GPON Optical Troubleshooter

Description

This procedure is designed to ensure that the optics are all within specification and properly maintained. There are classes of problems that are due to problems with the fiber, lasers, or receivers that result in bit errors but no hard failures. This procedure ensures that the optics are within specification isolate and resolve any issues.

Pre-Conditions:

• Ensure all RX Power High or Overload conditions are addressed first. These should be addressed using the procedure for that alarm type (refer to the Alarms List).
• Ensure all LOPL-LOS alarms associated with this ONT are cleared using the procedure for that alarm type (refer to ONT Loss Of Signal).
• Ensure that all Dying Gasp (refer to ONT Dying Gasp) and AC Power Fail alarms (refer to ONT AC Power Failure) on this PON are cleared or not relevant to the problem being worked. Dying Gasp and AC Power alarms indicate that there may be a power outage affecting the ONTs on this PON.

  Note: The 100 series ONTs, ONT120 or ONT140, do not generate an ONT Dying Gasp alarm. With all ONT Dying Gasp alarms (refer to ONT Dying Gasp), an ONT LOPL LOS alarm (refer to ONT Loss Of Signal) is generated prior to an ONT Dying Gasp. If an ONT LOPL LOS alarm is received from a 100 series ONT, the user should go to the ONT and press the reset button to determine if the ONT is working properly by observing the ONT LED activity once the reset button is depressed momentarily.]

Electrostatic Discharge (ESD)

Observe and follow all ESD instructions while handling Tellabs plug-in equipment.

Caution! The Tellabs plug-in equipment uses semiconductor devices that can be damaged by electrostatic discharge (ESD). Do not remove a plug-in from a protective packing box or equipment rack until time of installation or replacement. To equalize the static charge on the plug-in, establish a body path to ground using an ESD wrist strap connected to a ESD ground. If storing or shipping the plug-in becomes necessary, place the plug-in in a conductive shipping box to prevent static buildup.]

Apply the following precautions:

• Use grounded wrist straps, table mats, floor mats
• Handle static-sensitive devices by the nonconductive areas
• Place static-sensitive cards into an antistatic plastic bag
• Use protective ESD-sensitive packing and shipping bags, when returning repairable components

Laser Safety

Warning! Personnel handling fiber optic cables must be trained for laser safety.

Warning! Optical Fibers emit invisible laser radiation. Avoid direct exposure to the beam. Never look into the end of a Fiber or into a Fiber connector on a cable or a device. Permanent eye damage or blindness can occur quickly from laser radiation.

Fiber Optic Cable

Ensure that the fiber connectors are properly cleaned per local standards. Dirty connectors may result in inaccurate measurements.

Caution: Fiber optic cable installation and maintenance should only be performed by trained service personnel.

Caution:  Do not bend the fiber optic cable to a diameter smaller than 7.5cm/3 inches. Doing so may damage the fiber or prevent the signal from passing through properly.

Trouble Clearing

The probable causes of this alarm are: 

• Laser Failed or Degraded: It is possible that the OLT laser is out of specifications, has degraded over time, or has failed and needs to be replaced.
• ONT Laser Failed or Degraded: It is possible that the ONT laser is out of specifications, has degraded over time or failed and the ONT needs to be replaced.
• Fiber Damaged or Broken: It is possible that the fiber has been broken, been disconnected, the bend radius has been exceeded, or the fiber is damaged.
• Fiber is Dirty: Dirt on the fiber may cause issues with transmission or bit errors. It is essential that fiber be kept clean and free of oil and dirt for proper transmission.
• Splitter Problems: The splitter may have issues and may need to be replaced.
• Improperly Seated connectors: One possible cause of this alarm is that the fiber connectors have been improperly seated. Ensure that two audible clicks are heard when plugging in the fiber.
• Distance too Long: One cause of this problem could be that the fibers are too long and exceed the optical budget. Ensure, at installation, that the optical budget is within specifications. Fiber types and splitter types affect the optical budget. The table below should simply be used as a rule of thumb and should not substitute for an optical budget.
   

  OLT with B+ Optics (Standard Config)
  32-Way Split	20km
  8-Way Split	30km
  OLT with C+ Optics
  32-Way Split	30km

   

  Note 1: GPON Power readings typically need to be made using a power meter that has a through mode. Many power meters used for point-to-point optics do not have this option. The ONT only transmits when it is connected optically to the OLT and therefore the power can only be measured with a power meter that is in line with the OLT/ONT fiber.

  Note 2: GPON uses three different wavelengths. When measuring the optical power, please ensure that the optical meter is set for the correct wavelength.

  Wavelength (nm)	Use
  1550	Downstream RF Video
  1490	Downstream Data/ONT Management
  1310	Upstream Data/ONT Status

Optical Power 

The OLT Downstream Optical Power can be viewed from the EMS GUI. The OLT SFP GPON laser has the capability to monitor and report the output power. To view the downstream optical power:

1. In the Common Tree, expand the ONT and select the ESU. The Links screen is displayed.
2. With a row selected, right-click and select Property. The PON Port Property screen is displayed.
3. In the PON Port Property screen, select the ONT Status tab.
   
   
    
4. In the PON Port Property screen the RX Optical Power is shown for the selected ESU laser.

   Note: The EMS GUI only measures the RX Downstream optical power.  T0 measure the TX power use an optical power meter.

   Installed QOIU	Transmit Limits (dBm/mW)		Receive Limits (dBm/mW
   	Min.	Max.	Min.	Max.
   QOIU7	+1.5/1.413	+5.3/3.162	-27/0.002	-8/0.158
   ONT (typical)	0.5/0.891	+5/3.162	-8/0.158	-27/.002

5. If the power is within specification, then go to Step 9.

6. If the downstream power is zero (0) then either the fiber is broken, or the OLT Laser has failed. If the downstream power is low, either there is a problem in the optical plant or the OLT laser is not within specifications.
   1. Move to the OLT and measure the output at the OLT laser using an optical power meter, or retrieve it using the EMS GUI. If zero, or low at the OLT when compared to the Transmit limits, then the OLT laser or QOIU card is the source of the problem.
      1. Replace the QOIU GPON SFP laser (refer to Replace Laser (SFP/XFP) Module). Then re-measure the power to ensure the problem is resolved.
      2. If replacing the QOIU laser still did not resolve the issue, and the optical power is still out of specification, then replace the QOIU (refer to Replace a Quad Optical Interface Unit (QOIU7)).
   2. If the laser is within specification, then there is a problem in the fiber plant be- tween the far end equipment and the ESU. Go to Step 8.
7. If the Output Power is high at the ONT, it may be necessary to insert a 5 or10 dB pad in the path, typically at the OLT output, to ensure that the receive power falls within an acceptable range. The GPON optics are engineered for a minimum 13 dB loss across the fiber. If this 13 dB loss is not present, the ONT input threshold may be exceeded. Typically, RX Power High alarms (refer to GPON Optical Overload) are present if this is the case. The power should also be measured at the OLT to make sure the OLT side is within specifications.
8. Perform Fiber Plant Maintenance: This point has been reached because the optical power has been found to be low and the OLT laser is within specifications. The problem is within the fiber plant.
   1. Starting at the OLT work towards the ONT.
   2. Clean the fiber using local practices. The fiber should be cleaned every time it is disconnected. Fiber caps should be kept on fibers that are not in use to prevent scratching of the fiber and accumulation of dust.
   3. Measure the output power at the 1490 nm wavelength after cleaning the fiber.
      1. If the output power is ever measured as low, replace the fiber jumper for the previous segment with a known good fiber. Since the OLT output is good, the problem must be in the fiber jumper or possibly in the splitter.
      2. If the output power is still low after replacing the jumper, then replace the splitter or patch panel the fiber was connected to.
9. ONT Optical Problems: This procedure should be used when the optics at the input (downstream optical power is within specifications) are known, but the ONT is experiencing physical layer alarms on the optics.
   1. Measure the optical output of the ONT while it is attached to the OLT. A power meter that can measure in line must be used as the ONT does not transmit unless the OLT has given it a grant to transmit.

      Note: If the power output is measured from the ONT when the fiber is not connected to the OLT, then there is a problem with the ONT and the ONT should be replaced.

   2. If the ONT optical output is not within the specifications found below, then reset the ONT.
       

      Installed QOIU	Transmit Limits (dBm/mW)		Receive Limits (dBm/mW)
      	Min.	Max.	Min.	Max.
      QOIU7	+1.5/1.413	+5.3/3.162	-27/0.002	-8/0.158
      ONT (typical)	0.5/0.891	+5/3.162	-8/0.158	-27/.002

   3. If resetting the ONT resolves the issue, then make note of the ONT and the issue. If this ONT continues to experience problems in the future, then replace the ONT.
   4. If resetting the ONT did not correct the problem, then replace the ONT.