Fibre can slow down at night due to peak-time congestion, access-layer contention, or incorrect speed testing over WiFi. If PON is solid and LOS is off, the issue is not signal loss but network load or testing method. Proper wired testing is required before escalation.
What the Problem Means
If your fibre is slower at night but:
- PON is solid green
- LOS is off
- ONT shows normal status
Then the issue is not a physical signal fault.
In South Africa, slower night speeds are typically related to:
- Peak usage periods (6pm–10pm)
- Access-layer contention ratios
- Cabinet or exchange load
- Testing over WiFi instead of Ethernet
This is different from a red LOS or flashing PON condition.
Why It Happens
Common causes:
1. Peak-Time Congestion
Even on fibre, multiple homes share access infrastructure. During peak hours:
- More simultaneous streaming
- More downloads
- Higher cabinet utilization
This may reduce throughput.
2. WiFi Testing Errors
Many "slow fibre" complaints are caused by:
- 2.4GHz WiFi usage
- Distance from router
- Interference
- Mesh hop latency
WiFi speed ≠ fibre line speed.
3. Router CPU Saturation
At high traffic volumes:
- Entry-level routers may bottleneck
- NAT processing may slow throughput
ONT remains stable; internal equipment becomes the limiter.
4. Background Upload Saturation
Cloud backups or large uploads can:
- Saturate upstream bandwidth
- Reduce downstream performance
- Trigger buffer delays
Wired vs WiFi Speed Comparison Table
| Test Method | Accuracy for Line Speed | Common Result |
|---|---|---|
| Ethernet directly to router | High | True line speed |
| 5GHz WiFi (near router) | Moderate–High | Slight reduction |
| 2.4GHz WiFi | Low | Major reduction |
| Mesh node (wireless backhaul) | Low–Moderate | Variable speeds |
Always test using wired Ethernet before escalation.
Step-by-Step Diagnostic Process
Check ONT Lights
- PON solid + LOS off = signal stable.
Test Using Ethernet
- Connect laptop directly to router via cable.
Disable Background Downloads
- Pause updates, backups, streaming.
Run Multiple Speed Tests
- Test at:
- Afternoon (off-peak).
- Evening (peak).
Compare Results
- If only peak-time drop occurs but wired speed remains close to plan, this is congestion-related.
Check Neighbour Experience
- If multiple homes report similar issue, access-layer load may be high.
Definition: Access-Layer Contention
Shared bandwidth allocation between multiple homes connected to the same fibre distribution point.
During peak demand, available throughput per household may temporarily decrease without any physical signal loss.
South African Context
In dense suburbs and complexes:
- Evening streaming demand spikes
- Loadshedding shifts usage patterns
- Cabinet-level demand increases
If slowdown resolves after peak hours, it is not a fibre break.
When to Escalate
Escalate when:
- Wired Ethernet test shows persistent underperformance
- Speeds remain low outside peak hours
- Multiple neighbours affected
- ONT shows normal signal state
Do not escalate based on WiFi-only testing.
Typical investigation timelines:
- 24–48 hours for performance analysis
- Longer if capacity upgrades are required
When This Is NOT the Issue
If all of the following are true:
- LOS is red
- PON is flashing indefinitely
Then the issue is signal-related, not peak-time slowdown.
See our ONT light status guides