eCall In-band Modem

Description

The eCall In-band Modem is a key component of the eCall service that enables the transmission of the Minimum Set of Data (MSD) from the In-Vehicle System (IVS) to the Public Safety Answering Point (PSAP) over the voice channel of an emergency call. It is defined in 3GPP TS 26.267 and TS 26.269. The modem operates in full-duplex mode, meaning it can transmit and receive data simultaneously, and is designed to work reliably over cellular and PSTN networks, which may introduce impairments such as noise, echo, and compression.

The in-band modem modulates the MSD into audio signals that are transmitted within the voice band (typically 300-3400 Hz) of the emergency call. This allows the data to traverse the voice channel without requiring a separate data connection. The modem pair consists of a transmitter at the IVS and a receiver at the PSAP. The IVS modem encodes the MSD into a series of tones or symbols, which are then sent over the voice channel. The PSAP modem demodulates the received audio to recover the MSD. The process is designed to be robust against the various distortions and codecs used in voice networks.

The in-band modem uses a combination of modulation and error correction techniques to ensure reliable transmission. It employs a synchronization mechanism to align the transmitter and receiver, and uses forward error correction (FEC) to mitigate the effects of noise and interference. The modem also includes a handshake procedure to confirm successful reception of the MSD. If the initial transmission fails, the modem can retry. The full-duplex capability allows the PSAP to send acknowledgments or requests for retransmission while the IVS is transmitting.

The in-band modem is particularly important for eCall because it allows the MSD to be transmitted even in networks that do not support simultaneous voice and data, such as traditional circuit-switched networks. It also ensures that the MSD is transmitted as part of the emergency call itself, so no additional setup is required. The modem is specified to work with various voice codecs, including AMR and PCM, and is tested under a range of network conditions.

In later releases, with the introduction of IMS-based eCall, the MSD can also be transmitted via an IMS data channel (e.g., using SIP or MSRP). However, the in-band modem remains a fallback and is still used in many deployments. The in-band modem specifications have been maintained and updated to ensure interoperability and reliability.

Purpose & Motivation

The eCall In-band Modem was created to solve the problem of transmitting data from a vehicle to emergency services over a voice-centric network. Before its introduction, there was no standardized way to send the MSD reliably over the voice channel. The in-band modem provides a robust, low-complexity solution that works across different network technologies and does not require additional data bearers. It ensures that the MSD is delivered even in areas with limited data coverage or in older networks. This is critical for the success of eCall, as the MSD contains vital information for emergency response.