Distributed Fleet Management over GPRS with Solid FlowEngine

Transcription

1 Proc. 4th Internat. Conf. on Mobile Data Management (MDM'03), Industrial Track, Melbourne, Australia, January 21-24, Distributed Fleet Management over GPRS with Solid FlowEngine Ali Paasimaa Solid Information Technology Oy Merimiehenkatu 36D FIN Helsinki, Finland ali.paasimaa ( at ) solidtech.com Abstract With wireless connectivity becoming commonplace, and with the cost and footprint of computers steadily dropping, managers of public transport fleets are realizing that it is now possible to treat vehicles as part of the corporate network. Vehicles can be fitted with onboard computers that periodically establish connectivity to report back to the central site, and to download new information. GPRS is a good fit for this kind of environments where fairly large amounts data is transferred from time to time as bursts, and Solid FlowEngine in part provides platform independent functioning, enables transactions to be executed while offline and guarantees transactional consistency throughout the whole network. 1 Introduction Circuit-switched GSM (Global System for Mobile Communications) data services have been around for about 8 years, and up to now, the primary application of this radio network has been telephony. There is no doubt that real-time speech transfer over the air will retain its dominant role for many years to come, but also new applications for radio networks are emerging. Typically these new applications need higher data transfer rates than does voice, and they are able to utilize non-deterministic packet-switched data transfer, because for many of these applications, there is no fundamental need for the transfer rate to be constant and deterministic. Internet browsers, and other connectionless applications that have a need for occasional highspeed data replication, are good examples. Today, GPRS (General Packet Radio Service) is the sole packet-switched transfer protocol working within GSM networks 1 that was developed by taking into account needs of these emerging applications. Also, because it could be assumed that a growing fraction of future applications are somehow going to utilize the Internet, it was convenient to develop a protocol that supports communication over TCP/IP by default. 1 EDGE (Enhanced Data Rates for Global/GSM Evolution) is another packet-switched data connection protocol for GSM that in a way combines HSCSD (High Speed Circuit Switched Data) and GRPS but it is not being used by anyone yet because there are no commercial terminal devices available on the market. Oct 30, 2002

2 2 The Finnish company, Buscom Oy, is one of the first to deploy new GPRS mobile communication technology in their public transportation fleet management applications. They chose to embed the Solid FlowEngine to provide both the embeddable relational data manager, and also the synchronization capabilities it needed to operate the overall system, and to wirelessly collect and reconcile public transportation related transactions. This paper compares an IP-based GSM network protocol GPRS to traditional circuitswitched technologies, and introduces a Traffic Information Management system Buscom Palette which is built on Solid FlowEngine. 2 GPRS IP Based Mobile Network GPRS was developed to support wireless IP services and is currently the only packet-oriented service in GSM networks with deployed applications. It partly shares the same network infrastructure with GSM and HSCSD (High Speed Circuit Switched Data) and uses the same radio resources to transfer data over the air. The difference between a circuit-switched connection and a packet-switched connection is that the packet-switching connection enables radio resources to be used only when users are actually sending or receiving data. This ameliorates the utilization of the air network infrastructure up to 40 percent compared to the traditional circuit-switched techniques, which reserve the circuit even when no data transmission is taking place. The main goal of GPRS is to amend the shortcomings of traditional GSM by providing (in practice) about 2 to 4 times faster data transfer speed and an ability to more efficiently utilize radio resources. In addition to the above, GPRS also provides a wireless connection using the TCP/IP protocol 2. Within the GPRS network, an IP address can be assigned for each mobile terminal. To other IP networks, the GPRS network appears as just another Internet subnet where network elements are separated with unique IP addresses. Also, the mobility of GPRS terminals is totally transparent to other IP network such as the Internet. One restriction on GPRS is that it is non-deterministic. GPRS traffic has the lowest priority in the network and is hence bound to use the residue of the radio capacity that is not occupied by GSM or HSCSD. Thus, in current networks, depending on how much circuit-switched traffic there is on the air, the transfer speed of GPRS can change on the fly to be just about anything between the maximum and zero. However, for most GPRS applications this kind of behavior is tolerable as normally the transfer rate of GPRS averages well above GSM even during daytime 3. The GRPS protocol enables operators to take advantage of under-utilized, network resources, and thus it provides a way for operators to boost revenues collected from network usage. In Finland, depending on the operator, transferring 10 MBytes/month over GPRS costs around Circuit-switched GSM data costs around 0.16 /minute, and since the transfer rate of 2 Other protocols such as X.25 are also supported. 3 During night-time there is typically much less circuit-switched traffic on the air.

3 3 GSM is 1.2 KBytes/s, it costs about 22 to transfer 10 MBytes using GSM data. 4 Thus the cost of moving 10 MBytes over GPRS is about the same as GSM, but with GPRS the data is transferred about 2 4 times faster, depending whether data is sent up-link or down-link. 3 Solid Innovation for Mobile Replication Solid FlowEngine is a carrier-class distributed data management platform made up of two tightly coupled components: an embeddable, lightweight database management system and a patented synchronization technology Solid SmartFlow technology that guarantees the transactional correctness of the data in an intermittently connected network of nodes. The Solid FlowEngine replication scheme does not include distributed transactions but is an incremental and asynchronous replication concept, and it is a certain kind of variant of the Lazy Replica method [Wol01]. It guarantees global transactional consistency throughout the system and makes sure that the entire intermittently connected network converges towards a globally consistent state. The SmartFlow scheme has some conceptual similarities with the Two-Tier replication protocol presented by Gray et al. [GHOS96]. In Gray s method, transactions originated by replicas execute as distributed transactions that run against the master and the replica node simultaneously. In FlowEngine, replica transactions can be executed while offline from the network and be later propagated to the master for re-execution. Solid s patented Intelligent Transaction technology enables developers to write pre-programmable actions to handle central conflict resolution. It allows statements (stored procedures) residing inside the same transaction to exchange information about the success of their fragments of the transaction. The SmartFlow replication process consists of two steps: 1. Propagating transactions originating at the replica to the master, and 2. Refreshing changed data from the master to the replicas. This data may have been changed by other replicas or by any application using the master. The two operations can be decoupled so that it is possible to only refresh the replica with new data or to only propagate transactions to the master. The replication scheduling and design is programmed using Solid SQL extensions, and thus can be operated through standard ODBC and JDBC interfaces. Replication can be triggered by application events or a timer, and masters can be programmed to signal replicas when they have new data for them. Abnormally interrupted replication tasks (abnormal connection breakdowns or some other system failures) can simply be restarted after a system recovery. 4 (10000 KBytes / 1.2 KBytes / 60 seconds) x 0.16 = 22

4 4 4 Buscom Palette A Fleet Management System Built on Solid FlowEngine Buscom Oy s Traffic Information Management System, Buscom Palette, is a Public Transport Operator s ERP system. A transport operator owning a fleet of vehicles (e.g. buses and trams) uses Buscom Palette as a tool to plan and execute business strategies by designing and configuring the most efficient and profitable schedules, routes, tariffs, products, etc. For example, Buscom Palette can be used to record product-oriented passenger flows and to gather schedule-oriented vehicle location information that can be used to continuously manage fleet cost-effectiveness. The system is typically operated by the transport operator s staff at the depot and in vehicles, and by inspectors working on the field. Figure 1. Palette architecture. The Palette system comprises distinct software modules including Ticketing, Accounting and Statistics, and different hardware devices that function in vehicles and in the headquarters. The whole system, all the in-vehicle devices and the back-end depot system, is built on Solid FlowEngine and integrated with SmartFlow (Figure 1.). FlowEngine links both the in-vehicle applications and the central management system (Figure 2.). The back end software modules operate on Windows NT, using the FlowEngine master database via an ODBC interface to query, insert, update and delete business- and configurationrelated information. The FlowEngine replica database is embedded into the driver s device (the TRM200) which runs BlueCat Linux. A contactless smartcard reader terminal (the PR200) collects fares from passengers. Another proximity reader is embedded into the TRM200. In

5 5 addition to smart cards, TRM200 accepts also bank, chip, and credit cards as an instrument of payment. The TRM200 and PR200 in-vehicle devices function offline from the Internet most of the time. Using the embedded replica database, they are able to capture passenger-oriented fare information, as well as collecting schedule and vehicle usage information. Figure 2. Replication in the Palette system is conducted over GPRS. At end of day, or as convenient, transactions recorded in the in-vehicle database are aggregated and uploaded to the central depot system, by propagating the transactions to the FlowEngine master. The transaction stream includes payment and passenger transaction, other passenger statistics, schedule data, and vehicle usage information. The depot system handles payment transactions with banks and smartcard companies. Once uploaded into the master database, the collected statistics can be analyzed at the depot using the Palette system, in order to plan more profitable routes, schedules and products. Remote provisioning of the driver s TRM200 device is managed in the same way as moving any other data to the replica database. The transport operator configures the Palette system by planning and setting parameters at the depot side (e.g.. new stops, routes, schedules, drivers, blacklists, products, tariffs, etc.), and then, when a vehicle establishes the GPRS connection to the depot system, new parameters are automatically refreshed to the TRM200 device, after the replica originated transactions are run against the depot master. 5 Conclusions The existence of ubiquitous mobile packet networks and a new breed of consistency-preserving replicated databases makes the building of complex and dynamic mobile data systems

6 6 surprisingly feasible. The use of the IP-based mobile network system GPRS made it possible to utilize a general-purpose system like Solid FlowEngine. Thanks to the replication technologies built into Solid FlowEngine, the application developers did not have to be concerned with the redundancy and consistency of the data. Solid's SmartFlow takes care of maintaining the total consistency of data in the system, even in the presence of long-lasting offline periods of the fleet's mobile processing nodes. Buscom s innovative use of embedded database and GPRS technologies provides it with a lowmaintenance way to maintain control of a public transportation fleet while minimizing communications costs. Buscom's fleet management products have been sold to many European public transport operators. References [Wol01] A. Wolski. Embedding Data Recharging In Mobile Platforms. Real-Time & Embedded Computing Conference (RTEC'01), Milan, Italy, November 26-29, [GHOS96] Jim Gray, Pat Helland, Patrick E. O'Neil, Dennis Shasha: The Dangers of Replication and a Solution. SIGMOD Conference 1996: URLs Solid: FlowEngine Solid: FlowEngine User Guide Buscom: Palette