Communication System, Communication Method, and Child Station of Communication System

1. A communication system including a parent station and one or a plurality of child stations, said parent station comprising: a parent station communication unit that has a reference time and communicates with said plurality of child stations; one or a plurality of parent station power control units each of which determines whether said child station should be in a power saving mode in which an apparatus is periodically partially or wholly stopped or in a normal mode in which the apparatus is operated without being partially or wholly stopped, and instructs said child station to change a mode; and one or a plurality of parent station period measurement units each of which measures a stop period in which the apparatus of the child station is partially or wholly stopped and a non-stop period in which the apparatus of the child station is not stopped in the power saving mode, and said child station comprising: a child station communication unit that performs communication while synchronizing the reference time of said parent station with a local time of said child station; a child station power control unit that changes the mode between the power saving mode and the normal mode of the child station in accordance with the mode change instruction from said parent station; and a child station period measurement unit that measures the stop period and the non-stop period of said child station, and performs a correction for one or both of the stop period and the non-stop period in the power saving mode using an error obtained by calculating a difference generated during the power saving mode between the reference time of said parent station and the local time of the child station.

2. A communication system according to claim 1 , wherein said child station period measurement unit is configured to subtract a value of the error obtained by calculating the difference between the local time at the time of completion of the time synchronization with the parent station and the local time immediately before the completion of the time synchronization from a value of one or both of the stop period and the non-stop period of the child station in the power saving mode, thereby performing the correction.

3. A communication system according to claim 1 , wherein said child station period measurement unit is configured to calculate, as an estimated error, an error that occurs in accordance with a length of the subsequent stop period based on a value of the error that has occurred in the stop period of said child station, and subtract a value of the estimated error from the value of one or both of the subsequent stop period and the subsequent non-stop period, thereby performing the correction.

4. A communication system according to claim 1 , wherein said child station causes said child station communication unit to notify said parent station of a value of the error that occurs in the stop period of said child station, and said parent station causes, based on a value of the error notified by said child station, said parent station power control unit to calculate an error that occurs in accordance with a length of the stop period of said child station as an estimated error, and in the subsequent stop period of said child station, instructs said child station to return from the power saving mode to the normal mode after an elapse of time not less than the value of the error from completion of the stop period.

5. A communication system according to claim 1 , wherein the stop period or the non-stop period of said child station in the power saving mode comprises one of a period preset for said child station, a period until the local time of said child station passes an end time of the stop period or the non-stop period notified from said parent station to said child station, and a period until the local time of said child station passes the end time of the stop period or the non-stop period calculated by said child station from a start time of the power saving mode.

6. A communication method including a parent station and one or a plurality of child stations that communicate with the parent station, comprising: a reference time notification step of causing a parent station communication unit of the parent station, which has a reference time, to communicate with the plurality of child stations, thereby transmitting the reference time of the parent station to the child stations; a time synchronization step of causing a child station communication unit of each of the child stations to receive the reference time of the parent station and synchronize the reference time of the parent station with a local time of the child station; a mode change instruction step of causing one or a plurality of parent station power control units of the parent station to determine whether the child station should be in a power saving mode in which an apparatus is periodically partially or wholly stopped or in a normal mode in which the apparatus is operate without being partially or wholly stopped, and instruct the child station to change a mode; a mode change processing step of causing a child station power control unit of the child station to change the mode between the power saving mode and the normal mode of the child station in accordance with the mode change instruction from the parent station; a parent station period measurement step of causing one or a plurality of parent station period measurement units of the parent station to measure a stop period in which the apparatus of the child station is partially or wholly stopped in the power saving mode and a non-stop period in which the apparatus of the child station is not stopped; and a child station period measurement step of causing a child station period measurement unit of the child station to measure the stop period and the non-stop period of the child station, and to perform a correction for one or both of the stop period and the non-stop period in the power saving mode using an error obtained by calculating a difference generated during the power saving mode between the reference time of the parent station and the local time of the child station.

7. A communication system including a parent station and one or a plurality of child stations, said parent station comprising: a parent station communication unit that has a reference time and communicates with said plurality of child stations; one or a plurality of parent station power control units each of which determines whether said child station should be in a power saving mode in which an apparatus is periodically partially or wholly stopped or in a normal mode in which the apparatus is operated without being partially or wholly stopped, and instructs said child station to change a mode; and one or a plurality of parent station period measurement units each of which measures a stop period in which the apparatus of the child station is partially or wholly stopped and a non-stop period in which the apparatus of the child station is not stopped in the power saving mode, and said child station comprising: a child station communication unit that performs communication while synchronizing the reference time of said parent station with a local time of said child station; a child station power control unit that changes the mode between the power saving mode and the normal mode of the child station in accordance with the mode change instruction from said parent station; and a child station period measurement unit that measures the stop period and the non-stop period of said child station, obtains an error by calculating a difference generated during the power saving mode between the reference time of said parent station and the local time of the child station, and transmits the error to said parent station via said child station communication unit, said parent station making its parent station period measurement unit perform the correction for one or both of the stop period and the non-stop period in the power saving mode using the error received from said child station by said parent station communication unit.

8. A communication method including a parent station and one or a plurality of child stations that communicate with the parent station, comprising: a reference time notification step of causing a parent station communication unit of the parent station, which has a reference time, to communicate with the plurality of child stations, thereby transmitting the reference time of the parent station to the child stations; a time synchronization step of causing a child station communication unit of each of the child stations to receive the reference time of the parent station and synchronize the reference time of the parent station with a local time of the child station; a mode change instruction step of causing one or a plurality of parent station power control units of the parent station to determine whether the child station should be in a power saving mode in which an apparatus is periodically partially or wholly stopped or in a normal mode in which the apparatus is operate without being partially or wholly stopped, and instruct the child station to change a mode; a mode change processing step of causing a child station power control unit of the child station to change the mode between the power saving mode and the normal mode of the child station in accordance with the mode change instruction from the parent station; a parent station period measurement step of causing one or a plurality of parent station period measurement units of the parent station to measure a stop period in which the apparatus of the child station is partially or wholly stopped in the power saving mode and a non-stop period in which the apparatus of the child station is not stopped; and a child station period measurement step of causing a child station period measurement unit of the child station to measure the stop period and the non-stop period of the child station, and to obtain an error by calculating a difference generated during the power saving mode between the reference time of the parent station and the local time of the child station, and causing the child station communication unit to transmit the error to the parent station, said parent station making its parent station period measurement unit perform the correction for one or both of the stop period and the non-stop period is performed in the power saving mode using the error received from the child station by the parent station communication unit.

9. A child station of a communication system, comprising: a child station communication unit that performs communication while synchronizing a reference time of a parent station of the communication system with a local time of the child station itself; a child station power control unit that changes a mode between a power saving mode in which an apparatus is periodically partially or wholly stopped and a normal mode in which the apparatus is operated without being partially or wholly stopped in accordance with a mode change instruction from the parent station; and a child station period measurement unit that measures the stop period in which the apparatus of the child station is partially or wholly stopped and the non-stop period in which the apparatus of the child station is not stopped in the power saving mode, and performs correction for one or both of the stop period and the non-stop period in the power saving mode using an error obtained by calculating a difference generated during the power saving mode between the reference time of the parent station and the local time of the child station.

10. A communication method comprising: a communication step of causing a child station communication unit of a child station to perform communication while synchronizing a reference time of a parent station including in a communication system with a local time of the child station including in the communication system and connected to the parent station; a mode change step of causing a child station power control unit of the child station to change a mode of the child station between a power saving mode and a normal mode of the child station in accordance with a mode change instruction from the parent station; and a measurement step of causing a child station period measurement unit of the child station to measure a stop period in which an apparatus of the child station is partially or wholly stopped and the non-stop period in which the apparatus of the child station is not stopped in the power saving mode, and performs a correction for one or both of the stop period and the non-stop period in the power saving mode using an error obtained by calculating a difference generated during the power saving mode between the reference time of the parent station and the local time of the child station.