# What Causes Packet Loss on Fibre?

Source: https://urbanx.co.za/knowledge-hub/fno-troubleshooting/what-causes-packet-loss-fibre

**Answer:** Packet loss on fibre occurs when data packets fail to reach their destination. If PON is solid and LOS is off, packet loss is usually caused by router overload, Ethernet issues, congestion, or upstream network instability — not a physical fibre break.

Part of [FNO Troubleshooting & Fibre Diagnostics](https://urbanx.co.za/knowledge-hub/fno-troubleshooting) · 6 min read · by UrbanX Network Operations

## What the Problem Means

Packet loss means some data packets are dropped during transmission.

On fibre, packet loss does not automatically mean:

- The fibre cable is damaged
- There is a red LOS condition
- The ONT is faulty

If your ONT shows:

- PON solid green
- LOS off

Then optical signal is stable. Packet loss must be investigated at:

- Internal network level
- Access-layer congestion level
- Upstream routing level

## Common Causes of Packet Loss on Fibre

1. Router CPU Saturation

Entry-level routers can become overloaded when:

- Multiple devices stream simultaneously
- Large downloads occur
- QoS is misconfigured

Symptoms:

- Intermittent lag
- High ping spikes
- Packet drop under load

2. Ethernet Cable Fault

Damaged Ethernet cables can cause:

- Intermittent disconnections
- Dropped packets
- Speed negotiation issues

This is internal, not fibre-related.

3. Upload Saturation

Heavy uploads (cloud backups, torrents) can:

- Saturate upstream bandwidth
- Cause buffer delays
- Trigger packet drop

4. Access-Layer Congestion

During peak hours:

- Cabinet-level congestion may increase
- Shared infrastructure can drop packets under heavy load

This is temporary and not a fibre break.

5. Upstream Network Instability

If:

- PON is solid
- Speed tests are normal
- Packet loss appears only to specific services

The issue may be upstream routing, not physical fibre.

## Packet Loss Source Identification Table

| Symptom | Likely Cause | Fault Layer | Action |
| --- | --- | --- | --- |
| Red LOS + no connection | Optical signal loss | Access Layer | Escalate |
| PON solid + lag under load | Router overload | Internal | Reduce load |
| Packet loss only during uploads | Upload saturation | Internal | Pause uploads |
| Packet loss to one service only | Upstream route issue | Network Layer | Report |
| Intermittent disconnections | Ethernet cable fault | Internal | Replace cable |

## Step-by-Step Packet Loss Diagnostic Flow

1. **Check ONT Lights**
   - PON solid, LOS off = signal stable.
2. **Test via Ethernet**
   - Avoid WiFi for packet testing.
3. **Pause All Uploads**
   - Disable backups and large transfers.
4. **Restart Router (Not ONT First)**
   - Clear session state.
5. **Test Multiple Destinations**
   - If packet loss only affects one platform, issue may not be local.
6. **Compare Peak vs Off-Peak**
   - If worse at night, congestion likely.

If packet loss persists across all tests with stable ONT lights, escalation is justified.

## Definition: Packet Loss (Access vs Network Layer)

- **Packet Loss** — Occurs when transmitted data fails to reach its intended destination.
- **Access Layer** — At the access layer, packet loss may be caused by congestion or hardware faults.
- **Network Layer** — At the network layer, packet loss may be caused by upstream routing instability or external service issues.

> **INFO:** Packet loss is different from latency and different from LOS.

## South African Context

In dense urban areas:

- Evening streaming increases upstream demand
- Loadshedding shifts usage spikes
- Cabinet-level congestion may occur

Packet loss during peak periods does not automatically indicate fibre damage.

## When to Escalate

Escalate when:

- Packet loss persists during wired testing
- Occurs across multiple destinations
- Persists outside peak hours
- ONT lights show stable signal

Typical investigation timelines:

- 24–48 hours for access-layer testing
- Longer if upstream coordination required

Do not escalate based on WiFi-only packet loss tests.

## When This Is NOT the Issue

If all of the following are true, this is not your fault domain:
- LOS is red
- PON is flashing

Then packet loss is not the primary issue — signal integrity must be restored first.

## Frequently Asked Questions

**Can fibre have packet loss?**

Yes. Fibre reduces but does not eliminate packet loss.

**Does packet loss mean fibre is broken?**

No. Broken fibre usually causes red LOS and full outage.

**Can WiFi cause packet loss?**

Yes. Interference and weak signal increase packet drops.

**Why is packet loss worse at night?**

Peak congestion increases network load.

**Should I restart the ONT for packet loss?**

Only if signal lights indicate instability. Otherwise restart the router first.