Three base stations operating at different frequencies are handling the polling of the vehicles. The fourth base station is controlling the bus and tram stop displays. The base stations are connected to the central computer with modem lines. A fifth frequency reserved for data support of the on-board computers is used to modify and upgrade system parameters at night at the bus and tram depots. The traffic signal priority requests are sent over the sixth frequency direct from the vehicle to the signal controller cabinet in each junction using low power.
All data transmission is based on open protocols.
For instance the possibility for signal priority is limited not only to buses and trams of Helsinki City Transport. Any bus sending the right message on the right frequency to the traffic signal controller will get priority. This is a key point when the system will be enlarged in future. Manufacturers and costs Several companies have provided equipment to the system. The central system hardware and software, on-board computers, interfaces to signal controllers etc. are from Thoreb , Sweden.