Vehicle to and from control center
In intelligent transport systems the transfer of information between the control centre and operation vehicles can be divided into short and long distance communications. Long distance communications primarily concerns automatic vehicle location information and voice communications, while short distance communications mainly involve the transfer of route data and software updates. As reference data, which includes routes stops and schedule information, transaction and statistical data and software updates represent large quantities of data which cannot be handled or are difficult to handle on long distance radio systems, these are usually transmitted at close range at the depot. Typically this is done using wireless local area network (LAN) provided at the depot and sometimes at specific strategic positions across the network. Generally this is undertaken on an opportunistic basis with data being sent in packets and the on-board computer keeping track of successful data transferral.
Long distance communications occur during route operations and can be undertaken by a variety of technologies. The most widely implemented of these include digital or analogue private mobile radio (PMR) systems and GPRS (General Packet Radio Service) which operates over the GSM (global system for mobile communications) mobile network. In general the in-vehicle radio system is connected to the on-board computer which is connected to the on-board GPS system. The GPS receiver feeds location information to the on-board computer which transfers the information to the radio system (used for both voice and data transmission) which transmits location, speed and time information updates approximately every 30 seconds to the control centre.
Analogue PMR systems are not very common and are mainly used for voice communications, but they can handle data transfer. This generally includes vehicle location, speed and time data. The radio system can transfer more complex data but this uses up scarce capacity. Analogue PMR systems, like digital PMR systems are trunked radio systems which utilise a network of base stations (masts), a channel switching facility, control centre and mobile radios. In a trunked radio system a central computer operating on a control channel, which is a dedicated frequency, receives data packets from a mobile radio system which is requesting communication with a specific talk-group. All radios monitoring that group are instructed to automatically switch to an indicated frequency for transmission monitoring. After the initial user is finished speaking mobile radios of that talk-group return to monitoring the control channel. This packet switching system conserves limited radio frequencies by allowing multiple user groups to share radio frequencies without listening in on each-others conversations.
To a certain extent analogue PMR systems are considered out of data and have been superseded by TETRA (Terrestrial Trunked Radio) systems which are next generation digital PMR standard. TETRA systems use essentially the same infrastructure with the base station network continuously sending out signal to provide coverage for mobile radio systems in range, the network switch facility connected to the base stations via landline or microwave link, and control centre, which generally has priority communications, connected to the switch via landline. In the TETRA system the network switch is the heart of the system and the control is the centre point of communications. Several types of data communications are supported by TETRA, over the main control channel short data services and status messages are supported and on specifically assigned channels circuit switching data or packet switching data communications are supported.
Instead of a private network of base stations specifically for transport services, space can be leased from other networks operating base stations. This could also be done in addition to the private network to extend coverage and/or further supplemented where there are no base stations by using GPRS over the GSM network.
GSM operates using similar infrastructure to TETRA and is a full digital service. It was originally developed for voice communications but demand for data communications led to the integration of data transfer technologies such as GPRS which use the same infrastructure as GSM. In the GPRS system of data transfer when GPRS is not available an SMS is sent. When there is no coverage at all, data is stored and transmitted when coverage returns. Typically GPRS systems send location, speed and time data updates to the control centre every 10 to 60 seconds with location information being provided by the GPS system connected through the on-board computer. This type of system can offer voice communication, fuel consumption and engine status monitoring via connection to monitoring devices linked to the on-board computer, CCTV still images and a panic button facility.
Operations management is the primary application served by communications connection between operations vehicles and the control centre. With access to real-time data, management at the control centre can ensure higher efficiency levels in terms of vehicle utilisation, schedule management and route optimisation as well as a higher level of customer service. In addition to this the connection serves as a drivers’ aid, providing quality information such as the availability of connections at certain stops, which improves their service delivery.
Benefits and cautions
While mobile radio systems are often used, GPRS is also very popular. GPRS is an ‘always on’ service which utilises the existing mobile phone network and does not require additional infrastructure. However, the technology is also a ‘best effort’ system which has many users and as such there is competition for network space. This can lead to occasional dropped transmissions and timing delays. Despite this, GPRS is suitable for long distance data transmissions and for RTPI applications.
PMR systems are very reliable given adequate coverage of base station masts in operations that do not require channel access for everyone at the same time. Digital PMR solutions tend to offer a larger quantity of data and faster data transfer capabilities than analogue, however analogue systems can be appropriate for certain operations. Each system takes advantage of trunking technology that allows many users to be accommodated on a limited number of channels. PMR systems can offer faster data transfer and additional reliability due to the use of a dedicated network, but is somewhat offset by the large additional infrastructure requirement.
Relevant case studies
Dublin, Florence, Izmir, Johannesburg, Mysore, Sri Lanka, Zurich