In a livestock farm's control room, a PLC is managing the environmental control system—temperature and humidity sensors upload data every 30 seconds, ammonia concentration triggers exhaust fans automatically, and commands for roll-up doors and feeders arrive within milliseconds. Everything runs smoothly.
Then the surveillance cameras start streaming video, and the SCADA system simultaneously batches up production data for cloud upload. The PLC's real-time communication begins to lag.
This is not a hypothetical scenario. It is a bandwidth competition that happens every day on industrial networks—ordinary network traffic squeezing the space for PLC and other control equipment's real-time communication.
Synchronization communication between PLCs, and data exchange between PLCs and I/O modules, are extremely sensitive to latency and jitter. In real-time control systems, control commands must arrive within precise time windows—even a few milliseconds of delay on a high-speed production line or in an environmentally sensitive farming operation can result in delayed command execution, synchronization errors, or false triggers.
When video streaming, file transfers, and remote logging compete with control data on the same Ethernet network, network congestion is no longer a theoretical risk. Burst traffic from video streams can gobble up bandwidth while PLC I/O messages wait in queue—this is not "occasionally slower," this directly affects the determinism of control logic.
QoS (Quality of Service) in industrial networks assigns priority levels to different types of data, ensuring that critical control messages are transmitted first when the network is congested.
Industrial switch QoS typically relies on the following standards:
IEEE 802.1p CoS (Class of Service): Inserts a 3-bit priority field into the Ethernet frame header, dividing data into 8 priority levels (0-7, with 7 being the highest). This is a Layer 2 (data link layer) marking mechanism.
DSCP (Differentiated Services Code Point): Marks priority in the IP packet header, providing 64 priority levels (0-63). This is a Layer 3 (network layer) marking mechanism.
The switch places packets into different hardware queues based on these tags and uses scheduling algorithms to determine transmission order.
Strict Priority (SP): Processes the highest-priority queue first until it is empty, then moves to the next level. PLC communication is virtually never blocked, but in extreme cases low-priority traffic may receive no transmission at all. Best suited for scenarios with the highest real-time demands.
Weighted Round Robin (WRR): All queues receive service, but higher-priority ones are allocated more bandwidth proportionally. Critical data transmits first while ordinary traffic is not completely starved. Best for networks that need to balance multiple business types.
Hybrid (SP + WRR): The highest-priority queue uses strict priority while remaining queues use weighted round robin. This combination is the most stable choice in most industrial scenarios.
Here is a concrete configuration example using a typical production line:
Case: A large automotive manufacturing factory introduced industrial Ethernet switches and set PLC-to-MES (Manufacturing Execution System) communication traffic as the highest priority, AGV scheduling management traffic as medium priority, and ordinary data and logging as the lowest priority. After QoS deployment, PLC communication latency jitter dropped from ±50μs to ±5μs, and production line downtime decreased by 82%.
Configuration steps:
In livestock farming environments, LoRa wireless networking has become the dominant approach for sensor data acquisition—sheds are difficult to wire, the environment is harsh, and LoRa's long-distance coverage (15+ km in suburban areas), ultra-low power consumption (multi-year battery life), and strong anti-interference capability (spread spectrum technology resists electromagnetic noise) make it an ideal choice for sensor networks.
A typical solution architecture works like this: LoRa terminals collect sensor data inside sheds (temperature, humidity, ammonia concentration, CO₂), transmit via LoRa wireless to an industrial IoT gateway, and the gateway uploads to the cloud platform via Ethernet (or 4G/5G). The industrial Ethernet switch behind the gateway handles return traffic to the cloud and surveillance cameras.
This is where QoS comes into play—when camera video streams and data backhaul run simultaneously, PLC commands for environmental control equipment (exhaust fans, feeders, roll-up doors) must reach their destinations first. Without QoS, video streams could saturate bandwidth for several seconds, causing delayed environmental system response.
In practice: At a smart livestock farm, LoRa sensor data was routed through a gateway to the industrial network. The switch marked PLC control traffic as highest priority (DSCP 46) and surveillance video as medium-low priority. PLCs support remote programming and debugging, the cloud platform triggers automatic alarms on equipment anomalies, and all critical control commands remain unaffected by video backhaul.
QoS is not a silver bullet. End-to-end data prioritization requires supporting measures:
VLAN Segmentation: Divide PLC communication, surveillance video, and management data into separate VLANs to reduce broadcast and collision domains. QoS combined with VLAN provides stronger isolation.
Port Rate Limiting: Limit the outbound bandwidth on camera ports to prevent a single business from saturating the switch backplane bandwidth. For example, limit a camera port to 80Mbps, leaving headroom for PLC communication.
Storm Control: Set broadcast and multicast suppression thresholds to prevent switch CPU overload from network broadcast storms.
Industrial environments demand switches that can resist vibration, electromagnetic interference, and operate across wide temperature ranges. PUSR USR-ISG series industrial Ethernet switches support IEEE 802.1p/802.1Q VLAN tagging and QoS traffic prioritization, offering 5/8-port 10/100/1000M port configurations. They feature aluminum housing for fanless cooling, DIN-rail mounting design, operating temperature range of -40℃ to +85℃, 6KV lightning protection, and dual power supply redundancy. Certified under CE, FCC, ROHS, and 3C, they are suitable for smart factories, new energy, rail transit, and other industrial applications.
When selecting an industrial Ethernet switch, the following features deserve attention:
Industrial network QoS is not an "optional nice-to-have"—it is infrastructure for IT/OT converged networks. When control data and ordinary traffic share the same physical network, QoS is the key mechanism ensuring PLC real-time communication is not disrupted.
Configuring QoS is not complicated—identify critical traffic, apply tags, map to queues, and choose an appropriate scheduling algorithm. Once these steps are complete, PLC real-time communication gains determinism assurance, regardless of how video streams, log uploads, or file transfers fluctuate. Control commands arrive on time, every time.
