ONU Troubleshooting Guide: Fixing Signal Loss, Offline & Slow Speed Issues

The Most Common ONU Problems

Fiber ISP support teams deal with a predictable set of ONU issues. Understanding the root causes lets you resolve tickets faster and reduce truck rolls.

Diagnosing ONU Offline Status

When an ONU shows offline, first check the alarm type on the OLT. Different alarms indicate different root causes:

• LOSi (Loss of Signal) — no optical signal from ONU. The fiber is broken, connector is dirty, or ONU is physically disconnected
• DyingGasp— ONU lost power. The subscriber's power supply failed or there is a power outage
• LOFi (Loss of Frame) — ONU is receiving signal but cannot sync. Usually a hardware fault
• LOAMi — OAM connectivity lost. Could be firmware issue or ONU hardware degradation

# Check ONU status and alarms on Huawei OLT
display ont info 0 1 0 all
display ont alarm-info 0 1 0

# Check on ZTE OLT
show gpon onu state gpon-olt_1/1/1
show gpon onu alarm gpon-olt_1/1/1

# Check on VSOL OLT
show onu status gpon 0/1
show onu alarm gpon 0/1

Fixing Weak Optical Signal

Weak signal (-25 to -28 dBm) causes packet loss and slow speeds. The troubleshooting steps:

• Check ONU Rx power on the OLT — if below -27 dBm, there is excessive loss in the fiber path
• Clean connectors — dirty SC/APC connectors are the #1 cause. Use IPA wipes on both OLT and ONU connectors
• Check splitter — each 1:2 split adds ~3.5 dB loss. A 1:32 splitter adds ~17 dB. Verify the total loss budget
• Measure with OTDR — an Optical Time Domain Reflectometer shows exactly where the loss occurs in the fiber
• Check bend radius — fiber bent too tightly (less than 30mm radius) causes macro-bend loss
• Replace patch cord — damaged patch cords at either end are cheap to replace

Resolving Slow Speed Issues

When an ONU is online with good signal but the subscriber reports slow speeds, check these in order:

• OLT service profile— verify the DBA (Dynamic Bandwidth Allocation) profile matches the subscriber's package speed
• TCONT configuration — ensure the T-CONT type and bandwidth allocation are correct
• VLAN tagging — mismatched VLANs can cause traffic to be dropped or rate-limited
• PPPoE profile — check the RADIUS rate-limit attribute if using PPPoE over GPON
• ONU port mode — verify the LAN port is in the correct bridge/route mode
• Congestion — check if the PON port is oversubscribed (too many active ONUs on one splitter)

# Huawei — check service port and rate limit
display service-port port 0/1/0 ont 5
display ont traffic 0 1 0 5

# ZTE — check ONU bandwidth profile
show gpon onu profile gpon-onu_1/1/1:5
show gpon onu traffic gpon-onu_1/1/1:5

# VSOL — check ONU bandwidth
show onu bandwidth gpon 0/1 onu 5

ONU Registration Failures

If an ONU appears in auto-discovery but fails to register, check:

• Serial number format — ensure you are using the correct SN format for your OLT brand (hex vs ASCII)
• ONU type mismatch— the ONU model must match or be compatible with the OLT's supported ONU types
• Maximum ONU limit — each PON port has a maximum ONU count. Check if the limit is reached
• Authentication mode — verify the OLT is set to the correct auth mode (SN-auth vs password-auth)
• Firmware compatibility — very old or very new ONU firmware may not be compatible with your OLT version

How ISPbills Streamlines ONU Support

ISPbills reduces ONU troubleshooting time by giving your support team all the information they need in one screen:

• Real-time signal levels — see Rx/Tx power for any ONU without logging into the OLT CLI
• Historical signal trending — detect gradual signal degradation before it causes outages
• ONU status dashboard — online/offline status for all ONUs across all OLTs in one view
• Alarm notifications — get SMS/email alerts when an ONU goes offline or signal drops below threshold
• Remote reboot — restart an ONU remotely from the ISPbills dashboard
• Subscriber mapping — instantly see which subscriber is affected when an ONU has issues