Data Diode vs Firewall for OT Security: Top 5 One-Way Visibility Solutions in 2026

OT security teams keep asking the same question: data diode or firewall?

The answer shapes how traffic leaves your control network. It also decides whether your security tools can ever talk back into it.

Firewalls give you flexible, two-way rule sets. Data diodes give you physics: traffic moves in one direction only. Most OT programmes need both, at different points in the Purdue Model.

There is a third question that often gets missed. How do you feed your OT detection tools without opening a new path into Level 1 and Level 2? SPAN ports are bidirectional and drop packets under load. So the choice of monitoring access hardware matters as much as the perimeter device.

This guide compares the two perimeter controls. It then reviews five vendors that deliver one-way, passive traffic access for OT monitoring.

Data Diode vs Firewall: The Core Difference

A firewall inspects traffic and decides what to allow, in both directions. A data diode removes the return path in hardware, so nothing can flow back.

The practical rule is simple. If a zone must never accept inbound traffic, a firewall rule is a promise. A diode is a guarantee.

But a diode cannot pass acknowledgements, so remote control and two-way protocols stop working. That is why firewalls still sit on conduits where controlled two-way traffic is required.

Monitoring access is where the two ideas meet. A sensor that receives mirrored OT traffic should never be able to transmit into the production link. One-way access hardware solves that problem without the cost of a full gateway diode.

At a Glance: One-Way Visibility Solutions for OT Security

Throughput reflects each vendor's maximum supported visibility product speed. One-way and data diode products within each range may run at lower speeds.

1. Network Critical

Network Critical: Passive Fiber Optical TAPs, SmartNA-XL, INVIKTUS

Network Critical gives OT teams one-way, non-intrusive access to live traffic without touching PLCs or SCADA configuration. Its Passive Fiber Optical TAPs use no power and contain no active electronics. They split the optical signal, passing the live link at full speed and sending a copy to your tools.

Units support 1G to 100G, with up to 16 TAPs per 1RU. They ship preconfigured to the chosen split ratio. There is nothing to configure and nothing to maintain.

For copper links, the SmartNA-XL combines fail-safe TAP modules and packet broker features in one 1RU chassis. It supports 1G to 40G and 480 Gbps of visibility throughput. If the chassis loses power, live traffic keeps flowing through the hybrid TAP. It also aggregates slow copper links into fewer high-speed tool ports, so you buy fewer sensors.

Where a zone needs inline enforcement rather than monitoring, INVIKTUS adds a zero trust layer. It has no IP or MAC address, runs at full line rate and blocks unauthorised users by policy. Protected systems stay invisible to everyone outside their mapped path.

At the enterprise core, the SmartNA-PortPlus HyperCore scales to 400G. All output is standard PCAP, so any OT detection platform, SIEM or NDR tool can ingest it.

Proven results:

• BP: Passive fibre TAPs enabled centralised IT and OT monitoring across refineries spanning 10 to 12 buildings, without impacting live traffic
• Airbus: Fail-safe network TAPs captured mission-critical test rig data and helped complete all first flight test objectives on schedule
• A university: INVIKTUS made vital servers invisible to student devices while keeping access open for authorised staff

Best for: OT teams that need passive, fail-safe monitoring access plus a route to zero trust enforcement, from one vendor.

2. Garland Technology

Garland Technology: Hardware Data Diodes, Data Diode Network TAPs

Garland Technology runs the most explicit data diode line among network TAP vendors. Its Hardware Data Diodes protect switch SPAN ports. Mirrored traffic leaves the monitoring ports in one direction only. Physical hardware separation inside the unit blocks any return path into the switch.

Portable models regenerate or aggregate SPAN inputs. One aggregation model combines eight copper SPAN connections into a half-rack 1U unit. Modular chassis hold up to four TAP modules in 1U or twelve in 2U.

The P10GSFP+A Configurable Data Diode TAP extends one-way enforcement to 10G. It offers four SFP+ ports, five DIP-switch modes and no IP or MAC address. It is TAA compliant, with DC power and DIN rail mounting available.

Garland also sells Data Diode Network TAPs in passive single-mode and multi-mode fibre variants. Its wider passive fibre range, including the SelectTAP chassis, covers 1G to 400G. The company partners with several OT security vendors, including Nozomi Networks and TXOne.

3. Profitap

Profitap: Network Data Diodes, DiodeTAP module, C1D-100

Profitap approaches one-way access from two directions: standalone diodes and diode-protected TAPs. Its Network Data Diodes enforce physical one-way transfer, so monitoring tools cannot inject traffic into production. They have no IP address or management interface. Units fit a one-third 1U rack space and include redundant power supplies.

The DiodeTAP module sits inside Profitap's MOD-TAP chassis. It blocks light coming from the monitor ports, with more than 35 dB insertion loss on the TAP port input. That stops signal insertion from a compromised or faulty tool.

For control cabinets, the C1D-100 is a compact Fast Ethernet TAP. It has a DIN rail clip and runs on 20 to 30 VDC. It monitors 10/100 Mbps links and includes an integrated data diode function.

Profitap's wider fibre TAP range covers 1G to 400G, and the MOD-TAP chassis mixes module speeds in one unit. The company has a strong European field presence through certified resellers. Its industry pages also cover power and utilities, rail and maritime networks.

4. Gigamon

Gigamon: G-TAP M Series

Gigamon's G-TAP M Series is a modular family of passive fibre optical TAPs. They need no power source, no software and no special patch cords. Optical splitting lets you monitor full-duplex links without affecting production traffic.

The range covers 1G, 10G, 25G, 40G, 100G and 400G networks. Options include BiDi, breakout and unidirectional deployments. High-density chassis tap up to 36 full-duplex links in 1RU, or 18 in half a rack unit. Split ratios of 50/50, 60/40 and 70/30 are available.

Thin-film optics keep insertion loss low and consistent across transceiver vendors. That matters on multimode OT links with tight optical budgets. Several G-TAP M Series parts are listed as TAA compliant, which suits public sector and regulated utility buyers.

Gigamon has no dedicated OT data diode line. Its strength is access density feeding the wider Gigamon visibility platform. Gigamon reports a 51 per cent share of the deep observability segment, per 650 Group in Q1 2026. That platform uses subscription licensing, which affects total cost over a multi-year OT programme.

5. Keysight

Keysight: Copper Tough Tap, Vision packet brokers

Keysight's Network Visibility business builds on the Ixia product line. For ICS and OT, the Industrial Copper Tough Tap (TPAT2-CU3-T) targets harsh environments and extreme operating temperatures. It is TAA compliant and independently certified. The tap fails to wire, so traffic keeps passing if it loses power.

Keysight also offers passive fibre TAPs and high-density aggregator TAPs for consolidating traffic from distributed industrial sites. Its partnership with Dragos documents TAP and packet broker feeds into the Dragos Platform.

Higher up the stack, the Vision packet broker family supports 400G and 800G. Its FPGA-based architecture targets zero packet loss and has been validated by The Tolly Group. A drag-and-drop interface removes the need for CLI filter rules.

Keysight's OT motion is newer than its service provider business. Its Forescout technology partnership dates from January 2026. Visibility is one business unit inside a much larger test and measurement company.

How to Choose the Right One-Way Visibility Solution for Your OT Network

Decide Where You Need a Diode and Where You Need a Firewall

Map each conduit in your zone model first. Conduits that carry engineering access or remote control need controlled two-way traffic, so a firewall fits. Conduits that only export data, such as historian feeds or monitoring copies, suit one-way enforcement.

Protect the Monitoring Path, Not Just the Perimeter

Your OT sensors receive traffic from inside critical zones. If you feed them from SPAN, the switch port is bidirectional and can drop packets under load. Passive TAPs or data diode TAPs remove that risk at the access layer. Your OT network monitoring design should treat sensor connections as conduits in their own right.

Confirm Fail-Safe Behaviour Before You Deploy

Production downtime ends most OT projects before they start. Check how each device behaves when power fails or a tool is unplugged. Ask for answers to these questions:

• Does the live link stay up with zero power?
• Do copper TAPs use fail-safe relays or physical bypass?
• Can you remove the device without a production outage?
• Does an inline tool have protection if it fails?

Match the Hardware to the Environment

Control cabinets differ from data centre racks. Check operating temperature, DC power input and DIN rail options for each site. Fibre links in noisy electrical areas often favour passive optical TAPs, which have no electronics to fail.

Plan for Aggregation as Sensors Multiply

A single plant may need dozens of tap points. Without aggregation, each link needs its own sensor port. A hybrid TAP and packet broker combines access and traffic management in one chassis. That cuts rack space, power draw and sensor count at remote sites.

Model Three-Year Cost, Not Purchase Price

OT budgets are smaller than IT budgets, and site counts multiply every line item. Compare perpetual licensing against subscription models over three years. Include deployment time, because OT maintenance windows are often two to four hours. A device that needs a vendor engineer on site for every change will cost more than its list price suggests.

Frequently Asked Questions

What is the difference between a data diode and a firewall?

A data diode allows traffic in one direction only, enforced in hardware. A firewall uses software rules to allow or block traffic in both directions. A diode cannot be misconfigured into allowing inbound traffic. A firewall offers flexibility for zones that need two-way communication.

Can a data diode replace a firewall in OT security?

No, not everywhere. A data diode works where data only needs to leave a zone, such as historian or monitoring exports. Conduits carrying remote access, patching or control commands need two-way traffic, so they still need a firewall. Most IEC 62443 architectures combine both.

What is a data diode TAP?

A data diode TAP copies live traffic to a monitoring tool. It blocks any traffic from the tool back into the network. It protects the monitored link, not a whole network boundary. Passive network taps and dedicated data diode TAPs both serve this purpose in OT environments.

Is SPAN safe for OT security monitoring?

SPAN is not the safest option for critical OT links. Switch mirror ports are bidirectional, compete with switching workloads and drop packets during traffic spikes. Dropped packets leave gaps in detection and audit evidence. Hardware TAPs or data diodes give OT sensors a complete, one-way copy.

Do data diodes and TAPs work with OT detection platforms like Dragos or Nozomi?

Yes. TAPs and data diode TAPs pass a full copy of traffic. OT detection platforms then decode it at the protocol level. Standard PCAP output works with most ot cybersecurity and NDR tools. The access hardware stays vendor-neutral, so you can change detection tools later.

Where should a data diode sit in the Purdue Model?

Most OT teams place data diodes at the boundary between Level 3 operations and the Level 3.5 demilitarised zone. That lets historian and monitoring data leave without any inbound path. Smaller data diode TAPs sit lower, on Level 1 and Level 2 links feeding OT sensors. Firewalls then manage the conduits that need controlled two-way access.

Build Your One-Way OT Visibility Architecture With Network Critical

Choosing between a data diode and a firewall settles your perimeter. Your monitoring access decides whether OT sensors see every packet without opening a new path in.

Network Critical delivers that access with zero-power passive fibre TAPs, fail-safe hybrid taps and INVIKTUS zero trust enforcement. Perpetual licensing typically brings 3-year TCO 40 to 60 per cent below subscription-led incumbents. Drag-n-Vu configuration means typical deployments finish in under two hours, inside a standard maintenance window.

BP and Airbus already rely on this approach for critical operations. To map your conduits and request a free network audit, speak to the Network Critical team.