Common GPON network problems are mainly divided into several categories, including optical path failure, equipment hardware failure, ONU management abnormality, service performance problems and network protection failure. The following are specific analysis and treatment methods:
Optical path failure (highest proportion)
1. Loss of optical signal (LOS/LOSi)
Phenomenon:
Trunk fiber break (LOS): OLT cannot receive optical signal, and all ONUs under the entire PON port are offline.
Branch fiber break (LOSi): A single ONU is frequently offline, which may be accompanied by the “ONT signal degradation (SDi)” alarm.
Cause:
Fiber bending (bending radius <3cm), connector contamination, physical breakage of optical cable or damage to splitter port.
Solution:
Clean the fiber connector and replace the damaged fiber;
Use an optical power meter to detect the optical attenuation and ensure that the ONU receives light at -8dBm ~ -24dBm (ideal value -18dBm ~ -22dBm).
2. Excessive or critical optical attenuation
Phenomenon: ONU goes offline intermittently, and users are stuck on the Internet.
Cause:
Multi-level splitting superimposed loss (such as 1:16 splitter loss of about 15dB), too many fiber nodes or connector oxidation.
Solution:
Optimize the optical path structure to avoid serial splitting; ensure that the full-path attenuation of ODN is ≤28dB.
GPON optical power standard reference
| Detection point | Standard range (dBm) | Threshold value (needs to be processed) |
| OLT transmit power | +1.5 ~ +5 | < +1 or > +7 |
| ONU receive power | -8 ~ -24 | < -24 |
| Splitters output power | -10 ~ -15 | < -15 |
Equipment hardware failure
1. OLT board failure
Phenomenon:
Communication interruption of the board, chip overheating or repeated restarts, causing the entire board ONU to go offline.
Solution:
Hot-swap and restart the board;
Replace the faulty board or optical module.
2. ONU power supply abnormality
Phenomenon:
The “ONT power failure (DGi)” alarm is issued, but the actual power supply is normal.
Cause:
The AC power supply fluctuates or the power module is damaged.
Solution:
Restart the ONU power supply and replace the voltage stabilizing device.
3. Fan failure
Phenomenon:
Device overheating triggers protection shutdown, and the network management reports that the fan status is abnormal.
Solution:
Clean the dust on the fan or replace the fan module.
ONU management abnormality
1. ONU cannot be registered
Reason:
SN code conflict (ONU with the same SN already exists in the OLT);
OLT does not enable the “auto discovery” function;
Insufficient optical power or distance exceeds the limit (>20km, the ranging parameters need to be adjusted).
Solution:
Check SN code consistency, enable automatic discovery, and check the optical path.
2. OMCI configuration failed
Phenomenon:
“ONT does not receive OMCI configuration” alarm, service interruption.
Cause:
OLT and ONU versions are incompatible or software BUG.
Solution:
Upgrade OLT version or restart the board.
Service performance issues
1. User Internet access failed (error 678)
Cause chain:
The fault may occur in any link:
- ONU network port is not activated (network cable is loose);
- VLAN mapping error on OLT;
- BRAS authentication failed.
Solution:
Check each link: Check the network port indicator → check the OLT data configuration → contact the upper network maintenance.
2. Rogue ONU interference
Phenomenon:
A single ONU frequently disconnects, resulting in poor service quality for all ONUs under the same PON port.
Solution:
Pull out the fibers one by one on the splitter side to locate the faulty ONU and replace the device.
Failure of network protection mechanism
GPON improves reliability through redundant design, but improper configuration will lead to protection failure:
1. Type B protection (OLT side redundancy)
When the main and standby PON ports are switched, if the standby optical path attenuation is too large, the switchover fails.
2. Type C protection (full optical path redundancy)
In case of branch fiber failure, the ONU needs to support dual PON ports, otherwise the switchover cannot be performed.
Comparison of protection mechanisms
| Type | Protection range | Applicable scenarios | Limitations |
| Type B | Trunk optical fiber/OLT board | Enterprise core nodes | Does not protect against branch fiber failures |
| Type C | Trunk + branch optical fiber | Scenarios requiring high reliability (such as finance) | High cost, requires dual PON port ONU |
Daily maintenance suggestions
1. Optical path inspection:
Regularly test the optical attenuation of the redundant ports of the optical splitter and manage the optical fiber with labels.
2. Network management monitoring:
Pay real-time attention to the status of the OLT PON port (display ont optical-info command to detect optical power).
3. Environmental protection:
The temperature of the equipment room is controlled at 15℃~30℃, and the humidity is 40%~65%.
4. User-side self-test:
Teach users to observe the indicator light of the optical modem:
PON always green: normal;
LOS red light: report repair immediately.
Preventive operation example:
When restarting the device, follow the steps below:
Through the above-mentioned layered troubleshooting and maintenance, the failure rate of GPON network can be significantly reduced. For complex failures (such as OLT software BUG), it is necessary to cooperate with the equipment manufacturer to upgrade the system or replace the hardware.
