jones_supa writes "A couple of weeks ago hacker Oona Räisänen told about finding a 16 kbps data stream on FM broadcast frequencies, and her suspicion was that it's being used by the public transit display system in Helsinki, Finland. Now it's time to find out the truth. She had the opportunity to observe a display stuck in the middle of its bootup sequence, displaying a version string. This revealed that the system is called IBus and it's made by the Swedish company Axentia. Sure enough, their website talks about DARC and how it requires no return channel, making it possible to use battery-powered displays in remote areas. Other than that, there are no public specs for the proprietary protocol. So she implemented the five-layer DARC protocol stack in Perl and was left with a stream of fully error-corrected packets on top of Layer 5, separated into hundreds of subchannels. Some of these contained human-readable strings with names of terminal stations. They seemed like an easy starting point for reverse engineering..."