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Taking an existing large-scale realistic discrete event simulation model [1,2] of a mobile distributed system with more than 700 000 units we investigate the efficiency of control protocols for the software and data updates of on-board-units (OBUs) . Adding four different closed-loop control protocols to the existing simulation model we explain and investigate the technical implications of the control protocols: OBUs that allow incoming network connections can be reached at any time (Always-On or Sometimes-On). If the OBU is designed without a TCP/IP server, signaling is implemented either as polling (SometimesOn with polling) or using a dedicated GSM signaling channel (On-Demand-On). The efficiency is discussed as a combination of metrics gathered during simulation runs and compared with the OBUcontrolled update logic. These simulation results are subsequently used as input for the system development process.

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We analyze existing server-side log data of a large scale automatic toll system to measure the TCP roundtrip-time (RTT) as experienced by the communication between the central system and the on-board units (OBUs) deployed for tolling heavy-goods vehicles. The RTT is estimated from passive monitoring by parsing server-side log files and aggregating fleet-wide statistics over time. Using this data we compare the characteristics of the four different types of OBU and the three GPRS (2G) networks used. We find the RTT data to be consistent with existing, smaller samples and extend the observed RTT range by an order of magnitude. The OBU types exhibit a markedly different behavior, most notably for long RTTs, and we find one of the 2G networks to 'hum' at 50 Hz and harmonics.

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