ATM Modernization, Business & GA, Commercial

Product Focus: Air Traffic Control Switches

By George Marsh | February 1, 2001
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Most people think of a switch as something small and simple–something that can be manipulated by a finger. Not so in an air traffic control (ATC) center.

The ground-based megaswitches for ATC are huge. They hold dozens of functions and must meet aviation standards of reliability and safety. A busy, modern control tower (or center) needs a switching complex of heroic proportions to accomplish the following:

  • Route multiple VHF (and some UHF for military) channels between aviators and ground controllers;

  • Allow controllers selectable access to ground telecommunications;

  • Provide intercom and conferencing facilities between controllers; and

  • Allow controllers to access public mobile radios and speak with ground assets ranging from emergency vehicles to runway clearance teams.

At one time, a switch handling all this would have occupied a large room. Nowadays it likely resides in a single rack, with an associated workstation at each controller’s position. All communications traffic passes through an ATC switch for anything from a handful to several hundred air traffic controllers, depending on the size of the airfield or area being served.

Generally, switching has become digital. Today’s switches permit growth in functionality and have distributed architectures based on commercial processors.

The ATC switch does not handle the thousands of lines a telephone exchange handles, but is nevertheless more complex. Your local, standard telephone exchange does not have to accommodate radio as well as phone channels, nor provide integrity levels requiring dual or triple redundancy.

In times past, most ATC centers had two separate switches, one for radio communication and one for telephony. Called radio distribution control equipment (RDCE) and telephone distribution control equipment (TDCE), each of these channels probably had a backup waiting on hot standby, with changeover occurring automatically should a system fail. A few centers–including one of the latest, the UK’s New En Route Control Centre (NERC) at Swanwick–still prefer the two-by-two architecture. But most up-to-date centers have amalgamated RDCE and TDCE into a single integrated system.

Systems engineer Mike Price at the UK branch of one leading switch producer, Frequentis, says suppliers often face more problems with the TDCE than with RDCE. Despite interface differences, the latter tends to be more straightforward, assuming sufficient bandwidth is available to handle between 10 and 20 (typically) frequencies at 25 KHz (becoming 8.33 KHz) spacings. However, airport and control center telephone installations around the world have different standards, and often the details of these standards are shrouded in the mist of history. Various communications protocols, plus different mains and DC power supply standards all complicate the matter.

Today’s big names in ATC switching include Frequentis, in Austria; Litton Denro and Telegenix, in the United States; Australia’s Redflex; and Schmid Telecommunication, of Switzerland. All can handle advanced switching/routing functions. The competitive advantage lies in making switches that are more efficient and reliable to operate; modular, to meet diverse client requirements; more reconfigurable during service life; and of course more economical both to buy and to run.


One state-of-the-art system is Frequentis’ VCS 3020 voice communications system. It is operated worldwide, including in the United States (e.g. the Federal Aviation Administration’s Air Traffic Control System Command Center), Canada (both civil and military systems), UK (NERC at Swanwick), Sweden, and at Eurocontrol’s Maastricht center in the Netherlands.

The company is particularly proud of the VCS 3020’s user interface (UI), according to Norman Wilkins, Frequentis UK managing director. The UI concept was developed in collaboration with Eurocontrol and the UK’s Civil Aviation Authority (CAA).

For graphic interface, the 3020 marries a bright, sunlight readable (for tower cabs), wide angle viewable, flat-panel color display with touchscreen entry devices and a commercial off-the-shelf (COTS) processor board. Operating with LINUX or PanTools software and an internal serial databus, the VCS 3020 interface was designed to provide fast, logical access to all functions and information pages. Blinking, color-coded keys guide controllers–who may also be concentrating on a radar display–through pre-programmed operating procedures.

A VCS 3020 default (works standard) layout provides multiple pages for air/ground, ground/ground, or mixed operation, together with special features like a telephone book and setting windows.

Frequentis technicians can also tailor panel layout and operating procedures to specific client requirements. The interface can adapt its indication of radio, telephone and intercom resources to different operational situations–for example, normal, military and emergency ATC procedures. Functions that become irrelevant are grayed out.

To ensure error-free operation, the VCS 3020 requires minimum keystroking, says Wilkins. For example, the most important functions and communication resources are located on a default page and can be selected at a single touch.

Like most modern ATC switches, the VCS 3020 series integrates radio, telephone and intercom voice communication. Various digital and analog interfaces also are available.

The system is "non-blocking"–that is, an operator has access to all communication channels at any time regardless of system loading. Full digital voice processing from operator to line interface is included to ensure high speech quality.

To meet international standards for system availability, parallel running of a duplicate core switch is backed by fully redundant voice and data distribution resources. By rerouting radio and telephone traffic, operations can continue in the event of a link failure. Receiver "voting" ensures that the best available signal is selected. Built-in test equipment (BITE) in the VCS 3020 enables faults to be located to individual line replaceable units (LRUs).

The open, decentralized hardware architecture enables the VCS 3020 series to be integrated into any kind of ATC environment. Small systems can be scaled up to large (up to 2,048 ports for the VCS 3020X and more for the 3020XM, due to be launched this year) in modular steps; no need for software changes. Peripheral hardware is common irrespective of system size.

Mindful that ATC requirements evolve, Frequentis designed configurable system settings into the 3020 software. The VCS 3020 was one of the first ATC systems to have full networking capabilities, enabling tower, approach and en route control centers to be integrated into large single centers. (See for more.)

Litton Denro

In the United States, Paul Boyce of Litton Denro suggests that radio switching is becoming more complex because of multiple disparate analog and digital interfaces and the growing need to control radios remotely over long-haul microwave or satellite links. Remote radio communications equipment no longer is limited to simple voice and push-to-talk signaling, he says.

Litton Denro developed a configurable radio controller that can be connected directly to a voice communications system with its new RCE 200, a universal radio interface. The company made the switch’s control adaptable to various radio applications, be they single-channel, dual-control, split or paired, and whether under independent, shared or pre-emptive operation.

Litton Denro’s Model 400D VCS integrates radio, telephone, intercom and conferencing communications. Each controller can communicate in any combination with all other controllers, with all radio channels and with all telephone circuits. Controller position configurations (the access granted to each individual controller) can be determined either individually or globally by a supervisor, who can reprogram the system via a PC and local area network (LAN).

Architecture for the 400D is based on distributed microprocessors, and the system utilizes the pulse code modulated (PCM) time division multiplexed (TDM) digital switching familiar in telephony. Interestingly, control panels communicate with equipment racks via digital audio protocols to minimize cabling. Touch-entry display-based operator interfaces are available.

Developed to meet ICAO recommendations plus CCITT (telephony) and CCIR (radio) standards, the 400D has a Windows-based configuration and control subsystem, which provides equipment status monitoring, system management and a flexible configuration database. It includes receiver voting, networking (including Eurocontrol MFC [multifrequency coding] Network capability) and control of remote radio sites.

Litton Denro currently has three major FAA contracts that run through to 2005. And it has established its latest VCS, the enhanced terminal voice switch (ETVS), as a standard with the FAA. ETVS is similar to the 400D system but utilizes the "latest cost-effect technology," says Boyce. (See for information.)


New Jersey-based Telegenix Inc. entered the switching business in the early 1990s. The result is the PROCOM-1400 programmable voice communication control series (VCCS). It offers all the usual voice access facilities and can be built up in modules to suit the scale of application. And it shares the non-blocking switching and unlimited conferencing found in other switches. After developing the PROCOM-1400 for fixed-base applications, Telegenix proceeded by designing a sister product, the PROCOM-1500, a transportable system for a mobile environment.

Building blocks for the PROCOM units comprise one or more central communications control unit (CCU) card cages and a number of operator communication access units (CAU), which are connected to the CCU by single twisted-pair wires. Switching modules in the CCU include interface adapters for radio, telephone and digital circuits. Each circuit module has its own power converter and is hot swappable.

Distributed microprocessor supervision and control includes the user’s specific configuration resident programmed into EPROMs (erasable programmable ROM). PROCOM incorporates built-in test equipment; an optional PC extension provides status monitoring and reconfiguration facilities. The CAUs–which can be desk, wall, console or panel mounted–rely on a button and key interface. Operators have assignable, channel-select push buttons, a dialing dual-tone multifrequency (DTMF) keypad, audio level controls, and a loudspeaker.

Telegenix’s Al Henry says touch-screen monitors are now available for PROCOM. Another recent upgrade permits control from remote CAUs via wireline 64 kilobits per second (Kbits/sec) PCM and fibre-optic links. (See for details.)


Across the Pacific, Switchplus is Australia’s home-grown ATC switching solution, variants of which manufacturer Redflex also has sold into railway, utility and air-defense command and control, plus naval shipboard communications. A second-generation digital system launched in 1996, Switchplus differs from most of its competitors in having, instead of redundant data buses, single or dual/triple-redundant point-to-point links to transmit signals between interface units. Redflex claims this feature maximizes availability.

With the Redflex system, switch modules are connected to line interface modules by 8 megabits per second (Mbits/sec) serial links, ensuring rapid data transmission. Faster, 34 Mbits/sec inter-switch links are used between elements that are geographically separated, so that the disperse systems can operate as a single, large logical switch.

A generic signal protocol (GSP) developed by Redflex enables voice and data to be treated alike–allowing a call to be set up between analog or serial ports with the same call set-up mechanism. Here, the company differentiates from most other suppliers, who are adopting the Internet Protocol standard.

In common with other suppliers, however, Redflex provides touch-screen graphic displays, which can consolidate such functions as navaid monitoring, lighting control, meteorological data, and operational procedures. Used with application-specific C2AT software, Switchplus has been selected for Dubai International Airport and for use in air defense C2 projects by Lockheed Martin, GEC-Marconi, Celsius Tech, and Tenix Defence Systems, as well as by the Royal Australian Air Force. (See for more.)

Schmid Telecom

Back in Europe, another field-proven switching system is the ICS 200/60 from Schmid Telecom in Switzerland. Its distributed and redundant intelligent system architecture was designed with voice communications networking in mind, since air traffic management (ATM) will, says the company’s Daniel Weidell, increasingly depend on multiple centers. Schmid is betting that networks comprising several ICS 200/60 switches in different rooms, or different sites, will be preferred by some ATC providers over a large centralized switching entity.

The ICS 200/60 provides access to all the usual ATC telephone networks, including air traffic service networks based on MFC-R2 or IVA signaling. Digital interconnections with other ICS 200/60s or other VCSs are supported by 2 Mbits/sec E1 or 64 Kbits/sec links. Links to remote radio sites can be analog or digital. (See for additional information.)

The Future

In years to come, ATC switches will likely match prevailing semiconductor trends by becoming smaller, faster, more efficient and cheaper. Line cards of double the present density are envisaged. Even while they grow smaller and easier to install, switching systems will become more modular, self monitoring, more resilient, and easier to support. Adapting to the changing ATM environment will mean switches will support communication between air and ground via digital data links, both terrestrial- and satellite-based, along with integration of voice and data traffic onto various digital networks. These will include the emerging point-to-point system for air traffic services known as the Aeronautical Telecommunications Network (ATN), plus other digital links such as VHF data link (VDL), satcom, Mode S and even HF.

Under Voice over Internet Protocol (VoIP) technology, both voice and signal data will be sent as digital packets or "datagrams" of variable duration. This has major bandwidth and efficiency advantages over present telephony-style TDM/circuit switching in which message units are all of a standard fixed duration.

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