. The Faraday rotator however rotates the phase in the opposite direction (in relation to the direction of the ray) as it did before, that is, clockwise by the same 45°. That is the phase is rotated in the opposite direction . Thus there is no net change in polarisation. (Of course in practice there are losses due to reflections and imperfections in device manufacture.)

Optical Circulator Path from Port1 to Port2

As shown in the 3-port optical circulator, light travels from Port 1 to Port 2 as following:

  • 1. A ray input on Port 1 is split into two separate rays of orthogonal polarisations. The “ordinary” ray passes through without refraction but the orthogonally polarised “extraordinary” ray is refracted (upwards in the figure).
  • 2. Both rays proceed from left-to-right through the Faraday rotator and phase retardation plates. Both rays are rotated through 90°.
  • 3. The two rays then meet another birefringent walk-off block (block B) identical with the first. The effect of the phase rotation in the previous stage was to swap the status of the rays. The ray that was the ordinary ray in block A (and was not refracted) becomes the extraordinary ray in block B (and is refracted in block B). The extraordinary ray in block A (the upper path in the figure) becomes the ordinary ray in block B (and is not refracted in block B). The light is refracted and re-combined as shown. It is then output to Port 2.

Optical Circulator Path from Port2 to Port3

Coupling to fiber on input and output would normally use a lens of some kind. Typically a GRIN lens might be used here. The path from Port 2 to Port 3 is somewhat more involved:

  • 1. Light entering from Port 3 is split in block B.
  • 2. Travelling in the reverse direction the polarisation of both rays is unchanged.
  • 3. Birefringent block A now passes the upper ray unchanged but shifts the lower one further away. 4. The two rays are then re-combined using the reflector prism and the polarising beamsplitter cube.

Note: If you only connect Ports 1 and 2, the optical circulator can be used as an Optical Isolator. Indeed if you leave out the beamsplitter cube and the reflector prism, you have an excellent (very low loss) polarisation independent isolator. A path from Port 3 to Port 1 can be constructed by adding additional components; however, for most applications this is unnecessary as we don’t want the connection from Port 3 to Port 1 anyway.

Conclusion

There are many ways to construct optical circulators (both 3 and 4 port). All of these ways use combinations of components and similar principles as those described above. The biggest problem with optical circulators is that the components must be manufactured to very close tolerances and positioned extremely accurately. This causes the cost to be relatively high. However, you could find cost-effective Optical Circulators in Fiberstore.

Article Source: http://www.fiberopticshare.com/passive-optical-components-optical-circulator.html