Skip to main content

ED-137 and SIP-based Radio over IP

EUROCAE ED-137 is a family of interoperability standards for Voice over IP components used in Air Traffic Management (ATM). For radio applications, it defines more than ordinary SIP telephony: the IP session has to carry radio-specific behavior needed by voice communication systems, ground radio stations and related ATM components.

This makes ED-137 relevant to engineering teams building Radio over IP equipment for aviation, but it is important to identify the exact revision and volume before treating a generic SIP implementation as an ED-137 solution.

Current revision context​

EUROCAE currently lists ED-137C/1 — Volume 1: Radio as the published radio interoperability standard. EUROCAE's WG-67 radio subgroup also lists later maintenance material including Change 2 and a 2025 Corrigendum 1. Historical ED-137/1B is listed as superseded.

Official references:

Because the standards continue to evolve, a current aviation project should always verify the revision set required by the customer, procurement specification or interoperability environment.

Why ordinary SIP is not enough​

SIP itself is a session-control protocol. It can establish, modify and terminate multimedia sessions, and SDP can negotiate media parameters. That solves the basic IP call-control problem, but a radio system has additional state that a telephone endpoint usually does not expose.

Examples include:

  • push-to-talk state;
  • receiver/squelch indication;
  • transmitter selection;
  • radio priority;
  • frequency or channel context;
  • radio quality or supervision information;
  • behavior specific to ground radio station and voice communication system roles.

ED-137 defines how ATM components interoperate around these requirements. The standard therefore sits above and alongside normal SIP/RTP behavior, rather than replacing SIP and RTP entirely.

Typical ED-137 architecture​

A simplified radio path may contain:

  1. a Voice Communication System (VCS) or controller-side endpoint;
  2. an IP network;
  3. a Ground Radio Station (GRS) or gateway-side endpoint;
  4. the radio transmitter/receiver hardware.

SIP/SDP establish the session. RTP carries real-time audio and can also carry radio-specific information as defined by the applicable profile. The application still has to connect that protocol state to the physical transmitter, receiver and control logic.

Simplified ED-137 Radio over IP architecture with VCS, SIP/RTP network path, ground radio station gateway and radio equipment

ED-137B versus ED-137C​

ZMATRIX's SK400VOP embedded SIP stack includes historical implementation work targeting ED-137B. That is useful engineering background, but it should not be confused with automatic compliance with current ED-137C/1 requirements.

A migration or new project should treat ED-137B implementation experience as a starting point and perform a gap analysis against the precise current requirement set. That review may affect:

  • SIP behavior;
  • SDP attributes;
  • RTP extensions or control information;
  • supervision behavior;
  • error handling;
  • security expectations;
  • test and interoperability procedures.

Compliance, conformance and certification language​

For aviation-related work, wording matters. A product should not claim current ED-137 compliance simply because it uses SIP/RTP or because an older implementation targeted ED-137B.

A defensible engineering process should identify:

  • the exact ED-137 volume and revision;
  • mandatory versus optional features for the intended role;
  • deviations or implementation limitations;
  • interoperability targets;
  • the verification method expected by the customer or authority.

ZMATRIX therefore describes SK400VOP's ED-137B work as implementation history, not as certification against the latest ED-137C publications.

Choosing a software architecture for ED-137 work​

The radio application usually owns hardware-facing state, so a reusable communications stack is easier to integrate when it does not hide the media and control path behind a softphone-style application framework.

Useful properties include:

  • explicit SIP signaling APIs;
  • application-visible call events;
  • application-owned media handling;
  • controllable RTP packet processing;
  • target-independent networking callbacks;
  • a small enough footprint for dedicated embedded hardware.

That architecture allows an engineering team to implement radio-specific behavior in the product while using a reusable SIP foundation.

Security and lifecycle considerations​

ED-137 is not static. EUROCAE continues work within WG-67, including current radio maintenance and a separate cybersecurity subgroup. For long-lifecycle ATM equipment, the project architecture should therefore leave room for standards maintenance, security changes and future interoperability updates rather than freezing the protocol implementation around one historical profile.

Where SK400VOP fits​

SK400VOP can be evaluated as an embedded SIP/RTP foundation where its portability and callback model are useful. Any current ED-137 project would still require a requirements review against the target revision and role before making a compliance claim.

For an ED-137-related software evaluation, contact ZMATRIX with the required revision, endpoint role, target platform and interoperability environment.