Industrial Control System (ICS) networks operate under a fundamentally different set of priorities than enterprise IT networks. Uptime isn't just a performance metric in these environments – it's a safety and operational imperative. A conveyor line that stops unexpectedly, a valve that fails to respond, or a SCADA system that loses communication can trigger consequences ranging from lost production to physical harm. That reality shapes everything about how you approach network visibility in industrial settings.
The good news is that achieving comprehensive ICS network visibility without disrupting production is entirely possible. The key lies in choosing monitoring approaches that are inherently passive and non-intrusive. Network TAPs sit at the core of this approach, providing complete traffic copies to your security and monitoring tools without inserting any device into the live data path. Combined with intelligent network packet brokers that filter and distribute traffic efficiently, you can build a full-spectrum visibility architecture that your OT team can trust.
This guide covers everything you need to know: why ICS networks present unique visibility challenges, which monitoring approaches work in operational technology (OT) environments, and how to build an architecture that keeps both your security teams and your plant floor teams satisfied.
In a traditional IT network, rebooting a server or taking a switch offline for maintenance is an inconvenience. In an ICS environment, the same action can halt production lines, disrupt critical processes, or – in industries like energy, water treatment, or chemical manufacturing – create genuine safety hazards. This means that any monitoring solution you introduce must be vetted against one overriding question: what happens to the production network if this device fails?
This is why standard enterprise monitoring approaches often can't be transferred directly into ICS environments. Agents installed on endpoints, inline security appliances, or tools that actively probe devices may all carry unacceptable risk when the devices being protected control physical processes.
ICS networks frequently run protocols designed for reliability and determinism rather than security. Modbus, DNP3, PROFINET, EtherNet/IP, and IEC 61850 are common examples. These protocols often lack encryption and authentication, which makes them inherently vulnerable but also means that monitoring tools need to understand them to provide useful analysis.
Compounding the challenge, ICS devices themselves are often legacy hardware running embedded operating systems that can't be patched, updated, or have agents installed. A Programmable Logic Controller (PLC) from 15 years ago may still be controlling a critical process today. You can't put an endpoint agent on it, which means your visibility strategy must be entirely passive and network-based.
Historically, many ICS networks operated in relative isolation from enterprise IT networks and the internet. That isolation has eroded significantly as organizations adopt Industrial Internet of Things (IIoT) connectivity, integrate OT data with business intelligence platforms, and enable remote maintenance capabilities. Each new connection point between the OT and IT domains introduces potential lateral movement paths for threats – and requires corresponding visibility coverage.
The fundamental tension in ICS visibility is this: you need to see network traffic in order to detect threats and diagnose problems, but the tools you use to achieve that visibility must not themselves become a source of risk or disruption.
In enterprise environments, many organizations rely on switch port analyzer (SPAN) ports to copy traffic to monitoring tools. SPAN ports have a number of limitations in any environment, but in ICS networks these limitations become particularly problematic.
SPAN ports are software-configured on managed switches, which means they can be accidentally misconfigured, overridden, or disabled during routine switch maintenance. More significantly, SPAN port mirroring consumes switch processing resources. On switches that are already handling deterministic, time-sensitive industrial protocols, additional load can introduce the latency and packet drops that ICS processes absolutely cannot tolerate.
Key SPAN port limitations in ICS environments include:
Inline security appliances – firewalls, intrusion prevention systems, and similar tools – sit directly in the data path between network segments. When they work correctly, they inspect and filter traffic in real time. When they fail, reboot for an update, or become overwhelmed by traffic, the result can be a complete traffic interruption.
In a data center, a brief interruption while a security appliance reboots is manageable. On a factory floor where a PLC is waiting for a heartbeat signal from a SCADA system, the same interruption can cause an emergency stop or a process fault. This is why inline tool deployment in ICS networks requires careful planning and, critically, bypass protection.
The safest way to gain visibility into ICS network traffic is through passive network TAPs. Unlike SPAN ports or inline appliances, a passive TAP introduces no active component into the network path. It works by physically splitting the optical signal (for fiber links) or using transformer-based coupling (for copper), sending a complete, identical copy of all traffic to your monitoring tools while the original traffic continues uninterrupted.
Passive fiber TAPs use optical splitters to divide the light signal on a fiber link. A portion of the light budget continues to the original destination, while the remainder is directed to your monitoring tool. There is no electronics, no IP address, no MAC address, and no configuration required. The TAP is completely invisible to the network.
This architecture delivers several properties that are critical in ICS environments:
Many industrial networks still use copper Ethernet for device connectivity, particularly at the field device level where PLCs, remote terminal units (RTUs), and human-machine interfaces (HMIs) connect. Ethernet TAPs provide the same fundamental monitoring capability for copper links.
Active Ethernet TAPs use transformer-based hardware coupling to copy traffic without interrupting the live connection. They include fail-safe protection, meaning that if the TAP loses power, the copper link automatically passes through uninterrupted. This is the "fail-open" property that OT teams require: the monitoring infrastructure must never be the reason a production device loses connectivity.
Once you've established passive TAPs at your key network monitoring points, the next challenge is managing the traffic those TAPs generate and getting the right data to the right tools efficiently. This is where network packet brokers play a critical role.
A typical ICS network has multiple network segments: a process control network connecting PLCs and controllers, a supervisory network running SCADA systems, a historian network, and potentially an industrial DMZ separating OT from IT. Each segment may have multiple links that require monitoring.
Without aggregation, you'd need a dedicated monitoring tool for every TAP, which quickly becomes impractical. A packet broker aggregates traffic from multiple TAPs into consolidated streams, allowing a single tool to monitor multiple network segments simultaneously.
Not every tool needs to see every packet. Your ICS-specific intrusion detection system (IDS) needs to analyze industrial protocol traffic. Your anomaly detection platform needs to see device communications to build behavioral baselines. Your forensic capture appliance may only need to record traffic during specific time windows or incident investigations.
Packet brokers apply filtering rules that direct specific traffic types to the tools designed to analyze them. Common filtering criteria in ICS visibility deployments include:
At higher traffic volumes, a single monitoring tool may not be able to process every packet at line rate. Packet brokers support load balancing across multiple instances of the same tool type, ensuring session-aware distribution that keeps related traffic flows together. This is particularly important for ICS anomaly detection tools that build device behavior profiles over time – you need consistent session routing to avoid fragmenting the behavioral picture.
Some security functions simply can't be performed out-of-band. Deep packet inspection for certain threat types, or active blocking of known malicious commands, requires inline placement. If you need to deploy inline security tools in an ICS environment, bypass TAPs are what make that deployment safe.
A bypass TAP sits around an inline security appliance. It continuously sends heartbeat signals through the appliance. If the appliance stops responding – due to a software fault, reboot, power failure, or maintenance activity – the bypass TAP automatically redirects traffic around the failed appliance in milliseconds. The production network never loses connectivity.
When the appliance recovers, the bypass TAP detects the restored heartbeat and gradually reintroduces live traffic, allowing the appliance to re-synchronize its state before handling full production loads.
Key bypass TAP capabilities for ICS deployments include:
Knowing where to place TAPs is as important as understanding the technology itself. The right monitoring points give you comprehensive visibility without requiring a TAP on every single link.
The most valuable monitoring points in an ICS network are typically at zone boundaries, where traffic crosses from one security zone to another. These locations let you see all cross-zone communications, which is where both legitimate operational traffic and potential lateral movement threats will appear.
Priority monitoring locations include:
Within individual network zones, monitoring key device uplinks provides device-level visibility. Not every device requires dedicated monitoring, but critical controllers, primary SCADA servers, and safety system connections typically justify individual TAP coverage.
Your TAPs and packet brokers create the visibility layer, but the analysis and detection work happens in the tools you connect to that infrastructure. ICS environments require tools that understand industrial protocols natively.
Standard IT network analysis tools don't understand Modbus function codes, DNP3 application layer messages, or PROFINET cyclic data. OT-aware network analysis platforms decode these protocols and can detect anomalies like unexpected function codes, commands from unauthorized source addresses, or changes in polling frequency that might indicate reconnaissance activity.
Because many ICS devices communicate on regular, predictable schedules with consistent message patterns, anomaly detection is highly effective in OT environments. A PLC that suddenly starts communicating with an IP address it has never contacted before, or an HMI that begins issuing commands outside its normal operational parameters, represents a meaningful deviation worth investigating.
One of the most immediate benefits of ICS network visibility is automatic asset inventory. Many OT environments lack up-to-date records of every connected device. Passive traffic analysis can identify devices by their communication patterns, protocols, and source addresses, building a real-time asset register without sending a single probe packet to any device.
Benefits of passive asset discovery include:
ICS environments in critical infrastructure sectors face regulatory requirements that create specific visibility obligations. NERC CIP standards for the electric sector, IEC 62443 for industrial automation, and sector-specific guidance from the Cybersecurity and Infrastructure Security Agency (CISA) all emphasize the importance of network monitoring and anomaly detection.
While specific requirements vary by standard and sector, most ICS security frameworks share common monitoring expectations:
The compliance case for passive TAP-based visibility architecture is straightforward: you can demonstrate comprehensive monitoring coverage across all required network zones without accepting the operational risk of inline tools or the reliability limitations of SPAN-based approaches. Audit trails, traffic logs, and forensic captures can all be delivered from your visibility infrastructure to compliance reporting platforms.
Moving from concept to deployment requires addressing several practical questions specific to ICS environments.
Any modification to an ICS network requires careful change management. Even deploying a passive TAP, which carries no operational risk, needs to go through your organization's OT change management process. This typically involves:
Successful ICS visibility projects almost always involve close collaboration between OT engineering teams and IT security teams. OT teams understand the operational constraints, the sensitivity of specific devices, and the change management processes. IT security teams understand threat landscapes, monitoring tool capabilities, and visibility architecture principles.
The most common source of friction is timeline: OT teams often have very limited maintenance windows and long change approval cycles. Building trust by starting with out-of-band passive monitoring (which carries zero operational risk) before proposing any inline security tools is a consistently effective approach.
ICS environments often have different physical infrastructure than data centers. Industrial switches may be DIN-rail mounted in compact enclosures, running at temperatures and humidity levels outside typical data center specifications. When selecting TAP hardware for OT deployments, consider:
No. Passive fiber TAPs work by physically splitting the optical signal and require no processing, adding zero latency to the live network path. Active Ethernet TAPs for copper links use hardware coupling and include fail-safe designs, so a TAP power failure causes no network interruption. The monitoring traffic flows entirely out-of-band and cannot affect live network performance.
Yes. The TAP-based visibility approach specifically avoids any interaction with production devices. TAPs install on the physical links between devices, not on the devices themselves. No agents, no configuration changes on PLCs or SCADA servers, and no active probing of any device is required.
Many older ICS installations still use serial connections for device communication. Serial-to-Ethernet protocol converters are often already deployed to integrate legacy serial devices with modern network infrastructure. Monitoring the Ethernet side of those converters with standard network TAPs provides visibility into the underlying serial protocol traffic.
The fundamental difference is in protocol understanding and operational priorities. IT monitoring tools are designed for standard TCP/IP protocols and enterprise application traffic. ICS monitoring requires understanding industrial protocols like Modbus, DNP3, and PROFINET. Additionally, ICS monitoring must prioritize operational availability above all else – a monitoring approach that could potentially disrupt a production process is unacceptable regardless of its security value.
ICS network traffic volumes are generally much lower than enterprise IT networks. Industrial control traffic is often highly predictable: regular polling cycles, deterministic control messages, and periodic status updates. This low, predictable traffic volume makes ICS environments particularly well suited to comprehensive packet capture and full-session recording, which would be impractical at IT network speeds.
Building ICS network visibility infrastructure requires hardware specifically designed to deliver complete traffic access without introducing any risk to operational continuity. Network Critical has provided network visibility solutions to organizations in critical infrastructure, energy, manufacturing, and government sectors since 1997, with deployments where network reliability is an absolute requirement.
Our passive fiber TAPs deliver 100% traffic capture across fiber links from 1Gbps to 100Gbps with zero power dependency and no impact whatsoever on live network traffic. For copper Ethernet links running to PLCs, HMIs, and field devices, our Ethernet TAPs include fail-safe protection that ensures the production link passes through uninterrupted even if the TAP loses power.
The SmartNA-XL combines TAP and packet broker functionality in a compact modular chassis, supporting 1G/10G/40G speeds with advanced filtering, aggregation, and load balancing through the intuitive Drag-n-Vu management interface. For environments where inline security tools are required, our bypass TAPs provide automatic failover protection that keeps production traffic flowing regardless of what happens to the inline appliance.
Whether you're building initial visibility into a previously unmonitored OT network, extending coverage to new network zones, or supporting compliance requirements under NERC CIP, IEC 62443, or sector-specific frameworks, our team can help you design an architecture that delivers comprehensive monitoring without compromise to your production environment.