HTML5 Webook

54/182

An example of the transmission frame structure used in the testbed established around the Seika Kururin Bus route is shown in Fig. 7. A transmission frame is 60 seconds in duration and synchronized to time pluses from GPS satellites. It is composed of multiple time slots assigned to the test terminals.In the frame, we rst assigned four almost equally spaced four-second time slots to signage terminals installed in buses (S-B) to allow these terminals to transmit data frequently. When two or more buses exist in communica-tion range, the CSMA/CA scheme is used to autono-mously avoid data packet collisions. We then assigned time slots—similar to those assigned to the signage terminals in buses—to both signage terminals installed in facilities (S-F) and sensor terminals (Sen). We made this arrangement because both S-F and sensor terminals were stationary and can be installed outside communication range. If S-F and sensor terminals happen to enter communication range, the CSMA/CA scheme is used to establish autonomous communications between these terminals, as with the S-B terminals mentioned above. Because rooop terminals covered a larger area than the other types of terminals mentioned above and neighboring rooop terminals need to communicate with each other to relay data, we assigned a time slot of two seconds respectively to each rooop terminal. Rooop terminals were designed to transmit and receive bus location data, data collected by sensors and emergency information; but not to transmit or receive the “contents” data.Seika Kururin buses in operation transmit their loca-tion data to rooop terminals that are within their com-munication range. e rooop terminals then share the data among themselves via the network established. An example of measurements of delay in bus location data transmission is shown in Fig. 8. Without loss of generality, Seika Kururin buses transmitted their location data at ar-bitrary locations along the bus route to rooop terminals within communication range. e location data was then relayed to the T6 rooop terminal on top of Mukunoki Center (Fig. 6). erefore, the transmission delay is the time it takes for the bus location data to travel from a bus to T6 rooop terminal. Transmission delay of bus location data was mostly within 60 seconds, but occasionally ex-ceeded 120 seconds.e results of transmission delay data analysis are shown in Table 1. Transmission delay was within 60 seconds for 82.8% of datasets and within 120 seconds for 99.7% of datasets. In other words, the current location of a Seika Kururin bus can be collected within two minutes most of the time, and it can be detected within a minute 82.8% of the time.5Conclusionis paper described the D2D communication testbeds with no central control function we developed and their operation and demonstration. e testbeds deployed in Minato City, Tokyo and Seika Town, Kyoto have been operating without problems, eectively collecting and distributing local information and facilitating information sharing in local communities. In the suburban environ-ment of Seika Town, the simple D2D communication system used in our testbed enabled provision of a bus loca-tion notication service. In addition, the system can be operated free of communication fees and is therefore easy to introduce.Although we did not mention it in this paper, we were able to establish coordination between the D2D communi-cation system and external servers of Minato City during our demonstration experiment. We were also able to dis-tribute emergency information related to Minato City across the testbed area. In addition, the demonstration experiment in Seika Town conrmed the usefulness of the TabT1　Results of transmission delay data analysisDelay (seconds)5–3031–6061–120120–132Number of datasets4123981653Percentages42.1%40.7%16.9%0.3%Cumulative percentages42.1%82.8 /%99.7%100%端末間通信実証実験のシステム構成図Number of bus location datasets receivedDelay in bus location data reception (seconds)FiF8Example of measurements of delay in bus location data transmission2 Terrestrial Communication Technology Research and Development50　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 2 (2017)