JP7485648B2 - Communication device and communication system - Google Patents

Description

The present invention relates to a communication device and a communication system.

In recent years, IP (Internet Protocol) networks that utilize IP technology have adopted the Precision Time Protocol (PTP), a high-precision time synchronization method (see, for example, Patent Document 1).

Patent document 1 discloses a transmission device used in a time transmission system that transmits and receives time synchronization packets between time synchronization devices via a transmission device and synchronizes the time of the time synchronization devices based on the time information of the transmission and reception, the transmission device having a delay calculation unit that measures the delay time within the device until the time synchronization packet input to the device is output from the device, and a delay information writing unit that imparts the delay time within the device measured by the delay calculation unit to a packet following the time synchronization packet and outputs the packet with the delay time to the output destination of the time synchronization packet.

JP 2020-91135 A

When a communication device transmits and receives time synchronization packets between a master device and multiple slave devices, there is a problem in that delays within the communication device affect the time synchronization between the master device and the communication device, and ultimately affect the time synchronization between the communication device and the slave devices. In addition, when packets transmitted and received between slave devices are relayed by the communication device, and time synchronization is performed between the slave devices based on time synchronization packets transmitted from the master device, fluctuations in the packet transmission load on the communication path between the communication device and an external device cause fluctuations in the delay time within the communication device, resulting in a decrease in synchronization accuracy and large variation between the slave devices.

The object of the present invention is to provide a communication device and a communication system that can improve the accuracy of time synchronization compared to a case where delay time within the device is not taken into account.

[1] A communication device that transmits and receives packets for time synchronization between a master device and a plurality of slave devices,
a time synchronization processing unit that corrects the time of the communication device so as to synchronize with the time of the master device;
a delay time measurement unit that measures a first in-device delay time corresponding to a path length from when the communication device receives the time synchronization packet transmitted from the master device to when the communication device outputs the packet to the time synchronization processing unit, and a second in-device delay time corresponding to each path length from the time synchronization processing unit to when the communication device outputs the packet to the plurality of slave devices, for each output destination, and records the measured delay time in a delay time table ;
a specific packet generating unit that reads out the second internal delay time corresponding to an output destination of the packet for time synchronization from the delay time table, and generates a specific packet for time synchronization including the read out second internal delay time;
a packet transmission unit that transmits the generated specific time synchronization packet to the slave device as the output destination, based on the time of the communication device corrected by the time synchronization processing unit;
The time synchronization processing unit reads the first in-device delay time from the delay time table, and corrects the time of the communication device to be synchronized with the time of the master device, taking into account the first in-device delay time read .
[2] The communication device further relays packets transmitted and received between the plurality of slave devices,
The communication device according to [1], wherein the packet transmission unit transmits the time synchronization packet with priority over other packets.
[3] The delay time measurement unit periodically measures the first in-device delay time and the second in-device delay time and records them in the delay time table;
the specific packet generation unit periodically reads out the second internal device delay time from the delay time table, and generates the specific time synchronization packet including the read out second internal device delay time;
The communication device according to [1], wherein the time synchronization processing unit periodically reads out the first in-device delay time from the delay time table, and corrects the time of the communication device to be synchronized with the time of the master device, taking into account the read out first in-device delay time.
[4] The communication device according to [3], wherein the time synchronization accuracy, which is the difference in time between the slave device and the master device, is 200 ns or less.
[ 5 ] A communication system including a master device, a plurality of slave devices, and a communication device that transmits and receives packets for time synchronization between the master device and the plurality of slave devices,
the communication device comprises: a time synchronization processing unit that corrects the time of the communication device to be synchronized with the time of the master device; a delay time measurement unit that measures, for each output destination, a first in-device delay time corresponding to a path length from when the communication device receives the time synchronization packet transmitted from the master device to when it outputs the packet to the time synchronization processing unit, and a second in-device delay time corresponding to each path length from the time synchronization processing unit to when it outputs the packet to the plurality of slave devices, and records the measured delay time in a delay time table ; a specific packet generation unit that reads out the second in-device delay time corresponding to an output destination of the time synchronization packet from the delay time table, and generates a specific time synchronization packet including the read out second in-device delay time; and a packet transmission unit that transmits the generated specific time synchronization packet to the slave device of the output destination, based on the time of the communication device corrected by the time synchronization processing unit;
The time synchronization processing unit reads the first in-device delay time from the delay time table, and corrects the time of the communication device to be synchronized with the time of the master device, taking into account the first in-device delay time read .

According to the invention as set forth in claims 1, 3 to 5 , it is possible to improve the accuracy of time synchronization compared to a case in which delay times within an apparatus are not taken into consideration.
According to the second aspect of the present invention, even if the packet transmission load on the communication path between the communication device and the external device fluctuates, it is possible to suppress deterioration and variation in synchronization accuracy between slave devices.

FIG. 1 is a block diagram showing an example of a communication system according to an embodiment of the present invention. FIG. 2 is a block diagram showing an example of the configuration of the transmission/reception processing unit. FIG. 3 is a diagram illustrating an example of the delay time table. FIG. 4 is a diagram showing an example of a PTP packet format. FIG. 5 is a sequence diagram showing an example of a packet flow between devices. FIG. 6 is a sequence diagram showing an example of a packet flow within a communication device.

The following describes an embodiment of the present invention with reference to the drawings. In each drawing, components having substantially the same functions are given the same reference numerals and their repeated explanations are omitted.

Summary of the embodiment
A communication device according to this embodiment is a communication device that transmits and receives time synchronization packets between a master device and a plurality of slave devices, and includes a time synchronization processing unit that corrects the time of the communication device to be synchronized with the time of the master device, a delay time measurement unit that measures, for each output destination, a first in-device delay time from when the communication device receives a time synchronization packet transmitted from the master device to when it outputs the packet to the time synchronization processing unit, and a second in-device delay time from when the time synchronization processing unit outputs the packet to the plurality of slave devices, a specific packet generation unit that generates a specific time synchronization packet including the second in-device delay time corresponding to the output destination of the time synchronization packet, and a packet transmission unit that transmits the generated specific time synchronization packet to the output destination slave device based on the time of the communication device corrected by the time synchronization processing unit, and the time synchronization processing unit corrects the time of the communication device to be synchronized with the time of the master device, taking into account the first in-device delay time.

The master device may be not only a grandmaster device that operates as a grandmaster clock (hereinafter referred to as GM clock) as an example of reference time information that serves as a time source, but also a server device that has the function of a grandmaster device and transmits packets of video, audio, etc. The time synchronization packet refers to a packet used for time synchronization, and is applied to a system that employs, for example, PTP (Precision Time Protocol), but is not limited to PTP. The communication device may be, for example, a switching hub such as an L2 switch or an L3 switch, a router, a gateway, an IEEE 1588 BC (Boundary Clock), an optical switch, etc. The communication device may be one that transmits and receives only packets for time synchronization, or one that transmits and receives packets for time synchronization and relays packets transmitted and received between multiple race devices.

[Embodiment]
1 is a block diagram showing an example of a communication system according to an embodiment of the present invention. This communication system 1 includes a grandmaster device 2, a communication device 3, and a plurality of slave devices 4A, 4B (hereinafter, collectively referred to as "slave devices 4"). The grandmaster device 2, communication device 3, and the plurality of slave devices 4 are communicatively connected by an IP network such as Ethernet (registered trademark). Here, the grandmaster device 2 is an example of a master device. The grandmaster device 2 and the slave devices 4 are examples of external devices. The IP network is an example of a communication path.

In the figure, two slave devices 4 are connected to the communication device 3, but three or more may be connected. Also, in the figure, multiple slave devices 4 are connected to the grandmaster device 2 via one communication device 3, but multiple slave devices 4 may be connected to the grandmaster device 2 via multiple communication devices 3.

The grandmaster device 2 periodically generates PTP packets that include the GM clock as an example of reference time information that serves as the time source for the entire IP network, and transmits and receives the PTP packets to and from the communication device 3. Here, the PTP packets are an example of packets for time synchronization.

The communication device 3 functions as a boundary clock device that operates in PTP time synchronization with the GM clock of the grandmaster device 2. The communication device 3 uses, for example, an L2 switch that complies with PTP.

The slave device 4 functions as an ordinary clock device that operates in PTP time synchronization with the GM clock. The slave device 4A has a time synchronization processing unit 41a that performs time synchronization processing, and a clock unit 42a. Similarly, the slave device 4B has a time synchronization processing unit 41b and a clock unit 42b. The clock units 42a and 42b clock a clock signal from a clock source (not shown, for example, an oscillator, an oscillator, etc.) and output the time of the slave devices 4A and 4B.

(Configuration of communication device)
The communication device 3 includes a transmission/reception processing unit 30 , a time synchronization processing unit 31 , and a clock unit 32 .

The transmission/reception processing unit 30 has a port S30a for the grandmaster device 2, ports M30a and M30b for the slave devices 4A and 4B, and ports C30a and C30b for the time synchronization processing unit 31. Port S30a and port C30a are connected by a first transmission path Ra, port C30b and port M30a are connected by a second transmission path Rb, and port C30b and port M30b are connected by a third transmission path Rc. Note that the transmission paths Ra, Rb, and Rc in FIG. 1 are not shown as actual routes but are shown conceptually.

The time synchronization processing unit 31 has ports C31a and C31b, and transmits and receives PTP packets to and from the grandmaster device 2, and periodically (e.g., 8 times per second) performs time synchronization processing to correct the time of the communication device 3 to be synchronized with the time of the grandmaster device 2 based on the offset time Toff calculated by the formula (1) described later in the communication path of the IP network and the delay time due to the first transmission path Ra. By providing the two ports C31a and C31b, the time synchronization processing unit 31 can perform processing related to the grandmaster device 2 and processing related to the slave device 4 in parallel. Note that port C30b may be divided into two ports, one for slave device 4A and the other for slave device 4B.

The clock unit 32 measures a clock signal from a clock source (e.g., an oscillator, an oscillator, etc.) not shown in the figure and outputs the time of the communication device 3.

Figure 2 is a block diagram showing an example of the configuration of the transmission/reception processing unit 30. The transmission/reception processing unit 30 includes a delay time measurement unit 301, a specific packet generation unit 302, a packet transmission/reception unit 303 that transmits and receives packets, a delay time memory 304 that is composed of a RAM (Random Access Memory) or the like and stores a delay time table 304a (see Figure 3), and a packet memory 305 that stores packets. Here, the packet transmission/reception unit 303 is an example of a packet transmission unit.

The delay time measurement unit 301, the specific packet generation unit 302, and the packet transmission/reception unit 303 may be configured with hardware such as an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit). In addition, the functions of each of these units 301 to 303 may be realized by storing a program in a memory such as a ROM (Read Only Memory), reading the program into a RAM (Random Access Memory), and executing it with a CPU (Central Processing Unit).

The delay time measurement unit 301 periodically (e.g., 8 times/second) measures the delay time in each transmission path Ra, Rb, and Rc as an example of the delay time within the device. Note that the delay time within the device may also include waiting time, processing time, etc.

The specific packet generating unit 302 generates a PTP packet that records the delay time within the device (hereinafter referred to as a "specific PTP packet") and transmits it to the time synchronization processing unit 31. Details of the specific PTP packet will be described later. Here, the specific PTP packet is an example of a packet for specific time synchronization.

The packet transmission/reception unit 303 determines the type of packet sent from the grandmaster device 2 or the slave device 4 based on the packet type recorded in the Ether header 110 (see FIG. 4) described below, and transmits and receives PTP packets with priority over other packets.

The packet transceiver 303 also obtains the destination address from the packet's Ethernet header 110, and transmits the packet from ports S30a, M30a, and M30b to ports M2a, S4a, and S4b of the grandmaster device 2 and slave devices 4A and 4B that correspond to the destination address.

Furthermore, the packet transmission/reception unit 303 obtains from the timing unit 32 the time when the PTP packet transmitted from the grandmaster device 2 or the slave device 4 was received, and the time when the PTP packet was transmitted to the grandmaster device 2 or the slave device 4, and notifies the time synchronization processing unit 31.

The packet memory 305 stores not only PTP packets but also packets other than PTP packets. Storing and retrieving packets in the packet memory 305 is performed by the packet transmission/reception unit 303.

Figure 3 is a diagram showing an example of the delay time table 304a. The delay time table 304a has the items of "port number" and "delay time". In the "port number" field, port numbers 001, 002, and 003, which are assigned to ports S30a, M30a, and M30b, respectively, are recorded. The delay time measurement unit 301 periodically measures the delay time in each transmission path Ra, Rb, and Rc, and records the measurement results in the delay time table 304a in association with the identification information (e.g., port number) of the ports S30a, M30a, and M30b for external devices, as shown in Figure 3. Note that in Figure 3, the delay times are represented by Dta, Dtb, and Dtc, but in reality, measured values are recorded. Here, the delay time Dta is an example of a first device delay time, and the delay times Dtb and Dtc are examples of a second device delay time.

Figure 4 is a diagram showing an example of the format of a specific PTP packet. Specific PTP packets 100a to 100c (201, 202a, 202b, 203a, 203b, 204a, 204b) have an Ether header 110, a PTP header 120, and a time information storage section 130. The Ether header 110 records a destination address (e.g., MAC address) indicating the destination, a source address (e.g., MAC address) indicating the source, and a packet type indicating whether the packet is a PTP packet or a different packet. The PTP header 120 includes a message type 121 in which identification information corresponding to the type of message is recorded, and a correction field 122 in which information for correcting time such as an internal device delay time is recorded. The time information storage section 130 records the transmission time, reception time, etc. of the PTP packet. In Figure 4, the delay times are represented by Dta, Dtb, and Dtc, but in reality, measured values are recorded.

The specific packet generation unit 302 reads the delay times corresponding to port numbers 001, 002, and 003 from the delay time table 304a, generates specific PTP packets 100a to 100c, etc., in which the read delay times are recorded in the collection field 122, and transmits them to the time synchronization processing unit 31.

(Operation of this embodiment)
Next, an example of the operation of the communication system 1 according to the present embodiment will be described with reference to Fig. 5 and Fig. 6. Fig. 5 is a sequence diagram showing an example of the flow of packets between the devices 2, 3, and 4. Fig. 6 is a sequence diagram showing an example of the flow of packets within the communication device 3.

(1) Time synchronization processing between the grandmaster device 2 and the communication device 3 The specific packet generation unit 302 reads out the delay time Dta corresponding to the port number 001 from the delay time table 304a, and generates a specific PTP packet 100a in which the read out delay time Dta is recorded in the correction field 122. The specific packet generation unit 302 transmits the specific PTP packet 100a to the time synchronization processing unit 31, as shown in FIG.

As shown in FIG. 5(a), the grandmaster device 2 transmits a PTP packet 101 including a Sync message from port M2a to port S30a of the communication device 3. The packet transmitter/receiver 303 of the communication device 3 receives the PTP packet 101 including the Sync message transmitted from the grandmaster device 2, obtains the received time t2 from the clock unit 32, and notifies the time synchronization processing unit 31. The time synchronization processing unit 31 records the notified time t2.

As shown in FIG. 5(a), the grandmaster device 2 transmits a PTP packet 102 including a Follow-up message, which records the time t1 at which the PTP packet 101 including the Sync message was transmitted, to port S30a of the communication device 3.

As shown in FIG. 5(a), the packet transmitter/receiver 303 of the communication device 3 receives the PTP packet 102 transmitted from the grandmaster device 2. As shown in FIG. 6, the PTP packet 102 is stored in the packet memory 305 and then transmitted to the time synchronization processing unit 31 by the packet transmitter/receiver 303. The time synchronization processing unit 31 obtains and records the time t1 from the PTP packet 102. Here, the PTP packet 101 including the Sync message and the PTP packet 102 including the Follow-up message are examples of packets for time synchronization.

Next, as shown in FIG. 6, the time synchronization processing unit 31 generates a specific PTP packet 201 including a Delay_Request message from the specific PTP packet 100a transmitted from the specific packet generating unit 302, and transmits the generated packet to the transmission/reception processing unit 30. The specific PTP packet 201 is stored in the packet memory 305, and then transmitted by the packet transmission/reception unit 303 from port S30a to port M2a of the grandmaster device 2, as shown in FIG. 5(a).

The packet transmission/reception unit 303 obtains the time t3 at which the specific PTP packet 201 was transmitted from the clock unit 32 and notifies the time synchronization processing unit 31. The time synchronization processing unit 31 records the notified time t3.

Next, as shown in FIG. 5(a), the grandmaster device 2 transmits a PTP packet 103 including a Delay_Respose message, which records the time t4 at which the specific PTP packet 201 including the Delay_Request message was received, to port S30a of the communication device 3.

The PTP packet 103 is then stored in the packet memory 305 as shown in FIG. 6, and is then transmitted to the time synchronization processing unit 31 by the packet transmission/reception unit 303. The time synchronization processing unit 31 obtains and records the time t4 from the PTP packet 103.

Then, the time synchronization processing unit 31 calculates the offset time Toff by the following equation (1).
Toff=(Dms+Dsm)/2
= {(t2-t1) + (t4-t3)} / 2 ... (1)

Then, the time synchronization processing unit 31 acquires the delay time Dta from the collection field 122 of the specific PTP packet 100a, and performs time synchronization processing to correct the clock unit 32 based on the offset time Toff and the delay time Dta so that the time of the communication device 3 is synchronized with the time of the grandmaster device 2.

(2) Time Synchronization Processing Between the Communication Device 3 and the Slave Devices 4A and 4B Here, the communication device 3 operates as the master device.

The specific packet generation unit 302 reads the delay times Dtb and Dtc corresponding to port numbers 002 and 003 from the delay time table 304a, generates a specific PTP packet 100b in which the delay time Dtb is recorded in the collection field 122, and generates a specific PTP packet 100c in which the delay time Dtc is recorded in the collection field 122. The specific packet generation unit 302 transmits the specific PTP packets 100b and 100c to the time synchronization processing unit 31, as shown in FIG. 6.

As shown in FIG. 6, the time synchronization processing unit 31 generates specific PTP packets 202a and 202b including a Sync message from specific PTP packets 100b and 100c sent from the specific packet generating unit 302.

The specific PTP packets 202a and 202b are stored in the packet memory 305 as shown in FIG. 5(b), and are then transmitted by the packet transmitter/receiver 303 from port M30a to ports S4a and S4b of the slave devices 4A and 4B.

The time synchronization processing units 41a and 41b of the slave devices 4A and 4B record the times t6 and t6' at which they received specific PTP packets 202a and 202b containing the Sync message.

Next, as shown in FIG. 6, the time synchronization processing unit 31 generates specific PTP packets 203a and 203b including a Follow-up message from the specific PTP packets 202a and 202b, recording the time t5 at which the specific PTP packets 202a and 202b including the Sync message were transmitted, and transmits the generated packets to the transmission/reception processing unit 30.

The specific PTP packets 203a, 203b are stored in the packet memory 305 as shown in FIG. 5(b), and then transmitted by the packet transmitter/receiver 303 from ports M30a, M30b to ports S4a, S4b of the slave devices 4A, 4B. The time synchronization processors 41a, 41b of the slave devices 4A, 4B record the time t5. Here, since the communication device 3 functions as a boundary clock device, the time t5 recorded by the communication device 3 is a time synchronized with the time of the grandmaster device 2.

Next, as shown in FIG. 5(b), the slave devices 4A and 4B transmit specific PTP packets 401a and 401b including a Delay_Request message from ports S4a and S4b to port M30a of the communication device 3.

The time synchronization processing units 41a, 41b of the slave devices 4A, 4B record the times t7, t7' when they transmitted the specific PTP packets 401a, 401b containing the Delay_Request message. As shown in FIG. 5B, the communication device 3 transmits to the ports S4a, S4b of the slave devices 4A, 4B the specific PTP packets 204a, 204b containing the Delay_Respose message, which record the times t8, t8' when the PTP packets 401a, 401b were received. The time synchronization processing units 41 of the slave devices 4A, 4B record the times t8, t8'.

The time synchronization processing units 41a and 41b of the slave devices 4A and 4B calculate the offset time Toff based on the times t5, t6, t6', t7, t, 7', t8, and t8', and perform a time synchronization process to correct the time of the devices 4A and 4B to the time of the communication device 3 based on the delay time and offset time Toff recorded in the collection field 122 of the specific PTP packets 202a and 202b (or 203a and 203b). That is, the time synchronization processing unit 41a of the slave device 4A performs a time synchronization process to correct its own clock unit 42a based on the offset time Toff and the delay time Dtb corresponding to the port number 002 so that the time of the slave device 4A is synchronized with the time of the communication device 3. The time synchronization processing unit 41b of the slave device 4B performs a time synchronization process to correct its own clock unit 42b based on the offset time Toff and the delay time Dtc corresponding to the port number 003 so that the time of the slave device 4B is synchronized with the time of the communication device 3.

In Figures 5(a) and (b), as an example of time distribution using the Two-step clock method, time information is recorded in a PTP packet containing a Follow-up message and a PTP packet containing a Delay_Respose message. However, in the case of time distribution using the One-step clock method, time information may be recorded in a PTP packet containing a Sync message and a PTP packet containing a Delay_Request message.

(Actions and Effects of the Present Embodiment)
The communication system 1 according to the present embodiment provides the following functions and effects.
(a) By synchronizing the time of the communication device 3 with the time of the grandmaster device 2 taking into account the delay time within the device, it is possible to improve the accuracy of time synchronization compared to a case in which the delay time within the device is not taken into account.
(b) In the communications between the grandmaster device 2 and the slave device 4 and the communications device 3, PTP packets are given priority over other packets and the delay time within the device is notified to the transmission destination, thereby improving the time synchronization accuracy to about 1/5 of the 1 μs of the IEEE 1588 standard (200 ns). In addition, the variation in time synchronization accuracy, which was previously 700 ns to 60 ms, was reduced to 200 ns or less.
(c) Since the communication device 3 functions as a boundary clock device, it takes on the role of a master device instead of the grandmaster device, thereby reducing the load on the grandmaster device 2.

Although the embodiment of the present invention has been described above, the embodiment of the present invention is not limited to the above embodiment, and various modifications and implementations are possible. For example, in the above embodiment, the specific packet generating unit 302 generates a specific PTP packet that records the in-device delay time corresponding to the output destination, but it is also possible to generate a specific PTP packet that records the in-device delay time corresponding to all output destinations together with information indicating the output destination.

Reference Signs List 1: communication system, 2: grandmaster device, 3: communication device, 4, 4A, 4B: slave device, 30: transmission/reception processing unit, 31, 41a, 41b: time synchronization processing unit, 32, 42a, 42b: clock unit, 100a, 100b, 100c, 201, 202a, 202b, 204a, 204b: specific PTP packet, 101, 103, 401a, 401b: PTP packet, 110: Ether header, 120: PTP header, 121 ... message type, 122... collection field, 130... time information storage unit, 301... delay time measurement unit, 302... specific packet generation unit, 303... packet transmission/reception unit, 304... delay time memory, 304a... delay time table, 305... packet memory, C30a, C30b, C31a, C31b, M30a, M30b, M2a, S30a... port, Ra... first transmission path, Rb... second transmission path, Rc... third transmission path,

Claims (5)

A communication device that transmits and receives packets for time synchronization between a master device and a plurality of slave devices,
a time synchronization processing unit that corrects the time of the communication device so as to synchronize with the time of the master device;
a delay time measurement unit that measures a first in-device delay time corresponding to a path length from when the communication device receives the time synchronization packet transmitted from the master device to when the communication device outputs the packet to the time synchronization processing unit, and a second in-device delay time corresponding to each path length from the time synchronization processing unit to when the communication device outputs the packet to the plurality of slave devices, for each output destination, and records the measured delay time in a delay time table ;
a specific packet generating unit that reads out the second internal delay time corresponding to an output destination of the packet for time synchronization from the delay time table, and generates a specific packet for time synchronization including the read out second internal delay time;
a packet transmission unit that transmits the generated specific time synchronization packet to the slave device as the output destination, based on the time of the communication device corrected by the time synchronization processing unit;
the time synchronization processing unit reads out the first internal delay time from the delay time table, and corrects the time of the communication device so as to be synchronized with the time of the master device, taking into account the read out first internal delay time;
Communication device. the communication device further relays packets transmitted and received among the plurality of slave devices,
the packet transmission unit transmits the time synchronization packet with priority over other packets;
The communication device according to claim 1 . the delay time measurement unit periodically measures the first in-device delay time and the second in-device delay time and records the measured delay time in the delay time table;
the specific packet generation unit periodically reads out the second internal device delay time from the delay time table, and generates the specific time synchronization packet including the read out second internal device delay time;
the time synchronization processing unit periodically reads out the first internal delay time from the delay time table, and corrects the time of the communication device so as to be synchronized with the time of the master device, taking into account the read out first internal delay time.
The communication device according to claim 1 . The time synchronization accuracy, which is the difference between the time of the slave device and the time of the master device, is 200 ns or less.
The communication device according to claim 3. A communication system including a master device, a plurality of slave devices, and a communication device that transmits and receives packets for time synchronization between the master device and the plurality of slave devices,
the communication device comprises: a time synchronization processing unit that corrects the time of the communication device to be synchronized with the time of the master device; a delay time measurement unit that measures, for each output destination, a first in-device delay time corresponding to a path length from when the communication device receives the time synchronization packet transmitted from the master device to when it outputs the packet to the time synchronization processing unit, and a second in-device delay time corresponding to each path length from the time synchronization processing unit to when it outputs the packet to the plurality of slave devices, and records the measured delay time in a delay time table ; a specific packet generation unit that reads out the second in-device delay time corresponding to an output destination of the time synchronization packet from the delay time table, and generates a specific time synchronization packet including the read out second in-device delay time; and a packet transmission unit that transmits the generated specific time synchronization packet to the slave device of the output destination, based on the time of the communication device corrected by the time synchronization processing unit;
the time synchronization processing unit reads out the first internal delay time from the delay time table, and corrects the time of the communication device so as to be synchronized with the time of the master device, taking into account the read out first internal delay time;
Communications system.

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