FORM 2
THE PATENTS ACT, 1970 (39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
[See section 10, Rule 13]
RELAY APPARATUS;
MITSUBISHI ELECTRIC CORPORATION, A CORPORATION ORGANIZED AND EXISTING UNDER THE LAWS OF JAPAN, WHOSE ADDRESS IS 7-3, MARUNOUCHI 2-CHOME, CHIYODA-KU, TOKYO 100-8310 JAPAN
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

DESCRIPTION
Field
[0001] The present invention relates to a relay
apparatus constituting a ring network.
Background
[0002] The use of Ethernet (registered trademark) has been increased in industrial fields, and a network for connecting FA (Factory Automation) devices (hereinafter, "FA network") is constructed using Ethernet. In a case of achieving high-speed and highly accurate motion control in an FA network, each of nodes or relay apparatuses may be required to transfer a frame for time synchronization with a fixed delay so as not to cause delay fluctuation. [0003] An apparatus that executes store-and-forward system transfer control in which a frame is once stored in a memory and is then transferred cannot be applied to a system mentioned above. This is because a transfer delay of an Ethernet frame that has a variable frame length varies depending on the frame length.
[0004] A ring network that is a ring-shaped network also has the same problem. That is, even when each of apparatuses constituting the ring network can perform transfer with a fixed delay, a delay time may vary depending on a transfer direction, such as a clockwise direction or a counter-clockwise direction, based on the location of an apparatus or a communication line in which a failure has occurred.
[0005] As a conventional technique for solving the above-mentioned problems in a ring network, that is an invention described in Patent Literature 1. In the

invention described in Patent Literature 1, a master station and a plurality of slave stations are connected in the form of a ring-shape. The master station periodically transmits predetermined synchronization frames via two paths including a clockwise path and a counter-clockwise path, respectively, to the slave stations. The slave station includes a slave clock, a synchronization-valid-period setting unit that sets a synchronization-valid period per two paths, and a synchronization correction unit that corrects the slave clock using a synchronization frame only when a reception-ending time of the synchronization frame is included in the synchronization-valid period, thereby performing synchronization.
Citation List Patent Literature
[0006] Patent Literature 1: Japanese Patent Application Laid-open No. 2011-199420
Summary Technical Problem
[0007] However, in the invention described in Patent Literature 1, a master node or a slave node itself performs time synchronization, and therefore, there has been a problem that the technique thereof cannot be applied to a network having a configuration such that a communication apparatus such as an Ethernet switch intervenes therein.
[0008] The present invention has been achieved in view of the above circumstances, and an object of the present invention is to provide a relay apparatus that achieves time synchronization in a ring network.
Solution to Problem

[0009] In order to solve the above-mentioned problems and achieve the object, the present invention provides a relay apparatus constituting a ring network, comprising: a first queue that retains a specific frame transferred by the other relay apparatus from outside of the ring network to a right-handed path in the ring network, when receiving the corresponding frame; a second queue that retains a specific frame transferred by the other relay apparatus from outside of the ring network to a left-handed path in the ring network, when receiving the corresponding frame; and a delay adjustment unit that instructs the first queue or the second queue to provide a delay to the retained frame and subsequently output the retained frame with the delay, based on a frame transfer time from the other relay apparatus to the relay apparatus itself in the right-handed path and a frame transfer time from the other relay apparatus to the relay apparatus itself in the left-handed path.
Advantageous Effects of Invention
[0010] According to the relay apparatus of the present invention, a delay time of frame transfer is not changed even when path switching occurs in a ring network, and . therefore it is possible to achieve a ring network that enables time synchronization.
Brief Description of Drawings
[0011] FIG. 1 is a diagram illustrating an example of a communication system configured to include a relay apparatus according to the present invention.
FIG. 2 is a diagram illustrating a logical configuration of the ring network illustrated in FIG. 1.
FIG. 3 is a diagram illustrating an example of a ring

network in which a failure has occurred.
FIG. 4 is a diagram illustrating a logical configuration of the ring network illustrated in FIG, 3.
FIG. 5 is a diagram illustrating an example of a transmission processing unit of a switch apparatus.
FIG. 6 is a diagram illustrating an example of a reception processing unit of the switch apparatus.
FIG. 7 is a diagram illustrating an example of main parts of a switch apparatus to which a master node is connected.
FIG. 8 is a diagram illustrating an example of main parts of a switch apparatus to which a slave node is connected.
FIG. 9 is a sequence chart illustrating a measurement operation for a delay time.
Description of Embodiments
[0012] Embodiments of a relay apparatus.according to the present invention will be described below in detail with reference to the drawings. The present.invention is not limited to the embodiments. [0013] First embodiment.
FIG. 1 is a diagram illustrating an example of a • communication system configured to include a relay apparatus according to the present invention. [0014] In the communication system illustrated in FIG. 1, switch apparatuses 11 to 16 constitute a ring network 1. These switch apparatuses correspond to the relay apparatuses according to the present invention and are connected to other switch apparatuses via an Ethernet cable. A master node 21 is connected to the switch apparatus 11 via the Ethernet cable. Similarly, a slave node 22, a

slave node 23 and a slave node 24 are connected to the switch apparatus 12, the switch apparatus 13 and the switch apparatus 14, respectively, via the Ethernet cables. The master node 21 is a node that has a master time of the overall system and regularly transmits a frame for time synchronization. Upon reception of the frame for time synchronization transmitted by fixed delay forwarding from the master node 21 via the ring network 1, the slave nodes 22 to 2 4 use information on the master time included in the received frame to perform time synchronization with the master node 21.
[0015] A ring blocking port 30 is set in the switch apparatus 12. This ring blocking port 30 is a blocking port in Ethernet Ring Protection (ERP) defined in ITU-T G.8032. No user frame is transferred from this blocking port. That is, a logical configuration of the ring network 1 illustrated in FIG. 1 is as illustrated in FIG. 2. In this case, the master node 21 and the slave node 23, for example, communicate with each other through a communication path 41 illustrated with a dotted line. [0016] For example, in a case where disturbance 40 as illustrated in FIG. 3, that is, line disturbance occurs between the switch apparatus 13 and the switch apparatus 14, a new ring blocking port 31 and a new ring blocking port 32 are set and the ring blocking port 30 is released. Consequently, the logical configuration of the ring network 1 after occurrence of the disturbance is as illustrated in FIG. 4. Further, the master node 21 and the slave node 23 cannot continue the communication realized through the communication path 41 illustrated in FIG. 2, and are therefore switched to perform communication through a communication path 42 illustrated with a dotted line in FIG. 4.

[0017] A configuration of the switch apparatus 13 is described next. Although the switch apparatus 13 is described here as an example, the description can be also applied to other switch apparatuses.
J [0018] FIG. 5 is a diagram illustrating an example of ■ constituent elements (transmission processing unit) among constituent elements of the switch apparatus 13,- which process a frame to be transmitted to the master node 21. As illustrated in FIG. 5, the transmission processing unit
! includes delay adjustment queues 201 and 205, fixed delay queues 202 and 206, normal queues 203 and 207, reading control units 204 and 208, and a delay adjustment unit 209. [0019] The delay adjustment queue 201 performs queuing of a fixed delay frame for the master node 21 inputted from the slave node 23 (that is,, a fixed delay frame transmitted . and received between the slave node 23 connected to the switch apparatus 13 itself and the master node 21). The fixed delay queue 202 performs queuing of a fixed delay frame inputted from the other switch apparatuses constituting a ring network. The normal queue 203 performs queuing of a frame that does not have to be transferred with a fixed delay (hereinafter, referred to as "normal frame") among frames inputted from the slave node 23. The reading control unit 204 reads out frames from the delay adjustment queue 201, the fixed delay queue 202 and the normal queue 203 in a predetermined order, and outputs the frames to the clockwise side (the switch apparatus 12). The constituent elements described above are constituent elements that process frames to be transferred clockwise. [0020] The delay adjustment queue 205 performs queuing of a fixed delay frame for the master node 21 inputted from the slave node 23. The fixed delay queue 206 performs queuing of a fixed delay frame inputted from the other

switch apparatuses constituting the ring network. The normal queue 2 07 performs queuing of a normal frame. The reading control unit 208 reads out frames from the delay adjustment queue 205, the fixed delay queue 206 and the normal queue 207 in a predetermined order, and outputs the frames to the counter-clockwise side (the switch apparatus 14). The constituent elements described above are constituent elements that process frames to be transferred counter-clockwise.
[0021] The delay adjustment unit 209 sets delay times for the delay adjustment queue 201 and the delay adjustment queue 205 to adjust the delay times' to be fixed (so that the delay times until arrival of a transferred frame at the master node 21 are the same) between the case where the frame is transferred in the clockwise path and the case where the frame is transferred in the counter-clockwise path.
[0022] FIG. 6 is a diagram illustrating an example of constituent elements (reception processing unit) among the constituent elements of the switch apparatus 13, which process a frame transmitted from the master node 21. As illustrated in FIG. 6, the reception processing unit includes frame forwarding units 301 and 302, delay adjustment queues 303 and 304, a normal queue 305, a reading control unit 306, and a delay adjustment unit 307. [0023] The frame forwarding unit 301 receives a frame from the clockwise path (receives a frame from the switch apparatus 14). In a case where the received frame is a frame addressed to the slave node 23 under the switch apparatus 13, the frame forwarding unit 301 determines whether the received frame is a fixed delay frame or a normal frame. The frame forwarding unit 301 forwards the received frame to the delay adjustment queue 303 when it is

determined as a fixed delay frame, but to the normal queue 305 when it is determined as a normal frame. Further, in a case where the received frame is not a frame addressed to the slave node 23, the frame forwarding unit 301 forwards the frame to the clockwise path (transfers the frame to the switch apparatus 12). On the other hand, the frame forwarding unit 302 receives a frame from the counter¬clockwise path (receives a frame from the switch apparatus 12). In a case where the received frame is a frame addressed to the slave node 23 under the switch apparatus 13, the frame forwarding unit 302 determines whether the received frame is a fixed delay frame or a normal frame. The frame forwarding unit 3 02 forwards the received frame to the delay adjustment queue 304 when it is determined as a . fixed delay frame, but to the normal queue 305 when it is determined as a normal frame. Further, in a case where the received frame is not a frame addressed to the slave node 23, the frame forwarding unit 302 forwards the frame to the counter-clockwise path (transfers the frame to the switch apparatus 14) .
[0024] The delay adjustment queue 303 performs queuing of the fixed delay frame transmitted in the clockwise path, and the delay adjustment queue 304 performs queuing of the fixed delay frame transmitted in the counter-clockwise path. The normal queue 305 performs queuing of the normal frame. [0025] The reading control unit 306 reads out frames from the delay adjustment queue 303, the delay adjustment queue 304 and the normal queue 305 in a predetermined order, and outputs the frames to the slave node 23. The delay adjustment unit 307 sets delay times for the delay adjustment queues 303 and 304 to adjust the delay times to be fixed between the case where the frame from the master node 21 is transferred in the clockwise path and the case

where the frame is transferred in the counter-clockwise path.
[0026] For convenience of explanation, it has been described that the transmission processing unit and the reception processing unit each include the delay adjustment unit. However, only one delay adjustment unit, may be provided outside the transmission processing unit and the reception processing unit, so that one delay adjustment unit instructs the delay times to be given to the frame to the delay adjustment queues 201 and 205 of the transmission processing unit, and the delay adjustment queues 303 and 304 of the reception processing unit.
[0027] Operations of the switch apparatus according to the present embodiment are described next. As an example, operations of the switch apparatus 13 are described. Descriptions are made assuming that the switch apparatus 13 beforehand knows a delay time until arrival of a frame at the switch apparatus 13 when the switch apparatus 11 to which the master node 21 is connected transmits the frame clockwise (a delay time from transmission of the frame by the switch apparatus 11 to arrival of the frame at the switch apparatus 13), and a delay time when the switch apparatus 11 transmits the frame counter-clockwise. Note that the delay time until arrival of the frame at the switch apparatus 13 when the switch apparatus 11 transmits the frame clockwise is equal to a delay time until arrival of a frame at the switch apparatus 11 when the switch apparatus 13 transmits the frame counter-clockwise. The switch apparatus 13 transmits and receives a control frame for delay time measurement in each path, and measures a delay time in each path, in a case where the delay time is expected to change, for example, in a case where the ring network is constructed, a case where the number of the

switch apparatuses constituting the ring network is changed, or a case where a faulty switch apparatus is replaced. In the present embodiment, it is assumed that a delay time from the switch apparatus 13 to the switch apparatus 11 in the clockwise path is 2us and a delay time from the switch apparatus 13 to the switch apparatus 11. in the counter¬clockwise path is 4us,
[0028] In this case, the delay time in the clockwise path is shorter than the delay time in the counter¬clockwise path by 2us . Therefore, the delay adjustment unit 209 in the transmission processing unit illustrated in FIG. 5 instructs the delay adjustment queue 201 that receives and retains a fixed delay frame to be transmitted to the master node 21 in the clockwise path among frames inputted from the slave node 23, to perform fixed delay forwarding of 2us . That is, the delay adjustment unit 209 instructs the delay adjustment queue 201 to add a delay of 2us and output the frame with the delay. The delay adjustment unit 209 does not instruct the delay adjustment queue 205 to add a delay. Meanwhile, in the reception processing unit illustrated in FIG. 6, the delay adjustment unit 307 instructs the delay adjustment queue 304 that receives and retains a frame forwarded in the counter¬clockwise path, to perform fixed delay forwarding of 2us. [0029] In this manner, the switch apparatus 13 can fix the delay time until the frame transmitted from the slave node 23 arrives at the master node 21, irrespective of the transfer path. Further, the switch apparatus 13 can fix the delay time until the frame transmitted from the master node 21 arrives at the slave node 23, irrespective of the transfer path.
[0030] In the network configured as illustrated in FIG. 2, the fixed delay frame transmitted from the slave node 23

is transferred to the master node 21 in 4us . Meanwhile, in a state where disturbance occurs as illustrated in FIG. 4, the fixed delay frame transmitted from the slave node 23 is in a state where the frame is transferred to the master node 21 in 4us that is the same delay as that in the counter-clockwise path, because a delay of 2us is added for the frame in the delay adjustment queue 201 .of the switch apparatus 13. Further, the fixed delay frame from the master node 21 to the slave node 2 3 arrives at the switch apparatus 13 clockwise in a normal state (a state illustrated in FIG. 2 before occurrence of the disturbance) and the delay time in this case is 4us. Meanwhile, in the state where the disturbance occurs (the state illustrated in FIG. 4), the frame arrives at the switch apparatus 13 with a delay time of 2us and a delay of 2us is added for the frame in the delay adjustment queue 304. Therefore, a transfer time is the same, that is, 4us . [0031] The operations of the switch apparatus 13 described in the present embodiment are operations in a case where no delay adjustment is performed in the switch apparatus 11 to which the master node 21 is connected. Therefore, the switch apparatus 13 is configured to perform delay time adjustment both when the switch apparatus 13 transmits a frame to the switch apparatus 11 and when the switch apparatus 13 receives a frame from the switch apparatus 11. In a case where the switch apparatus 11 has a function of adjusting a delay time, it suffices that the switch apparatus 13 adjusts the delay time in either of a transmission process and a receiving process of the fixed delay frame. For example, in a case where the switch apparatus 11 performs the addition of the delay of 2 us described above in the receiving process of the fixed delay frame, the switch apparatus 13 is configured not to adjust

a delay time in the transmission process of a frame. However, in a case where the switch apparatus 11 to which the master node 21 is connected performs delay adjustment, it is necessary to manage a delay time to each of the switch apparatuses to which the slave nodes are respectively connected and to give delays having different values to the respective switch apparatuses as frame transmission destinations. This makes the process complicated and increases the load. Therefore, it is desirable for each of the switch apparatuses to which the slave nodes are connected to perform delay adjustment both at the time of transmission and at the time of reception. [0032] Although the switch apparatus 13 has been described, same descriptions can be also applied to the other switch apparatuses to which the slave nodes are connected (the switch apparatuses 12 and 14, and so on). [0033] As described above, the switch apparatus according to the present embodiment is configured to retain in advance information on a difference between a delay time when a fixed delay frame is transmitted clockwise in a ring network and a delay time when a fixed delay frame is transmitted counter-clockwise in the ring network, and to perform delay adjustment in accordance with a transfer path of the fixed delay frame. More specifically, when a fixed delay frame is transmitted in a path having a small delay and when the fixed delay frame is received in a path having a small delay, delay is added. Due to this configuration, fixed delay forwarding of a fixed delay frame can be continued even in a case where disturbance occurs in the ring network and then a path is changed. Further, although an operation for time synchronization is performed in a master node or slave node itself in the conventional technique, it is possible to perform fixed delay forwarding

even in a case where the transfer is performed via a communication apparatus such as an Ethernet switch, so that time synchronization can be achieved.
[0034] In a case where synchronization between a local time managed individually by each slave node and a master time managed by the master node is to.be realized, it suffices that each switch apparatus performs fixed delay forwarding of a frame for time synchronization the maser node has transmitted to each slave node. That is, it suffices that each switch apparatus performs delay adjustment at least in a case where that switch apparatus receives a frame for time synchronization to be forwarded . to a slave node connected to that switch apparatus. [0035] The present embodiment has been described. assuming a case where a frame for time synchronization is transferred by fixed delay forwarding. However, the present embodiment is also applicable to a case of fixed delay forwarding of a frame other than the frame for time synchronization. [0036] Second embodiment.
In the first embodiment, descriptions have been made assuming that the switch apparatus 11 to which the master node 21 is connected knows beforehand the transfer delay time in each of the paths (the clockwise path and the counter-clockwise path). Next, an embodiment is described in which a transfer delay time is dynamically acquired to realize fixed delay forwarding. Note that the overall configuration of the communication system is identical to that in the first embodiment (see FIG. 1) . [0037] FIG. 7 is a diagram illustrating an example of main part of the switch apparatus 11 to which the master node 21 is connected. Although the switch apparatus 11

obviously includes the same constituent elements as the transmission processing unit and the reception processing unit (see FIG. 5 and FIG. 6) included in the switch apparatus 13 described in the first embodiment, the constituent elements are omitted for simplifying the explanations in FIG.7. FIG. 7 shows only constituent elements related to an operation for dynamically acquiring a transfer delay time.
[0038] As illustrated in FIG. 1, the switch apparatus 11 includes a synchronization-frame snoop unit 402, a delay measurement unit 403 and .a-switch unit 404, as a configuration for acquiring a transfer delay time. Every time the synchronization-frame snoop unit 4 02 receives a frame, the snoop unit 402 performs snooping of the received frame to confirm whether the received frame is a synchronization frame addressed to a slave node. The delay measurement unit 403 measures a delay time in each path based on a snooping result in the synchronization-frame snoop unit 402. The. switch unit 404 transmits and receives a frame to and from other switch apparatuses. For example, the switch unit 404 transmits and receives a. delay measurement request and a delay measurement answer which are described later.
[0039] In the switch apparatus 11 illustrated in FIG. 7, a port (not illustrated) to which the master node 21 is connected is assumed to receive a synchronization frame 401 regularly transmitted by the master node 21. In this case, the synchronization-frame snoop unit 402 detects that the master node 21 is connected to the synchronization-frame snoop unit 402 itself (the switch apparatus 11) and the synchronization frame 401 is regularly transmitted. Upon detection of a fact that the synchronization frame 401 is regularly transmitted from the master node 21, the

synchronization-frame snoop unit 402 notifies the delay measurement unit 403 of the fact. Upon reception of this notification, the delay measurement unit 403 instructs the switch unit 404 to, when the switch unit 404 receives a frame measuring the delay time, forward the frame to the delay measurement unit 403 itself.
[0040] FIG. 8 is a diagram illustrating an example of main part of the switch apparatus 13 to which the slave node 23 is connected. Similarly to FIG. 7, descriptions of constituent elements identical to the transmission processing unit and the reception processing unit (see FIG. 5 and FIG. 6) described in the first embodiment are omitted for simplifying the explanations.
[0041] As illustrated in FIG. 8, the switch apparatus 13 includes a switch unit 501, a synchronization-frame snoop unit 503 and a delay measurement unit 504. As is apparent from comparison of FIGS. 8.and 7, the switch apparatus 13 and the switch apparatus 11 are identical in basic configuration. However, they are different in operation of each constituent element. That is, each switch apparatus performs an operation when a master node is connected thereto, which is different from an operation when a slave node is connected thereto.
[0042] In the switch apparatus 13 to which the slave node 23 is connected, when the switch unit 501 receives a synchronization frame transmitted from the master node 21 from the clockwise path or the counter-clockwise path, the switch unit 501 transfers the synchronization frame to the synchronization-frame snoop unit 503. Upon reception of the synchronization frame from the switch unit 501, the synchronization-frame snoop unit 503 instructs the delay measurement unit 504 to measure a delay time. Upon reception of the instruction to measure the delay time, the

delay measurement unit 504 measures the delay time between the switch apparatus 13 and the switch apparatus 11 connected to the master node 21 in accordance with a sequence illustrated in FIG. 9. That is, the delay measurement unit 504 transmits and receives a frame for delay time measurement to/from the switch apparatus 11 in accordance with the sequence of FIG. 9. .More specifically, the delay measurement unit 504 transmits a delay measurement request and receives a delay measurement answer as a response to the delay measurement request. The frame transmission and reception in accordance with the sequence of FIG. 9 is performed in both the clockwise path and the counter-clockwise path. At this time, the- synchronization-frame snoop unit 503 executes control in such a manner that a,synchronization frame 502 is not transferred to the slave node side until completion of the delay time measurement in the delay measurement unit 504. When the-delay time measurement is completed, the delay measurement unit 504 notifies the synchronization-frame snoop unit 503 of the completion, and the synchronization-frame snoop unit 503 that has received this notification resumes an operation of transferring the synchronization frame 502 to the slave node side.
[0043] With reference to FIG. 9, the delay-time measurement operation by the delay measurement unit 504 is described in detail.
[0044] First, as illustrated in FIG. 9, the delay measurement unit 504 of the switch apparatus 13 retains a local time (TO) and transmits a frame provided with information on this local time (TO) as a delay measurement request to the switch apparatus 11. Subsequently, the delay measurement unit 403 in the switch apparatus 11 that has received the delay measurement request returns a frame

provided with an internal processing tirae (Tm) as a delay measurement answer. Note that the switch apparatus 11 transmits the delay measurement answer in an orientation from which the switch apparatus 11 has received the delay measurement request. That is. in a case where the switch apparatus 11 has received the delay measurement request in the clockwise path, the switch apparatus 11 transmits the delay measurement answer in the counter-clockwise path. For example, in a case where the switch apparatus 11 has received the delay measurement request from the switch apparatus 13 via the switch apparatus 12, the switch apparatus 11 returns the delay measurement answer via the switch apparatus 12. Upon reception of the delay measurement answer, the delay measurement unit 504 of the switch apparatus 13 confirms a reception time (Tl) and calculates a delay time Td based on the following expression (1).. The delay measurement unit 504 notifies the delay adjustment unit 209 and the delay adjustment unit 307 (see FIGS. 5 and 6) of the calculated delay time Td to cause the respective units to retain the delay time Td, or to cause a storage unit (not illustrated) to store the delay time Td therein. Alternatively, the delay measurement unit 504 may retain the delay time Td and make the notification in response to a request from the delay adjustment unit 209 or the delay adjustment unit 307.
Td=(Ts-Tm)/2=(Tl-TO-Tm)/2 ---(l) [0045] The delay measurement unit 504 calculates the delay times Td both for the clockwise path and for the counter-clockwise path.
[0046] As described above, according to the present embodiment, the switch apparatus is configured to automatically detect a master node and measure delay times both in a clockwise path and in a counter-clockwise path.

Therefore, even in a case where disturbance occurs in a network having a ring configuration, it is possible to achieve fixed delay forwarding.
[0047] Further, by performing an operation of measuring the delay time Td described in the present embodiment in constant periods, updating the delay time Td, which is impossible in the first embodiment, can be realized, and the fixed delay forwarding can be continued even when the delay time is changed.
[0048] Note that, in order to implement the present embodiment, the ring blocking port 30 transfers a synchronization frame in the same manner as a frame for controlling an Ethernet ring, and a switch apparatus to which a master node is connected discards the synchronization frame that has been forwarded around the ring when receiving the synchronization frame.
Industrial Applicability
[0049] As described above, the relay apparatus according to the present invention is useful in implementing fixed delay forwarding of a frame in a ring network.
Reference Signs List
[0050] 11, 12, 13, 14, 15, 16 switch apparatus; 21 master node; 22, 23, 24 slave node; 30, 31, 32 ring blocking port; 40 disturbance; 41, 42 communication path; 201, 205, 303, 304 delay adjustment queue; 202, 206 fixed delay queue; 203, 207, 305 normal queue; 204, 208, 306 reading control unit; 209, 307 delay adjustment unit; 301, 302 frame forwarding unit; 401, 502 synchronization frame; 402, 503 synchronization-frame snoop unit; 403, 504 delay measurement unit; 404, 501 switch unit.

We Claim :
1. A relay apparatus constituting a ring network, comprising:
a first queue that retains a specific frame transferred by the other relay apparatus from, outside of the ring network to a right-handed path in the ring network, when receiving the corresponding frame;
a second queue that retains a specific frame transferred by the other relay apparatus from outside of the ring network to a left-handed path in the ring network, when receiving the corresponding frame; and
a delay adjustment unit that instructs the first queue or the second queue to provide a delay to the retained frame and subsequently output the retained frame with the delay, based on a frame, transfer time from the other relay apparatus to the relay apparatus itself in the right-handed path and a frame transfer time from the other relay apparatus to the relay apparatus itself in the left-handed path.
2. The relay apparatus according to claim 1, wherein
when a required time from transmission of the specific frame by the other relay apparatus to the right-handed path until the specific frame is stored in the first queue and outputted from the first queue is assumed to be a first required time, and a required time from transmission of the specific frame by the other relay apparatus to the left-handed path until the specific frame is stored in the second queue and outputted from the second queue is assumed to be a second required time,
the delay adjustment unit instructs the first queue or the second queue so as to make the first required time and

the second required time equal to each other.
3. The relay apparatus according to claim 1 or 2, wherein the first queue and the second queue retain a frame of which a transmission source is a master node that manages a master time and which has to be forwarded with a fixed delay.
4. The relay apparatus according to claim 1, 2 or 3, further comprising:
a third queue that retains a specific frame that is transferred by the relay apparatus itself to a right-handed path in the ring network and is transferred by the other relay apparatus to the outside of the ring network, when receiving the corresponding frame.from the outside of the ring network; and
a fourth queue that retains a specific frame that is transferred by the relay apparatus itself to a left-handed path in the ring network and is transferred by the other relay apparatus to the outside of the ring network, when receiving the corresponding frame from the outside of the ring network, wherein
the delay adjustment unit further instructs the third queue or the fourth queue to provide a delay to the retained frame and subsequently output the retained frame with the delay, based on the frame transfer time from the other relay apparatus to the relay apparatus itself in the right-handed path and the frame transfer time from the other relay apparatus to the relay apparatus itself in the left-handed path.
5. The relay apparatus according to claim 4, wherein when a required time from storing of the specific

frame in the third, queue until the frame is transmitted in the right-handed path and arrives at the other relay apparatus is assumed to be a third required time, and a required time from storing of the specific frame in the fourth queue until the frame is transmitted in the left-handed path and arrives at the other relay apparatus is assumed to be a fourth required time,
the delay adjustment unit instructs the third queue or the fourth queue so as to make the third required time and the fourth required time equal to each other,
6. The relay apparatus according to claim 4 or 5, wherein the third queue and the fourth queue retain a frame of which a destination is a master node that manages a master time and which has to be forwarded with a fixed delay.
7. The relay apparatus according to any one of claims 1 to 6, further comprising a delay-time measurement unit that sets the other relay apparatus as a destination of a frame for measurement of a frame transfer time and transmits the frame for measurement of a frame transfer time to the right-handed path and the left-handed path, and calculates a frame transfer time from the other relay apparatus to the relay apparatus itself in the right-handed path and a frame transfer time from the other relay apparatus to the relay apparatus itself in the left-handed path based on a time at which the frame for measurement of a frame transfer time is transmitted to the right-handed path and a time at which the frame for measurement of a frame transfer time to the left-handed path, and a time at which a response frame to the frame for measurement of a frame transfer time is received in the right-handed path and a time at which the

response frame is received in the left-handed path.