Optical Troubleshooting
Introduction
The purpose of this document is to describe the optical characteristics of the Tellabs PON equipment and define troubleshooting techniques for isolating issues with the optics in PON systems.
Applies To
This document applies to all Tellabs PON-based products.
Document Number
ENG-012925
Optical Troubleshooting
This document is designed to give a workflow for isolating problems within the optical portions of a PON network. While the PON optics tend to be very reliable, since PON is an all-fiber network, from time to time you will experience issues with the optics. This troubleshooter will help you quickly fault isolate many of the common optics issues.
BIP Errors
The PON optical signal maintains a mechanism for detecting errors both upstream and downstream on the PON. Any frame arriving with an error will be detected by the BIP error mechanism. BIP or Bit Interleave Parity is an 8-bit byte that is used to calculate a parity value for the bits in the frame. If any bits are errored, it will detect the errors, and they will be reported as BIP errors. It is important to understand that if no BIP errors occur, then there is no issue with the optical link and the problem is elsewhere.
If there are no BIP errors, we know that:
- OLT Laser is good
- ONT Laser is good
- Fiber plant for this ONT is good.
- No other ONTs on the PON are affecting this ONTs transmissions.
- The user needs to look elsewhere for any packet loss issues.
The BIP Error statistics can be found on the EMS GUI by going to the common tree, select the ONT which you suspect of experiencing issues, and right-clicking on the ONT and selecting properties. From there, select the ONT Status Tab to see the ONT BIP error information.
Two statistics are kept for each direction on the PON to that ONT:
- BIP Errors - The number of Bit Interleaved Parity errors seen on the link since the last link establishment.
- BIP Error Rate - A rate calculated over a 15-minute interval
In a normal well-maintained network, you would not expect to see very few BIP errors within the network. It is not uncommon to see a few errors in the upstream counter that occur at the startup of the link. After startup is complete, you should almost never see errors. As an example, this ONT in the example above has been running for 30 days with no errors at all. Also, looking at the error rate can be helpful. If the BIP error rate is zero or very close to it, then the PON is operating within spec and error-free across the PON optical link.
If the user doesn't see errors in the link, then the user needs to look else where in the system, and the problem is not with the PON optical interface.
Remote Troubleshooting Errored Links
There are several ways to remotely determine the likely nature of the failure. Since this is almost always a simpler and faster way to diagnose the nature of the failure remotely.
One of the other measures of optical quality is the Optical Receive Power. This measurement is made by the ONT in the downstream (OLT to ONT) direction. This measurement is not nearly as accurate as measuring with a meter but does give a good idea of the receive power. Depending on the components installed and ambient temperature, the ONT measurement can be off by as much as 3dB, but that is rare.
The downstream optical power as measured by the ONT should be in the range of:
| Minimum Power | Maximum Power | |
|---|---|---|
| > -25dBm | < +9dBM | If the value is out of range, then the problem is confirmed. |
The received signal must be in the range of +9dBm to -25dBm for proper operation. Most typical short-range systems should not be lower than about -21dBm if all components are in good shape. Most deployed systems are less than a few hundred meters in length and should have large amounts of optical budget and have very, very low error rates.
Optical Overload
Optical overload happens when the optical power that is being received exceeds the overload threshold of the laser. Essentially the receiver at the far end is being overdriven, and the signal becomes distorted and BIP errors begin to occur. If the optical signal is overloaded, additional attenuation is needed. This is accomplished in one of two ways:
- Swapping the splitter for one with a higher split ratio.
- Adding inline Attenuator modules at the appropriate place.
If all the ONTs on the PON show overload or near-overload conditions, then the attenuator should be a UPC style attenuator and be placed at the OLT faceplate. UPC-style attenuators are typically blue. This will add that amount of attenuation to ALL ONTs on the PON.
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If only a single ONT or a few ONTs are overloaded, the user can add an APC-style splitter to the Splitter output where it goes to the ONT.
It is important to match the style of attenuator to the cable type as mismatching will cause many optical issues. The green or APC modules have the fiber end face beveled at 7 degrees. The blue or UPC modules are flat. Mismatching the connection will cause very little of the fiber end face to be in contact and cause issues with the link.
The correct amount of attenuation should be applied to get the optical signal within the required range. Typically, these attenuators can easily be found online, and they typically come in 5dB, 10dB, and 15dB. Make sure to measure again or look at the ONT signal power after the addition to ensure the signal at all ONTs are within spec by consulting the table above.
Another way to see this type of problem is to look for alarms at the EMS. As these alarms tend to oscillate in and out of alarm it is good to look at the alarm history with the GPON Optical Overload and GPON Rx Power Low filters, allowing you to see all optical alarms in the recent alarms' history. If you see the optical overload alarm, this indicates the signal was too high on the PON.
GPON RX Power Low
The GPON RX Power Low alarm indicates that the PON received power is low at the ONT location. This can be due to the optical network design, problems with the OLT laser transmitter or the ONT laser receiver. If the signal is below the recommended -25dBm or the alarm has been seen on this ONT recently, then there is likely a problem with one of the components above and, as such, requires a site visit to determine the exact cause.
It is recommended that you check for the following:
- Connectors not fully plugged in, reseat and listen for an audible click indicating it is fully seated.
- Clean the connectors to ensure the issue is not due to dust or contamination in the connectors.
PON FEC
The PON standard puts a number of tools at your disposal for resolving problems remotely. The PON equipment supports a feature known as FEC or Forward Error Correction. FEC on the Tellabs PON system is a configurable feature that allows the OLT and ONT to correct errors on the PON port. This can enable clean operation over the PON link even though errors are occurring. GPON supports upstream error correction (only one direction is corrected). The 10G XGS-PON system allows for FEC error correction in both directions. If you enable this and the errors clear up, then this could be a short-term solution to enable error-free operation. This does not change any physical characteristics and the PONs may still have alarms due to low signal levels or signal overload conditions. For network designs where you are reaching the limits of the PON system either due to long distances (>20km), or high split ratios, FEC may be required at all times to achieve error-free performance.
To enable FEC on a link, go to the PON profile you are currently using on the port and click the FEC button on the profile, or clone the existing profile and change the FEC value and apply to the link. FEC is typically on by default for 10G links.
PON Troubleshooting
If the source of the issue cannot be determined from the remote measurements and alarms, it may be necessary to visit the OLT and ONT to take further measurements.
Optical Meters
There are several types of meters that can be used for testing the PON:
- Optical Power Meters - Simple meters that measure optical power, and typically are not very wavelength sensitive. These only measure continuous signals and can only measure the downstream power. These typically only have one connector.
- GPON/XGS-PON Power Meters - Meters are capable of measuring both the downstream continuous laser signal and the upstream bursts of a given ONT. These meters have two connectors and can measure both upstream and downstream power.
- GPON/XGS-PON Testers - These can typically measure power, some can serve as an ONT, some can perform traffic testing through the system.
The user should ensure that your power meter supports SC connectors to match those of PON. The user will likely need a pigtail and bulkhead to handle connecting to the system when testing.
Launch Power and RX Power
There are two values that can be measured for the lasers.
- One is the launch power and indicates the power level of the laser at the laser source. There are no ODN losses here.
- Second is the received power at the far end of the circuit. This measures the power just as it enters the OLT or ONT and is after all the losses in the Optical Distribution Network (ODN). This includes connectors, cabling, splitters, etc.
The user should always try to keep the optics within the parameters specified below. These numbers have a built-in margin and, if adhered to, should result in good error-free operation.
|
Max Launch |
Min Launch Power |
Target RX Power |
RX Power Overload (at laser input) |
Comments |
|---|---|---|---|---|
| +5dBm | +1.5 dBm | > -25dBM | -8dBm | GPON Power Specifications |
| +5dBm | +2 dBm | > -25dBm | -9dBm | XGS-PON Power Specifications |
Troubleshooting Out of Range Values
These are the high-level guidance for handling various optical conditions:
| Condition | Action |
|---|---|
| Power at ONT or OLT received power is too high | Add attenuation, preferably at the OLT or use a higher split ratio. |
| Power at ONT or OLT Launch power is too high | OLT laser should be replaced with a spare, ONT issues are likely to mean ONT replacement. If close to the range, but the far end is error-free, the user can elect to keep |
| Power at ONT or OLT, Launch power is too low | OLT laser or ONT should be replaced. Will cause errored links and issues over time. |
| Large numbers of BIP errors but power is within range. | Look at BIP errors on all ONTs. If All are errored, replace ONT laser. If BIP errors are isolated to a single ONT, then replace ONT. If BIP errors are isolated to a single splitter, replace that splitter. |
Optical Budget
The Optical Budget defines the amount of optical loss that can be in the ODN or Optical Distribution Network. It includes all the connectors, splices, and the losses in the Splitter.
When designing the optical network you should be aware of the optical budget and use tools to ensure that you never violate the optical budget.
When contracting suppliers to provide and install cables, ensure that they provide reports showing the optical quality of each cable run.
The optical budget includes all components within the optical path, including the OLT connectors, fiber run, splitter and connectors, and ONT connectors.
The optical budget for the GPON is 28dB and the optical budget for the XGS-PON is 30dB. In addition, Tellabs recommends 3dB of margin (additional optical power at the receiver) to account for quality of connectors, splicing, variances in manufacturing, and aging over time.
The total optical loss is calculated by adding up:
- All the connector losses
- OLT fiber losses
- Splitter Losses
- ONT fiber losses
Summing all this will give the total optical loss.
The total optical budget is determined by subtracting the minimum receiver sensitivity from the optical launch power.
The optical loss + engineering margin < optical budget for proper error-free operation.
The following shows an example of loss calculation and whether it fits within the optical budget. This calculation uses a 28dB budget and a 3dB margin.
Ensure when designing the ODN that the design meets the optical budget and has a healthy margin to ensure resilience in the face of issues such as dust, aging lasers, etc.