When an emergency call connects, the operator needs more than a voice: where exactly is the crash, which direction was the vehicle travelling, what propels it, what is its identity. In eCall, those answers arrive as the Minimum Set of Data — a compact, standardized message that travels over the very same voice call. This article opens the MSD field by field, explains how the 3GPP in-band modem carries it reliably through a speech channel, and shows how next-generation eCall moves the same data onto SIP/IMS as mobile networks go all-IP.
Defined by EN 15722, the MSD is deliberately tiny: no more than 140 bytes, small enough to transmit in seconds over a narrow channel. Its ASN.1 schema, encoded with packed encoding rules for bit efficiency, contains the information a dispatcher can act on immediately:
Circuit-switched mobile calls give the IVS a single audio channel. Rather than require a parallel data connection — which legacy networks and roaming situations could not guarantee — eCall encodes the MSD as audio-band signals and sends it inside the call itself. 3GPP TS 26.267 specifies the modem: after the PSAP answers, the two modems synchronise with known training sequences, the IVS transmits the uplink MSD frame (the 140-byte payload becomes 1,120 bits, protected by CRC and a rate-1/3 turbo code so it survives a noisy, fading channel), and the PSAP returns acknowledgement. Successful transfer takes only a few seconds, after which the channel is released to clean two-way voice. Redundancy and repetition rules ensure the data gets through even at poor signal levels where speech is already degraded.
A PSAP modem decodes the burst, checks the CRC and decodes the ASN.1 message, presenting it on the dispatcher's screen — map position, VIN, propulsion, trigger type — while the voice conversation begins. End-to-end standards (EN 16454) and test specifications (3GPP TS 26.269, ETSI TS 103 412) guarantee that any compliant IVS reaches any compliant PSAP across vendors and borders, which is what makes the system interoperable Europe-wide rather than a collection of national islands.
As networks retire circuit switching in favour of VoLTE/VoNR and IP multimedia subsystem (IMS) cores, carrying modem tones over voice becomes a transitional technology. NG-eCall (3GPP TS 23.167 and related standards) places the emergency call as an IMS SIP session and carries the MSD as a structured SIP message body — the MIME media type application/EmergencyCallData.eCall.MSD — alongside negotiated audio and, in future, richer media such as images or video. The same EN 15722 MSD content is preserved, so PSAP data models stay stable while transport modernises; the IVS negotiates whether the network supports NG transport and falls back to the circuit-switched in-band path where it does not.
The MSD burst, modem synchronisation and the voice minutes that follow all draw on the T-Box's internal backup pack once vehicle power is lost. Weijiang Power designs wide-temperature NiMH and lithium backup cells and custom packs sized for the complete eCall energy profile — transmit pulses, data transfer, two voice calls across the 66-minute R144 sequence — with the internal-resistance control, welded construction and compliance documentation automotive programs require. Send us your terminal's current profile and target standards, and we will engineer the power behind a message that must never fail to arrive.
