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TechNotes-Troubleshooting GPON ONU Fails to Go Online 02

This document describes how to troubleshoot the fault that GPON ONUs fail to go online.
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Troubleshooting GPON ONU Fails to Go Online

Troubleshooting GPON ONU Fails to Go Online

What Is a GPON ONU?

The GPON ONUs described in this document include ONTs and MDUs.

These two types of products belong to the optical network unit (ONU) in the GPON network. Therefore, these two types of products are referred to as GPON ONUs in this document.

ONT Introduction

The ONT is applied to Huawei FTTH (Fiber To The Home) solution.

Figure 1-1 FTTH Solution Network
  • The ONT is placed in the user's home and provides data, voice, RF, and Wi-Fi services for users through the user-side interface.
  • The ONT can be managed through the Web interface.

MDU Introduction

MDU is applied to Huawei FTTB (Fiber To The Building) , FTTC (Fiber To The Curb) solution.

Figure 1-2 FTTB/FTTC Solution Network

  • FTTB: The OLT is connected to the ONU in the outside of the building through optical fibers. The ONU is connected to each user through twisted pairs to provide voice, data, and video services for users in the building.
  • FTTC: The OLT is connected to the ONU in the roadside cabinet through optical fibers. The ONU is connected to each user through twisted pairs to provide voice, data, and video services for users in the cell.
  • The MDU can be managed through the CLI.

What Is a GPON ONU Failing to Go Online?

A failure to go online is also called a registration failure. After the ONU is powered on, it initiates registration to the OLT and establishes a management channel with the OLT. The OLT can manage and configure services only after the ONU goes online.

Fault Locating

On the OLT, run the display ont info command to query whether the ONU Run State is "offline"
huawei#display ont info 0 3 0 0   
-----------------------------------------------------------------------------   
F/S/P                   : 0/3/0   
ONT-ID                  : 0   
Control flag            : active   
Run state            : offline
......
NOTE:

According to the query result, the Run state of the ONU is "offline", that is, the ONU is failure to go online.

GPON ONU Online Process

After the ONU is powered on, it initiates registration to the OLT, the ONU can go online only after the registration succeeds. The OLT can manage and configure services only after the ONU goes online.

Figure 1 shows the process of bringing a GPON ONU online.

Figure 1-3 GPON ONU Online Process
  1. After the O1 Initial state and O2 Standby state are complete, enter the O3 Serial number state.
  2. In the O3 Serial number state:
    1. The OLT sends an serial number (SN) request to the ONU.
    2. The ONU responds to the SN request message sent from the OLT.
    3. Upon receiving the SN response from the ONU:
      • If the SN of the ONU is not configured on the OLT (SN authentication is not required), the OLT allocates a temporary ONU ID to the ONU and enters the O4 Ranging state.
      • If the SN of the ONU is configured on the OLT (SN authentication is required), the OLT compares the SN reported by the ONU with the configured SN. After the comparison succeeds, the OLT enters the O4. If the comparison fails, the initial status of O1 is returned.
  3. After the ranging operation is complete in the O4 Ranging state, the system enters the O5 Operation state.
  4. In the O5 Operation state:

    • If the GPON ONU does not need to perform password authentication, configure a GEM port for carrying the OMCI message on the ONU, and then enable the ONU to go online. In addition, the OLT reports an ONU online alarm to the CLI or NMS.
    • If the GPON ONU need to perform password authentication:
      1. The OLT sends a password request message to the ONU.
      2. The ONU responds to the password request from the OLT and sends the password to the OLT.
      3. The OLT compares the password returned by the ONU with the locally configured password.
        • If the password returned by the ONU is the same as the password configured on the OLT, configure the GEM port for carrying the OMCI message on the ONU, enable the ONU to go online, and report the ONU online alarm to the CLI or NMS.
        • If the password returned by the ONU is different from the password configured on the OLT, the OLT reports a password error alarm to the CLI or NMS. Even if the ONU auto-discovery function is enabled on the PON port, the OLT does not report the ONU auto-discovery. The OLT sends the Deactivate_ONU-ID PLOAM message to deregister the ONU.

Troubleshooting GPON ONU Fails to Go Online

If GPON ONU fails to go online, locate the fault based on the fault scope. The possible causes are as follows:

Figure 1-4 GPON ONU Fails to Go Online Fault Location Roadmap

Troubleshooting the OLT Device Faults

The hardware faults of the OLT include the OLT, service boards, and optical modules.

Fault Locating

If an ONU fails to go online, locate the fault based on the fault scope.

If multiple ONUs fail to go online, the fault locating focuses on the upper-layer OLT. Common faults are as follows:
  • If all the ONUs connected to the GPON port of the OLT are faulty, the GPON port or optical module of the OLT board is faulty.
  • If all the ONUs connected to the GPON board of the OLT are faulty, the GPON board is faulty.
  • If all the ONUs connected to the OLT are faulty, the OLT is faulty.

Handling Procedure

  1. Rectify the fault on the GPON port and GPON optical module of the OLT.

    On the OLT, run the display port state command to query the information of GPON port 0/1/1.
    huawei(config-if-gpon-0/1)#display port state 0   
    ----------------------------------------------------------------------------     
    Port Information                                                                 
    ----------------------------------------------------------------------------   
    F/S/P                        0/1/0   
    Port state                   Online
    ......
    ----------------------------------------------------------------------------     
    Optical Module State                                                             
    ----------------------------------------------------------------------------     
    Optical Module status        Offline                                              
    Laser state                  Fail    
    ......                                     
    • Port state is "Online", it indicates that the GPON port is normal.
    • Optical Module status is "Offline", it indicates that the optical module is not in position or faulty. Remove and insert the optical module to rectify the fault. If the fault persists, replace the optical module.
    • Laser state is "Fail" , it indicates that the optical module laser faulty. Run the undo shutdown portid command to enable the laser of a GPON port, if the laser status cannot be restored, replace the optical module.

  2. Check whether the GPON service board of the OLT is faulty.

    On the OLT, Run the display board 0 command to query the information about the GPON service boards. Services can be carried on the board only when the board is in the "Normal" status.
    huawei>display board 0                                                       
    -------------------------------------------------------------------------        
    SlotID  BoardName  Status          SubType0 SubType1    Online/Offline           
    -------------------------------------------------------------------------        
    0                                                                                
    1       H901GPSFE  Failed                               Offline
    ......               
    
    The board Status is "Failed", The board is in the failed state. The causes for the board failure are as follows:
    • An offline board is added. In this case, insert a board of the same type into the corresponding slot. Several minutes later, check whether the board is in the normal state.

    • Loading the board program fails. In this case, run the display io-packetfile command to check whether the corresponding software package of the board is loaded to the flash memory of the control board. If the corresponding software package is not loaded, load the corresponding board software. If the fault persists after the preceding operations, contact Huawei for technical support.
    NOTE:

    The common abnormal states of service boards are as follows:

    • Board status is "Auto_find" : The board in the auto-find state. The board can be used only after you run the board confirm command to confirm the board.
    • Board status is "Mismatch" : The board is in the type mismatch state. Generally, the type of the board added offline is inconsistent with the actual board type. You need to run the board deletecommand to delete the type of the board in the slot. Then, the system will automatically recognize the board type.
    • Board status is "Prohibited" : The board is in the prohibited state. Before finding, diagnosing, and rectifying a fault of the faulty board, run the board prohibitcommand to prohibit the board. Then the board status is displayed as Prohibited. You can run the undo board prohibitcommand to un-prohibit the board.

Troubleshooting the GPON ONU Device Faults

The hardware faults of the GPON ONU include the hardware fault, GPON upstream port fault, and optical module fault.

Fault Locating

If an ONU fails to go online, locate the fault based on the fault scope.

If a single ONU fails to go online, the fault location focuses on the faulty ONU. The common faults are as follows:
  • The ONT is faulty. The following table lists the status of the indicators on the ONT, If the PON indicator and LOS indicator blink slowly (once /2 seconds), the PON terminal hardware is faulty.
    Table 1-1 PON and LOS indicators of the ONT

    PON

    LOS

    Meaning

    off

    off

    The PON terminal is prohibited by the upper-layer device or the optical power is abnormal.

    Blinks twice a second

    Blinks twice a second

    Blinks twice a second

    off

    The PON terminal attempts to set up a connection with its upper-layer device.

    Steady on

    off

    A connection is set up between the PON terminal and its upper-layer device.

    off

    Blinks once two seconds

    The PON terminal is does not receive optical signals.

    Blinks once two seconds

    Blinks once two seconds

    The hardware is faulty.

  • The MDU is faulty. Check the status indicator of the GPON port on the MDU. If the LINK and AUTH indicators are steady green and the optical fiber is normal, the MDU may be faulty.
    Status indicator of the GPON port

    Indicator

    Color

    Status

    Meaning

    LINK

    Green

    on

    The link functions properly and a connection is set up on the port.

    Green

    off

    The link malfunctions and no connections are set up on the port.

    AUTH

    Green

    on

    An authentication is successful.

    Green

    Blinking (on for 0.25s and off for 0.25s repeatedly)

    An authentication is being performed.

    Green

    off

    No authentication is being performed.

  • The upstream port or optical module of the MDU is faulty. On the MDU, run the display port state command to query the uplink port states, if the Laser state is "Off", it indicates that the optical module is disabled.
    huawei(config-if-gponnni-0/0/0)#display port state   
    ----------------------------------------------------------------------------   
    F/S/P                        0/0/0   
    Laser state               Off
    Port state                   Down
    ......

Handling Procedure

  1. Rectify the fault on the ONT.

    • Restart the ONT and check whether the fault is rectified.
    • Replace the ONT and check whether the fault is rectified.

  2. Rectify the fault on the ONU.

    • If the optical module Laser state is "Off", run the laser auto command to set the optical module to the automatic mode.
    • If the fault is rectified after the optical module is replaced, the hardware of the optical module is faulty.

Troubleshooting the GPON ONU is Deactivated

After the GPON ONU is deactivated, the GPON ONU receives only optical signals and does not transmit optical signals. As a result, the GPON ONU fails to go online.

Fault Locating

In the OLT, run the display ont info command to query the information of ONU, In the query result, the value of Control flag is "deactive", it indicates that the ONU is in the deactivated state.

Example: To query the information about ONU 0 connected to GPON port 0/1/1, do as follows:
huawei(diagnose)# display ont info 0 1 1 0    
Command:           display ontinfo 0 1 1 0   
-----------------------------------------------------------------------------    
F/S/P                   : 0/1/1
ONT-ID                  : 0
Control flag            : deactive
Run state               : offline
......  
NOTE:

Control flag: Indicates the control status of the ONT. The value can be active or deactive. After the ONT is deactivated, the ONT receives only optical signals and does not transmit optical signals. As a result, the ONT fails to go online.

Handling Procedure

  1. Check the reason why the ONU is in the deactivated state.

    • If the value is set to "deactive", you can determine whether to adjust the value according to the actual situation. If the value needs to be adjusted, run the ont activate command on the OLT to activate the ONU.
    • If the misoperation is set to "deactive", run the ont activate command on the OLT to activate the ONU.

  1. On the OLT, run the display ont info command to check whether the ONU Run State is "Online".

    • If yes, it indicates that the ONU fails to go online because the ONU is deactivated.
    • If not, it indicates that the ONU fails to go online because the ONU is not deactivated. Check other causes.

Troubleshooting the GPON ONU Authentication Failure

When the GPON ONU authentication information (such as the authentication mode, SN and password) is incorrect, the GPON ONU fails to be authenticated. In this case, the GPON ONU fails to go online.

Fault Locating

It is found that the authentication information configured on the OLT is inconsistent with the authentication information configured on the ONU.

  1. On the OLT, run the display ont info command to query the authentication information about ONU.
    Example: To query the authentication information about ONU 0 connected to GPON port 0/1/1, do as follows:
    huawei(diagnose)# display ont info 0 1 1 0    
    Command:           display ontinfo 0 1 1 0   
    -----------------------------------------------------------------------------    
    F/S/P                   : 0/1/1
    ONT-ID                  : 0
    Authentic type          : password-auth                                             
    Discovery mode          : always-on mode                                            
    Discovery state         : on                                                        
    Password                : huawei123        //Indicates the authentication password of the current ONU configured on the OLT.
    ...... 
  2. Query the authentication information about MDU.
    On the MDU, run the display password command to query the authentication information about MDU.
    huawei(config-if-gponnni-0/0/0)#display password  
    GPON ONU password: huawei    //Indicates the authentication password of the current ONU.
  3. Query the authentication information about ONT.
    On the ONT, check the authentication password configured for the ONT on the ONT web page.
    1. Log in to the Web configuration interface.

    2. In the navigation tree on the left, choose System Tools > ONT Authentication. In the pane on the right, you can query the authentication mode for the registration of the ONT on the OLT.
      Figure 1-5 ONT authentication
  4. According to the query result, the password configured on the OLT is huawei123, but the password configured on the ONU ishuawei, authentication fails due to inconsistent authentication information.

Handling Procedure

The authentication information configured on the OLT and ONU must be the same, including the authentication mode and password.

NOTE:

The methods for modifying the authentication information of the MDU and ONT are different.

  1. Modifying the MDU Authentication Mode and Authentication Information.

    On the MDU, run the password command to set the password used by the current device to register with the OLT when the device functions as a GPON ONU.

  2. Modifying the ONT Authentication Mode and Authentication Information.

    On the ONT, You can modify the information in the following way:
    1. Log in to the Web configuration interface.
    2. In the navigation tree on the left, choose System Tools > ONT Authentication. In the pane on the right, you can change the authentication mode for the registration of the ONT on the OLT.
      Figure 1-6 ONT authentication
    3. Click Apply.

  3. Modify the ONU authentication mode and authentication information configured on the OLT.

    • On the OLT, run the ont modify portid ontid authtype sn-auth [ sn-value ] [ password-auth { password-value | hexpassword-value } ] command modify the GPON ONU authentication mode and authentication information.

  4. On the OLT, run the display ont info command to check whether the ONU Run State is "Online".

    • If yes, it indicates that the ONU fails to go online because the ONU authentication fails.
    • If not, it indicates that the ONU fails to go online because the ONU authentication fails. In this case, check other causes.

Troubleshooting the ODN Faults

The ODN contains all lines and devices between the optical line terminal (OLT) and optical network unit (ONU), such as backbone fibers, optical splitters, and branch fibers.

Fault Locating

When the ODN line is faulty, the transmission between the ONU and the OLT is faulty. As a result, the ONU fails to go online.

Figure 1-7 ODN Fault

You can determine the possible causes of a fault based on the fault scope:

  • The optical fiber is faulty:
    • If all ONUs fail to go online, the backbone fiber and the optical component may have faults. The OLT generates an alarm 0x2e11a001 The feeder fiber is broken or OLT can not receive any expected optical signals(LOS).
    • If a single ONU fail to go online, the branch fiber and the optical component may have faults. The OLT generates an alarm 0x2e112007 The distribute fiber is broken or the OLT cannot receive expected optical signals from the ONT(LOSi/LOBi).
  • Check the optical power. It is found that the attenuation of the optical power is too large or the signal is unstable, and the packet loss is severe:
    • The connector of the optical fiber connector is dirty. As a result, the optical attenuation increases.
    • The bending radius of the optical fiber is too small. As a result, the optical fiber attenuation is too large.
    • The fiber splicing quality is poor. For example, the splicing point has bubbles. As a result, the transmission of optical signals is unstable and packet loss occurs.

Handling Procedure

For ODN line faults, refer to the following troubleshooting methods and suggestions.

  1. If the optical power attenuation is too large, perform the following operations:

  2. If the fiber connector is dirty or damaged, perform the following operations:

Appendix Common ODN Fault Locating Methods

Checking the Optical Power

Checking the optical power is a common method for ODN troubleshooting. By checking and analyzing upstream and downstream optical power, you can determine whether the optical line quality is good.

Analyzing the Optical Power

In optical power analysis, the actual optical attenuation is compared with the theoretical value to determine the quality of the optical line and locate the abnormal attenuation point in the optical line.

The optical power attenuates after being transmitted through the optical components or optical fibers. Normally, the actual attenuation is close to the theoretical value. If the actual attenuation is much larger than the theoretical value, abnormal attenuation point exists in the optical line.

Figure 1-8 shows the ODN optical line
Figure 1-8 ODN optical line

Actual optical attenuation = Upstream optical power on one side of the test point - Upstream optical power on the other side of the test point. Alternatively, Actual optical attenuation = Downstream optical power on one side of the test point - Downstream optical power on the other side of the test point. These two calculated values are the same. For details about how to measure the optical power, see Measuring the Upstream Optical Power Using the Optical Power Meter and Measuring the Downstream Optical Power Using the Optical Power Meter.

For example, to calculate the actual optical attenuation of two-level optical splitter A in Figure 1-8, do as follows:
  • Method 1: Actual optical attenuation of two-level optical splitter A = Upstream optical power of the IN port on two-level optical splitter A - Upstream optical power of the OUT port on two-level optical splitter A
  • Method 2: Actual optical attenuation of two-level optical splitter A = Downstream optical power of the OUT port on two-level optical splitter A - Downstream optical power of the IN port on two-level optical splitter A
NOTE:
  • If the upstream optical power is used for calculating the optical attenuation of an optical splitter, only the ONU to be tested is powered on. That is, other ONUs connected to the same optical splitter are powered off. This ensures accurate optical power.

    In the preceding example, only ONU 0, ONU 1, or ONU 2 is powered on and the other two ONUs are powered off.

  • It is recommended that you use the downstream optical power for calculating the optical attenuation because the downstream optical is easy to measure.
  • You can run commands to query the optical power if it cannot be measured onsite. However, the optical power of an optical splitter cannot be queried using the CLI. For details, see Querying the Optical Power Using the CLI. When the CLI is used for querying the optical power, the query result is accurate and stable if a great volume of data is transmitted; the query result has a maximum difference of 2 dB from the actual optical power if a small volume of data is transmitted. Therefore, it is not recommended that you run commands to query the optical power.
Table 1-2 lists theoretical optical attenuation.
Table 1-2 Theoretical optical attenuation

Component

Type

Average Loss (dB)

Connection point

Fusion splicing

≤ 0.1

Active connector (fiber adapter)

≤ 0.3

Mechanical splicing or quick connector

≤ 0.5

Optical splitter

1:64

≤ 20.5

1:32

≤ 17.5

1:16

≤ 13.8

1:8

≤ 10.6

1:4

≤ 7.5

1:2

≤ 3.8

Optical fiber

1490 nm / 1577 nm (1 km)

≤ 0.23

1310 nm / 1270 nm (1 km)

≤ 0.35

Table 1-3 describes possible faults and causes if the actual optical attenuation is much larger than the theoretical value.
Table 1-3 Possible faults and causes

Fault

Possible Cause

Connection point (such as mechanical splicing point, fusion splicing point, active connector, and quick connector)

  • Cores at the two ends of an optical fiber of the mechanical splicing point or the fusion splicing point are not aligned.
  • The fusion splicing point has air bubbles.
  • The active connector or the quick connector is faulty or not clean.

Optical splitter

The optical splitter is faulty or the fiber adapter for the optical splitter is not clean.

Optical fiber

  • The connector endface of an optical fiber is unclean, scratched, or indented.
  • The optical fiber connector is too tight or loose.
  • Optical fiber connectors of different types are connected.
  • The optical fiber is excessively bent.
  • The optical fiber is damaged.
  • The multi-mode optical fiber is used.

Querying the Optical Power Using the CLI

If the optical power cannot be measured onsite, you can query the Tx and Rx optical power of the device using the CLI. It is not recommended that you use the CLI to query the optical power because there is a difference between the query result and actual value.

Context

NOTE:
When the CLI is used for querying the optical power, the query result is accurate and stable if a great volume of data is transmitted; the query result has a maximum difference of 2 dB from the actual optical power if a small volume of data is transmitted. Therefore, it is recommended that you use the optical power meter to measure the optical power onsite.
Table 1-4 OLT commands for querying the optical power

Item

Command

Rx optical power of the OLT

display ont optical-info

Tx optical power of the OLT

display port state

Rx optical power of the ONU

display ont optical-info

Tx optical power of the ONU

display ont optical-info

Table 1-5 MDU commands for querying the optical power

Item

Command

Rx optical power of the ONU

display port state

Tx optical power of the ONU

display port state

Measuring the Upstream Optical Power Using the Optical Power Meter

This topic describes how to measure the upstream optical power using the optical power meter.

Prerequisites

  • The OLT and the ONU are powered on.
  • The PON port is enabled.
Tools and Materials
  • A new SC/PC single-mode patch cord not longer than 1 m is recommended.
  • Burst optical power meter: Measures the upstream and downstream optical power without disconnecting a working optical line. This power meter is usually used in external tests. It generally measures the optical power at the ingress for optical signals and displays the result on its screen. Optical signals attenuate from their entry into the optical power to their departure from the optical power, which affects the measurement result. However, this attenuation is not considered in actual external tests.
Impact on the System

Services carried on the optical line will be interrupted.

Precautions

Never look into the optical port or the connector of an optical fiber without eye protection. Never put the optical port towards the flammables.

Clean the connector of an optical fiber after testing the optical power by referring to Cleaning the Connector of an Optical Fiber. This is because if a contaminated optical fiber is connected to a functional optical fiber connector, the connector will be contaminated, which leads to abnormal attenuation and reflection and therefore affects the quality of the optical line.

Procedure

  1. Configure the measurement parameters of the burst optical power meter.

    • Optical power unit: dBm
    • Wavelength (nm): 1310
    NOTE:

    A functional ONU does not proactively send optical signals. An ordinary optical power meter measures only the upstream optical power or downstream optical power at one time and the optical line is disconnected in measuring. Therefore, a bust optical power meter is required in measuring the upstream optical power.

  2. Use the patch cord to connect the bust optical power meter to the measurement point in an ODN link to measure the optical power.

    Points for measuring the upstream optical power are connection points (such as mechanical splicing points, fusion splicing points, active connector, and quick connector) in an ODN link, IN and OUT ports of an optical splitter, OLT PON ports, and ONU PON ports.

    The following figure shows measurement of the upstream optical power using an ONU PON port as an example.

  3. View and record the optical power read from the burst optical power meter.

    NOTE:
    • If the value on the optical power meter changes within a range of 0.2 dBm, take the average value.
    • If the value on the optical power meter changes in a range wider than 0.2 dBm, there is a possibility that the optical fiber is not properly connected, the optical fiber is excessively bent, or the optical fiber connector is unclean.
    • Do not bend the optical fiber. A bent optical fiber may affect the test result.

  4. Remove the burst optical power meter after measurement and reconnect the optical line.
  5. Analyze the quality of the optical line. For details, see Analyzing the Optical Power.

Measuring the Downstream Optical Power Using the Optical Power Meter

This topic describes how to measure the downstream optical power using the optical power meter.

Prerequisites

  • The OLT and the ONU are powered on.
  • The PON port is enabled.
Tools and Materials
  • A new SC/PC single-mode patch cord not longer than 1 m is recommended.
  • Burst optical power meter or common optical power meter
    • Burst optical power meter: Measures the upstream and downstream optical power without disconnecting a working optical line. This power meter is usually used in external tests. It generally measures the optical power at the optical signal ingress and displays the result on its screen. Optical signals attenuate from their entry into the optical power to their departure from the optical power, which affects the measurement result. However, this attenuation is not considered in actual external tests.
    • Ordinary optical power meter: Measures only the upstream or downstream optical power at one time and the optical line is disconnected in measuring.
    NOTE:

    When measuring the downstream optical power, disconnect the optical line and connect the optical power meter to the measurement point. That is, you do not need to connect the optical power meter to the optical line. The optical power meter can be the bust optical power meter or ordinary optical power meter.

Impact on the System

Services carried on the optical line will be interrupted.

Precautions

Never look into the optical port or the connector of an optical fiber without eye protection. Never put the optical port towards the flammables.

Clean the connector of an optical fiber after testing the optical power by referring to Cleaning the Connector of an Optical Fiber. This is because if a contaminated optical fiber is connected to a functional optical fiber connector, the connector will be contaminated, which leads to abnormal attenuation and reflection and therefore affects the quality of the optical line.

Procedure

  1. Configure the measuring parameters of the optical power meter.

    • Optical power unit: dBm
    • Wavelength (nm): 1490

  2. Connect the optical power meter to the measurement point.

    Points for measuring the downstream optical power are connection points (such as mechanical splicing points, fusion splicing points, active connector, and quick connector) in an ODN link, IN and OUT ports of an optical splitter, OLT PON ports, and ONU PON ports.
    • If the measurement point is the IN port of an optical component (such as the IN port of an optical splitter or the ONU PON port), remove the optical fiber of the measurement point and connect the optical fiber to the optical power meter.
    • If the measurement point is the OUT port of an optical component (such as the OUT port of an optical splitter or the OLT PON port), remove the optical fiber of the measurement point and use the patch cord to connect the optical power meter to the measurement point.

    The following figure shows measurement of the downstream optical power using an OLT PON port as an example.

  3. View and record the optical power read from the optical power meter.

    NOTE:
    • If the values on the optical power meter change within a range of 0.2 dBm, use the average value.
    • If the values on the optical power meter change in a range wider than 0.2 dBm, there is a possibility that the optical fiber is not properly connected, the optical fiber is excessively bent, or the optical fiber connector is unclean.
    • Do not bend the optical fiber. A bent optical fiber may affect the test result.

  4. Remove the optical power meter after measurement and reconnect the optical line.
  5. Analyze the quality of the optical line. For details, see Analyzing the Optical Power.

Using the OTDR to Locate Abnormal Attenuation Points on the Optical Line

The optical time domain reflectometer (OTDR) is usually used for locating abnormal attenuation points on the optical line.

Concept

The OTDR is used to test parameters such as the optical fiber curve, return loss, fusion splicing loss, reflection ratio, and length/attenuation/break of the optical fiber on the cable line. The OTDR issues a laser pulse signal to the tested optical fiber, and receives the reflected optical signal on the optical port in certain intervals. Based on the optical power of the Rayleigh scattering and Fresnel reflection, the OTDR shows the signal trail of the whole optical fiber. In this way, the loss of different parts on the optical fiber and the fiber end position can be determined based on the test result.

Figure 1-9 shows the procedure for testing the performance (such as the loss) of the fiber line by using the OTDR.
Figure 1-9 OTDR test procedure

Rayleigh scattering

When the optical fiber is heated during manufacturing, thermal agitation causes uneven atom compression, which leads to uneven material density, and further leads to uneven refraction ratio. When the optical fiber is cooled, the unevenness is fixed and it arises optical scattering, which is called Rayleigh scattering. The Rayleigh scattering is an inherent feature of the optical fiber. Points that can generate Rayleigh scattering exist on the entire optical fiber and they are continuous.

Fresnel reflection

The Fresnel reflection generally occurs on discrete interfaces such as the connector and adapter. It is caused by air gap, misalignment or refraction mismatch. The Fresnel reflection is a discrete reflection and it is generated on some discrete points of the fiber. The reflection points generally include the fiber connector (at the gap between the glass and the air), smooth mirror cross section that blocks the optical fiber, and the fiber end.

OTDR dynamic range

The OTDR dynamic range is a physical quantity used to test the maximum capacity of events on the fiber line. It determines the longest fiber distance that the OTDR can measure. If the OTDR dynamic range is small and the tested optical fiber is with high loss, the remote end may be displayed as noise in the OTDR curve.

Deadzone

A deadzone is two events that are close to each other but still can be measured, namely, the resolution of two events. The deadzone of the OTDR is a certain range within which the OTDR curve cannot reflect the fiber line status due to the impact of Fresnel reflection. Attenuation deadzone is the part of OTDR trail whose measured data is covered by a strong reflection. Event deadzone is the minimum distance between two reflection events when they can still be distinguished. In this case, the distance between two events can be measured, but the loss of each of them cannot be measured.

Event

An event on the optical fiber is anything (apart from normal scattering of the optical material itself) that causes loss or reflection. The event includes all kinds of connections and damages (such as bends, cracks or breaks). An event can be reflective or non-reflective. A reflective event occurs when some pulse energy is reflected (for example on a connector), and it generates a peak signal on the trail. A non-reflective event occurs on the optical fiber at the part where some loss is generated but no reflection occurs, and it generates an angle on the trail.

Optical attenuation

The attenuation of the optical fiber is the power loss occurred when optical signals travel along the optical fiber. The unit of the attenuation (A) is dB, and the attenuation can be calculated using the following formula: A = 10 x lgP1/P2. In the formula, A is the attenuation, P1 is the optical power of the input end, and P2 is the optical power of the output end.

Impact on the System

Services carried on the optical line will be interrupted.

Precautions

  • Select the OTDR whose test wavelength is the same as communication wavelength of the tested system.
  • Select test instruments that are of good performance.
  • Select the OTDR with a relatively large memory.
  • Select the OTDR with USB port or network cable to facilitate data reading.
  • Select the OTDR that is with a relatively long power supply duration and then prepare for power supplying.
  • Do not replace the test instruments during the test to prevent great change of the test value.
  • Record the parameter settings and test results of the instrument in detail during the test. After the test, collect and save the record data for reference in subsequent maintenance.
  • Before storing the instruments, fully charge their batteries to extend the life cycle of the batteries. If the instruments are idle for a long time, charge and discharge the batteries at least once every three months.
  • Clean the connector of an optical fiber after testing the optical power by referring to Cleaning the Connector of an Optical Fiber. This is because if a contaminated optical fiber is connected to a functional optical fiber connector, the connector will be contaminated, which leads to abnormal attenuation and reflection and therefore affects the quality of the optical line.
Never look into the optical port or the connector of an optical fiber without eye protection. Never put the optical port towards the flammables.

Procedure

  1. Connect the tested line.
    1. Disconnect the light source of the tested fiber line.
    2. If the OTDR optical port does not match the connector of the tested optical fiber, prepare a 300-2000 m transitional patch cord, with one end matching the OTDR optical port and the other end matching the connector of the tested optical fiber.

      NOTE:
      If the OTDR optical port matches the connector of the tested optical fiber, an additional 300-2000 m optical fiber can be used to process the deadzone and to test the insertion loss of the terminal connector. The additional optical fiber includes the following two types: the transmitting optical fiber and receiving optical fiber. The transmitting optical fiber is connected between the OTDR optical port and the connector of the tested optical fiber. It is used to cover the front deadzone so that the front part of the tested optical fiber is in the linear stable zone of the OTDR curve, and to measure the insertion loss of the front connector. The receiving optical fiber is connected to the end of the tested optical fiber. It is used to prevent the Fresnel reflection peak from affecting the measurement of the event that occurs close to the end of the tested optical fiber, and to measure the insertion loss of the rear connector.

    3. Clean the OTDR optical port and the fiber connector by using alcohol.

      NOTE:
      Avoid other detergents or refractive index matching liquid that dissolve the adhesive in the fiber connector.

    4. Connect the tested line, as shown in Figure 1-10.

      Figure 1-10 Connecting the tested line

      1. Transitional patch cord

      2. Connector

      3. Connection point

  2. Set parameters.
    1. Set the mode.

      The following modes can be used as required: automatic, manual and fault locating.

      NOTE:
      • The automatic mode is used in common tests.
      • The manual mode is used in the following situations:
        1. The event point is determined and located incorrectly in a short-distance (within tens of meters) or ultra-long-distance test.
        2. Test results of the same optical fiber are different in different tests.
      • The fault locating mode is used to fast locate obvious fault positions.

    2. Set the wavelength.

      Generally, the test wavelength is the same as the communication wavelength of the tested system. For example, if the system opens a wavelength of 1550 nm, the test wavelength must also be 1550 nm.

    3. Set the pulse width.

      The pulse width varies with the distance of the tested optical fiber. The shorter the distance, the smaller the pulse width. Figure 1-11 shows reference values for setting the pulse width.

      Figure 1-11 Reference values for setting the pulse width

    4. Set the measurement range.

      The best measurement range is between 1.5 times and twice the length of the tested optical fiber.

      NOTE:
      • In actual tests, perform the automatic test that is of the maximum measurement range first to locate the faulty section, and then select a proper measurement range that is larger than and closest to the tested distance. In this way, the accuracy of the OTDR is utilized sufficiently.
      • Keep the measurement range twice the length of the tested optical fiber to prevent second reflection at the fiber end.
      • If the measurement range is shorter than twice the length of the tested optical fiber, second reflection peak of the fiber end may occur on the even test curve, hence causing the ghost, which leads to a feint that the optical fiber line is faulty.

    5. Set the average time.

      Generally, the average time is around 30s. The recommended average time is 20s or 30s.

    6. Set fiber parameters.

      The refraction ratio n and backscattering coefficient η can be set based on the ratio and coefficient provided by the manufacturer. If the provided values cannot be obtained, use the default values of the instrument.

      NOTE:
      • If different sections of optical fibers are of different refraction ratios, set the refraction ratio by section to reduce test errors caused by inaccurate settings.
      • If the refraction ratio error is 0.001, the measured distance error can be 0.7 m/km.

    7. Set the event threshold.

      The setting of the event threshold depends on your interested events. Table 1-6 lists reference values for setting the event threshold.
      Table 1-6 Reference values for setting the event threshold

      Threshold

      Minimum Value (Unit: dB)

      Default Value (Unit: dB)

      Maximum Value (Unit: dB)

      Fusion splicing point

      0.01

      All

      1.99

      Reflection

      -98.00

      All

      -11.00

      Fiber end

      3.00

      Automatic detection

      20.00

  3. Test the performance of the optical fiber.
    • Test the fiber curve.
      1. Select Start to perform the test.
      2. The test result is the fiber curve.
    • Test the return loss.
      1. Enter the test interface after testing the fiber curve.
      2. Select Return Loss. Then two marking lines A and B are displayed on the interface.
      3. Move the marking lines A and B to delimit the area for testing the return loss.
      4. Select Return Loss Test to obtain the return loss of area A-B.
    • Test the fusion splicing loss.
      1. Enter the test interface after testing the fiber curve.
      2. Select 4-pt SPL. Then four marking lines a, A, B and b are displayed on the fiber curve.
      3. Move marking lines a and A to the start point and end point of the linear area before the tested event respectively, and move marking lines B and b to the start point and end point of the linear area after the tested event respectively, as shown in Figure 1-12.
        Figure 1-12 Setting the marking lines for a fusion splice event

        1. Tested fusion splice event

      4. Select Fusion Splicing Loss and perform the test. The test result is the loss value of the fusion splice event.
    • Test the reflection ratio.
      1. Enter the test interface after testing the fiber curve.
      2. Select 4-pt SPL. Then three marking lines a, A and B are displayed on the fiber curve.
      3. Move marking lines a and A to the start point and end point of the linear area before the tested event respectively, and move marking line B at the peek of the tested event, as shown in Figure 1-13.
        Figure 1-13 Setting the marking lines for a reflection event

      4. Select Reflection Coefficient and perform the test. The test result is the reflection ratio of the reflection event.
    • Test the length/attenuation/break of the optical fiber.
      1. Enter the interface of the event list after testing the fiber curve.
      2. Check the length of the optical fiber displayed in the event list.
      3. Check whether the fiber curve is consecutive. If the curve is consecutive, the optical fiber is not broken. Otherwise, the optical fiber is broken.
      4. Check the attenuation of the optical fiber between two points by reading the vertical level difference between them directly from the fiber curve.
  4. Save the test data.

    • Common saving mode of the test data includes the OTDR curve mode and figure mode.
    • To save the test data using a USB disk, insert the USB disk directly into the USB port on the OTDR and then export the files.
    • To save the test data using a network cable, connect the computer to the OTDR by using the network cable and then export the files.

  5. Analyze the data.
    1. Check the trail whose vertical axis displays the optical power and horizontal axis displays the distance. The test trail shows the optical power of the return signal relative to the distance.

      NOTE:
      • Normally, the slopes of each section (such as single or multiple spools of optical cables) on the entire curve are basically the same in an OTDR test.
      • A greater slope in a section indicates a greater attenuation in it.
      • If the entire curve is anomalous with large change of slopes or is bent or bracket-shaped, the quality of the optical fiber degrades severely.

    2. Analyze the event.

      Figure 1-14 shows common events.

      Figure 1-14 Common events

      1. Mechanical splicing point

      2. Connection point of the patch cord

      3. Fusion splicing point

      4. Connector

      5. Fiber end

      Figure 1-15 shows common curves of fiber faults.

      Figure 1-15 Common curves of fiber faults

      1. Macro-bend

      2. Break

      3. Extrusion

      4. Incorrect connection or ghost

      NOTE:
      • Ghost: The position of the ghost is generally an integer multiple of the distance from a strong-reflection event to the instrument, and no loss occurs at the position. The ghost can be cleared by selecting a short pulse width or adding attenuation to the front part (such as the OTDR output end) of the strong-reflection even. If the event causing ghost occurs at the fiber end, make a small bending to increase the attenuation for the optical signal that reflects back to the start part.
      • Positive gain: Some connectors are displayed as amplifiers, and the power level seems to be increased by certain gain. Positive gain is caused by the situation that more backscattering optical signals are reflected in the optical fiber after the fusion splicing point than the fiber before the fusion splicing point. Actually, the fusion splicing loss occurs at the fusion splicing point of the optical fiber. The actual loss can be obtained as follows: Perform multiple measurements from the other end opposite to the end where amplifiers are displayed to obtain the measured loss values of the fusion splicing point, and then calculate the average loss; the difference between the gain and the average loss is the actual loss of the fusion splicing point.
      • Fiber connection: Main factors that affect cable safety are mechanical damages. An excessively large connection loss does not affect the connection intensity. Therefore, in the cases that some connecting loss values are excessively large with about 1% of them exceeding the standard value, and the values do not decrease after multiple re-connecting, the fiber connection can still be determined as qualified.

Checking Whether the Optical Fiber Is Damaged Using the Red Pointer

The red pointer, also called visual fault locating meter or visual fault detector, sends red light to check whether the optical fiber has red light leak to locate the damage point of an optical fiber.

Context

You can directly see the position with red light leak by using the red pointer. For onsite observation, it can only be used for locating the damage point of an optical fiber in a short distance.

An optical fiber is generally damaged on the bare fiber, coiled fiber or fusion splicing point.

Precautions

Never look directly into the optical fiber connector or the laser transmit port on the optical port board without eye protection. Never put the optical port towards the flammables.

Procedure

  1. Place the red pointer on the endface of an optical fiber and send red light.
  2. Check whether the optical fiber has red light leak. If the red light leaks, the fiber is damaged.



  3. Replace or re-splice the optical fiber that has red light leak.

    • Replace the optical fiber if its bending is excessively large.
      NOTE:
      The bending diameter of an optical fiber must be longer than 6 cm.
    • Splice the optical fiber again if air bubbles exist at the splicing point.

Cleaning the Connector of an Optical Fiber

This topic describes how to clean the connector of an optical fiber. Frequent insertion and removal or not taking dustproof treatment for a long time causes the connector to be unclean and deteriorated, which compromises the quality of the line. Therefore, you need to take measures to prevent dust and periodically clean optical fiber connectors, including the connector endface of an optical fiber, optical port of an optical module, and fiber adapter.

Prerequisites

Prepare the cleaning tools before cleaning, and follow the instructions in "Precautions".

Context

A large number of optical fiber connectors are used in optical transmission, which are easy to be contaminated in OM. The dust particles that can be seen by a microscope affect the quality of optical signals. As a result, the system performance deteriorates and network stability is affected. For two connected optical components, dust particles may damage the surface of the optical fiber. If the cladding or edge of an optical fiber has dust particles, the cores of two connected optical fibers may not be exactly aligned. As a result, the quality of optical signals is affected.

A 1 μm dust particle on a single-mode optical fiber blocks 1% optical signals and therefore leads to 0.05 dB attenuation loss. A 9 μm dust particle is hard to be seen without a microscope but it completely blocks the core of an optical fiber. Therefore, even an extremely small contaminant that can only be found by an instrument such as a microscope may block the connector of an optical fiber. Besides dust particles, the following contaminants need to be cleaned away:
  • Grease (usually brought by hands)
  • Condensation residues
  • Powder (evaporation residues of water or solvent)

Such contaminants will also damage optical components and are more difficult to clean away than dust particles. To clean optical components, you must follow the corresponding steps.

Tools and Materials

The following lists commonly used cleaning tools and materials:

  • Optical power meter: used for testing whether the laser on the connector of an optical fiber is disabled.
  • Lint-free wipe: a piece of long silk cotton specially used for cleaning the connector endfaces of an optical fiber.
  • Lint-free swab: used for cleaning the optical port of an optical module, and a fiber adapter. It has two specifications: ф2.5 mm and ф1.25 mm. You can select one according to the port type (use the lint-free swab with ф2.5 mm for the ports of SC and FC types, and use that with ф1.25 mm for the ports of LC and MTRJ types).
  • Protective cap: used on the connector of an optical fiber, optical port of an optical module, and fiber adapter.
  • Cleaning tool box: used for placing lint-free wipes and protective caps. Place lint-free wipes and protective caps separately from other tools.
  • Cleaning reagent (alcohol): used for cleaning the connector of an optical fiber. It is flammable and therefore must be safely stored and kept clean.
  • Optical fiber endface magnifier: a microscope (400*) used for checking whether the connector endface of an optical fiber is clean and smooth.

Impact on the System

An optical module must be powered off before its port is cleaned. In this case, services carried on the optical port will be interrupted.

Precautions

  • Never look into the optical port or the connector of an optical fiber without eye protection. Never put an optical port towards the flammables.
  • Never clean an optical fiber connector when the laser is on.
  • ESD discharge damages the equipment. To remove or insert a pluggable optical module before or after cleaning, wear an ESD wrist strap or ESD gloves.
  • Put a protective cap into the cleaning tool box immediately after taking it off. Place unused protective caps in the cleaning tool box, or in the ESD bag for sealed storage. Clean protective caps quarterly (it is recommended to clean them by using an ultrasonic cleaner).
  • Keep your hands clean and dry before cutting a lint-free wipe, and place unused lint-free wipes in the clean ESD bag or the cleaning tool box for sealed storage.
  • After the cleaning, cover the connector of the optical fiber, optical module, and fiber adapter that will not be immediately used with protective caps.

Procedure

  • Clean the connector endface of an optical fiber.
    1. Power off the laser of the connector before cleaning. Disconnect the optical fiber (at both ends) to be cleaned.
    2. Use the optical power meter to test the optical power and ensure that no optical signals are sent from the connector of the optical fiber.
    3. Clip a piece of lint-free wipe into 32 small pieces of the same size.
    4. Use a dry lint-free wipe (two-layer) to wipe the connector endface of the optical fiber along one direction once. For a seriously contaminated connector, use a lint-free wipe (two-layer) dipped with a little cleaning reagent to wipe the connector endface of the optical fiber along one direction once, and then use a dry lint-free wipe (two-layer) to wipe it along one direction once again for ensuring that the connector endface is dry

      NOTE:
      • A lint-free wipe can be used only once. Use the portion of the lint-free wipe that is not touched by your hands.
      • You can use the optical fiber end magnifier to check the cleaning and abrasion condition of an optical fiber connector.

    5. After the cleaning, do not touch the connector. Connect the optical fiber (at both ends) immediately. Cover the optical connectors that will not be immediately used with protective caps.
    6. Power on the laser.
  • Clean the optical port of an optical module.
    1. Power off the laser of the optical module before cleaning. Disconnect the optical fiber (at both ends) from the optical module.
    2. Use the optical power meter to test the optical power and ensure that no optical signals are sent from the port of the optical module.
    3. Wear an ESD wrist strap or ESD gloves to remove a pluggable optical module.
    4. Select lint-free swabs with a suitable diameter according to the type of the optical port. Dip a swab with the cleaning reagent, insert the swab into the inside of the optical port, and then clean it by rotating the swab 360 degrees in one direction along the inner wall of the optical port.

      NOTE:
      The lint-free swab with ф2.5 mm is used for the ports of SC and FC types and that with ф1.25 mm is used for the ports of LC and MTRJ types.

    5. Insert a dry swab of the same type into the inside of the optical port and clean it by rotating the swab 360 degrees in one direction along the inner wall of the optical port.
    6. After the cleaning, connect the optical fiber (at both ends). Cover the ports of the optical modules that will not be immediately used with protective caps. Wear an ESD wrist strap or ESD gloves to insert a pluggable optical module.
    7. Power on the laser.
  • Clean a fiber adapter.
    1. Power off the laser of the optical port before cleaning. Disconnect the optical fiber (at both ends) from the fiber adapter.
    2. Use the optical power meter to test the optical power and ensure that no optical signals are sent from the connector of the fiber adapter.
    3. Select lint-free swabs with a suitable diameter according to the type of the fiber adapter. Dip a swab with the cleaning reagent, insert the swab into the socket inside the fiber adapter, and then clean it by rotating the swab 360 degrees in one direction along the inner wall of the fiber adapter.

      NOTE:
      The lint-free swab with ф2.5 mm is used for the ports of SC and FC types and that with ф1.25 mm is used for the ports of LC and MTRJ types.

    4. Insert a dry swab of the same type into the socket inside the fiber adapter and clean it by rotating the swab 360 degrees in one direction along the inner wall of the fiber adapter.

      NOTE:
      Use an ultrasonic cleaner to clean fiber adapters when there are a large quantity of them.

    5. After the cleaning, connect the optical fiber (at both ends). Cover the fiber adapters that will not be immediately used with protective caps.
    6. Power on the laser.
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Updated: 2019-09-19

Document ID: EDOC1100071261

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