Operational Technology (OT) networks control the physical world. They operate the pipelines, power grids, manufacturing lines, water treatment facilities, and transportation systems that modern society depends on. For decades, these networks ran in isolation, separated from corporate IT infrastructure by design. That separation was considered security enough. It isn't anymore.
As OT environments have converged with IT networks and the broader internet, they've become a prime target for cyberattacks. Unlike a compromised database server, a compromised OT system can cause physical damage, environmental harm, and, in the most serious cases, endanger lives. OT network monitoring gives security and operations teams the visibility to detect threats, investigate incidents, and maintain operational integrity, without disrupting the processes the network controls.
This guide covers what OT network monitoring is, how it differs from conventional IT monitoring, why it's become essential, and what effective OT visibility infrastructure looks like in practice.
OT network monitoring is the continuous collection, analysis, and management of traffic flowing across operational technology environments. These environments include Industrial Control Systems (ICS), Supervisory Control and Data Acquisition (SCADA) systems, Distributed Control Systems (DCS), and the Programmable Logic Controllers (PLCs) and Remote Terminal Units (RTUs) that interface directly with physical equipment.
OT networks exist across nearly every critical infrastructure sector:
What these environments share is a reliance on real-time control systems where availability and reliability take priority over almost everything else. Monitoring these networks requires a fundamentally different approach than monitoring a corporate data center.
Effective OT monitoring collects traffic from across the OT network and delivers it to analytical tools without interfering with operational processes. These tools typically include:
The challenge is getting traffic to these tools reliably and safely, which is where purpose-built OT visibility infrastructure becomes critical.
To understand why OT monitoring requires a specialized approach, you need to understand what makes OT networks structurally different from conventional IT infrastructure.
In IT environments, the standard security hierarchy places confidentiality at the top, followed by integrity, then availability (the CIA triad). In OT environments, availability comes first. A production line that goes offline, a pump that stops, or a power circuit that trips can have immediate physical and financial consequences. Any monitoring approach that introduces risk to availability is unacceptable.
This means that techniques acceptable in IT environments, such as connecting monitoring tools inline where they could introduce latency or become single points of failure, are often off-limits in OT. Passive monitoring approaches that tap traffic without touching the live data path are the preferred solution.
IT hardware typically has a refresh cycle of three to five years. OT hardware often runs for decades. It's common to find PLCs, RTUs, and SCADA systems in production that were installed in the 1990s or early 2000s. These devices:
Traditional IT monitoring methods that rely on software agents installed on endpoints simply don't work in these environments.
OT networks run protocols that most enterprise monitoring tools don't understand natively. These include Modbus TCP, DNP3, PROFINET, EtherNet/IP, BACnet, IEC 61850, and OPC-UA, among others. Each protocol has specific structure, timing requirements, and behavior norms that differ from standard IT protocols like HTTP, DNS, or SMTP.
Meaningful OT monitoring requires tools that can parse these protocols and distinguish normal command sequences from anomalous or malicious ones. Getting traffic to those tools accurately and completely is the foundational requirement.
Many OT networks were designed for reliability and simplicity rather than security. Network segmentation, which is standard practice in IT environments, has historically been minimal in OT. This means that once an attacker gains a foothold in one area, lateral movement can be straightforward.
Monitoring these flat architectures requires coverage across broad network segments, with the ability to aggregate traffic from many points and deliver it to centralized analysis tools.
The threat landscape for OT environments has changed dramatically over the past decade. Three converging trends have made OT monitoring a security and operational priority.
The concept of the "air gap," keeping OT systems physically isolated from external networks, was never perfectly maintained. Today it's largely a myth. Industrial sites connect OT systems to enterprise networks for production data, remote access for maintenance, supply chain integration, and cloud-based analytics. Each connection creates a potential attack path.
This convergence makes OT networks reachable through IT networks. Attackers who compromise an employee's laptop or breach a corporate network can, in many organizations, pivot directly into OT environments from there.
Nation-state actors and cybercriminal groups have demonstrated both the capability and willingness to target industrial control systems. Incidents affecting power grids, water utilities, and manufacturing facilities have illustrated the real-world consequences of inadequate OT security. Without monitoring in place, these attacks can operate undetected for weeks or months before the effects become apparent.
Regulatory frameworks covering critical infrastructure security have expanded significantly. Organizations operating in energy, water, transportation, and other critical sectors face increasing requirements to demonstrate active monitoring, anomaly detection, and incident response capability across OT environments. The ability to produce traffic records and incident timelines depends on having monitoring infrastructure in place before an event occurs.
Understanding the goal is straightforward. Achieving it without compromising OT operations requires careful attention to several specific challenges.
The fundamental requirement for any network monitoring system is access to traffic. In IT environments, Switch Port Analyzer (SPAN) ports are a common method for mirroring traffic to monitoring tools. In OT environments, SPAN ports are often unreliable, may drop packets under load, and can introduce processing overhead on managed switches that affects timing-sensitive control traffic.
Network TAPs provide a better solution for OT environments. Passive fiber TAPs in particular operate with zero power dependency, introduce no latency, and have no active components that can fail. They copy 100% of traffic to monitoring tools without touching the live data path in any way that could affect it. For environments where uptime is non-negotiable, this passive approach is the only acceptable method for traffic access.
OT sites can be geographically spread across large facilities. A single manufacturing plant may have dozens of network segments serving different production cells, utilities systems, and control rooms. Aggregating traffic from all of these into a coherent monitoring architecture requires infrastructure that can consolidate streams from multiple TAP points.
Network packet brokers handle this aggregation function, combining traffic from multiple sources into organized streams for delivery to monitoring and security tools. They also provide filtering capability, so time-sensitive industrial protocol traffic can be separated and directed to specialized OT analyzers, while general IP traffic goes to standard security tools.
Some OT security architectures place inspection tools inline, between network segments. If an inline tool fails or requires maintenance, it becomes a single point of failure for the traffic path it monitors. Bypass TAPs address this problem by continuously monitoring the health of inline tools and automatically redirecting traffic around a failed appliance. The protected link stays up; the failed tool is isolated for maintenance without impacting operations.
This is particularly important in OT environments where planned maintenance windows are infrequent and unplanned outages are unacceptable.
While OT protocols have historically transmitted in cleartext, modern OT deployments increasingly incorporate Transport Layer Security (TLS) and encrypted tunnels, particularly for remote access and cloud connectivity. Monitoring tools need visibility into this traffic to detect threats that use encryption to conceal malicious activity.
Effective OT monitoring doesn't happen by connecting a single tool to a single network point. It requires a layered visibility architecture that provides comprehensive coverage across all segments.
Before deploying any monitoring infrastructure, you need an accurate picture of what exists. This means:
This mapping exercise defines the scope of your visibility requirements and informs TAP placement decisions.
Once segments are mapped, you deploy network TAPs at points where visibility is required. For OT environments, passive fiber TAPs are often the preferred choice on fiber links because they require no power and have no active electronics that could introduce risk. For copper Ethernet links, active Ethernet TAPs provide full-duplex traffic capture with aggregation capability.
TAP placement priorities in OT environments typically include:
Traffic from multiple TAP points flows into a network packet broker, which provides several functions essential for OT monitoring:
The SmartNA-PortPlus provides all of these functions in a compact 1RU chassis, supporting port speeds from 1G through to 100G. Its Drag-n-Vu management interface enables drag-and-drop configuration of traffic flows, making it straightforward to add new monitoring tools or reconfigure existing traffic paths as the OT environment evolves.
With aggregated and filtered traffic available, you can direct appropriate streams to specialized tools:
The packet broker manages this distribution, ensuring each tool receives exactly the traffic it needs without being overwhelmed by irrelevant data.
Traffic visibility addresses the monitoring requirement. But OT security also requires controlling which users, devices, and applications can communicate with which systems. This is the access control layer.
INVIKTUS from Network Critical provides zero-trust access control for OT and other critical networks. Unlike conventional security appliances that are identifiable on the network, INVIKTUS operates with no IP or MAC address, making it invisible to any attacker who has already gained network access. It validates every user, device, and application against configured policies before granting access, regardless of where the request originates.
For OT environments, where legacy systems cannot be hardened at the endpoint level and where any unauthorized command to a controller carries physical risk, this kind of policy-based access validation provides a critical additional layer of protection.
Even organizations that recognize the need for OT monitoring frequently encounter avoidable problems during implementation.
SPAN ports are convenient but unreliable. They drop packets when switches are under load, they add processing overhead to managed switches, and many older OT switches don't support SPAN at all. For traffic that needs to reach security tools 100% of the time, passive TAPs are the correct solution.
Many organizations deploy monitoring at the point where IT and OT networks connect, then assume that any threat originating inside the OT perimeter is benign. Internal threats, whether from compromised vendor laptops, insider activity, or malware that has already crossed the boundary, require internal monitoring coverage to detect.
Inline security appliances that fail can take down the traffic path they're protecting. In OT environments, this kind of unplanned outage can have serious operational consequences. Every inline tool in an OT environment should be protected by a bypass TAP that can reroute traffic automatically if the tool becomes unresponsive.
OT environments can generate substantial traffic volumes across many protocols. Monitoring tools sized for average conditions often struggle during peak loads or following network changes. Packet brokers with load balancing and filtering capability ensure tools receive manageable, relevant traffic rather than everything at once.
You can use some IT tools for OT monitoring, but they need to support OT protocols to be genuinely useful. Standard IT tools won't decode Modbus, DNP3, or PROFINET traffic, which means they'll miss the operational context needed to detect industrial-specific threats. Most effective OT monitoring deployments combine OT-specific protocol analysis tools with broader IT security platforms like SIEMs, using a packet broker to deliver the right traffic to each.
Passive network monitoring is the answer. By deploying network TAPs on the links that connect legacy OT devices to the network, you can capture all traffic to and from those devices without installing anything on them or touching their configuration. This makes passive TAPs the only viable monitoring method for the large number of legacy OT devices that exist across industrial environments.
The most important OT protocols vary by industry, but common ones include Modbus TCP (widely used in manufacturing and utilities), DNP3 (common in energy and water utilities), PROFINET (manufacturing), EtherNet/IP (manufacturing and process control), BACnet (building management), IEC 61850 (power systems), and OPC-UA (cross-industry data exchange). Your monitoring tools should support the protocols present in your specific environment.
Many critical infrastructure regulations require evidence of active monitoring, anomaly detection, and the ability to produce incident timelines. Network TAPs provide a legally defensible, complete traffic record because they capture 100% of packets rather than sampled or potentially incomplete SPAN data. This complete record is valuable both for compliance reporting and for forensic investigation following a security incident.
OT environments present unique challenges for network visibility, but the core requirement is straightforward: you need complete, reliable access to traffic without introducing any risk to the operational systems that traffic controls. Network Critical has provided network visibility solutions to organizations in critical infrastructure sectors including oil and gas, energy, and manufacturing since 1997.
Our passive fiber TAPs provide total traffic capture with zero power dependency and no active components, making them ideal for OT links where any point of failure is unacceptable. Our SmartNA-PortPlus packet broker aggregates, filters, and distributes traffic from across your OT environment to the specialized tools that analyze it, with Drag-n-Vu management software making configuration fast and error-free. For organizations requiring inline security coverage with guaranteed uptime, our bypass TAP solutions ensure inline tools never become single points of failure.
Whether you're building OT visibility from scratch, extending coverage into previously unmonitored segments, or integrating OT monitoring with your broader security operations center, our team can help you design an architecture that delivers complete coverage without compromising the operational integrity your environment depends on.