TITLE OF THE INVENTION
COMMUNICATION CONTROL EQUIPMENT
FIELD OF THE INVENTION
5 The present invention relates to communication control
equipment used in a distributed control system.
BACKGROUND OF THE INVENTION
In a control system that includes distributed plural
10 pieces of communication control equipment, time
synchronization between the communication control equipment
maybe requiredinaccordancewithanapplicationthatrealizes
the control. In a distributed control system connected to a
network, time synchronization is possible by transmission and
15 reception of time synchronization packets via the network.
Atime synchronization method using a network includes
NTP (Network Time Protocol) , SNTP (Simple Network Time Protocol) ,
IEEE 1588, etc.
Here, an execution procedure of a time synchronization
20 protocol of IEEE 1588 is explained using Fig. 14 and Fig. 15.
IEEE 1588 is applied to master-slave configurations.
Fig. 14 explains the execution procedure of the time
synchronizationprotocol of IEEE1588. Fig. 15 shows exchange
of messages between a master and a slave in IEEE 1588.
2 5 First, amaster transmits a Syncmessage to a slave (SO60) .
At this time, the master records a transmission time tl
of the Sync message (S061) .
Upon receiving the Sync message, the slave records a
reception time t2 thereof (S062).
5 The master notifies the transmission time tl of the Sync
message to the slave by use of any of the following methods
(S063). Inoneofthemethods, informationonthetransmission
time tl is placed on the Sync message. In another method,
i n f o r m a t i o n o n t h e t r a n s m i s s i o n t i m e t l i s p l a c e d o n a F o l l o w- U p
10 message following the Sync message.
Then, the slave transmits a Delay - Reqmessage to themaster
(S064).
At this time, the slave records a transmission time t3
of the Delay - Req message (S065).
15 Uponreceivingthe Delay - Reqmessage, themaster records
a reception time t4 thereof (S066).
The master places information on the reception time t4
of the Delay - Req message on a Delay - Resp message and notifies
the slave of the reception time t4 (S067).
2 0 The slave that has received the Delay - Resp message
calculates a communication delay and atime difference between
the master and slave from the times tl, t2, t3, and t4 (S068).
The calculation of communication delays is executed on
the premise that the forward communication delay and backward
25 communicationdelay are equal to one anotherbetween themaster
and s l a v e . Therefore, a one-way communication delay t d is
expressed by Formula (1) .
t d = ( ( t 4 - t 3 ) + ( t 2 - t1) ) / 2 (1)
A t i m e d i f f e r e n c e t d i f f between t h e master and s l a v e is
5 expressed by Formula (2) .
t d i f f = ( ( t 4 - t 3 ) - ( t 2 - t 1 ) ) / 2 (2)
The s l a v e is time-synchronized with t h e master by use
of t h e t d i f f .
When t h e network includes intermediary equipment such
10 a s a network switch, a p r e c i s i o n of t h e t i m e synchronization
w i l l d e c r e a s e . I E E E 1 5 8 8 i s b a s e d o n t h e p r e m i s e t h a t a f o r w a r d
delay and backward delay of a time synchronization packet a r e
equal t o one another o r t h a t , even when t h e r e is a t i m e d i f f e r e n c e
between t h e delays, t h e t i m e d i f f e r e n c e is w e l l known.
15 When a network is shared by p l u r a l pieces of communication
c o n t r o l equipment a s i n a d i s t r i b u t e d c o n t r o l system, time
synchronization packets a r e queued within intermediary
equipment such a s a network switch, j i t t e r s occur i n a time
d i f f e r e n c e between t h e forward and backward delays. That is,
20 s i n c e t h e forwardandbackwarddelays o f a t i m e synchronization
packet d i f f e r s from one another and a time d i f f e r e n c e between
t h e forwardandbackwarddelays is unknown, t h e premise of IEEE
1588 c o l l a p s e s .
In IEEE 1588-2008, "IEEE Standard f o r a Precision Clock
25 SynchronizationProtocolforNetworkedMeasurementandControl
Systems," a transfer delay of a packet within a network device
can be measured by use of an end-to-end TC (Transparent Clock)
orpeer-to-peerTC. Byreflectingthismeasuredtransferdelay
in data in the packet, it is possible to calculate a difference
5 between the forward and backward communication delays. However,
to use these methods, a network device corresponding to the
end-to-end TC or peer-to-peer TCmay be required. Such a network
device is high incost anddoes not become widespread comparing
to general network devices, so they are difficult to obtain,
10 and thus difficult to use.
In JP-A No. 2010-74600, plural delay estimation packets
are transmitted at a predetermined transmission interval,
reception intervals and predeterminedtransmission intervals
are compared to estimate a queuing delay in a network device.
15 However, in this method, since atime synchronization protocol
is executedinboth amaster anda slave, communication control
equipment compliant with a standard cannot be used in both the
master and slave. Therefore, it is difficult to configure a
system using communication control equipment compliant with
20 the standard. Additionally, since a delay estimation packet
is transmitted, an amount of transmission and reception of
packets other than time synchronization packets are small.
Accordingly, anamountofinformationtransmittedandreceived
in the system is small.
2 5 It is desirable to provide communication control
equipment that is useful for a network configuration using
generalnetworkintermediaryequipment andthat is able to time
synchronize with other communication control equipment
connectedbyanetworkwithoutreducinganamountofinformation
transmitted and received in a system.
SUMMARY OF THE INVENTION
For addressing the above-mentioned disadvantage,
communication control equipment of the present invention has
the following characteristics. The communication control
equipment ofthe present invention is connected to aplurality
of communication control equipment via a network and is
configured to be time-synchronized with the plurality of
communicationcontrolequipmentbyusingatimesynchronization
procedure using a communication including at least request
packets and acknowledgement packets. The communication
control equipment comprises a receiving-interval measurement
section configuredto measure a receiving interval of request
packets fromthepluralityofcommunicationcontrolequipment;
and a queuing-occurrence determination section configured to
detect conflict oftherequest packets fromanyoftheplurality
of communication control equipment on a basis of the receiving
interval of the request packets measured by the
receiving-interval measurement section. The communication
controlequipmentisconfiguredtodeterminewhethertotransmit
an acknowledgement packet to the communication control
equipment that has transmitted the request packets based on
a detection result of the queuing-occurrence determination
section.
The communication control equipment of the present
invention is useful in a network configuration using network
intermediary equipment, and is able to time synchronize with
other communication control equipment connected by a network
without reducing an amount of information transmitted and
received in the system.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows an example of a distributed control system
configured using communication control equipment of an
embodiment of the present invention;
Fig. 2 shows a hardware configuration of the communication
control equipment of an embodiment of the present invention;
Fig. 3 shows a functional configuration of communication
control equipment of Embodiment 1;
Fig. 4 shows the action when packets conflict in network
intermediary equipment;
Fig. 5 shows a functional configuration of a communication
unit in Embodiment 1;
Fig. 6 shows a procedure of time synchronization
processingofthecommunicationcontrolequipmentofEmbodiment
1;
Fig. 7 shows a functional configurationof communication
control equipment of Embodiment 2;
Fig. 8 shows a procedure of time synchronization
5 processingofthecommunicationcontrolequipmentprovidedwith
a grouping management section of Embodiment 2;
Fig. 9 shows a functional configuration of the
communication unit using a domain of IEEE 1588 standard of
Embodiment 2;
10 Fig. 10 shows an example of group management of the
communication units based on delay information;
Fig. 11 shows an example of a communication flow when
packets which are not for a time synchronization protocol are
not intended for the communication control equipment;
15 Fig. 12 shows a functional configuration of the
communication unit in Embodiment 3;
Fig. 13 is a flowchart to show operation of the
communication unit in Embodiment 3;
Fig. 14 shows a procedure of executing a time
20 synchronization protocol of IEEE 1588;
Fig. 15 shows exchanges of messages between a master and
a slave in IEEE 1588;
Fig. 16 shows an example of a distributed control system
using a time synchronization control device;
2 5 Fig. 17 shows a functional configuration of the time
synchronization control device;
Fig. 18 shows a functional configuration of the network
intermediary equipment in Embodiment 4;
Fig. 19 shows a configuration of a distributed control
5 system in Embodiment 5;
Fig. 20 shows a configuration of a distributed control
system in Embodiment 6;
Fig. 21 shows a configuration of a distributed control
system in Embodiment 7;
10 Fig. 22 shows an example wherein a communication flow
which is not a time synchronization procedure conflicts with
a time synchronization packet transmitted from the
communication control equipment to the communication unit;
Fig. 23 shows an example wherein a communication flow
15 which is unrelated to a time synchronization is present; and
Fig. 24 shows exchanges of messages between a master and
a slave.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
2 0 Communicationcontrolequipmentaccordingtoembodiments
ofthe present invention is explainedusingthe drawings. The
communication control equipment of the present invention is
included in a distributed control system and is connected to
othercommunicationcontrolequipmentvianetworkintermediary
25 equipment. In the following embodiments, other communication
control equipment is expedientially called a "communication
unit" to distinguish the interested communication control
equipment from other communication control equipment. When
a master-slave configuration is adopted as in IEEE 1588, the
communication control equipment serves as a master and the
communication unit serves as a slave. When a server-client
configuration is adopted as in NTP, the communication control
equipment serves as a server and the communication unit serves
as a client. The communication control equipment of the present
invention can be a master and a slave in a master-slave
configuration and can be a server and a client in a server-client
configuration.
The communication control equipment of the present
invention is time-synchronized with other communication
control equipment by use of a time synchronization procedure
using a communication that includes at least request packets
and acknowledgement packets. In the time synchronization,
conflict of request packets from other communication control
equipmentis detected, andit is determinedwhether to transmit
an acknowledgement packet to the communication control
equipmentthathastransmittedthe request packets on thebasis
of the detection result.
In the explanation of the following embodiments, IEEE
1588 is executed as a time synchronization protocol. In the
time synchronization of a slave with a master in the following
embodiments, a Delay Req message of IEEE 1588 is a - message for
a slave to request a master to execute a time synchronization,
andaDelay - Respmessageisamessageforamastertoacknowledge
a slave. Therefore, a packet of the Delay- R eq message serves
5 as a request packet, and a packet of the Delay- R esp message
servesasanacknowledgementpacket. Apacketofasyncmessage
is a request packet for a time synchronization, i.e., a
re-request packet.
In the present invention, NTP or SNTP may be executed
10 as atime synchronizationprotocol. For example, in executing
NTP or SNTP, a server acknowledges a request from a client.
That is, the client transmits a request packet to the server
and the server transmits an acknowledgement packet to the client.
15 EMBODIMENT 1
Fig. 1 shows an example of a distributed control system
configured using communication control equipment 120 of this
embodiment. The distributed control system includes the
communication control equipment 120, communication units 123,
20 a network 122, and network intermediary equipment 121. The
communication control equipment 120 and communication units
123 and the communication units 123 themselves are connected
to and communicate with each other via the network 122 and network
intermediary equipment 121.
2 5 Examples of the network intermediary equipment 121
includeanetworkswitch, arouter, agateway, anOpenFlowswitch
in OpenFlow, etc. Examples of the network 122 include IEEE
802.3, various industrial networks, IEC 61784, IEC 61158, etc.
Fig. 2 shows a hardware configuration of the communication
control equipment 120 of this embodiment. The communication
c o n t r o l e q u i p m e n t 1 2 0 i s p r o v i d e d w i t h a C P U 1 0 1 , atransceiver
IC 102, a memory 108, a non-volatile memory 109, and a bus 110.
The CPU 101 transfers a program from the non-volatile
memory109 to thememory 108 andexecutes this program. Programs
tobe executedinclude anoperating system (hereinafter called
"OS") and an application program that operates on an 0s.
The transceiver IC 102 has a function of communication
with the network 122. The transceiver IC 102 receives a
communication request from a program executed by the CPU 101
and communicates with the network 122. An example of the
transceiver IC 102 includes an IC such as a MAC (Media Access
Control) chip of IEEE 802.3 standard, a PHY (physical layer)
chip, a multi-chip of MAC and PHY, FPGA, CPLD, ASIC, and a gate
array. The transceiver IC 102 may be contained in the CPU 101
or a chip set that controls an information route inside a
computer.
The memory 108 is a temporary storage region to operate
the CPU 101, and stores, e. g., an OS and application programs
transmitted from the non-volatile memory 109.
The Non-volatile memory 109 is a memory for information
to store anOS, application programs, device drivers, aprogram
for operating the CPU 101, and additionally execution results
of the programs. Examples of the non-volatile memory 109
include a hard disk drive (HDD), a solid-state drive (SSD),
5 anda flashmemory. Aneasily-detachable externalstoragemay
be used as the non-volatile memory 109. As such an external
storage, a flexible disk (FD), an optical disk such as CD and
DVD, and a flash memory such as a USB memory and a compact flash
(registered trademark) can be used, for example.
10 The bus 110 connects the CPU 101, transceiver IC 102,
memory 108, and non-volatile memory 109. Examples of the bus
110 include PC1 bus, an ISA bus, a PC1 Express bus, a system
bus, and a memory bus.
[Explanation of each function of communication control
15 equipment]
Fig. 3 shows a functional configuration of the
communication control equipment 120 of Embodiment 1 of the
present invention. The communication control equipment 120
isprovidedwithatime-synchronizationprocessingsection130,
20 a communication section 131, a receiving-interval measurement
section 134, and a queuing-occurrence determination section
Thetime-synchronizationprocessingsection130executes
time synchronization. Examples of the time synchronization
25 protocol to be performed include IEEE 1588, NTP, and SNTP. As
mentioned above, in explanation of this embodiment, IEEE 1588
is executed as the time synchronization protocol. However,
NTP or SNTP also may be executed as the time synchronization
protocol.
The time-synchronization processing section 130 may be
realized by software executed by the CPU 101 or realized as
a h a r d w a r e l o g i c w h e n t h e t r a n s c e i v e r I C 1 0 2 i s c o n f i g u r e d u s i n g
FPGA or CPLD. Alternatively, the time-synchronization
processing section 130 may be configured using both software
performedbytheCPU101andahardwarelogicofthetransceiver
IC 102. In this case, for example, measurement of a transmission
time and reception time of a packet or generation of a packet
format can be processed in the transceiver IC 102.
The communication section131has atransmission section
132 and a receive section 133 and transmits and receives
predetermined packets in the time synchronization protocol.
The transmission section 132 transmits the predetermined
packets in the time synchronization protocol. The receive
section 133 receives the predetermined packets in the time
synchronization protocol. The communication section 131,
transmissionsection132, a n d r e c e i v e s e c t i o n 1 3 3 c a n i l l u s t r a t e
realization by the transceiver IC 102.
The receiving-intervalmeasurementsection134measures
receivingintervals of packets receivedinthe receive section
133 ofthecommunicationsection131. Tomeasurethe receiving
intervals, the receiving-interval measurement section134 has
a function that measures a time of reception of a packet by
the receive section 133. The receiving intervals of packets
may be measuredby calculating a difference between reception
5 times of two any packets received consecutivelybythe receive
section 133. When conforming to the standard of IEEE 1588,
a receivingintervalof packets may bemeasuredby calculating
adifferencebetweenreceptiontimesoftwotimesynchronization
packets (forexample,Delay - Reqmessage) receivedbythereceive
10 section 133.
The receiving-interval measurement section 134 may be
realized by software performed by the CPU 101 or as hardware
in the transceiver IC 102. Alternatively, the
receiving-interval measurement section 134 maybe realizedby
15 the combination of the software executed by the CPU 101 and
the transceiver IC 102. In this case, for example, the
transceiver IC 102 may measure reception times of packets, and
t h e s o f t w a r e p e r f o r m e d b y t h e C P U 1 0 1 m a y d e t e r m i n e a d i f f e r e n c e
between the reception times to calculate a receivinginterval.
20 In this case, the transceiver IC 102 discloses information for
calculating the receiving interval to the CPU 101. This
informationmaybemaintainedusingaregisterofthetransceiver
IC 102, and the transceiver IC 102 may write the information
in a predetermined region of the memory 108.
2 5 The queuing-occurrence determination section 135
determines whether a time synchronization packet is queued in
a packet buffer in the network intermediary equipment 121.
Hereinafter, occurrenceofawaittimeduetoqueuingofpackets
in the buffer in the network intermediary equipment 121 is called
5 "conflict."
It is determined whether packets conflict from a
comparison between a receiving interval T of actual packets
measured in the receiving-intervalmeasurement section and
a receiving interval Tr calculated using one or more of a
10 communication processing time Tp of a packet, an inter-frame
gap IFG, and a communication processing time Ts of the network
intermediary equipment 121. The receiving interval Tr is
calculated from a packet size and an equipment configuration.
The communication processing time Tp of any packet is
15 determined by Formula (3).
Tp = Sp x 8 [bit/byte] /Th (3)
Here, Sp is a packet size (expressed in the unit "byte")
and Th is a communication throughput. For example, the
communication processing time Tp at the time of transmitting
20 a packet of 64 byte on the network of IEEE 802.3 standard of
100 Mbps is determined by Tp = 64 [byte] x 8 [bit/ byte] /I00 [Mbps]
= 5.12[ps].
The inter-frame gap IFG is a packet interval defined on
the network. IEEE 802.3 standard defines an interval from
25 transmission of a frame onto a communication route to
transmission of a following frame as an inter-frame gap (Inter
frameGap, IFG). the inter-framegapisdefinedas 96bittime,
and the inter-frame gap IFG can be determined by Formula (4).
IFG = 96[bit]/Th (4)
5 For example, on the network of IEEE 802.3 standard of
100 Mbps, IFG = 96[bit] /I00 [Mbps] = 0.96[ps].
The processing time Ts of the network intermediary
equipment 121 depends on a content of processing of each piece
of network intermediary equipment 121. The content of
10 processing ofthe network intermediary equipment 121includes
extraction of destination port information and forward
processing of packets in the network intermediary equipment
121. The processing time Ts of the network intermediary
equipment 121 may use a value indicated in a data sheet and
15 manual of the network intermediary equipment 121, and may use
a value determined by measuring a forwarding delay of the network
intermediary equipment 121 experimentally.
As an example of a method to measure a forwarding delay
of the network intermediary equipment 121 experimentally, two
20 communication ports are provided in a certain communication
terminal, a test packet is transmitted from one communication
port and then received by another communication port via the
network intermediary equipment 121. A communication delay at
this time (for example, a time of reception of a start of a
25 packet minus a time of transmission of a top of the following
packet) minus the communication processing time Tp of the packet
by Formula (3) is the processing time Ts of the network
intermediary equipment 121.
The communication processing time Ts of the network
intermediaryequipment12lmaychangedependingonaprocessing
portion proportional to a packet size, whether a packet is
present before and after a concerned packet, and an amount of
information on a managed communication route. Therefore,
estimation is necessary using various conditions. An example
of a method to estimate the processing time Ts of the network
intermediary equipment 121 is shown in Formula (5).
Ts = A + kSp + B + mC (5)
A shows a fixed delay for packet forwarding, and k is
a proportional factor to the packet size Sp. B shows a delay
in case a packet is present before a concerned packet in the
packet buffer of the network intermediary equipment 121. C
is an amount of information on a communication route, and m
is a proportional factor to the information amount C.
When plural pieces ofthe networkintermediaryequipment
121 are present between the communication control equipment
120 and communication units 123, the processing time Ts uses
Ts of the network intermediary equipment 121 next to the
communication control equipment 120 and a total value or the
maximum of Ts of the network intermediary equipment 121 on the
communication route, for example.
Fig. 4 shows t h e actionwhenpackets c o n f l i c t i n t h e network
intermediary equipment 121. A b u f f e r 142 is one f o r packets
i n t h e n e t w o r k i n t e r m e d i a r y e q u i p m e n t 1 2 1 . T h e b u f f e r 1 4 2 u s e s
a F i r s t - i n F i r s t - o u t (FIFO) type data s t r u c t u r e . Now, it is
assumed t h a t a packet 140 is being t r a n s m i t t e d and a packet
1 4 1 following t h e packet 140 is queued i n t h e b u f f e r 142. A t
t h i s time, t h e packet 1 4 1 is delayed i n transmission and has
a wait t i m e due t o t h e preceding packet 140 i n t h e b u f f e r 142.
That is, t h e packet 1 4 1 c o n f l i c t s with t h e packet 140 i n t h e
b u f f e r 142.
A t t h i s t i m e , itisassumedthatthecommunicationcontrol
equipment 120 i s connected with t h e output s i d e shown i n Fig.
4 . Froma r e c e i v i n g i n t e r v a l b e t w e e n t h e p a c k e t 1 4 0 andpacket
1 4 1 i n t h e communication c o n t r o l equipment 120, it can be
determined t h a t t h e packet 1 4 1 has c o n f l i c t e d with t h e packet
140. That is, when t h e packet 1 4 1 and packet 140 c o n f l i c t with
one another, t h e processing f o r t h e communication c o n t r o l
equipment 120 t o receive t h e packet 1 4 1 is delayed by a
communication processing time of t h e packet 140.
When it is determined t h a t c o n f l i c t h a s occurred from
a reception time of t h e s t a r t of t h e packet, t h e r e c e i v i n g
i n t e r v a l T r b e t w e e n t h e p a c k e t 1 4 0 a n d p a c k e t 1 4 1 i s determined
by Formula (6) .
T r = Tp + IFG + T s (6)
The communication processing time Tp of t h e packet is
determined by Formula (3). The inter-frame gap IFG is
determined by Formula (4) . The processing time Ts of the network
intermediary equipment 121 is determined by Formula (5).
Alternatively, the receiving interval Tr between the
packet 140 and packet 141 may be determined by Formula (7 ) without
the processing time Ts of the network intermediary equipment
121. The processing time Ts of the network intermediary
equipment121is omissibleinaccordancewiththeconfiguration
of the network intermediary equipment 121.
Tr = Tp + IFG (7)
Alternatively, when it is determined that conflict has
occurred by use of a reception time of the end of the packet
and a reception time of the start of the following packet, the
receiving interval Tr between the packet 140 and packet 141
is determined by Formula (8) or Formula (9) .
Tr = IFG + Ts (8)
Tr = IFG (9)
In Formula (9), the processing time Ts of the network
intermediary equipment 121 is omitted.
When the packet 141 conflicts with the packet 140, the
amount of the packets on the network is large, and reception
intervals of the packets are short. In case of no conflict,
the amount ofthe packets is small, andthe reception intervals
of the packets are long. Therefore, when the packet 141 does
n o t c o n f l i c t w i t h t h e p a c k e t 1 4 0 , the actual receivinginterval
T of the packets should be longer than the receiving interval
Tr determined by Formula ( 6 ) , Formula ( 7 ) , Formula (8), or
Formula (9). Therefore, when a margin (degree of margin) is
set as a and the actual receiving interval T satisfies Formula
(lo), it is determined that the packet 141 have not conflicted.
T > Tr + a (10)
The margin a can be any value (positive numerical value
or zero) and is predefined.
[Functional configuration of communication unit]
Fig. 5 shows a functional configuration of each of the
communication units 123 of this embodiment. Each of the
communication units 123 is provided with the
time-synchronization processing section130 andcommunication
section 131.
Thetime-synchronizationprocessingsection13Oexecutes
atime synchronization protocol with the communication control
equipment120 and time synchronizes the communication unit123
with the communication control equipment 120. NTP, SNTP, or
IEEE 1588 can be used as the time synchronization method.
In the master-slave configuration as in IEEE 1588, the
communication control equipment 120 serves as a master and the
communication unit 123 serves as a slave.
The time-synchronization processing section 130 of the
communication unit 123, which is a slave, calculates the
communication delay td and the time difference tdiff between
themasterandslaveby Formula (1) andFormula (2) respectively
after reception of the Delay - Resp message (SO68 of Fig. 14) .
Upon reception of the Sync message or Follow Up -U p message, on
the basis of the communication delay td and time difference
5 tdiff between the master and slave, the time-synchronization
processingsection130 synchronizesthe communicationunit123
with the communication control equipment 120 in time.
The communication section 131 has the transmission
section 132 and the receive section 133 and transmits and
10 receives predetermined packets in the time synchronization
protocol. Since the functions ofthe transmission section132
andreceive section 133 are the same as those of the communication
section 131 of the communication control equipment 120, their
explanation is omitted.
15 Fig. 6 explains a procedure of the time synchronization
of the communication control equipment 120. Since IEEE 1588
is executed as the time synchronization protocol in this
embodiment as mentioned above, operation of the communication
control equipment 120 in executing the IEEE 1588 protocol is
20 shown in Fig. 6. In this embodiment, as mentioned above, the
communication control equipment 120 serves as a master in the
IEEE 1588 protocol.
First, the communication control equipment 120 serves
as amaster ofthe IEEE1588 (S001). In IEEE1588, inaccordance
25 with a Best Master Clock (hereafter called "BMC") algorithm,
information (Announce message) is exchanged between the
communication control equipment 120 and each of the
communication units 123, and it is determined whether to serve
as a slave or a master in accordance with contents of this
information. In this embodiment, parameters of the
communication control equipment 120 and each of the
communication units 123 are previously set so that the
communication control equipment 120 serves as a master. For
example, amethodofmakingthecommunicationcontrolequipment
120 serveas amaster includes amethodofincreasingapriority
of information on exchange of Announce messages to make the
communication control equipment 120 serve as a master.
Alternatively, the configuration of the communication
unit123maybemadetobethe same as that ofthe communication
control equipment 120 without particularly setting the
parametersintheBMCalgorithm. Inthiscase, forconvenience,
as a result of executing the BMC algorithm, the unit serving
as a master is the communication control equipment 120, and
the unit serving as a slave is the communication unit 123.
Next, the time-synchronization processing section 130
transmits the Sync message to the communication unit 123 by
a m u l t i c a s t i n a c c o r d a n c e w i t h t h e p r o t o c o l o f I E E E 1 5 8 8 (S002).
I n t h e s p e c i f i c a t i o n o f I E E E 1 5 8 8 , atwostepmodethattransmits
the Follow - Up message after transmission of the Sync message
is defined. This embodiment is explained on the premise of
the one step mode that transmits only the Sync messages, but
an advantageous effect of this embodiment is acquired even in
the two step mode as in the one step mode.
Next, the time-synchronization processing section 130
waits for reception of a packet (Delay - Req message) (S003).
Upon reception of the Delay - Req message, the
receiving-interval measurement section 134 of the
communication control equipment 120 measures a receiving
interval between the Delay - Req message and a most recently
receivedpacket (SOO4) . When the Delay - Reqmessage is the first
receivedpacket, the receiving interval between the Delay - Req
messageandthemostrecentlyreceivedpacketcannotbemeasured.
Therefore, in this case, the receiving interval is set to 0,
and the queuing-occurrence determination section 135
determines that no conflict has occurred.
Next, the queuing-occurrence determination section 135
of the communication control equipment 120 determines whether
the conflict of the packets has occurred (S005).
When it is determined that the conflict of the packet
has occurred in S005, the time-synchronization processing
section 130 of the communication control equipment 120 stores
acommunicationpartnerthathastransmittedtheconflictpacket
in thememory108 (S006). The communication partner is stored
with an IP address or MAC address or an identifier (a clock
identifier, a domain, etc.) identifiable on IEEE 1588, for
example.
When it is determined that the conflict of the packets
has not occurred in S005,
section130transmitsthe Delay - Respmessage andcontinuesthe
5 time synchronization protocol (S007).
The time-synchronization processing section 130
determines whether packets have been received from all the
communication partners after SO06 or SO07 (S008). When the
packets have not been received from all the communication
10 partners, reception of a packet (Delay - Req message) is waited
When it is determined that packets have been received
from all the communication partners at S008, the
time-synchronizationprocessingsection13Odetermineswhether
15 there is a communication partner that has transmitted the
conflict packet (S009) (there is the communication partner
storedinS006). Inconsiderationof occurrenceofpacket loss
on the way, when a predetermined time has elapsed without
receivingapacket, theprocessmayproceedsto SO10 fromS008.
20 If a predetermined time is 0, this means re-execution of time
synchronization is conducted at the time of determining a
conflict without waiting of all packets received.
Whenthereisacommunicationpartnerthathastransmitted
a conflict packet at S009, a delay has occurred due to queuing
25 in the network intermediary equipment 121. In this case, the
I
I t
communication control equipment 120 re-executes the time
synchronization protocol procedure.
When a multicast message is transmitted at this time as
wel1asthefirsttransmittedSyncmessage,al1thecommunication
units 123 reply again. This is not desirable because the packets
mayconflictwithinthenetworkintermediaryequipment121again.
Therefore, it is necessary to communicate only with the
communication partner determined to have transmitted a
conflicted packet. Examples of the method of realizing this
communication include a unicast communication that
communicates only with specific communication partners, a
communication that defines and uses a different multicast
address, and a communication that uses grouping by VLAN by use
ofthenetworkintermediaryequipment121correspondingtoVLAN
functions.
the time-synchronizationprocessing section130 changes
a destination of a communication partner to communicate only
with the communication partner determined to have transmit a
conflict packet by use of any of those communication methods
(Solo). Then, thetime-synchronizationprocessingsection130
again transmits the Sync message tothe communication partner
determined to have transmitted the conflict packet, and
re-executes the time synchronization protocol (S002).
When there is no communication partner that has
transmittedaconflictpacketatSOO9,thetime-synchronization
processing section 130 clears a record of the communication
partners that have transmitted conflict packets (SOll).
Next, the time-synchronization processing section 130
determines whether a termination condition has been satisfied
(S012) . The termination condition may be satisfied by an
explicit instruction of a user or a system manager or may be
set using an elapsed time after the start of the system or the
number of executions of the time synchronization protocol.
When the termination condition has been satisfied, the
communication control equipment 120 terminates the time
synchronization processing.
When the terminating condition has not been satisfied,
the communication control equipment 120 waits only for a
predetermined time (S013), and re-executes the time
synchronization protocol (S002).
The time synchronization protocol is re-executedto the
communicationpartnerdeterminedtohavetransmittedaconflict
packet in the procedure shownin Fig. 6, but it is not necessary
to re-execute the protocol. When there is no re-execution of
the protocol, the communicationcontrol equipment120 executes
a predetermined processing because of failure of the time
synchronization with the communication unit 123. Examples of
such a processing include notification of the failure of the
time synchronization and an identifier of the interested
communication unit 123 to a predetermined destination and
notification ofthe failure ofthe time synchronization tothe
outside by a predetermined indication method (for example, a
monitor connected to the communication control equipment 120,
an LED lamp installed in the communication control equipment
120, or analarmemitted fromthe communication control equipment
120).
Additionally, a time span from transmission of the Sync
messagetoreceptionofthe first Delay - Reqmessageismeasured.
When this time span exceeds a predetermined time, it is
determined that a packet of the first Delay - Req message has
conflicted. Then, the time synchronization protocol may be
re-executedwith apartnerthathastransmittedthis Delay - Req
message. The predetermined time may be defined by measuring
the time span until reception of the Delay - Req message
experimentally without generating other communication flows.
Thewait time a t S 0 1 3 m a y b e d e t e r m i n e d s u c h t h a t a t i m i n g
of transmission of the Sync message at SO02 becomes a
predetermined period or may be changed every time. For example,
the wait time at SO13 may be changed every time in accordance
with the number of conflicts of packets of the Delay - Reqmessages
detectedbythe queuing-occurrence determination section135.
When the number of changes from SO10 to SO02 is large (namely,
the number of conflicts of packets is large), the first
time-synchronized communication unit123 has a longtime span
until the next synchronization and thus the processing time
thereof is long without synchronization. Therefore, the time
synchronizationmaybe executedas soon as possible. When the
number of changes from SO10 to SO02 is small, each of the
communicationunits123isconsideredtohavebeensynchronized
relatively precisely, and therefore the wait time may be set
rather longer.
The procedure of Fig. 6 is an application of the present
invention to the delay request-response method of IEEE 1588.
The present invention is applicable also to the peer Delaymethod
of IEEE 1588.
The peer delay method measures a route delay between
adjacent nodes by use of a Pdelay - Req message, a Pdelay - Resp
message, anda Pdelay-Resp-Follow - Up message (ifnecessary),
and time information is transmitted to each slave by the Sync
message. Therefore, the Delay - Req message is unnecessary in
the peer delay method.
The present invention may be applied to the peer delay
method to measure a route delay by use of the Pdelay - Req,
Pdelay - Resp, and Pdelay - Resp - Follow - Up (if necessary). That
is, at SO02 of Fig. 6, not the Sync message but the Pdelay - Req
message is transmitted. At SO03 of Fig. 6, reception of the
Pdelay - RespmessageinsteadoftheDelay - Reqmessageiswaited.
In the two step mode of the peer delay method, correspondence
to the Pdelay - Resp - Follow - Up message is made as well as that
to the Follow - Up message.
When connecting between the plural communication units
1 2 3 t h a t m e a s u r e a r o u t e d e l a y b y u s e o f t h e n e t w o r k i n t e r m e d i a r y
equipment, a conflict of delay measurement packets within the
network intermediary equipment 121 can be detected as in Fig.
6 by measuring a receiving interval of the Pdelay - Respmessages.
When the conflict is detected, correspondences such as
re-transmission of the Pdelay - Req message and no-use of time
information in the received Pdelay - Resp message are
illustrated.
As explained above, the communication control equipment
1 2 0 o f t h i s e m b o d i m e n t c a n b e u s e d f o r t h e n e t w o r k c o n f i g u r a t i o n
usingthenetworkintermediaryequipment121, andcandetermine
the existence of conflicts of packets within the network
intermediary equipment from a receiving interval of
acknowledgementmessagesofthe time synchronizationprotocol.
When a conflict of the packets occurs, the time
synchronizationprocedure is re-executed, andwhennoconflict
of the packets occurs, the time synchronization protocol is
continued without change. Also in the network configuration
using the network intermediary equipment 121, the time
synchronizationprotocolisrealizable. Therefore, thesystem
of plural time-synchronized communication units sharing the
networkcanbeconfigured. Alarge-scale time synchronization
system can be configured by relaying and extending networks
via the network intermediary equipment. According to this
embodiment, the time synchronization system can be configured
using the communication units 123 based on IEEE 1588 standard
without the communication units 123 having a special
configuration.
EMBODIMENT 2
InEmbodiment2, the communicationcontrol equipment120
is further provided with a grouping management section, and
executes the time synchronization protocol by use of a group
ofthe plural communicationunits 1 2 3 t o r e d u c e t h e p r o b a b i l i t y
of conflict of the time synchronization packets of the
communication units 123. In the figures used in Embodiment
2, since the same reference numerals as Embodiment 1 show the
same functions, components, etc. explained in Embodiment 1
unless otherwise indicated, their explanation is omitted.
Fig. 7 shows a functional configuration of the
communication control equipment 120 of Embodiment 2 of the
present invention. The communication control equipment 120
of Embodiment 2 is provided with the components of the
communication control equipment 120 of Embodiment 1 shown in
Fig. 3, andalsoisprovidedwitha groupingmanagement section
150 and a network-configuration information storage section
151.
Based on information on the network configuration, the
grouping management section 150 divides the plural
communication units 123 into groups, and provides identifiers
to the groups of the communication units 123. In accordance
with the number of transmissions of the Delay - Resp messages,
success or failure of the time synchronization with each of
the communication units 123, etc., the groups of the
communication units 123 are notified to the
time-synchronization processing section 130. For example, as
success or failure of the time synchronization, the groups of
the communication units 123 are notified to the
time-synchronizationprocessingsection130basedonthenumber
of successes or failures of the time synchronizations or an
elapsed time after the last time synchronization is complete.
The network-configuration information storage section
151 stores information on the network configuration used in
order that the grouping management section 150 may group the
communication units 123. The information on the network
configuration includes the number of the devices configuring
thenetwork, informationonthesedevices (suchasperformances
of components configuring each device, processing times of
hardware and software of the devices, and jitters of the
processingtimes, forexample), thenetworktopology, thenumber
of hops to each device, and a measured communication delay to
each device.
Fig. 8 explains a procedure of the time synchronization
processingofthe communication control equipment120 provided
with the grouping management section150. The difference from
thetimesynchronizationprocedureofthecommunicationcontrol
equipment 120 of Embodiment 1 shown in Fig. 6 is such that a
procedure SO20 of determining a group to which the Syncmessage
istransmittedisadded. Thedifferenceisalso suchthat there
is no procedure SO10 of changing a destination of a communication
partner and that SO20 is re-executed after a wait by a previously
predetermined time after SO09 (S013).
InS020,thetime-synchronizationprocessingsection130
queries the grouping management section 150 for the group of
the communication units 123 used to be destinations. The
grouping management section 150 that has received the query
notifies an identifier of the group used as the destinations
to the time-synchronization processing section 130. The
grouping management section 150 may notify identifiers of one
or plural communication units 123 instead of the identifier
ofthegroup. For example, whenthenumberofthe communication
units123usedasthecommunicationpartnersofthecommunication
control equipment 120 is small (for example, one), notifying
of the identifiers of the communication units 123 is better
than grouping and defining of the small number of the
communication units 123.
AtS002,thetime-synchronizationprocessingsection130
transmits the Sync message to the group of the destinations
byuseofthe identifier ofthe groupnotifiedfromthegrouping
management section 150 at SO20 (or identifier of the
communication unit 123).
Examples of a method of expressing identifiers of the
groups include a method of using VLAN in domain IEEE802.1Q
defined by IEEE 1588 or using a multicast address defined by
IP (Internet Protocol), IEEE 802.3, or other communication
protocols. In addition, any method of communicating with
plural communication terminals based on a communication
protocol on a network may be used.
Fig. 9 shows a functional configuration of the
communication unit 123 using a domain of IEEE 1588 standard.
The communication unit 123 in Embodiment 2 is provided with
the components of the communication unit 123 in Embodiment 1
shown in Fig. 5, and also is provided with a synchronized-time
selection section 161. In IEEE 1588, operation on a protocol
and a timescale can be defined independently using domains.
In this embodiment, synchronized times are managed for every
domain within the communication unit 123. Therefore, the
synchronized-timeselectionsection161selectsonefromplural
synchronized times 160.
A reference of selection of the synchronized time 160
by the synchronized-time selection section 161 includes an
i d e n t i f i e r o f t h e s y n c h r o n i z e d t i m e 1 6 0 , anexecutionsituation
of a synchronized time protocol, or an attribute of a master
of IEEE 1588 of a corresponding domain (time precision etc. ) .
For example, the synchronized time 160 of a domain in which
an execution frequency of the synchronized time protocol is
high or the synchronized time 160 of a domain in which a time
precision of a master is the highest is selectable.
Amethodof communicatingwiththe communicationcontrol
equipment 120 by multiplexing a domain in one communication
unit 123 may include a method of configuration of routing control
information on the network intermediary equipment 121 (for
example, changing a route for each domain) or a method of
identifying a domain number on an IEEE 1588 packet in the
communication unit 123.
Amethodof identifying the groups usingVLAN in IEEE802.1Q
is explained. The VLAN configured by one or more of the
communicationunits123andthecommunicationcontrolequipment
120 is defined by the number of defined groups.
At SO02 shown in Fig. 8, the time-synchronization
processingsection130setsanidentifierofVLAN (VID) notified
from the grouping management section 150 at SO20 to a tag for
VLAN, and transmits the Sync message.
In this case, as the network intermediary equipment 121,
one corresponding to VLAN functions is used. In addition,
before executing the procedure of Fig. 8, grouping of VLAN is
previously set to each piece of the network intermediary
equipment 121.
Alternatively, the groupingmanagement maybemade using
an IP and a multicast address in IEEE 802.3. In this case,
a multicast address is defined for each group. Then, by use
ofa settingrequiredforrealizationofthemulticastaddress,
IGMP (Internet Groupingmanagement Protocol) etc. areexecuted
5 t o t h e n e t w o r k i n t e r m e d i a r y e q u i p m e n t 1 2 1 , communicationunits
123, and communication control equipment 120.
Next, an example of a grouping management method in the
groupingmanagement section150 is explained. It is desirable
that the grouping management has a smaller probability of
10 conflict of packets in the network intermediary equipment 121
on the network if possible.
For example, the method of grouping management can be
used in reference to a communication delay between the
communication control equipment 120 and the network
15 intermediaryequipment121andtoacommunicationdelaybetween
the communication control equipment 120 and each of the
communication units 123. In the preparation for the system
configuration, when the time synchronization protocol of IEEE
1588 etc. is executed by one-to-one between the communication
20 control equipment 120 and each of the communication units 123,
thecommunicationdelaycanbedeterminedintheprocedureshown
already described in Fig. 15.
Alternatively, the groupingmanagement maybemadebased
on the network configuration. For example, on the basis of
25 the number of and performance of the pieces of network
intermediary equipment 121 between the communication control
equipment 120 and the communication units 123, a communication
performance between the pieces of network intermediary
equipment 121, a performance of the network, or network media,
5 the grouping management can be made. Examples of the
communication performance between the pieces of network
intermediary equipment 121include a communication throughput
(10 Mbps, 100 Mbps, or 1 Gbps) of IEEE 802.3, whether a type
of communication media is a wired communication or wireless
10 communication, and a type of a telecommunication standard.
Asmentionedabove, on thebasis ofthe delay information
and the network configuration, parameters for grouping each
ofthe communication units 123 canbe determined. On the basis
of these parameters, the predetermined number of groups may
15 be acquired, and the grouping management of the communication
units123maybemade s o t h a t t h e c o m m u n i c a t i o n u n i t s 1 2 3 h a v i n g
the same parameter are distributed to different groups. For
example, the grouping management of the communication units
123 is made sothat the communication units 123 havingthe same
20 communication delay are distributed to different groups.
Fig. 10 shows an example of grouping management of the
communication units based on the delay information. In the
distributed control system shown in Fig. 10, the communication
control equipment 120 and six communication units 123a to 123f
25 are connected to the network 122 via the network intermediary
equipment 121.
Communication delay times between each of the
communication units 123a to 123f and communication control
equipment 120 are respectively set to 1 ms, 2 ms, 2 ms, 3 ms,
5 3 ms, and 3 ms. When the communication units are expressed
by reference numerals 123a to 123f attached to the communication
units, and the number ofthe groupsis set to three, one example
of the grouping management of the communication units shows
a group of (123a, 123c, 123d), a group of (123b, 123e), and
10 (123f) . When the number of the groups is set to two, one example
of the grouping management shows a group of (123a, 123c, 123d)
and a group of (123b, 123e, 123f). However, in this case, it
is expectedthatthe communicationunit123dand communication
unit123ehave ahighpossibilityofconflictof replypackets,
15 and a resynchronization procedure is executed to one of the
communication unit 123d and communication unit 123e. The
communication delay between the communication control
equipment 120 and each of the communication units 123 may contain
acommunicationprocessingtime (includingjitters) ofhardware
20 and software on each of the communication units 123. When the
communication processing time includes jitters, the grouping
management is illustrated on the basis of the worst value or
an average value.
As another method of the grouping management, on the basis
25 of the last communication result or the transmission history
of the Delay - Resp messages, the group management may be made.
For example, the communicationunits 123thathavetransmitted
the time synchronizationpackets determinedtohave conflicted
in the last time synchronization procedure are arranged in an
array inchronological order. Then, every other communication
unit 123 is selected fromthis array. Alternatively, when the
number of the remaining communication units 123 whose time
synchronization is not complete is equal to or below a
predeterminednumberpreviouslydefinedas aresultofthelast
time synchronization procedure, a method of selecting a
previously defined predetermined number of the communication
units 123 from these remaining communication units 123 and
grouping the selected communication units 123 may be used. This
is because it is easier to execute the time synchronization
in each of the communication units 123 when the system is
compatible.
Alternatively, a method of grouping management based on
the delay information illustrated using Fig. 10 and a method
of grouping management based on the result of the last time
synchronization procedure maybe combined. For example, from
the group candidates groupedbythe groupingmanagement method
illustrated using Fig. 10, a group that contains all the
communication units 123 in the group configured based on the
last result may be notified to the time-synchronization
processingsection130. Inthismethod, for example, evenwhen
the communication units in which the time synchronization is
not complete are the communication units 123b and 123f, and
the communication unit in which the time synchronization is
already complete is the communication unit 123ef the group of
(123b, 123e, 123f) can be notified.
A timing of grouping management of the communication units
123 (grouping the communication units 123) may be at the
preparationbefore actual operations for time synchronization
communication and a system application or at a dynamic timing
determined in accordance with available information during
system operation.
When a communication flow other than the time
synchronization procedure is present, a receiving interval
between each packet may change. In this case, by maintaining
information about communication flows other than the time
synchronization procedure in the network-configuration
information storage section 151, the queuing-occurrence
determination section 135 can determine conflict of packets
in the network intermediary equipment 121.
Whenpacketsotherthanones forthe time synchronization
protocol are intended for the communication control equipment
120, the packet size Sp is a packet size of a packet other than
one for the time synchronization protocol in Formula (3) and
Formula (5) to determine a receiving interval between a packet
other thanone forthe time synchronizationprotocolandatime
synchronization packet. When a receiving interval is
determined using a reception time of the end of a packet and
a reception time of the start of the next packet as in Formula
(8) and Formula ( 9 ) , the data size is not influenced.
Fig. 11 shows an example of the communication flow when
a packet other than one for the time synchronization protocol
is not intended for the communication control equipment 120.
As shownin Fig. 11, it is assumedthatthereis a communication
flow 170 of the time synchronization protocol from the
c o m m u n i c a t i o n u n i t 1 2 3 b t o t h e communication control equipment
120, and that there is a communication flow 171 of other than
the time synchronization protocol fromthe communication unit
123c to the communication unit 123a. Further, it is assumed
that the communication control equipment120 andcommunication
unit 123a are time-synchronized with each other. In Fig. 11,
different reference numerals 121a and 121b are attached to the
pieces of network intermediary equipment.
A packet of the communication flow 170 may conflict with
a packet of the communication flow 171 in the network
intermediary equipment 121a and 121b. It is difficult to
estimate a delay of the packet of the communication flow 170
in the networkintermediaryequipment121a and121bduetothis
packet conflict by use of a past technology, but it is possible
to estimate this delay in this embodiment. It is possible to
estimate this delay by synchronizing the communication unit
123a and the communication control equipment 120 and then by
acquiring a reception time of the packet of the communication
flow 171 in the communication unit 123a.
When the communication control equipment 120 receives
the packet of the communication flow 170, the communication
control equipment 120 queries the communication unit 123a for
reception of the packet of the communication flow 171. The
communication control equipment120 queries the communication
unit 123a for the reception time when the communication unit
123ahas receivedthepacket. Whenthecommunicationunit123a
has received the packet of the communication flow 171, and a
difference between the reception time of this packet and the
reception time of the time synchronization packet in the
communication control equipment120 is the receiving interval
T of Formula (lo), andwhen this receivingintervalT satisfies
Formula (lo), thetimesynchronizationprocedureisre-executed
because the conflict has occurred.
Thus, even when the different equipment or device (the
communication control equipment 120 and communication unit
123a) receive packets, it is possible to estimate whether the
conflict has occurred by obtaining the receiving interval
between the two packets of the different equipment or device.
Aconditionthatthe communication control equipment120
queries the communication unit 123a is reception of the time
synchronization packet. The conditionmay be every reception
or a case where a receiving interval between a reception of
a packet and a reception of its precedingpacket is longerthan
a predeterminedvalue. Alternatively, execution ofthe query
may be determined based on time information in the time
synchronization packet. After the communication unit 123 and
the communication control equipment 120 are synchronized, a
communication delay relative to a synchronized unit can be
estimated also in the communication control equipment 120.
When the estimated value is larger than an average to date,
it is thought that the conflict with the communication flow
171 has occurred. At this time, the network-configuration
information storage section 151 canmaintain information such
as a packet size of a communication flow intended for each
communicationunit123, apacket size ofthe communication flow
171, a c o n n e c t i n g r e l a t i o n o f t h e c o m m u n i c a t i o n u n i t s 1 2 3 , etc.
Fig. 22 shows an example of the communication flow when
a communication flow other than the time synchronization
procedure occurs between the communication units 123, and
conflictswiththetimesynchronizationpackets (Syncmessages)
transmitted from the communication control equipment 120 to
the communication units 123. Fig. 22 shows the communication
flow 171 from the communication unit 123c to the communication
unit 123b and the communication flow 170 of the time
synchronization protocol from the communication control
equipment 120 to the communication unit 123b.
In such a case, the Sync message of the communication
flow170 ofthe time synchronizationprotocolmay conflict with
the packet of the communication flow 171 in the network
intermediary equipment 121a and 121b. A forward delay and
backward delay of the communication flow 170 of the time
synchronization protocol are not exactly equal. In such a case,
forexample, thecommunicationunit123cmaytransmitthepacket
of the communication flow 171 to the communication unit 123b
within a predetermined time after receiving the Sync message.
In this way, the conflict of the packets within the network
intermediary equipment 121 is avoidable.
As a countermeasure forthe caseinwhicha communication
flow other than the time synchronization procedure is present,
there is also a method of introducing a TC (Transparent Clock
defined by IEEE 1588 standard) explained in Embodiment 6
mentioned later into a portion where other communication flow
171 flows.
As explained above, the communication control equipment
120 is provided with the grouping management section 150 to
group the communication units 123 for reduction of the
probability of conflict of the packets, and the time
synchronizationcanbeexecutedevenintheconfigurationusing
the network intermediary equipment 121.
EMBODIMENT 3
Embodiment 3 shows the devised communication units 123
in Embodiments 1 and 2. In the figures used in Embodiment 3,
since the same numerals as Embodiments 1 and 2 are the same
as those of the functions, components, etc. explained in
Embodiments 1 and 2 unless otherwise indicated, their
explanation is omitted.
Fig. 12 shows a functional configuration of the
communicationunit123inEmbodiment3. Thecommunicationunit
123 in Embodiment 3 is provided with the components of the
communication unit 123 in Embodiment 1 shown in Fig. 5 and further
a time-synchronization execution control section 180.
The time-synchronization execution control section 180
controls transmission timings of packets of the
time-synchronization processing section 130. The
transmissiontimingsmaybecontrolledbasedontheinformation
notifiedfromthe communicationcontrolequipment120, orbased
on operation of the time-synchronization processing section
130.
Fig. 13 is a flowchart showing operation of the
communication unit 123.
First, areceptionof the Syncmessage is waitedfor (SO3O) .
When the Sync message is received, the reception time
t2 of the Sync message is measured and stored (S031).
Next, thetime-synchronizationexecutioncontrolsection
180 controls a transmission timing of the Delay - R eq message
(S032). Specifically, wait is made by a predetermined time.
After elapse of the predetermined time, the Delay - Req
message is transmittedtothe communication control equipment
120 (S033).
Next, the transmission time t3 of the Delay - Req message
is measured and recorded (S034).
Next, wait is made until reception of a time
synchronization packet (S035).
When the time synchronization packet is received, it is
determined whether the received packet is the Sync message
(S036) .
When the received packet is the Sync message, it is determined
that the communication control equipment 120 has re-executed
the time synchronization procedure, and the procedure of SO31
to SO35 is executed.
At S036, when the received time synchronization packet
is not the Sync message, the synchronization processing is
executed (S037). The synchronizationprocessingprocedure is
as shown in Fig. 15 described above. Namely, by use of the
time tl (transmission time of the master) in the Sync message
or Follow - UpmessagenotshowninFig.13, thetimet4 (reception
time of the Delay - Req message in the master) in the Delay - Resp
message, andtheabovetimest2 andt3, the time synchronization
with the communication control equipment 120 is executed.
Characteristically, the communication unit 123 in this
embodiment cancontrolthetransmissiontimingofthe Delay - Req
message by use of the time-synchronization execution control
section 180 (SO32). That is, the time-synchronization
execution control section 180 can control the wait time from
the reception of the Sync message to the transmission of the
Delay - Req message. The probability of conflict of time
synchronization packets of the other communication units 123
can be reduced using these characteristics.
The wait time until the transmission of the Delay - Req
message may be determined by the communication unit 123 based
on the information notified from the communication control
equipment 120 or independent of the communication control
equipment 120.
The communication control equipment 120 can control the
waittimeofthetime-synchronizationexecutioncontrolsection
180 by use of a receiving interval of the Delay - Req messages.
For example, when the communication control equipment 120
determinesthat the packets have conflicted fromthe receiving
interval of the Delay - Req messages, the wait time of the
communicationunit123thathastransmittedtheconflictpacket
can be made long or short by a predetermined time.
The communication control equipment 120 can control the
waittimeofthetime-synchronizationexecutioncontrolsection
180 by IEEE 1588 standard. In IEEE 1588, atransmittingperiod
of the Delay - Req message of a slave is controllable based on
a transmitting period of the Sync message of a master.
Specifically, the slave transmits the Delay - Req message once
in a predetermined period in which the master transmits the
Sync message. For example, by use of this specification, when
5 the communication control equipment 120 determines that the
p a c k e t h a s c o n f l i c t e d , t h e t r a n s m i s s i o n t i m i n g o f t h e D e l a y - Req
message is controlled by increasing or decreasing the
transmission frequency of the Delay - Req message of the
communicationunit123thathastransmittedtheconflictpacket.
10 Whenthecommunicationunit123 receivestheSyncmessage
after transmission ofthe Delay - Reqmessage, the communication
unit 123 can estimate the communication control equipment 120
to determine that the packet has conflicted to re-execute the
time synchronization procedure. Therefore, there is also a
15 method of making short the wait time of the time-synchronization
execution control section 180 by a predetermined time when the
Sync message is received after transmission of the Delay - Req
message. Alternatively, the communication control equipment
120 may notify the communication units 123 explicitlythatthe
20 conflict of the packet has been detected.
All the communicationunits123 donot needtobe provided
with the time-synchronization execution control section 180
of this embodiment. When at least some communication units
123 are provided with the time-synchronization execution
Inthiscase, identifiersofthecommunicationunits123provided
with the time-synchronization execution control section 180
may be maintained in the network-configuration information
storage section 151. The communication control equipment120
is able to determine the communication units 123 provided with
the time-synchronization execution control section 180 by use
of the identifiers of the communication units 123 maintained
in the network-configuration information storage section151,
and able to transmit an instruction of a wait time to each
time-synchronization execution control section 180 of these
communication units 123.
Information required for the Sync message transmitted
at the re-execution of the time synchronization by the
communication control equipment 120 can be embedded in the
Delay - Reqmessagetransmittedfromthecommunicationunits123.
Examples of such information include the information about the
identifiers of the communication units 123 concerning
d e s t i n a t i o n s o f t h e S y n c m e s s a g e a n d a g r o u p o f t h e d e s t i n a t i o n s
determined at SO20 of Fig. 8. For example, on the Delay- R eq
message transmitted by the communication unit 123, domain
information of IEEE 1588 standard to which this communication
unit 123 belongs is placed. When the communication control
equipment12Odeterminesthatthepacketfromthiscommunication
unit 123 has conflicted, the communication control equipment
120 is able to again transmit the Sync message by use of the
domain information placed on the Delay - Req message. In this
method, since the communication control equipment120 does not
need to query the grouping management section 150 for a group
of destiantions, the communication control equipment 120 is
able to transmit the Sync message in a short time.
In this case, the communication control equipment 120 may notify
grouping management information such as domain information to
the communication unit 123 previously.
The transmission timing of the Delay - Req message of the
communication unit 123 can be also controlled by other
communication units 123 and by not only itself or other
communication units 123 but a dedicated device and the network
intermediaryequipment121. For convenience, inthe following
explanation, a device that controls transmission timings of
the time synchronization packets of the communication units
123 is called a "time synchronization control device 190."
Fig. 16 shows an example of a distributed control system
using the time synchronization control device 190. The time
synchronization control device 190 is connected to the
communication control equipment 120 and communication units
123a to 123d via the network 122.
Fig. 17 shows a functional configuration of the time
synchronization control device 190. The time synchronization
control device 190 is provided with the communication section
131 shown in Fig. 3, the network-configuration information
storagesection151showninFig.7,andthetime-synchronization
execution control section 180 shown in Fig. 12. When the time
synchronization control device 190 monitors packets on the
network and detects the communication unit 123 that is executing
the time synchronization procedure while transmitting a
conflicting time synchronization packet, the time
synchronization control device 190 delays or advances
transmission of the Delay - Reqmessage fromthis communication
unit 123 by a predetermined time. Alternatively, the
communication control equipment 120 may explicitly notify the
time synchronization control device 190 that the time
synchronization packet has conflicted.
The monitoring of packets is executed by reception of
packets on the network by setting the receive section 133 of
the communication section 131 in a Promiscuous mode. After
confirmation that the communication unit 123 has transmitted
the Delay - Req message, when the time synchronization control
device 190 confirmsthat the Syncmessage has been transmitted
to the same communication unit 123, the time synchronization
control device 190 estimates the communication control
equipment 120 to determine that the packet has conflicted and
to re-execute the time synchronization procedure, and
determinesthattheconflictofthetimesynchronizationpackets
has occurred.
Examples ofamethodof controlling transmission timings
of the Delay Req messages of other communication - units 123
include amethodofmakingtransmission of other communication
units 123pause for apredeterminedtimebyuse of a PAUSE frame
in IEEE 802.3 and amethod of using a dedicatedprotocol. When
5 using the dedicated protocol, at least the information on the
comunicationpartnersandthetimeinformationforcontrolling
the transmission timing (wait time, transmission time, and so
on) are used. Alternatively, when a packet of the dedicated
protocol is received, the transmission may be made. In this
10 case, the communication unit 123 may be configured so that the
transmission is not executed until reception of the packet of
this dedicated protocol.
The time synchronizationcontroldevice190 doesnot need
to control all the communication units 123, but may control
15 some communication units 123.
The time synchronization control device 190 is alsoable
to control the transmission timings of the Delay- R eqmessages
o f t h e p l u r a l c o m u n i c a t i o n u n i t s 1 2 3 o n t h e b a s i s o f a t t r i b u t e s
ofthecomunicationunits123andthesynchronizationprocedure
20 results to date. For example, a priority is set in each of
the communication units 123, and a wait time of the communication
unit 123 can be set in accordance with the priority.
Alternatively, thewait time canbe adjustedinaccordancewith
theconflictfrequencyofthepacketsinthetimesynchronization
25 procedure to date and a precision of the clock of the
communication unit 123. In adjustment of the wait time in
accordancewiththeconflictfrequencyofthepackets, ashorter
wait time is set in the communication unit 123 transmitting
a larger number of packets causing no conflict, for example.
5 I n a d j u s t m e n t o f t h e w a i t t i m e i n a c c o r d a n c e w i t h t h e p r e c i s i o n
ofthe clock, a shorterwaittimemaybe set inthe communication
unit 123 having a higher clock precision, for example. The
information required to control the transmission timings of
the Delay - Req messages is managed and maintained by the
10 network-configuration information storage section 151 in the
time synchronization control device 190.
As explained above, also in this embodiment, the time
synchronization can be executed in the distributed control
s y s t e m u s i n g t h e n e t w o r k i n t e r m e d i a r y e q u i p m e n t 1 2 1 . Further,
15 the probability of the conflict of packets can be reduced by
controlling the transmission timings of the packets by use of
the communication units 123, and the load on the calculation
and control of the communication control equipment 120 also
can be reduced by distributed processing.
2 0
EMBODIMENT 4
Embodiment 4 is such that, in the network intermediary
equipment 121, the processing times of the packets are made
steady to reduce a difference between the forward andbackward
25 communication delays due to the conflict of the packets. In
the figures used in Embodiment 4, since the same numerals as
Embodiments 1 to 3 are the same as those of the functions,
components, etc. explained in Embodiments 1 to 3 unless otherwise
indicated, their explanation is omitted.
When the time synchronization packet is queued in the
networkintermediaryequipment121, forwardandbackwarddelays
ofthetimesynchronizationpacketaredifferentfromoneanother
to generate different communication delays. In this embodiment,
a large processing delay is used if needed and a difference
between forwardandbackwarddelays ofthetime synchronization
packet is reduced. However, to improve the communication
performance of the system, it is desirable to reduce a wait
time of the time synchronization packet within the network
intermediaryequipment121. Thus, inthisembodiment, thewait
time is reduced using the conflict determination results of
packets by the communication control equipment 120 etc.
Fig. 18 shows a functional configuration of the network
intermediary equipment 121 in this embodiment. The network
intermediary equipment 121 includes a packet identification
section 200, a route control section 201, a buffer 202, the
buffer 142 shownin Fig. 4, the time-synchronization execution
control section 180 shown in Fig. 12, and a clock section 203.
Three buffers 142 (input buffer) are connected to the input
side of the route control section 201, and three buffers 142
(output buffer) are connected to the output side thereof.
The packet identification section 200 identifies a type
of a packet basedon predetermined characteristics ofpackets.
The route control section 201 selects an output destination
from the buffers 142 on the basis of information stored in a
packet and forwards the packet thereto. The buffer 202
maintains packets.
The clock section 203 measures times. The packet
identification section 200, route control section 201, and
time-synchronization execution control section 180 acquire
times from the clock section 203. In Fig. 18, lines showing
relationshipbetween the clock section 203 and other sections
(packet identification section 200, route control section 201,
and time-synchronization execution control section 180) are
not shown.
The operation procedure of the network intermediary
equipment 121 in this embodiment is explained. First, a
characteristic of apackettobe identifiedis set in the packet
identification section 200. The characteristic ofthe packet
to be identified may be set via the network 122 from the
communicationcontrolequipment120orcommunicationunits123,
or may be directly set in the network intermediary equipment
121.
In direct setting in the network intermediary equipment 121,
input means not shown in Fig. 18 maybe required. The information
to be set is information to identify a communication flow between
the communication control equipment 120 and the communication
units 123, and includes a transmission source address, a
transmission destination address, a domain in IEEE 1588, an
identifier of VLAN, and a multicast address.
5 When a packet is input into the buffer 142 (input buffer)
of the network intermediary equipment 121, the packet
identificationsection200determineswhetheracharacteristic
ofthispacketmatchesthepreviouslysetcharacteristic. When
the characteristic of this packet matches the previously set
10 characteristic,thepacketidentificationsection200notifies
the route control section 201 and the time-synchronization
executioncontrol section180 ofthe completionofthematching
and the matched condition (characteristic). A the same time,
the packet identification section 200 acquires a time Tin at
15 which the packet is input into the buffer 142 from the clock
section 203, and notifies the route control section 201 of the
time Tin.
On reception of the notification from the packet
identification section 200, the route control section 201
20 forwardsthe packet tothe buffer 202 without transmittingthe
packet to the buffer 142 (output buffer). The packet
transmitted to the buffer 202 is set as a packet 141a. The
output buffer to which the packet 141a is to be transmitted
is set as a buffer 142a.
2 5 It is necessary to determine a time of maintaining the
packet 141a i n t h e buffer 202 i n c o n s i d e r a t i o n of a delay time
due t o o t h e r packets. The delay t i m e due t o o t h e r packets is
a communication processing t i m e of a preceding packet t h a t is
present i n t h e b u f f e r 1 4 2 andapackettransmittedtothebuffer
5 142 a f t e r reception of t h e packet 141a.
After forwarding t h e packet t o t h e b u f f e r 202, t h e route
c o n t r o l s e c t i o n 201 n o t i f i e s t h e time-synchronization
execution c o n t r o l s e c t i o n 180 of t h e number and data s i z e s of
packets i n t h e b u f f e r 142a and t h e number and data s i z e s of
10 packets forwarded t o t h e b u f f e r 142a a f t e r reception of t h e
packet 141a.
On t h e b a s i s of t h e n o t i f i e d information, t h e
time-synchronizationexecutioncontrol section c a l c u l a t e s
a communication processing time d o f t h e s e packets, and
15 c a l c u l a t e s a wait time Tw by s u b t r a c t i n g t h e communication
processing t i m e d from a remaining t i m e R u n t i l a t a r g e t output
t i m e Tout. That is, t h e wait time Tw can be determined by Formula
(11). The t a r g e t output time Tout is determined by adding a
predeterminedstayingtimeatothetime Tin. The s t a y i n g t i m e
20 a may be a f i x e d value or changed between forward and backward
of a packet.
TW = R - d (11)
The remainingtime R i s a t i m e ( R = Tout - ( c u r r e n t t i m e ) )
f r o m a c u r r e n t t i m e t o t h e t a r g e t o u t p u t t i m e T o u t . Thecurrent
25 t i m e is acquirable from t h e clock s e c t i o n 203. The
communicationprocessingtimedcanbedetermined, forexample,
by Formula (3).
The time-synchronization execution control section 180
notifies the calculatedwait time Tw to the route control section
5 201. Sincethecommunicationprocessingtimedchangesineach
reception of following packets, the time-synchronization
execution control section 180 calculates the wait time Tw, and
notifies it to the route control section 201.
The route control section 201 forwards the packet 141a
10 t o t h e b u f f e r 1 4 2 a w h e n t h e w a i t t i m e T w e l a p s e s fromthecurrent
time.
It shouldbe considered here that, when a certain following
packet is forwarded to the buffer 142a, the remaining time to
maintain the packet 141a may elapse. For example, it is
15 necessary to retain the packet 141a for more 50 ps, and a
processing time for the following packet is 60 ps. In such
a case, when the following packet is forwarded to the buffer
142a, the stayingtime a in the network intermediary equipment
121 cannot be fixed. Therefore, an example in which such a
20 following packet is not forwarded to the buffer 142a, but
maintained (blocked) until forwarding of the packet 141a may
be shown as a countermeasure.
Then, by the wait time Tw notified from the
time-synchronization execution control section 180, the route
25 control section 201 maintains the packet in the buffer 202.
After elapse of the wait time Tw, the packet is output to the
buffer 142 in accordance with the output destination. The route
control section 201 uses the clock section 203 to determine
a time to output the packet to the buffer 142.
5 An example of the method of determining a time for the
route control section 201 to output a packet and outputting
the packet to the buffer 142 includes a method of determining
the time Tout by adding the above wait time to the time Tin
acquired from the clock section 203 and notifying the elapse
10 of the wait time Tw from the clock section 203 to the route
control section 201atthetimeTout. Alternatively, the route
control section 201 may acquire the time until elapse of the
wait time Tw from the clock section 203 at a predetermined
interval.
15 An example of a method of setting the staying time a
in the time-synchronization execution control section 180
includes notification of the wait time Tw to the
time-synchronization execution control section 180 from the
communication control equipment 120 that has detected the
20 conflict of a packet via the network 122. Alternatively, to
achievethesettingmethod,thetime-synchronizationexecution
controlsection180monitorspacketsflowingthroughthenetwork
intermediary equipment 121, and a predetermined time for a
communication combination inwhich the Delay - R eqmessage flows
25 and then the Sync message flows in the reverse direction is
made long o r s h o r t .
T h e n e t w o r k i n t e r m e d i a r y e q u i p m e n t 1 2 l m a y b e configured
t o acquire t h e s t a y i n g t i m e a o r wait time Tw from t h e o u t s i d e .
The staying t i m e a o r wait t i m e Tw may be acquired by providing
certainconnectionmeansinthenetworkintermediaryequipment
121. The network intermediary equipment 1 2 1 m a y t r a n s m i t t h e
staying t i m e cr or wait t i m e Tw t o t h e outside through
communication by use of t h e network 122.
Forexample, afterrecognitionofapredeterminedpacketformat,
t h e staying time a or wait t i m e Tw is s t o r e d i n t h e packet.
A system manager can change t h e configuration of t h e system
or t h e network i n c o n s i d e r a t i o n o f t h e i n f l u e n c e on t h e system
by use of t h a t information when t h e s t a y i n g t i m e a or wait time
Tw is p a r t i c u l a r l y long.
The packet i d e n t i f i c a t i o n s e c t i o n 200 can be used f o r
i d e n t i f i c a t i o n of a communication flow.
As explained above, according t o t h i s embodiment, by
reducingthedelaydifferenceduetotheconflictinthenetwork
intermediaryequipment121, t h e distributedcontrolsystemcan
be configured t o execute t h e t i m e synchronization. The
degradation of t h e communication performance of t h e whole
network can be reduced by optimizing t h e wait t i m e of t h e network
intermediary equipment 121.
25 EMBODIMENT 5
Embodiment5is such that, whenplural routes are present
through the network intermediary equipment 121 between the
communicationcontrolequipment120andthecommunicationunits
123, the probability of conflict of packets is reduced by
5 changing the communication routes. In the figures used in
Embodiment 5, since the same numerals as Embodiments 1 to 4
are the same as those of the functions, components, etc.
explained in Embodiments 1 to 4 unless otherwise indicated,
their explanation is omitted.
10 Fig. 19 shows a configuration of a distributed control
system in this embodiment. In the distributed control system
of Fig. 19, the communication control equipment 120, two
communication units 123a and 123b, and four pieces of
communication control equipment 120 are connected via the
15 network 122.
The communication routes between the communication unit
123a and the communication control equipment 120 are a route
210a and a route 210b. The communication routes between the
communicationunit123bandthecommunicationcontrolequipment
20 120 are also the route 210a and route 210b. At this time, when
the communication unit 123a and the communication unit 123b
properly use the communication route 210a and communication
route 210b, the probability that packets conflict within the
network intermediary equipment 121 can be reduced.
2 5 This is realizablebyusinga staticrouting, a dedicated
routing protocol, MPLS (Multi-Protocol Label Switching), etc.
Group identification identifiers (multicast address etc.) may
be properly used for each route. Alternatively, in a controller
in an OpenFlow protocol, the rule to properly use destination
ports by distinguishing destinations maybe set to realize the
reduction of the conflict.
The route may be changed between the Sync message and
Delay - Reqmessage, namely, betweena forwardrouteandbackward
route. For example, when the Sync message is transmitted from
the communication control equipment 120 to the communication
unit 123a, the communication route 210a may be used, and when
the Delay Req message is transmitted from - the communication
unit 123a to the comq-tunication control equipment 120, the
communication route 210b may be used. At this time,
communication delays in the communication route 210a and
communication route 210b are acquired by experimental
determination etc., and used at the time synchronization.
Thecommunicationroute210aandcommunicationroute210b
can be properly used based on the determination result of
conflictofpacketsbythe communicationcontrolequipment120.
For example, when the communication control equipment 120
determines that packets have conflicted, the network can be
set to use a different communication route while setting the
interested communication unit 123a in a different group.
In this embodiment, there is an advantageous effect that
the time synchronization system can be configured using the
network intermediary equipment 121.
EMBODIMENT 6
Embodiments 6 is such that the network is configured by
a hybrid of the network intermediary equipment 121 of the
Transparent Clock (hereafter abbreviated as "TC") defined by
IEEE 1588 standard and the network intermediary equipment 121
that is not the TC. In the figures used in Embodiment 6, since
the same numerals as Embodiments 1 to 5 are the same as those
of the functions, components, etc. explained in Embodiments
1 t o 5 u n l e s s o t h e r w i s e i n d i c a t e d , t h e i r e x p l a n a t i o n i s o m i t t e d .
The TC stores a processing delay in the network
intermediary equipment121in each packet. The communication
unit 123 that has received this packet can determine a difference
between a forward delay and a backward delay by use of information
about these processing delays.
Fig. 20 shows a configuration of a distributed control
system in this embodiment. In the distributed control system
of Fig. 20, the communication control equipment 120, three
communication units 123a to 123c, the network intermediary
equipment 121, and a TC 220 are connected via network 122.
When attention is directed to transmission of the
Delay - Req message from each of the communication units 123a
to 123c, packets of a communication flow 221b anda communication
flow 221c may conflict in the TC 220, and the communication
flow221a, communication flow221b, andcommunication flow221c
may conflict in the network intermediary equipment 121.
However, even when the packets conflict in the TC 220,
jitters of the delays due to the conflict can be corrected by
the function of the TC. On the other hand, in the network
intermediary equipment 121, the jitters of the delays is
difficult to correct.
In this embodiment, the communication control equipment
120 measures receiving intervals of the packets of the
communication flows 22la to 22lc to determine whether the packets
have conflicted by use of Formula (10).
At this time, after receiving the time synchronization
packet of the communication flow 22la, the communication control
equipment 120 receives packets of one or both of the
communication flow 221b and communication flow 221c. When
determining that these packets have conflicted from receiving
intervals of the packets, the communication control equipment
120 re-executes the time synchronization processing to the
communication unit that has transmitted the packet of the
communication flow that has conflicted with the packet of the
communication flow 221a.
Similarly, after receiving the packets of one or both
of the communication flow 221b and communication flow 221c,
the communication control equipment 120 receives the packet
of the communication flow 221a, and also when determiningthat
the packet of the communication flow 221a has conflicted, the
communication control equipment 120 re-executes the time
synchronizationprocessing. This isbecause the packet ofthe
communication flow 221a and the packet of at least one of the
communication flow 221b and communication flow 221c are
estimated to have conflicted within the network intermediary
equipment 121.
On the other hand, when the packet of the communication
flow 221b and the packet of the communication flow 221c conflict,
itisnotnecessarytore-executethesynchronizationprocessing
bythe functionofTC220. Since the communicationdelaywithin
the TC 220 is acquirable by the function of the TC 220, correction
is possible basedon the communication delay in the TC 220 even
when these packets conflict. The communication delays in the
TC 220 are stored in the time synchronization packet in the
communication flow 221b and communication flow 221c.
Fig. 24 shows exchanges of messages between a master and
a slave, and is the same as Fig. 15. However, Fig. 24 shows
a communication delay Ttcg stored in a forward packet (Sync
message), acommunicationdelayTtcrstoredinabackwardpacket
(Delay - Req message), and a one-way communication delay td.
The one-way communication delay td and time difference
tdiffareexpressedbyFormula (12)andFormula (13) respectively
by use of the times tl, t2, t3, and t4 shown also in Fig. 14
and Fig. 15.
td = ((t4 - tl) - (t3 - t2) - Ttcr - Ttcg)/2 (12)
I
I tdiff = ((t4 - (Ttcr - Ttcg) + tl) - (t3 + t2))/2 (13)
I
I
At this time, the communicationdelaytddoes not contain
5 a communication delay of the TC 220.
When there is no conflict at a multicast in transmitting
the Sync message, (Ttcr - Ttcg) can be considered as a delay
occurred by conflict within the TC 220.
At this time, the network-configuration information
10 storage section 151 maintains network configuration
information suchas c o n n e c t i o n o r d e r o f t h e T C 2 2 0 , t h e n e t w o r k
intermediary equipment 121, and the communication unit 123,
and uses the information to determine the above-mentioned
conflict of the packets.
15 Fig. 23 shows an example of communication flows when
a communication flow unrelatedto the time synchronization is
present. Two TCs (TC 220a and TC 220b) are present in Fig.
23. As shown in Fig. 23, communication flows 170a and 170b
of time synchronization packets are present from the
20 communicationunits 123a and123btothe communication control
equipment 120, respectively. The communication flow 171
unrelated to the time synchronization is present from the
communication unit 123c to the communication unit 123d.
The packet of the communication flow 171 may conflict
25 with the packet of the time synchronization flow 170a or 170b
in the TC 220a or 220b. Since the communication control
equipment120 cannot detect the influence ofthe communication
i flow 171 on the time synchronization flow 170a or 170b, the
I time synchronization is typically difficult.
5 However, as shown in Fig. 23, the delay information in
the TC 220a and TC 220b is storable in the time synchronization
packets by providing the TC 220a and TC 220b in the portion
into which the communication flow unrelated to the time
synchronization flows. Therefore, it is notnecessarytotake
10 the conflicts within the TC 220 into consideration. Only the
conflicts within the network intermediary equipment 121 may
be determinedby themethoddescribedinthe above embodiments.
Therefore, the communication control equipment 120
measures receiving intervals of the time synchronization
15 packets of the communication flows 170a and 170b, and may
continue the time synchronization protocol upon determination
that there is no conflict. The communication units 123a and
123bmay synchronize with the communication control equipment
120 by use of Formula (12) and Formula (13) .
2 0 According to this embodiment, overheads due to the
re-execution of the time synchronization processing can be
reducedbyusing the TC. Since the TCsmaybe installedpartially
in the system, the number of the dedicated TCs installed may
be small advantageously.
2 5
EMBODIMENT 7
Embodiment 7 is such that a probability of conflict is
reduced by changing the communication control equipment 120
used as a master of IEEE 1588. In IEEE 1588, one of pieces
5 of communication control equipment configuring a distributed
control system serves as a master (see SO01 of Fig. 6). In
this embodiment, instead of a piece of communication control
equipment that serves as amaster, another piece of communication
control equipment serves as a master. In the figures used in
10 Embodiment 7, since the same reference numerals as Embodiments
1 to 6 are the same as those of the functions, components, etc.
explained in Embodiments 1 to 6 unless otherwise indicated,
their explanation is omitted.
Fig. 21 shows a configuration of a distributed control
15 system in this embodiment. In the system shown in Fig. 21,
pieces of communication control equipment 120a to 120e are
provided as equipment to execute the time synchronization
protocol. Eachpiece of communication controlequipment120a
to 120e has the configuration shown in Figs. 3 or 7, and connected
20 via the network intermediary equipment121af 121b, and121c.
In this embodiment, the role ofthemaster in IEEE 1588 is moved
between the pieces of communication control equipment 120a to
120e. That is, the communication control equipment that is
a master is made to be a slave, and one of the pieces of
25 communication control equipment that are slaves is made to be
a master. Not all the equipment connected to the distributed
control systemneed to be the communication control equipment
120 (that is, all the equipment connected to the distributed
control system do not need to have functions of the master).
5 A procedure of moving the master role from one
communication control equipment to another is explained.
First, the communication control equipment 120c is the
master, and the synchronization processing is executed in the
procedure of Fig. 6. The number of network intermediary
10 equipmenttorouteeachpieceofcommunicationcontrolequipment
120a, 120b, 120d, and 120e (the number of the network
intermediary equipmenttothe communication control equipment
120c that is the master) is two. Therefore, when the processing
15 are comparable and the conditions of the communication routes
are comparable, four packets from the communication control
intermediary equipment 121b when all the acknowledgement
timings overlap.
2 0 In this embodiment, when the communication control
equipment 120c detects this conflict, the master equipment is
changedtothe communication controlequipment120a. When the
communication control equipment 120a becomes the master, the
numberofnetworkintermediaryequipmentforrouting (thenumber
25 of network intermediary equipmenttothe communication control
equipment 120a that is the master) is distributed among the
remaining communication control equipment 120b to 120e that
serve as the slaves. As a result, the probability of conflict
of the packets can be reduced.
5 A method of changing the master may include a method of
using a dedicated protocol that identifies at least the
communicationcontrolequipment120afterthechangeoramethod
ofintentionallyadjustingindexvaluesdeterminedtodetermine
the master among the communication control equipment 120a to
10 120e in a Best Master Clock (hereafter called "BMC") algorithm
of IEEE 1588. In BMC, there are parameters definable by a user
and useful. A method of exchanging the user definable
parameters among the communication control equipment 120a to
120e by use of a dedicated protocol can be also used.
15 For example, a method of selecting a new master includes
a method of setting and selecting indexes in the communication
control equipment 120a to 120e to select one of the pieces of
communication control equipment that has the highest index.
The calculation of the indexes can use the number of network
20 intermediary equipment among the communication control
equipment 120a to 120e, communication delays, a communication
band, or a precision of a clock. For example, a method of
determiningdistributionsofthenumberofnetworkintermediary
equipment among the communication control equipment 120a to
25 120e to select the communication control equipment having the
largest distribution as the master is useful.
The master may be changed when the topology changes due
to obstacles of a STP (Spanning Tree Protocol), a RSTP (Rapid
Spanning Tree Protocol), a communication route, etc.
5 According to this embodiment, by changing the master,
the overheads to re-execute the time synchronization can be
reduced, and the time synchronization can be executed by the
distributed control system using the network intermediary
equipment.
10 The present invention is not limited to the above
embodiments, butincludesvariousmodifications. Forexample,
the above embodiments are explained in detail to understand
the present invention easily, but are not limited to ones
including all the explained components. It is possible to
15 replace part of the configuration of one embodiment with that
of another embodiment. It is also possible to add, to the
configuration of one embodiment, the configuration of another
embodiment. It is possibleto execute addition, deletion, and
replacement of another configuration to part of the
20 configuration of each embodiment.
Part or all of the above configurations, functions,
processing sections, processing means, etc. may be realized
using hardware by designing part or all thereof, e. g., by use
ofanintegratedcircuit. Theaboveconfigurations, functions,
25 etc. maybe realizedusing software by interpreting andexecuting
a program that realizes each function by use of a processor.
Programs to realize each function, tables, files, information
such as measurement information and calculation information
can be stored in a recording device such as a memory, a hard
5 disk, and a SSD (solid state drive) or recording media such
as an IC card, an SD card, and DVD.
The control lines and information lines required forthe
explanation are shown. All the control lines and information
lines are not shown for products. Actually, it may be considered
10 that generally all the components are connected to each other.

WHAT IS CLAIMED IS:
1. Communication control equipment, connected to a
pluralityof communication controlequipmentvia a networkand
configured to be time-synchronized with the plurality of
5 communicationcontrolequipmentbyusingatimesynchronization
procedure using a communication including at least request
packets and acknowledgement packets, comprising:
a receiving-interval measurement section configured to
measure a receiving interval of request packets from the
10 plurality of communication control equipment; and
a queuing-occurrence determination section configured
to detect conflict of the request packets from any of the
plurality of communication control equipment on a basis ofthe
receiving interval of the request packets measured by the
15 receiving-interval measurement section,
whereinthecommunicationcontrolequipmentisconfigured
to determine whether to transmit an acknowledgement packet to
the communication control equipment that has transmitted the
request packets based on a detection result of the
20 queuing-occurrence determination section.
2. Thecommunicationcontrolequipmentaccordingtoclaim
wherein the queuing-occurrence determination section
25 determines that there is no conflict of the request packets
when the receiving interval of the request packets measured
by the receiving-interval measurement section is equal to or
m o r e t h a n a r e c e i v i n g i n t e r v a l o f t h e r e q u e s t p a c k e t s c a l c u l a t e d
at least by using a packet interval defined in the network.
wherein, when the queuing-occurrence determination
section determines that there is no conflict of the request
10 packets, the communication control equipment transmits the
acknowledgement packet tothe communication control equipment
that has transmitted these request packets.
4.Thecommunicationcontrolequipmentaccordingtoclaim
15 1,
wherein, when the queuing-occurrence determination
section detects the conflict of the request packets, the
communication controlequipmenttransmits a re-request packet
for requesting a time synchronization to the communication
20 control equipment that has transmitted these request packets
of the plurality of communication control equipment.
wherein, when the communication control equipment does
not receive the request packets within a predetermined time,
the communication control equipment transmits a re-request
packet for requesting a time synchronization to the plurality
of communication control equipment.
5
6 . Thecommunicationcontrolequipmentaccordingtoclaim
wherein, when the queuing-occurrence determination
section detects the conflict of the request packets, the
10 communication control equipment transmits a re-request packet
for requesting a time synchronization at least to the
communication control equipment that has transmitted these
request packets.
7. Thecommunicationcontrolequipmentaccordingtoclaim
wherein the queuing-occurrence determination section
determines that the request packets has conflictedwhen atime
span exceeds a predetermined time, the time span being a time
20 until the communication control equipment first receives the
request packets from the plurality of communication control
equipment after the receiving-interval measurement section
transmits a re-request packet for requesting a time
synchronization to the plurality of communication control
25 equipment.
wherein the communication control equipment changes a
5 time span until the communication control equipment transmits
a re-request packet for requesting a time synchronization,
according to the number of conflicts of the request packets
detected by the queuing-occurrence determination section.
1 f
further comprising a grouping management section
configured to divide the plurality of communication control
equipment into groups,
15 whereinthecomunicationcontrolequipmentisconfigured
to determine whether to transmit request packets to the groups
dividedbythegroupingmanagementsection, basedonadetection
result of the queuing-occurrence determination section.
2 0 10. The communication control equipment according to
claim 9,
wherein the grouping management section is configured
todividethepluralityofcomunicationcontrolequipmentinto
groups based on communication delays with the plurality of
25 communication control equipment.
11. The communication control equipment according to
claim 9,
wherein the grouping management section is configured
5 todividethepluralityofcommunicationcontrolequipmentinto
groups based on any one or more of the number of transmissions
of acknowledgement packets, the number of success time
synchronizations with the plurality of communication control
equipment, the number of failure time synchronizations with
10 the plurality of communication control equipment, and an elapsed
time after completion of the time-synchronizations with the
plurality of communication control equipment.
12. The communication control equipment according to
15 claim 9,
further comprising a network-configuration information
storage section configuredto store configuration information
of the network,
wherein the grouping management section is configured
20 todividethepluralityofcommunicationcontrolequipmentinto
groups based on the configuration information of the network
stored by the network-configuration information storage
section.
13. The communication control equipment according to
claim 12,
wherein the configuration information of the network
stored by the network-configuration information storage
section includes one or more of the number of the equipment
5 includedinthenetwork, informationontheequipment, anetwork
topology, the number of hops to the equipment, communication
delaystotheequipment, aperformanceofintermediaryequipment
included in the network, a communication performance of the
network, and network media.
10
14. The communication control equipment according to
claim 9,
wherein the grouping management section is configured
todividethepluralityofcommunicationcontrolequipmentinto
15 groups based on a transmission history of acknowledgement
packets to the plurality of communication control equipment.
15. The communication control equipment according to
claim 9,
2 0 wherein, whenthenumberofthepluralityofcommunication
control equipment that has failed to be time-synchronized is
equal to or less than a predetermined number, the grouping
management section divides the plurality of communication
control equipment into groups by selecting a predetermined
25 number of communication control equipment from the plurality
of communication control equipment that has failed to be
time-synchronized.
16. The communication control equipment according to
5 claim 4,
wherein the communication control equipment that has
transmitted the request packets of the plurality of
communication control equipment communicates with other
communication control equipment within a predetermined time
10 after receivingthe re-request packet for requestingthe time
synchronization.
17. The communication control equipment according to
claim 1,
15 wherein the communication control equipment is included
in a network configuration employing network intermediary
equipmentconfiguredto store a stayingtime of apacketwithin
the network intermediary equipment into the packet,
wherein the queuing-occurrence determination section
20 consecutively receives first requestpacketstransmitted from
a first communication control equipment and second request
packets transmitted from a second communication control
equipment via the network intermediary equipment, and
wherein the queuing-occurrence determination section
25 determines that there is no conflict of the second request
packets when the queuing-occurrence determination section
determines that the first communication control equipment and
the second communication control equipment do not share a
communication route tothe network intermediary equipment and
5 that there is no conflict of the first request packets.
18. The communication control equipment,
substantially as herein described with reference to
accompanying drawings and examples.