ED137 VoIP Air Traffic Management Software with RoIP

Air Traffic Management (ATM) voice communication systems that utilize costly TDM circuits can be migrated to use Voice over Internet Protocol (VoIP) technology for lower cost and maintenance. The European Organization for Civil Aviation Equipment (EUROCAE) has defined ED-137 standards for air-ground and ground-ground communications. This allows end users to select products adhering to these standards from different vendors instead of relying on a single-sourced custom solution and still be confident of meeting operational and interoperability requirements. The Single European Sky ATM Research (SESAR) initiative, the International Civil Aviation Organization (ICAO) Doc 9896, and the Federal Aviation AdministrationVoice over IP Communication Enterprise (VoICE), cemented ED-137 as the future worldwide standard for the use of VoIP in Aviation and Air Traffic Management. But the uses of ED-137 do not end at aviation, they extend to numerous radio over IP use cases, including UAVs and other unmanned vehicles, rail, law enforcement, shipping and more.

ED137 Air Traffic Management Software

VOCAL has implemented an air traffic management software radio solution that complies with ED-137 1B and 1C standards. It is ideally suited for ATM radio manufacturers/system integrators looking for software to complement their hardware/software offerings. Vendors designing embedded GRS equipment will find VOCAL’s ROIP software as well as our AoIP/RoIP Reference Design invaluable for optimized time to market, offering great out of the box support for a variety of use cases, but with the flexibility of customization that all VOCAL products can offer.  Yet the same base software libraries and APIs can be configured and deployed to create a VCS(CWP) and only those features needed can be selected. This allows for small footprint installations and efficient use of processing resources, minimizing capital costs.

ed-137 modeule placement diagram

ED-137 Supported Features

• Best Signal Selection (BSS)
• Climax time delay (CLD)
• Remote Radio Control single frequency (RRC single)
• Remote Radio Control paired frequency (RRC paired)
• Request for Measurement (RMM)
• Request for Measurement Answer (RMM MAM)
• WG67 key-in package
• Permissions Lists
• Linked Sessions
• SELCAL tones
• Multicast of aircraft audio to multiple VCSs
• Summation of multiple VCS audio streams to radio
• Prioritization of VCS audio streams

ED-137 RTP TX and RX headers

VOCAL’s software internal architecture provides the ability to build multi-functional products through the partitioning of IP bandwidth and separation of hardware interfaces. For instance, running ED-137 air-ground software connected to radios on some channels in conjunction with ground-ground communication connected to telephony equipment on other channels. Each external interface has its own distinct addressing, allowing for parallel execution and co-existence. Reliability, scalability and interoperability are paramount in systems that rely on ED-137, and our systems excel in these areas. VOCAL systems are used every day worldwide, across the entire spectrum of communications, from subsea to space, as part of critical infrastructure systems. VOCAL software can run on multiple operating systems including its own LANsEND embedded kernel and can be built to run on a wide variety of computing platforms. It can be configured as a standalone product or incorporated into another application as a library.

The ED-137 module provides a flexible programming API for controlling, monitoring, and configuring all communication aspects. The API provides easy access to manipulate the RTP headers and receive updates for SIP, SDP, and RTP changes. An easy-to-use web interface is standard on our standalone solutions as is a command line interface for performing quick routine debugging and configuration tasks. If it is not customization, but a more turn key COTS solution that is needed, VOCAL has a variety of ED-137 solutions that fit the bill, from embedded systems to high density servers.

The NATO Standardization Office has published AComP-5634 “IP Access to Half Duplex Radio Networks” or IOP-HD, also referenced as STANAG 5634, which uses ED-137b as a mechanism to allow IP networks of one nation to access half-duplex Radio Networks of another nation.

VOCAL’s ED-137 radio solution is optimized for performance on both standard X86/X64 architectures as well as DSP and RISC architectures from TI, ADI, ARM, AMD, Intel and other leading vendors. Custom designs are also available for our clients.  Contact us for a demo and to discuss your specific ED-137 radio application requirements.


Speech Codecs

VOCAL implements a number of low bitrate speech codecs that are well suited for low-bandwidth satellite communications. One such coder is MELPe, which can run at low to very low bitrates (2400 / 1200 / 600 bps).  VOCAL has implemented MELPe on a variety of platforms for use in low-bandwidth systems.  We have also engineered VoIP transcoder systems for the purpose of relaying a standard VoIP terrestrial telephone call over satellite links. In this design, the end user application/configuration is unchanged due to the satellite link, other than being routed through a gateway.  The gateway transcodes from a common VoIP codec to MELPe and the far end gateway transcodes back to the original end-to-end negotiated codec.  SIP with compression (SIPCOMP) is used to keep the session control traffic to a minimum.

Beyond MELPe, the Tactical Secure Voice Cryptographic Interoperability Specification (TSVCIS) was developed to provide interoperability between waveforms utilizing different vocoder rates, levels of encryption and forward error correction. This codec has a number of advantages for satellite communication.  One of the main features of TSVCIS is its bandwidth scalability.  TSVCIS can operate in wideband and narrowband modes with data rates of 8000 to 16000 bps for wideband and 600 to 2400 bps for narrowband. Full compatibility with MELP allows for seamless transcoding to match the restricted bandwidth of satellite channels.

Jitter Buffering

Bandwidth constraint is not the only satellite-induced impairment that causes the performance of standard protocols to suffer.  Large amounts of jitter, due to transmission scheduling in combination with low bandwidth and high link utilization can also put a strain on systems.  VOCAL’s adaptive jitter buffering algorithms for RTP and similar packet-based transmissions is designed to excel in these conditions.

Fax and Data Modem

Other classes of telecommunications – such as fax and data modulations – also have issues with jitter as well as latency.  ITU-T T.38, used for transmitting fax over IP, was not specifically designed for the type of delays that satellite can introduce.  Additionally, using T.38 over satellite can often have the side effect of adding a second T.38 link (as a tandem T.38) after a terrestrial T.38 service.  The ITU design of T.38 did not take any tandem T.38 topologies into account as use cases.  VOCAL has designed its T.38 fax over IP systems to augment standard T.38 to combat satellite jitter and delay, and resolve many timing issues introduced by tandem T.38 links.  VOCAL’s T.38 remains compatible with other endpoints, gateways and fax machines, while improving performance and robustness of satellite communications – a delicate balance.

VOCAL’s SIP Analog Modem Server (SAMS) and Analog Modem Adapter (AMA) allow M2M modems to communicate on IP networks. VOCAL has used these technologies to allow satellite links to replace standard POTS lines; allowing legacy equipment to be used in very isolated locations, where there is no other communications method available.  Additionally, VOCAL’s V.150.1 Modem over IP Gateway allows for transparent in-network conversion of data modem communications to an IP-based data stream that is resilient to high levels of delay, jitter, and packet loss as well as being more bandwidth efficient overall.  With its robustness against large round-trip latencies, VOCAL’s modem stack present in its SAMS, AMA, and V.150.1 products, is also capable of terminating modem calls placed over voice-band satellite links.

Session Initiation Protocol (SIP)

Many of today’s satellite solutions use the Session Initiation Protocol (SIP) for session establishment and control.  There are times though, when a protocol like SIP is also not ideal for satellite use.  VOCAL has used a variety of methods to make SIP systems compatible with satellite transmission. SIPCOMP can be used for compression of SIP to significantly reduce bandwidth.  Custom modifications to SIP timing, and minimization of headers has been used to great effect when SIPCOMP is not available for a given the use case.  VOCAL has implemented custom SIP to satellite gateways that act as a SIP B2BUAs but with a standard SIP/RTP network on one side, and an extremely minimal configuration on the other. The satellite side is designed using low-bitrate codecs and custom signaling that uses only a few bits for signaling.

Custom Engineering

In addition to our already existing technologies for satellite, VOCAL offers custom engineering and design services for satellite a wide variety of communications systems. Our highly experienced communications engineers can design systems to meet your requirements using our existing licensed components as well develop new technologies to solve your satellite communication problems.

More Information


VOCAL’s optimized software is available for the following platforms. Please contact us for specific satellite solutions and supported platforms and performance data.
Operating Systems
  • Texas Instruments – C6000 (TMS320C62x, TMS320C64x, TMS320C645x, TMS320C66x, TMS320C67x), DaVinci, OMAP, C5000 (TMS320C55x)
  • Analog Devices – Blackfin, TigerSHARC, SHARC
  • PowerPC
  • MIPS – MIPS32, MIPS64, MIPS4Kc
  • ARM – ARM7, ARM9, ARM9E, ARM10E, ARM11, StrongARM, ARMCortex-A8/A9/A15/A3x/A5x/A7x, Cortex-M3/M4/M7/M33
  • Intel / AMD – x86, x64 (both 32 and 64 bit modes)
  • Linux, uClinux, BSD, Unix
  • Microsoft Windows ACM / RTC / CE / Mobile
  • Apple iOS / iPhone / iPad & MacOS
  • Google Android
  • Green Hills Integrity
  • Micrium μCOS
  • Symbian
  • Wind River VxWorks
  • eCOS / eCOSPro