IP technology offers benefits, but staying on top of your voice game is crucial

Case 1: Not all surprises are pleasant

This seemingly simple case highlights the importance of network visibility tools in ATC environments. It involves "surprise calls”, where a voice communications system (VCS) attempts to connect with a radio every 5 seconds, only to be rejected with a "603 Decline" response.

Take a look at the image below:

The unexpected activity

Imagine this scenario: the VCS call log shows a series of failed call attempts, occurring at regular intervals. Each line represents a call rejected by the radio with a "603" code. This code signifies the radio's refusal to accept the call.

Unveiling the truth

Further investigation revealed that the source of these calls was an active testing system. While the test calls themselves may not be harmful, they present a problem; namely, that the air navigation service providers (ANSPs) were completely unaware of their existence.

This lack of visibility raises a red flag and demonstrates the potential for blind spots within the ATC network. Without proper monitoring tools, ANSPs are vulnerable to unidentified issues with the potential to compromise their critical communications infrastructure. This case underscores the importance of proactive, real-time monitoring. By implementing robust oversight measures, ANSPs can ensure optimal performance, avoid unexpected surprises, and maintain complete control over their ATC communications, thereby enhancing overall safety and efficiency.

Case 2: When the blame game goes nowhere

This case highlights the complexities of ensuring user experience while meeting regulatory requirements in ATC VoIP deployments. Here, the focus is on mean opinion score (MOS), a crucial metric defined by the ITU-T and referenced in EUROCAE Document ED-136.

The challenge: maintaining a high MOS

ED-136 mandates an MOS score above 4.0 as a measure of acceptable voice quality. This scenario explores a situation where MOS levels repeatedly fell below this threshold, as shown in the image below. The green bars represent an acceptable MOS, while the yellow and red ones indicate a drop in quality below the standard.

Identifying the root cause

While low MOS is a red flag, the root cause might not always lie within your own infrastructure. This case highlights the importance of network visibility tools. Delving deeper into the data reveals that the network provider's performance is the culprit behind the MOS issues.

The evidence: packet loss paints a clear picture

Further analysis, as shown in the image below, reveals significant packet loss (3%) between the radio site and the central site. This level of packet loss is unacceptable in a controlled and paid network environment.

Taking action and problem resolution

With the evidence analysis reveals, ANSPs can effectively address the issue. Armed with clear data, they can initiate discussions with the network provider and demand improved performance to ensure consistently high MOS levels.

Lessons learned

This case study has two key takeaways:

1. MOS monitoring is essential. Continuously monitoring the MOS of actual live calls ensures compliance with ED-136 and an optimal user experience.

2. Network visibility is the key to identifying root causes. Proactive monitoring tools like R&S®AVQA empower ANSPs to pinpoint the source of performance issues, facilitating effective communication with network providers.

Case 3: The worst case scenario – one-way audio

Imagine this: you're an ANSP controller, coordinating vital air traffic; you call a colleague at an airport for coordination and hear nothing. Dead silence. This could be the result of an innocuous technical glitch, incompatible codecs.

Here's how it happens: during call setup, the ANSP and airport negotiate a codec, a digital code that converts sound into data, and then back again. In this scenario, the ANSP offered two options, G.711 A-law and µ-law. The airport may have mistakenly selected µ-law, for instance, when their equipment was actually transmitting in A-law.

The result was a classic communications breakdown. Expecting µ-law, the ANSP received either garbled data or silence from the A-law transmission. This could have had disastrous consequences for air traffic control.

Luckily, in this specific case, they were using advanced technology (R&S®AVQA) that identified the codec mismatch as the culprit, enabling a swift fix. This highlights the importance not only of having compatible codecs but also robust monitoring systems to catch unexpected issues before they turn into major problems.

Case 4: The confused gateway

Do you fully trust your network equipment vendors? Regardless of how you answer the question, the following case shows how important it is to perform a detailed check to ensure your equipment meets basic standards. The network operations team of an ANSP observed that some air-ground communications were abysmal; specifically, those carried out during times when the network was idle. The R&S®AVQA system showed an estimated MOS of 1.0 – it doesn’t get any worse than that. R&S®AVQA’s estimated MOS (eMOS) calculates an MOS even if there are no audio communications just to ensure that any potential impacts on user experience don’t go unnoticed. The actual audio communications quality in this example was perfect.

A drill-down to investigate the issue revealed that apparently some 90% of all packets sent by a gateway were lost, while those travelling in the opposite direction – from the radios to the gateway – were flawless.

So, why were 9 out of 10 packets lost only in one direction and only during idle times? It turned out that the gateway was not increasing the RTP sequence number as expected. The sequence number is used to detect packet loss and is supposed to increase by one with every packet sent. If the receiver detects a gap in the sequence number, it may conclude by definition that there was packet loss. However, the gateway violated IETF RFC 3550 and EUROCAE ED-137, since it did not increase the sequence number by one during idle times, but by 10 on average. This is why any receiver – including monitoring systems like R&S®AVQA – will report 90% packet loss.

While the specific radios used treated the malformed RTP stream with a high level of tolerance, this should not be taken for granted. Unexpected traffic can lead to unexpected behavior, and standards-conformance of network elements should always be monitored closely.

Ensuring all systems go for ATC's digital takeoff

Digital technologies have been shaking things up across various industries and markets for quite some time. Even though air traffic control rightfully tends to be a bit conservative, it is crucial for ATC to embrace the trend of digital innovation. Doing so brings scalability, resilience, economic sustainability, environmental efficiency and predictability. Adding robust digital systems boosts operational stability and helps the industry master ever-changing scenarios with ease. Ultimately, this transformation will make air traffic control safer, more efficient and more adaptable.

Air navigation service providers (ANSPs) need to amp up their operational flexibility with cutting-edge solutions like virtual centers, digital remote towers and cross-border air traffic control. A key part of these advancements is switching to IP based systems, the bedrock of modern digitalization in air traffic management. Moving to IP infrastructures comes with its own set of challenges, especially for voice communications that demand uninterrupted, real-time transmission. Migration plans need to be meticulously crafted to avoid any hiccups in operational continuity and quality. Practical VoIP implementations can get tricky and involve a mix of technologies and adaptations for smooth integration.

During the IP migration process and even afterward, in regular operations, it is fairly common to face issues like the ones mentioned in the real-life case studies above. Problems can pop up that involve third-party network providers, interconnection partners, or even your own radio teams, and it's crucial to quickly and effectively answer the critical questions of 'what', 'when', 'where' and 'why.' This is where an advanced VoIP monitoring solution like R&S®AVQA can come in handy. With 24/7 monitoring of actual traffic, you'll always have answers to these vital questions.