What is Network Congestion?
Network congestion occurs when more data packets are sent across a network than the network can handle, leading to delays, packet loss and performance problems. In OT environments, congestion can severely disrupt real-time communication, with the risk of process errors or downtime.
Congestion is especially dangerous in networks with deterministic behaviour, where timing is crucial.
⚠️ Symptoms of network congestion
| Symptom | Effect on industrial systems |
|---|---|
| Increased latency | Cycle Time becomes unpredictable |
| Jitter | Variable delays between messages |
| Packet loss | Loss of critical process data or control commands |
| Time-outs or disconnects | Devices fail to respond or drop offline |
| Overloaded switches/routers | Buffers fill up, causing traffic to be dropped |
🧠 Causes of network congestion
| Cause | Description |
|---|---|
| Too much traffic on a single segment | E.g. when combining IT and OT traffic |
| Insufficient network segmentation | No separation between real-time and non-real-time traffic |
| Poorly configured switches | No QoS, insufficient buffering or wrong priorities |
| Broadcast storms | Excessive ARP/multicast traffic without filtering |
| Large file transfers or backups | IT traffic crowding out OT traffic |
🧪 Consequences in OT environments
- Disrupted communication between PLCs, HMIs and sensors
- Delays in Motion Control or fail-safes
- Unreliable logging or SCADA visualisations
- Activation of fail-safe mode or emergency stop due to missing signals
OT systems are often not designed for congestion tolerance, unlike IT networks.
✅ Mitigating congestion
| Measure | Explanation |
|---|---|
| Network segmentation | Separate real-time OT traffic from non-critical data flows |
| Quality of Service (QoS) | Prioritise real-time protocols such as ProfiNET, EtherCAT |
| VLANs for OT traffic | Logical separation of networks at Layer 2 |
| Bandwidth monitoring | Detect peak load or anomalous behaviour in time |
| Rate limiting and storm control | Limit the impact of broadcast or multicast traffic |
| Switches with sufficient performance | Use industrial switches with hardware buffering and real-time features |
🔁 Congestion vs. bandwidth limitation
| Term | Description |
|---|---|
| Congestion | Traffic build-up due to a temporary excess of traffic |
| Bandwidth limitation | Structural limit on the amount of data per unit of time |
| Jitter | Consequence of congestion: variation in delay |
📌 In summary
Network congestion is the invisible enemy of real-time communication. In OT networks, it must be actively managed through segmentation, QoS and monitoring — not merely by adding bandwidth.