Some things are so glacially slow that you almost don’t see them coming until they are there. eCall is probably such a thing and here’s a quick overview in case you go back to sleep again on the topic and need to refresh your memory quickly when it comes up again in a couple of years.
eCall is a “new” feature that the EU requires to be built into new cars by the end of April 2018 (i.e. another 2 years from now). Apart from the potential privacy issues arising from requiring a device in a car by law that couples a GPS receiver with a GSM and/or UMTS module, the great idea behind eCall is that a car can automatically make an emergency call after an accident without manual intervention of the driver or a passenger who might not be able to do so. In addition to setting up a speech path between the inside of the vehicle and a person at an emergency response center the eCall device can also send up to 140 bytes of information such as location, travel direction, vehicle identification, etc. to the emergency center. While the emergency call part of the solution reuses the GSM/UMTS EMERGENCY SETUP procedure for voice calls that has been in the standard since its early days, the data transmission part is what makes eCall different.
What does “Inband” mean?
Most documents on the topic mention that the up to 140 bytes are sent “inband” in the speech path but little else. While it might be strange at first to send data in the voice channel it has the big advantage that no additional equipment is required in the network. At first I assumed that the solution would probably use “audible tones” for message transmission. But I guess that approach is too simple and too slow as transmitting up to 140 bytes (i.e. 1120 bits) would take quite some time that way. This is obviously not acceptable in practice and the specification requires that the message is sent within 4 seconds. As a consequence a somewhat more complicated modulation scheme is used that has been designed to pass through an AMR voice channel. During transmission the voice channel at the destination side is muted. 3GPP TS 26.267 contains the details. It’s interesting to note that the first version of the document is from back in 2009 and hasn’t changed much since then, hence my somewhat sarcastic introduction to the topic.
Push and Pull
Also noteworthy in this regard is that the data is not sent “blindly” but only after a request from the emergency center. If the emergency center does not request data the eCall device can also send an “invitation” to the emergency center to request the data. Also it can be repeated if it was not correctly received.
eCall and LTE/IMS/VoLTE
What surprised me a bit is that no eCall successor for IMS/LTE seems to be in the works at the moment. In other words, there is no way GSM can be switched off in the next 15 years. Bummer!
And here are a few eCall terms that are mentioned in the spec and also other document on the topic
IVS: In-Vehicle System (= The eCall device in the car)
PSAP: Public Safety Answering Point (= The emergency center the answers the call)
MSD: Minimum Set of Data (= the eCall Message). It is transmitted in either 1.3 seconds (fast mode) or 2.6 seconds (robust mode)