America’s Telecommunications Upgrade, Part Two: The Military's Challenging Transition to VoIP

The Armed Forces, long reliant on copper-based TDM, face unique challenges in adopting America’s new IP-based telecommunications infrastructure. 

Since ancient times, technological innovation associated with military communications has profoundly affected both tactical and strategic capabilities, strongly impacting defense-related policy decisions and enabling the integration of operations within the armed forces. 

As the United States commercial copper telecommunications infrastructure is retired in favor of high-bandwidth fiber-optic systems, among the most significant and challenging transitions facing modern military communications is the ongoing shift from its traditional Time-Division Multiplexing (TDM) telecommunications technology to Voice over Internet Protocol (VoIP) systems. 

This month, we'll explore the historical significance, challenges, and future implications of the Department of Defense's (DoD) transition from TDM to VoIP, including the technological and policy-driven factors influencing this shift and the role of cybersecurity in protecting VoIP-based networks, and how the DoD is ensuring that communication remains uninterrupted while navigating the complexities of this transformation. 

TDM and VoIP: an Introduction

Time-Division Multiplexing (TDM) was commonly used with legacy copper-based telephone systems to handle several voice calls within a fixed switching fabric. It allows multiple channels to share a single trunk connection by dividing the trunk into “time slots;” a separate time slot is assigned to each of the various channels, enabling them to share the same physical structure without interference. By sharing a single trunk, TDM maximizes the capacity of physical infrastructure. While TDM is primarily associated with landlines and wired systems, it has also been used in microwave, satellite, and radio communications for specific applications. 

Unlike TDM, VoIP operates over IP-based networks, allowing voice data to be transmitted in packets, as are emails and web traffic. VoIP enables organizations to consolidate voice, video, and data communications into a single IP network infrastructure. And because VoIP uses existing internet and data networks, installation and maintenance costs are reduced. VoIP is not bound to physical infrastructure; personnel can communicate securely from remote locations using mobile or satellite-based networks. IP-based telecommunications systems enable the warfighter to keep the same phone number, even when moving to a different command, post, station, or base using VoIP endpoints and soft clients. 

TDM in the Military: Born of Audacious Necessity

The British Wireless Set No. 10

The military use of TDM dates to the 1940s, when the British Army’s Wireless Set No.10 became the world's first multi-channel, microwave-relay telephone system. As the Allied armies on the Western Front maneuvered their way across Europe and coordinated their final, massive drives to destroy the Third Reich, a growing network of landlines and No.10 sets—sending multiplexed messages as far as 50 miles at a time using microwave relays—eventually stretched from Germany back to Supreme Headquarters, Allied Expeditionary Force in London. In the words of Field Marshal Bernard Montgomery, commander of the 21st Army Group and all Allied ground forces during the invasion of Normandy.

By using a chain of No. 10 Set Stations, I was able to maintain my tactical HQ as far forward as I did and still have contact with London. The value of being able to retain personal contact over my Armies in these circumstances cannot be overestimated. 

This was an extraordinary and exponential increase in the ability of military units to communicate, an innovation driven by the harsh necessities of modern warfare. 

But like all legacy technologies, TDM is being retired, making way for broadband IP-based systems' more scalable, cost-effective, and data-driven capabilities. The transition from TDM to VoIP is vital to the DoD’s broader modernization initiatives, which emphasize interoperability, resilience, and real-time data exchange across global operations. The demands of these initiatives have stretched aging TDM systems to their limits. Commercial service providers are quickly retiring their support for the legacy TDM infrastructure, and maintaining TDM systems is expensive and resource-intensive. The military’s adoption of VoIP has become both an operational priority and a strategic necessity, and it represents a fundamental transformation of the DoD’s vast communications infrastructure. 

The DoD’s shift from TDM to Voice over Internet Protocol (VoIP) is compelling the Armed Forces’ communications organizations simultaneously to address entirely new security concerns, navigate the complexities of complying with new regulatory mandates, and ensure that the changeover does not disrupt mission-critical operations. 

Time-Division Multiplexing-based networks were based on dedicated circuits. These systems were inherently stable, offering near-continuous uptime, even during power failures. This stability was a critical advantage for military installations and command structures, which depend on secure, uninterrupted communications and reliable, crystal-clear voice transmissions. 

Unlike TDM systems, however—engineered with a strong emphasis on operational reliability—VoIP is entirely dependent on the stability of its underlying digital network to function. Without meticulous contingency planning, service interruptions in a DoD VoIP’s IP network—which can be caused by such seemingly minor events as the misapplication of a software patch—could potentially leave portions of the U.S. Armed Forces temporarily facing communications disruptions ranging from degraded voice-call intelligibility to complete interruptions in service.

And while the dedicated nature of legacy TDM systems made them inherently secure against cybersecurity threats, VoIP’s architectural reliance on IP networks opens it to threats such as phishing, Trojan horses, denial-of-service attacks, and other cybersecurity threats. Encryption, firewalls, and real-time security measures are becoming essential to monitoring and protecting modern voice networks from malicious actors operating in digital contexts. 

So, when the U.S. Air Force awarded KriaaNet a contract in 2023 to modernize VoIP infrastructure at Maxwell Air Force Base, the upgrade involved navigating stringent cybersecurity requirements to ensure the resulting systems met the standards of modern IP-based networks. KriaaNet’s press release highlighted the complexity of the undertaking: “Ensuring mission success … in the air, space, and cyberspace required robust cybersecurity measures to mitigate risks absent in TDM systems.” 

Military commands and installations have always been intended to function as unified entities, with telecommunications services centrally managed to ensure reliability, security, and efficiency. Legacy, copper-based TDM networks provided this hierarchical, united communications structure at the installation level. TDM systems also offered seamless integration with 911 emergency response, secure voice (STE/STU) calls, and alarm systems. Unlike VoIP, TDM networks do not require an external power source. 

Currently, however, many of the VoIP systems replacing TDM on military bases comprise a patchwork of independent systems with no unified network to ensure base-wide interoperability. Command organizations manage their own primary IP networks, and subordinate and base-tenant units often deploy separate, isolated systems, sometimes using commercial ISP services to connect to higher commands. VoIP access to emergency-response capabilities, such as 9-1-1 and 9-8-8 crisis services, depends on proper IP network configurations. Yet, many bases lack the standards or personnel to guarantee the accuracy of these configurations. 

A 2025 Military Embedded Systems article highlighted the challenges of migrating associated elements like conference bridge, ancillary equipment (SYSLOG, backups, network time protocol, domain name systems), 911 services, and voicemail to VoIP:  

Everything else that interacts with the telephone system has to be migrated onto the same IP-based network. This reality creates an enormous amount of work, followed by a necessity of having all DoD entities interconnecting properly, between different agencies, battalions, program offices, and more. 

Every military branch has attempted to implement the transition with bold, large-scale campaigns—often with mixed results. At Fort Leonard Wood, for example, the Army’s Installation Information Infrastructure Modernization Program (I3MP) recently completed a TDM-to-VoIP initiative that converted almost 20,000 users to a VoIP system based on the DoD’s “Assured Services Session Initiation Protocol” (AS-SIP). 

Almost immediately, the new system began experiencing a series of technical complications. Users’ call attempts terminated in "NO CALL," "FAILED CALL," and "NO AUDIO" errors. Investigation revealed that the problems originated from the complexity of AS-SIP and a shortage of appropriately trained technical personnel tasked with the transition, as noted in the Army’s report on the situation: “A scarcity of the necessary technical knowledge, skills, and expertise complicated the effort.” 

Meanwhile, the Air Force’s operation “Switch Blitz”—a campaign to replace outdated telephone switches across the entire branch—is on pace to finish the installation of 75 new systems this year. “These analog switches are often over 30 years old and way past their life expectancy,” said Maj. Mary Vasta, Switch Blitz program lead. “The communication squadrons have worked magic to keep them alive this long, but there are no more parts available to fix them.” The costs have been significant, but the Air Force’s program has achieved remarkable results. “In two years, we’ve spent $53 million, and we only have 17 legacy telephone switches left to convert to VoIP,” Vasta said. 

For decades, TDM served as a reliable and effective voice communication system within the military. Still, the shift to VoIP—despite the practical difficulties its implementation presents—provides the military with a much more adaptive, cost-effective, and secure telecommunications infrastructure. The transition is in progress, inevitable, and mission-critical for the future of America’s Armed Forces.