1. A time synchronization method for a communication system configured to perform protection switching, the communication system including a first network in which an OLT (Optical Line Terminal) (1) and an ONU (Optical Network Unit) (10) are connected via a plurality of physical lines including a working line and a backup line and a second network connected to the ONU, wherein a synchronization command is transmitted by the OLT to the second network via the first network,
the time synchronization method comprising:
a step in which the OLT (1) transmits a synchronization signal to the ONU (10) and the ONU (10) synchronizes a clock of the ONU (10) with a clock of the OLT (1) using the received synchronization signal;
a step in which the OLT (1) transmits a synchronization command including designated time information and timing information to the ONU (10);
a switching step for performing a protection switching in which the OLT (1) uses, when detecting a communication failure of an uplink signal in the working line, the backup line as a new working line instead of the working line; and
a time synchronization step in which the ONU (10) executes, when not detecting a switching condition for the protection switching based on a downlink signal transmitted from the OLT (1), synchronization processing for correcting the time information included in the synchronization command based on the clock of the ONU (10) and the timing information and transmitting the synchronization command to the second network;

and when detecting the switching condition for the protection switching or receiving a switching notification from the OLT (1), then suppresses an output of time information after the synchronization processing based on the synchronization command received before the protection switching,
wherein, in the time synchronization step, the ONU (10) determines, based on information concerning a transmission source of the synchronization signal, the suppression of the output of the time information.
2. The time synchronization method according to claim 1, wherein, in the time synchronization step, the ONU (10) protects the clock before the protection switching from the synchronization by the synchronization signal after the protection switching.
3. The time synchronization method according to claim 1, wherein, in the time synchronization step, the ONU (10) corrects the time information based on information concerning an error of the synchronization signal caused by the protection switching.
4. A control device (2) of a slave station apparatus (10) connected to a master station apparatus via a first network, which is configured using redundant lines, and configured to receive a synchronization signal for synchronizing with the master station apparatus (1) and a synchronization command including time information, wherein the control device (2) performs synchronization processing for extracting the time information and timing information from the synchronization command and correcting the time information based on the timing information and output time of a local clock of

the slave station apparatus (10); and the control device is further configured to suppress, when detecting line switching in the first network or receiving a signal for notifying switching from the master station apparatus (1) in communication with the master station apparatus (1), an output of the synchronization message based on the synchronization command received before the line switching;
wherein the control device (11) executes, based on information concerning a transmission source of the synchronization signal, the suppression of the output of the synchronization message.
5. The slave station apparatus (10) comprising the control device according to
claim 4, further comprises:
an interface device connected to the second network and configured to transmit, as a synchronization message of the second network, the time information corrected by the control device (11).
6. The slave station apparatus (10) according to claim 5, further comprising another clock configured to continue to measure, even after the line switching, time based on the synchronization signal before the line switching.
7. The slave station apparatus (10) according to claim 5, wherein the control device (11) corrects the time information based on information concerning an error of the synchronization signal caused by the line switching.

8. The slave station apparatus (10) according to any one of claims 5 to 7, wherein the control device (11) detects the line switching based on a communication failure of the first network.
9. The slave station apparatus (10) according to any one of claims 5 to 7, wherein the control device (11) detects the line switching based on a switching notification from the master station apparatus (1).

10. The slave station apparatus (10) according to any one of claims 5 to 9, wherein the clock is an MPCP (Multi-point control Protocol) counter, and the synchronization signal is a time stamp received using the MPCP.
11. The slave station apparatus (10) according to any one of claims 5 to 10, wherein the first network is a PON (Passive Optical Network).
12. A control device of a master station apparatus (1) configured to transmit time information to a second network connected to a slave station apparatus (10) via a transceiver connected to a first network, which is configured using redundant lines, wherein the control device (2) transmits, to the slave station apparatus (10) via the transceiver, a synchronization signal used for synchronization of transmission and reception timing on the first network, performs, when a failure of an uplink signal in a working line among the redundant lines occurs, line switching for using a backup line as a new working line, transmits, to the slave station apparatus (10) via the transceiver, a

synchronization command including time information, timing information in the first network, and identifier identifying the new working line, and when the line switching is performed, compensates for the time information with a delay time of the new working line.
13. The master station apparatus (1) comprising the control device according to claim 12 comprises the transceiver.
14. The master station apparatus (1) according to claim 13, wherein
the transceiver performs communication of a wavelength division multiple system by using, for downlink transmission, an optical signal having wavelength different from an uplink optical signal, and
the control device (2) corrects the time information based on a delay time obtained by correcting a round trip time of the first network based on an effective refractive index of the uplink optical system and the downlink optical signal.
15. The master station apparatus (1) according to claim 13 or 14, wherein the
control device (2) corresponds to each of the plurality of transceivers and is divided into a
working control device and a backup control device, and, according to execution of the
line switching, the backup control device functions as a new working control device and
performs communication in place of the working control device.

16. The master station apparatus (1) according to any one of claims 13 to 15, wherein, when performing the line switching, the control device (2) transmits a line switching notification via the transceiver.
17. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of claim 1.