Three sim cards for data transfers in one vehicle

Three sim cards for data transfers in one vehicle

Complex Information Systems in Public Transportation

Is it necessary to use up to three SIM cards for data transfers in public transportation vehicles or is one SIM card enough? This article deals with the solution of this problem and with connections with a complex public transportation information system and also with the way of interconnecting these systems (dispatching, depots, stops, vehicles, passengers, and transportation technologies for the transportation itself). The article also describes individual types of communication and technology.

We have seen instances when in public transportation vehicles three SIM are used for data transfers. Are you asking why? One is used by the vehicle surveillance system and for communication with the dispatching, another one is used by the check-in system that transfers sold tickets data, uploads schedules and transfers files such as „blacklist“, „whitelist“ etc., the last one provides public internet and it is sometimes connected with advertisements and running LCD passenger displays. Even though these systems are supposed to be independent, they can use similar communication routes and protocols. Is it necessary to use this solution when it increases vehicle operation expenses? Is it possible to avoid it?

The goal of this article is to show that it is indeed possible even with the complicated interconnection of all systems for controlling public transportation and informing the passengers about its current state including controlling customer check-in systems and transportation routes and updating advertisement data in vehicles as well as at the stops. That is the reason why it is necessary to talk about and universally build complex information systems using various communication routes. It is also necessary to unite these systems inside vehicles.

Pic. no.1:The system of outer vehicle communication using GSM/GPRS/UMTS network.

Pic. no.1:The system of outer vehicle communication using GSM/GPRS/UMTS network.

What Are Specific Data Streams in a Complex Transportation Control System?

Data streams of a complex communication system must be divided as follows:

Controlling Transportation and Supplying Data Regarding Its State to the Passengers

The control center of a system is usually  a central dispatching of an IDS or a dispatch office of a transporter  (a CED server). Dispatchers need basic information regarding the state of transportation to control it – overtakes, delays, the number of passengers in  vehicles or at stops (if possible). Based on this information they try to provide and control transportation regularity (here we are not dealing with the reasons of these states). Using various ways of informing passengers about the state of transportation is becoming a new dispatching function.

Channels for informing passengers include:

  • stop information panels or intelligent signs ( displaying regular departures as well as delays).
  • web portals presenting departures from individual stops (can be used in mobile devices) including “web” virtual signs .
  • vehicle information panels and mainly solutions of line connections (basically a “dynamic” stop sign inside the vehicle).

For the purposes of this function it is possible to obtain vehicle position information from a GPS receiver (or exceptionally using localization loops or infrared beacons), process it and then transfer it using either (a combined) private radio network of a DP or GPRS/UMTS/3G technologies (regional transportation systems).

Passenger Information Data and Transportation Control Data Preparation

Preparing data for complex transportation systems can be divided into a number of levels:

  • Solving transportation as a whole, i.e. the basic principles of functioning, where the usual  result is creating schedules. Regional transfer systems based on line connections are usually built to lower expenses as much as possible. However, this cheaper solution increases control quality and system coordination demands.
  • Connected with this is preparation of data for the driver and the passengers, for stop panels (on- and offline mode) and if needed for other information channels – web presentations on the internet. Each of these presentations has a different form depending on the type of information channel.
  • Additional settings – controlling the transportation route by using  automatic switch throwing, crossroads preference etc.. Here the relation to the schedule and the description of the route suffices.
  • Passenger check-in – i.e. assigning appropriate price list(s) to individual vehicle drives.
  • Advertisement and general information data in vehicles – i.e. also their possible connection with information regarding the route.

The way data is transferred to vehicles is also a part of this data preparation. This usually happens in a depot (garage or centre) using the WiFi technology via a depot communication system at a set time, resp. after starting the vehicle.

Data Regarding Passenger Check-in

Checking in of passengers requires another communication system. Communication with checking systems usually happens once a day in a depot. Payment data are often extracted using an independent WiFi channel or less frequently using memory cards (contact or contactless). Newly built systems should have a data channel at their disposal even during the drive itself. Such a channel should have data permeability of at least the GPRS (transfer speed of tens of kb/s).

Additional systems for transportation

Since public transportation provides services for many people it is suitable to integrate other information systems into the main system. These integrated systems include:

  • Advertisements in vehicles and at stops (mainly LCD vehicle panels, update data demand is up to 500 Mb).
  • Warning and notifying systems for unexpected situations when integration of these systems increases passenger safety. Using them requires a slow data channel and a possibility of using voice to inform the passengers.
  • General information from a city or a region (it is necessary to solve putting this information in the system).
  • Information systems for people with impaired vision.
  • Internet in vehicles and at stops.
Pic. no.2: A complex public transportation information system.

Pic. no.2: A complex public transportation information system.

Technical Devices Used for Communication in the System

The basis of the whole system is a dispatching SW with a unified data and phonic interface capable of coordinating the above described communications or providing transfer space in media with limited band width (see below). The “CED server” itself can consist of a number of servers – communication, application, a GPS server, a server for uploading/downloading data into/from vehicle databases etc.. These servers are organized using one or more computers depending on the size of the transportation company or the IDS system. The “cloud” technology can be used as well. Local networks with sufficient capacity are used for communication. The network and security solution is more of a problem here (firewall, data coding, etc.).

The basic way of communicating with public transportation vehicles can be divided depending on the “served” space – regional and municipal transportation.

Regional public transportation covers vast spaces which means that there is no other reasonable solution than using services of mobile operators including GSM (voice – mobile phone) / GPRS (a data service with the speed of couple tens of kbit/s) / UMTS (3G) (fast data transfers, up to couple Mbit/s) or newly the LTE (theoretically more than 100 Mbit/s – currently starting in the Czech Republic). Using GPRS/3G networks is suitable everywhere where check-in data will be updated online (e.g in the case of internet tickets or payments using bank cards in vehicles, tec.). If the transporter wants to offer public internet and passenger WiFi it is necessary to use a UMTS (3G) network. If these technologies are used it is possible to use them to partly update vehicle informatics or to provide information regarding the current state of transportation to the passengers and the driver. They are also of limited use for data communication with intelligent stops. As said at the beginning of this article – this can be realized using from one to three connections GSM/GPRS/UMTS (3 SIM cards) or using only one connection capable of doing everything.

Regarding municipal transportation companies it is suitable to use a private radio network because of its independence, low operation expenses and its functionality during natural disasters. In the case of a DP it is advantageous to use a combined network with data transfers  (e.g. vehicle position information every 10 seconds) and voice transfers performed by one radio station. The disadvantage of this is low speed – 1,2 / 2,4 kbit/s which is sufficient for position surveillance for the purposes of transportation control (not for checking). A similar situation holds for the original TETRA (Terrestrial Trunked Radio) which demands more base stations than a standard DP radio network (transfer speed of 7,2 kbit/s). This does not hold for the last version of TETRA which supports communication speeds of 115,2 kbit/s or even 691,2 kbit/s when a broadened 150 KHz channel is used.

In depots (garages or centers) communication with vehicles is performed using WiFi networks. Now these networks are usually used for data updates and for downloading information regarding the drive (logs, tachograph records, check-in records, etc.) and they replace communication in vehicles that do not have any other connection. Using WiFi provides a “broad” data channel (usually more than 10 Mbit/s) but only in the limited space of a depot. It is suitable mainly for vehicle LCD display data updates (e.g. up to 500 MB). Data updates using WiFi networks in vehicles happen as follows:

  • waking up a parked vehicle on demand  – it is necessary to solve switching on of communicating components power supply and their gradual switching on according to the executed action.
  • now only during arrivals or departures to/from the depot – suitable for smaller amounts of data because the time for the action is restricted by the system running time before it switches off or before the departure from the depot. Another problem is at what time a vehicle systems update should be executed.

Other kinds of communication include communication with switches including automatic switch throwing, with crossroads control units because of preference, with electronic information stop panels, with command receivers for people with impaired vision, etc.

Overview of Communications inside Vehicles and with Vehicles

The above described system must be connected with communication inside public transportation vehicles including both inner (via vehicle busbars) and outer (radio or light waves) communication.

Inner vehicle communication:

Pic. no.3:Communication outside (antennas) and inside (busbars) a vehicle

Pic. no.3:Communication outside (antennas) and inside (busbars) a vehicle

  1. An IBIS standard busbar with the speed of 1200 bit/s and an integrated ability to record outer and inner direction indicators (an outdated 7-bit busbar).
  2. The RS 485 busbar with various communication speeds  – a cheap vehicle data transfer solution but without a defined standard of communication protocols (suitable for company solutions).
  3. Ethernet – a modern busbar for a LAN network with defined general data transfer protocols – requires more complex vehicle busbar distribution and is a little more expensive than the preceding two.
  4. CAN – a technologic busbar suitable for vehicle control processes. It is used mainly for transferring data into a vehicle tachograph.
  5. A camera system interconnected with an analogue conductor and other busbars.

Outer vehicle communication:

Outer communication is usually performed using radio signals (infra beacons – data transfers using infrared light – not used very often nowadays). We are talking about:

  1. GPS (Global position system) signal receivers used for vehicle position surveillance with 2,5 m accuracy (e.g. the SIRF star IV technology) or worse,
  2. WiFi using 2,4 GHz or 5,8 GHz frequency used for fast data transfers in depots,
  3. short range communication for “waking up” of vehicle on-board computers in depots or for communicating with the route during the drive (“interference free” FHSS communication), one of the characteristics of this type is low input power,
  4. command receivers for people with impaired vision working on the 86,790 MHz frequency,
  5. a PMR (private mobile radio) private radio network for municipal transportation companies allowing data and voice transfers and also crossroads preference,
  6. mobile operator technologies– GSM/GPRS/UMTS (3G) and newly also the LTE – for communicating with a central dispatching, a clearing center and public internet used mainly in regional transportation,
  7. switch throwing technologies used by municipal public transportation companies (there is a number of different systems in the Czech Republic),
  8. communication with crossroads control units that can be integrated in a transportation company radio network or solved using an independent radio modem to allow direct vehicle – crossroads control unit communication.

What Is the Solution Considering  the Current Development of Vehicle Data Communication?

Let us summarize data stream sizes needed for individual systems:

  1. Vehicle position surveillance and communication with the dispatching – the amount of data depends on how long the vehicle is used on a given day (position data, service messages regarding the state of the system, departure messages, logging in and logging out) – the amount of data after 16 hours can be up to 700 kB and it basically depends on a chosen vehicle surveillance time interval (from 6 seconds to 20 seconds). Small amounts of data that end up on an IDS or transportation company server (8 – 20 bytes) are sent regularly.
  2. Another system is a check-in system. This system requires data transfers once in a few days or in the case of systems using contactless chip cards multiple times in one day. Data is transferred in the form of files that must contain at least control counts as a safety measure or in the case of chosen files also coding. Data amounts depend on the way the vehicle is run and they can range from  tens of kilobytes  to 1 MByte (depending on the way the vehicle is used).
  3. The third system is public internet in vehicles – here data amounts are difficult to estimate and they can reach hundreds of Mbytes a day. This means that this system is the most demanding regarding data.

To put it simply, data streams needed for these systems are organized as can be seen in the following picture. The picture shows a virtual network belonging to a transportation company or to an IDS. At the access point (APN) of such a network, data streams are divided into streams for public internet and streams going into assigned systems – dispatching, check –in, advertisements or news shown on vehicle LCDs. Because mobile operators demand payments for vehicles or stop panels having their own IP addresses it is a good idea to build a so called “radius server” that assigns IP addresses according to a set key and GSM modem addresses – IMEI (a 15-digit number).

Pic. no.4: A solution of „Communication using one SIM card“ including public internet in vehicles.

Pic. no.4: A solution of „Communication using one SIM card“ including public internet in vehicles.


If all the above data services are put into one it is necessary to ensure respecting of the priority of messages and services with public internet having the lowest priority. The first two services  – vehicle surveillance (the highest priority) and checking – are directed via an inner data network inside a mobile operator network to a server of a transportation company or an IDS with no time restrictions while public internet is emitted right from the vehicle by setting a SIM card or through a “special” data tunnel. It goes without saying that the vehicle must be equipped appropriately to be able run systems like this.

If a system is equipped in such a way its capacity is sufficient to cover all demands of new complex transportation systems. Thanks to public internet prices (unlimited data) data created by vehicle surveillance and check-in is negligible in comparison with data flowing from public internet. Such systems must ensure priority of the individual services for fast data transfer signal coverage (UMTS or LTE) is insufficient in regions with lower population densities. In such regions, public internet data demands could render vehicle position surveillance impossible.

This conception basically does not need a depot WiFi network since all data can be acquired immediately. The only problem can be passenger LCD display updates if long high-quality videos (up to 100 Mbytes) are uploaded, or downloading data from security cameras. In the case of these systems depot WiFi networks are still very useful. Another advantage of depot WiFi networks is the existence of a backup communication system.