When a controller calls out a heading change or a pilot reads back a clearance, every syllable has to land. In air traffic control (ATC), voice communication is not a background process; it is the backbone of the entire operation. So when networks shift from old-school circuit-switched lines to Voice over IP (VoIP), the question of how to keep tabs on quality becomes urgent.
This article breaks down the two main monitoring strategies, active and passive, and walks through the trade-offs of each. The goal is to help ATC stakeholders and Air Navigation Service Providers (ANSPs) make smarter decisions about what to deploy and when.
From Dedicated Lines to Shared Networks
For decades, ATC voice communication ran on dedicated circuits. Rigid, purpose-built connections that offered predictability and reliability. Moving to VoIP changes the game. IP-based infrastructure brings flexibility, cost savings, and easier integration across distributed control centers. It also makes room for data services running on the same shared network.
But shared networks come with a different set of risks. In circuit-switched environments, a dedicated path meant consistent performance. IP networks are packet-switched, meaning voice data is broken into packets that travel independently through the network, and can hit turbulence along the way. Packet loss, jitter, and latency can all chip away at voice quality.
On top of that, when ATC voice traffic shares infrastructure with other applications, Quality of Service mechanisms become critical to making sure voice always gets priority. Without tight control, a busy network moment can quietly degrade a controller-pilot exchange.
These new dynamics make monitoring more important than ever. Modern ATC environments need real-time visibility into network behavior. Not just alerts when something breaks, but ongoing insight into trends, bottlenecks, and potential issues before they become actual problems.
The Standard Setting the Bar: ED-138 and What Comes Next
EUROCAE Working Group 67 SG-6 developed ED-138, the standard that defines network requirements and performance targets for VoIP in Air Traffic Management (ATM) systems. First published in 2009, ED-138 established the foundational rules for how VoIP should behave in the safety-critical ATC context.
The industry has evolved significantly since then, and a revision, ED-138A, is currently in progress. The update reflects how much the technology landscape has changed. Rather than prescribing specific protocols, ED-138A focuses on defining broader network behavior requirements, giving ANSPs greater flexibility to adapt to diverse and evolving infrastructure configurations.
A key driver behind this revision is the growing prevalence of shared network architectures, where VoIP traffic coexists with other ATM applications. ED-138A also expands the scope of the standard’s supervision interface, broadening coverage to include a wider range of protocols used in ATM environments, not just VoIP. The result is a more holistic framework that addresses network performance, safety, and security together.
By widening its lens, ED-138A aims to give ANSPs a more adaptable toolkit for implementing and managing voice communication systems in an increasingly complex digital environment.
Active Monitoring: Testing the Network on Your Terms
Active monitoring takes a hands-on approach. Instead of waiting for real traffic to reveal problems, it deliberately injects synthetic test traffic into the network to assess performance under controlled conditions. Think of it as a fire drill. You’re not waiting for a fire; you’re testing whether everything works before one starts.
How it works
Active monitoring tools generate test data packets that simulate real network traffic. These packets travel across the network, and the system measures performance metrics along the way. In VoIP contexts, this often means deploying artificial endpoints that replicate real devices – Controller Working Positions (CWPs), radios, Voice Communication Systems (VCS) – and using them to simulate voice calls and assess quality.
Key capabilities of active monitoring include:
- Synthetic traffic generation: Creates test packets that mimic actual network load
- End-to-end path testing: Evaluates the complete communication route, including all network elements in between
- Customizable scenarios: Allows administrators to simulate specific conditions, including high-stress situations
- Scheduled and repeatable tests: Automates regular testing and enables trend analysis over time
- Consistent baselines: Each test is a controlled experiment, making it easy to compare results over time
Strengths
The biggest advantage of active monitoring is its independence from live traffic. It can identify problems even in low-usage periods or before a new service goes live, which is particularly valuable for pre-deployment validation and periodic health checks. When test results are consistent and stable, you get solid evidence that the network is ready for real-world conditions.
Active monitoring also shines for end-to-end performance measurement. Simulating traffic across an entire communication path can catch issues that might not surface until an actual call is in progress.
Limitations
That independence from live traffic is also the main constraint of active monitoring. Since it uses synthetic data, there’s no guarantee that test results fully reflect the experience of real users on real calls. Issues can emerge in parts of the network not included in the test scenarios or in the gaps between scheduled test runs.
Additionally, active monitoring provides an end-to-end view, which is great when everything looks fine, but less useful when something goes wrong, and you need to pinpoint exactly where in the network the problem lives. It also introduces additional traffic into the network, which, if not carefully managed, could affect service performance.
Passive Monitoring: Listening In on What’s Actually Happening
Passive monitoring takes the opposite approach. Rather than generating test traffic, it taps into the network and captures what’s already flowing through it. It’s an observer, watching real communications between real users as they happen.
How it works
Passive monitoring tools analyze network traffic without adding any load or affecting performance. They run continuously in the background, capturing data around the clock. For VoIP traffic, this means extracting metrics such as delay, jitter, and packet loss from actual calls, giving you a picture of what users are actually experiencing.
Key capabilities of passive monitoring include:
- Non-intrusive operation: Captures and analyzes live traffic without generating any additional network load
- 24/7 real-time visibility: Provides a continuous, live view of network and service performance
- Full traffic coverage: All user traffic can be monitored, covering diverse services and usage patterns
- Detailed metric extraction: Pulls performance data, including delay, jitter, and packet loss, directly from real traffic
- Midpoint analysis: Enables precise isolation of where in the network an issue is occurring
Strengths
Because passive monitoring works with real traffic, the data it produces is always authentic. There’s no gap between what you’re testing and what users are actually experiencing. It’s particularly well-suited for ongoing service quality oversight and for supporting customer care teams, since it captures actual interactions rather than simulated ones.
One of passive monitoring’s most valuable capabilities is its ability to provide midpoint metrics. Unlike end-to-end measurements, midpoint data can pinpoint the specific segment of the network where a problem originated, making troubleshooting significantly faster and more targeted.
Setup and maintenance are typically straightforward, and since it doesn’t introduce any additional traffic, there’s no risk of disrupting the services being monitored.
Limitations
The flip side of relying on live traffic is that passive monitoring can only detect issues as they occur in real usage. It won’t catch a problem that appears only under specific conditions unless those conditions actually arise during normal operations. Recreating the exact circumstances of a detected issue for further analysis can also be challenging.
That said, these limitations are largely offset by the data’s authenticity. Real-world insights from real-world conditions are inherently more meaningful than any simulation can replicate.
Active vs. Passive at a Glance
| Active Monitoring | Passive Monitoring | |
| Traffic source | Synthetic. Generated by the monitoring tool | Real. Actual user traffic on the live network |
| Network impact | Adds test traffic to the network | Zero additional load |
| When issues are caught | Can detect issues before they affect users | Identifies issues as they occur in real usage |
| Coverage | Limited to configured test scenarios and paths | All live traffic, across all services and paths |
| Metric type | End-to-end. Full path performance | Midpoint. Pinpoints specific network segments |
| Best for | Pre-deployment validation, lab testing, availability checks on standby resources | Continuous service quality monitoring, troubleshooting, customer care support |
| Key limitation | May not reflect real-world conditions; cannot locate issues within specific network segments | Only detects issues present in live traffic; cannot simulate specific scenarios on demand |
Which Approach Should You Use?
The short answer: both, used strategically.
Active and passive monitoring are not competing solutions; they’re complementary. Each addresses gaps left open by the other, and the most resilient ATC monitoring setups leverage both together.
Active monitoring is the natural fit for controlled testing environments – labs, pre-deployment checks, and scheduled health assessments. It’s especially useful for verifying that standby or backup systems are ready to carry traffic when needed, even when no real users are on those paths. If you need to simulate an extreme load condition or test a configuration change before going live, active monitoring provides a controlled environment.
Passive monitoring, on the other hand, is the right choice for ongoing operational oversight. Once a system is live, passive monitoring provides continuous, unobtrusive visibility into real service quality. The kind of data that supports both proactive performance management and reactive incident response.
In practice, many ANSPs use active monitoring to validate their infrastructure periodically and passive monitoring to keep a constant finger on the pulse of real-world performance. Together, they provide the full picture: the reassurance of controlled testing combined with the authenticity of live traffic analysis.
As ATC communication systems grow more complex and more interconnected, the stakes around monitoring continue to rise. Whether you’re navigating the shift to VoIP, aligning with updated standards like ED-138A, or simply working to maintain the clarity that controllers and pilots depend on every day, having the right monitoring strategy in place is not optional; it’s foundational.