JP2002374255A - Redundant network system - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To prevent processing on redundant data in an upper layer without increasing the processing load on the upper layer. SOLUTION: It is determined whether or not a history corresponding to a received frame is registered in a history information storage unit 16. If the history is not registered, the history of the received frame is registered in the history information storage unit 16 and relaying is enabled. To notify the upper layer of the frame. When registered in the history information storage unit 16, the history corresponding to the frame received from the transmission source apparatus of the received frame earlier than this is removed from the history information storage unit 16, the relay is disabled, and the received frame is deleted. Is discarded without notifying the upper layer. Since the frame is notified to the upper layer only when the received frame is not registered in the history information storage unit 16, the notification of the redundant frame to the upper layer is avoided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a duplex network system in which a line between devices is duplicated and the same data is transmitted to these two lines.

[0002]

2. Description of the Related Art Conventionally, in a duplex network system in which a line is duplicated, a packet is received from only one of the lines. For example, when a packet is not received for a predetermined time, it is determined that the line is abnormal. To switch the line and receive packets from the other line.

[0003] When a packet is received from both lines, if the line is normal, the same packet is also transmitted from the other line within a predetermined time from when the packet is received from one line. Should receive it,
A packet received earlier is notified to an upper layer that is a user of this packet, and processing such as discarding a packet received later is performed. Also, for example, a serial number is assigned to a packet and stored as a history, based on which a redundant packet is detected, and a packet in which redundancy is detected is not notified to an upper layer.

[0006] Of the packets stored as history, only packets for which redundancy is detected are deleted from the history, or a packet in which a predetermined time has elapsed after being recorded as history is deleted from the history. The history is managed by performing a general management. Further, a line error is detected by detecting a frame error, and an error is detected by detecting that communication is not established for a certain period of time.

[0005]

However, as described above, the line is duplexed, a packet is transmitted from the transmitting device to both lines, and the receiving device receives the packet from both of the duplicated lines and transmits the frame. In the case of ensuring the reliability of the above, as described above, in the method of notifying the packet received earlier to the upper layer and discarding the packet received later, regardless of the contents of the packet Since the packet is notified to the upper layer, in some cases, the redundant packet may be notified to the upper layer and the redundant packet may be processed.

[0006] Generally, processing for redundant packets is performed on the upper layer side, which causes the complexity of processing on the upper layer side and an increase in load. Also, as a history management method, in a method in which a packet that matches a newly received packet is deleted from the history, if the same packet as the packet stored in the history is not received again, Since the history remains without being deleted from the history, if a packet from one line is lost for some reason, the history is not deleted and there is a problem that resources may be exhausted. Further, in the case of using the method of managing the time of the history, there is a problem that the processing load is large because it is necessary to manage each packet.

Therefore, the present invention has been made in view of the above-mentioned conventional unsolved problem, and reduces the processing load on the upper layer side and avoids performing processing on redundant data. It is an object of the present invention to provide a redundant network system capable of performing such operations.

[0008]

In order to achieve the above object, a duplex network system according to the first aspect of the present invention duplicates a line between devices and transmits the same data from a transmitting device to each line. In the duplex network system as described above, the transmission-side device adds transmission source information for identifying the transmission-side device to the data to be transmitted, and transmits the data. History storage means for storing identification information consisting of information for identifying received data, and searching for, when data is received, whether the identification information corresponding to the received data is stored in the history storage means. Receiving only when the identification information corresponding to the received data is not stored in the history storage means as a result of the search by the searching means. Relay means for relaying the received data to an upper layer; and, as a result of the search by the search means, when the identification information corresponding to the received data is not stored in the history storage means, the identification information of the received data is stored in the history storage means. Means, and when the identification information corresponding to the received data is stored in the history storage means, before the data of the identification information stored in the history storage means is received, the transmission source of the data is received. History management means for deleting all identification information corresponding to data received from the apparatus.

According to the first aspect of the present invention, when data is received from one of the lines, the receiving device identifies the received data and the transmission source device of the received data in the history storage means. Search is performed to determine whether or not the identification information is stored. Only when there is no corresponding identification information in the history storage means, the received data is notified to the upper layer, and identification information for identifying the received data and the transmission source device of the received data is transmitted. It is stored in the history storage means. On the other hand, when the identification information corresponding to the received data is stored in the history storage unit, the identification information stored in the history storage unit includes the identification information corresponding to the data received from the transmission source device of the received data. The identification information corresponding to the received data before receiving the received data is deleted.

Therefore, even if data is received from both lines, if the data is stored in the history storage means, this data is not notified to the upper layer, so that the same data received from both lines is transmitted to the upper layer. You will not be notified. Further, when the identification information of the received data is stored in the history storage unit, before receiving the received data, the identification information corresponding to the data received from the transmission source device is transmitted from the history storage unit. Since the history is deleted, the history of the history storage unit can be easily managed.

In a redundant network system according to a second aspect of the present invention, the history management means adds line information indicating from which of the lines the data is received to the identification information to the identification information. Stored in a history storage unit, and the receiving side device detects the abnormality of the line based on the line information added to the identification information corresponding to the data received from the same device, stored in the history storage unit. Is provided.

In a redundant network system according to a third aspect, the abnormality detecting means determines that a line different from the line is abnormal when the number of lines specified by the line information is equal to or more than a predetermined number. It is characterized by that. According to the second and third aspects of the present invention, line information indicating which of the two lines the received data is from is added to the identification information and stored in the history storage means. You. Then, of the identification information stored in the history storage means, a line abnormality is detected based on the line information added to the identification information corresponding to the data received from the same transmission source device.

That is, when a plurality of pieces of identification information corresponding to data received from the same transmission source device are stored and these are data received from the same line, data is received from the other line. It means that no abnormality has occurred on the other line. Therefore, for example, when data received from the same line exists continuously for a predetermined number or more, it is possible to determine that the other line is abnormal.

Further, in the redundant network system according to a fourth aspect, the history storage means is formed of a ring buffer for each transmission source device. In the invention according to claim 4, since the history storage means is formed in a ring buffer structure for each transmission source device, the identification information corresponding to the previously received data is easily deleted by operating the pointer. It becomes possible.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram showing an example of a redundant network system to which the present invention is applied.
Communication apparatuses 1 and 2 are connected via a duplex line L including lines L1 and L2.

The communication devices 1 and 2 are formed identically,
As shown in FIG. 1, while processing is performed on frames received from the lines L1 and L2, the lines L1 and L2 are processed.
A transmission / reception processing unit 12 for generating a frame to be transmitted to the transmission / reception unit 12 and performing predetermined processing on a frame received by the transmission / reception processing unit 12 (hereinafter referred to as a received frame), An upper layer 14 for generating data to be transmitted to the communication device, a history information storage unit 16 for storing the history of the received frames received by the transmission / reception processing unit 12, and history information stored in the history information storage unit 16. And a line abnormality detector 18 for detecting an abnormality of the duplicated line L based on the

Then, the transmission / reception processing unit 12 outputs the line L1
Alternatively, when a frame is received from L2, it is searched whether or not the received frame is registered as history information in the history information storage unit 16. If the received frame is not registered as history information, the received frame is relayed to the upper layer 14 and Registered as history information. On the other hand, when the received frame is registered in the history information, the received frame is discarded, and the history information stored in the history information storage unit 16 is transmitted from the communication device of the transmission source of the received frame before the received frame. Delete all received frames.

In the transmission / reception processing unit 12, the upper layer 14
When the data to be transmitted is notified from, a frame for transmitting the data to a designated communication device is generated, and the generated frame (hereinafter, referred to as a transmission frame) is transmitted to the lines L1 and L2. The frame is configured, for example, as shown in FIG. In other words, as shown in FIG. 2, the communication apparatus includes a physical frame header 51, a protocol header 52, and data 53 to be transmitted. It includes an address 52a, a source address 52b for specifying a communication source of a frame, an ID 52c updated every time a frame is transmitted, and a sum value 52d which is a checksum value of a transmission frame.

On the other hand, the history information storage section 16 manages an ID-SUM history 61 having a ring buffer structure formed for each transmission source communication device and this ID-SUM history 61, as shown in FIG. And a filter management table 62. The ID-SUM history 61
As shown in FIG. 3, a plurality of histories (32 in this case) can be stored for a plurality of communication devices for each communication device. Then, for each ID-SUM history 61 corresponding to each communication device, the number of entries 71 representing the number of histories registered in this ID-SUM history 61;
The oldest pointer 72 indicating the oldest history, the next registration pointer 73 indicating the history to be registered next, the ID-SUM history 6
ID field 74 in which identification information for identifying the history registered in No. 1 is set, and ID 52c in which the ID 52c included in the protocol header 52 of the transmission frame is set.
5. SUM 76 in which sum value 52d included in protocol header 52 of the transmission frame is set, lines L1 and L2
, A Port 77 in which information indicating which of the frames has been notified is set.
The ID 75, SUM 76, and Port 77 are set for each history specified in the ID field 74.

The filter management table 62 includes:
The number of entries 62a of the transmission source communication device registered as the ID-SUM history 61, the address 62b for specifying each communication device, and the ID-S corresponding to this communication device
ID-SUM history pointer 6 indicating storage location of UM history
2c, the address 62b and the ID-SUM
The history pointer 62c is formed for each communication device registered as the ID-SUM history.

Next, the operation of the above embodiment will be described.
Now, assuming that data is transmitted from the communication device 1 to the communication device 2, the upper layer 14 of the communication device 1 generates data to be transmitted and notifies the transmission / reception processing unit 12 of the data. The transmission / reception processing unit 12 first creates a transmission frame as shown in the flowchart of FIG. 4 illustrating an example of processing at the time of transmission (step S1). That is, as shown in FIG. 2, the protocol header 52 is updated as the destination address 52a including the address of the communication device 2, the source address 52b including the address of the communication device 1, and each time the frame is transmitted by the transmission / reception processing unit 12. An ID 52c and a sum value 52d, which is a checksum value of the transmission frame, are set, and a protocol header 52 and a predetermined physical frame header 51 including these are added to the data 53 to generate a transmission frame.

Then, the generated transmission frame is transmitted to the line L1.
(Step S2), and then to the line L2 (step S3). On the other hand, in the communication device 2 on the receiving side, the transmission / reception processing unit 12 receives the frames from the lines L1 and L2, and performs processing according to the flowchart of FIG.

First, a filtering process is performed (step S1).
1). In this filtering process, first, as shown in FIG.
With reference to the filter management table 62 of the history information storage unit 16, it is searched whether or not the address 62b corresponds to the source address 52b added to the received frame (step S21). At this time, the communication device 1
If no frame has been received from the communication device 2 before, there is no address corresponding to the communication device 2 in the filter management table 62.

Therefore, the process proceeds from step S22 to step S23, where an area for storing the ID-SUM history 61 corresponding to the communication device 2 is secured in the history information storage unit 16,
The information specified here is registered as an ID-SUM history, an ID-SUM history 61 corresponding to the communication device 2 is generated, and the address of the communication device 2 and its ID-SU
Filter management table 6 based on the storage location of M history
2 address 62a and ID-SUM history pointer 62
Set c.

Next, the process proceeds to step S24 to make the relay valid. On the other hand, if the frame has been received from the communication device 2 before, since the frame has been registered in the filter management table 62, the process proceeds from step S22 to step S25, and the ID-SUM history 61 corresponding to the communication device 2 is stored. By referring to the history specified by the oldest pointer 72 and the history immediately before the history specified by the next registration pointer 73, a search is performed to determine whether a history corresponding to the ID 52c and the sum value 52d of the received frame exists. I do. If there is no corresponding one, the process proceeds from step S26 to step S27, where the ID 52c and the sum value 52d of the received frame are registered as the history specified by the next registration pointer 73. Then, the number of entries 71, the next registration pointer 7
Update 3 Then, the relay is enabled (step S2
8).

On the other hand, the communication device 2 which is the transmission source of the received frame
ID of the received frame in the ID-SUM history 61 corresponding to
If there is a history corresponding to 52c and the sum value 52d, the process moves from step S26 to step S29, and the history corresponding to the ID 52c and the sum value 52d of the received frame and the history of the frame received from the communication device 2 before this. From the ID-SUM history 61 of the communication device 2.
Then, the number of entries 71 and the oldest pointer 72 are updated. Then, the relay is invalidated (step S30).

When the registration / deletion of the history in the history information storage unit 16 and the validity / invalidity of the relay are set in this way, the flow returns to FIG. 5 and the process shifts to step S12 to determine whether the relay is valid. I do. When the relay is valid, the process shifts to step S13 to notify the upper layer 14 of the received frame. On the other hand, if the relay is invalid, the process proceeds to step S14, where the received frame is discarded without notifying the upper layer 14 of the received frame.

FIG. 7 shows the update status of the ID-SUM history 61 in the history information storage unit 16 of the communication device 2 on the receiving side. In the initial state of the ID-SUM history 61, as shown in FIG. 7A, the number of entries 71, the oldest pointer 72, and the next registration pointer 73 are zero, and nothing is stored in each history. .

From this state, first, from the line L1 to the ID5
2c receives the frame of “01”, the transmission / reception processing unit 12 processes this, and the ID-S corresponding to the communication device 2
Register it in the UM history 61. As a result, as shown in FIG. 7B, the ID-SUM history 6
In the position of the history designated by the next registration pointer 73 of 1, in this case, the position of the history 0, “01” designated by the ID 52 c as the ID 75, the sum value 52 d such as “SUM 1” as the SUM 76 is registered, and as the Port 77 “1” that specifies the line L1 is registered. Further, the number of entries 71 is updated to "1", the next registration pointer 73 is updated to "1", and the oldest pointer 72 remains "0".

Subsequently, when the frame whose ID 52c is "02" is received again from the line L1, the transmission / reception processing unit 12 processes the frame in the same manner, and as shown in FIG.
The received frame ID 52c and the sum value 52d are represented by ID-S
An area designated by the next registration pointer 73 in the UM history 61;
In this case, while registering in the area of the history 1, port7
7 is registered as the line L1, and the number of entries 71 is "2";
The next registration pointer 73 is updated to “2”.

Subsequently, the ID 52c is set to "0" from the line L2.
When receiving the 2 ″ frame, the transmission / reception processing unit 12 processes this. The ID-SUM history 61 corresponding to the communication device 1 is specified by the next registration pointer 73 from the history 0 specified by the oldest pointer 72. Of the previous history, that is, history 1 of history 0 and history 1
75 is registered as “02” and SUM is registered as “SUM2”, so that the transmission / reception processing unit 12 disables the relay. Then, the received frame is discarded.
As shown in the figure, the oldest pointer 72 of the ID-SUM history 61 corresponding to the communication device 1 is set to the ID 52c of the received frame.
Then, the history following the history that matches the sum value 52d, in this case, since the history 1 matches, the history is updated to indicate the history 2. Also, the number of entries 71 is reduced by the number of times the oldest pointer 72 is advanced, in this case, by “2”, and set to “0”.
Therefore, since the number of entries 71 is "0" and the oldest pointer 72 is equal to the next storage pointer 73, the state is equivalent to a state where no history exists, that is, a deleted state.

Subsequently, when the transmission / reception processing unit 12 receives a frame whose ID 52c is "03" from the line L1, FIG.
As shown in (e), the ID 52c and the sum value 52d of this frame are registered in the history 2 designated by the next registration pointer 73, and the number of entries 71 and the next registration pointer 73 are updated. From this state, when an abnormality occurs in the line L2 and the frame is not received from the line L2, the frame is received only from the line L1, and the ID-SUM history 61 corresponding to the communication device 2 includes the line L1 Are sequentially registered. The number of entries 71 and the next registration pointer 73 are sequentially updated in accordance with this, and “port L
1 "will be registered.

Here, the line abnormality detecting section 18 detects an abnormality based on the history information in the history information storage section 16. That is, as shown in the flowchart of FIG.
First, in step S31, the addresses of all the communication devices registered in the history information storage unit 16 are checked by, for example, searching in the order of the addresses of the communication devices registered in the filter management table 62. It is determined whether or not it has been performed. If all the checks have not been completed, the process proceeds to step S32, where the ID-SUM corresponding to the communication device of the next address is determined.
Regarding the history 61, whether the ID-SUM history 61 is full, that is, as the number of entries 71, ID-SU
It is determined whether or not the number of histories that can be registered in the M history 61 (32 in this case) is registered. And ID-SU
If the M history 61 is not full, the process returns to step S31 to check the ID-SUM history 61 corresponding to the communication device at the next address. And ID-SU
If the M history 61 is full, the process moves to step S33, and for each history registered in the ID-SUM history 61, the Port 76 is referred to for all the histories.
It is determined whether ort77 is the same. And
If they are not the same, the process returns to step S31 to check the ID-SUM history 61 corresponding to the communication device of the next address.

Then, Port 77 in all histories
If they are the same, the process proceeds to step S34, where it is determined that the line different from the line specified by the Port 77 registered in the history is abnormal, and processing such as notifying the upper layer is performed. Do. That is, when an abnormality such as disconnection occurs in one of the lines, the frame can be received only from one of the lines. Therefore, a frame from a normal line is registered in the history. At this time, since frames are not received from both lines, even if frames are sequentially received, The history of the ID-SUM history 61 is not deleted but is stored sequentially.

Therefore, by checking the transmission source line of each frame in the ID-SUM history 61, a line abnormality can be easily detected. Thus, in the communication device on the transmitting side, the lines L1 and L2
When the same frame is transmitted, the communication device on the receiving side receives the frame from the lines L1 and L2, but the communication device on the transmitting side includes an ID 52c for identifying this frame and a sum value in the frame. 52d is added and transmitted. The communication device on the receiving side registers an ID for identifying this frame and its sum value in the ID-SUM history 61 when notifying the received frame to the upper layer 14. When a new frame is received, it is determined whether or not the ID added to this frame and its sum value are stored in the ID-SUM history 61. The received frame is notified to the upper layer 14 and the ID-S
When the received frame is stored in the UM history 61, the received frame is discarded without being notified to the upper layer 14, so that the same frame is not notified to the upper layer 14.

At this time, the history information storage unit 16 forms an ID-SUM history 61 for each transmission source address, that is, for each transmission source communication device. The history of the received frame is deleted from the ID-SUM history 61. Here, when the communication via the lines L1 and L2 is normally performed, the communication device on the receiving side uses the line L1
And the same frame from L2, the history of the frames registered in the ID-SUM history 61 is sequentially deleted. If only a frame from one line can be received for some reason, the history of this frame is the ID-SUM history 6
However, when a frame is received from both lines, the history of the frame received before this frame from the communication device that transmitted the frame is also ID.
-Unnecessary history does not remain in the ID-SUM history 61 because it is deleted from the SUM history 61.
In addition, since it can be realized without performing time management for each history, it can be realized without a significant increase in load.

Also, the history stored in the ID-SUM history 61 is deleted only when the same frame is received from both lines, and the I-SUM for identifying the frame is deleted.
Since the port 77 indicating from which line the frame was received together with the D75 and the SUM 76 is also registered as a history, by referring to the port 76 regarding the history left in the ID-SUM history 61, a line abnormality can be detected. It can be easily detected.

Further, since the ID-SUM history 61 is formed in a ring buffer for each communication device, time management of the history can be easily performed by manipulating the pointer, and the previously received frame can be obtained. Can be easily deleted. Further, the transmission / reception processing unit 12 processes the same frame received from the lines L1 and L2, and notifies the upper layer 14 of only the frame. Can be realized without increasing the load on the upper layer 14.

In the above embodiment, the case where the ID 75, the SUM 76, and the Port 77 are stored as the history has been described.
And SUM 76 need not be stored. For example, when transmission of a transmission frame is performed in units of transmission frames generated by the transmission / reception processing unit 12, since a frame having the same ID 75 is not transmitted, the ID 75 and Po
rt77 may be stored. For example, a transmission frame is divided and the same ID 52 is assigned to the divided frame.
In a system in which transmission is performed while loading c, the same ID 52c is added to each divided frame. Therefore, when only IDs are stored, the same ID 52c is added to a plurality of divided frames. Therefore, it is determined that the divided frames are the same even in different divided frames. Therefore, if the ID 75c and the SUM 76 are stored, even if the ID 52c is the same, the SUM 52d is different, so that it is possible to identify even a divided frame to which the same ID is added.

In the above-described embodiment, the case where the present invention is applied to a system to which the communication devices 1 and 2 are connected has been described. However, the present invention is not limited to this, and may be applied to a case where a plurality of communication devices are connected. Needless to say, it can be applied. Also, the present invention can be applied to a network system in which each communication device is configured in any of a tree type, a bus type, and a ring type.

Further, in the above embodiment, the case where the history information storage section 16 is formed in a ring buffer structure has been described. However, the present invention is not limited to this, and it is sufficient if a plurality of histories can be stored. However, by using a ring buffer structure as the history information storage unit 16 and performing pointer operation, history management and deletion can be easily performed.

Also, in the above embodiment, the ID-
In the case where the number of registrable histories is registered in the SUM history 61, a case has been described where the abnormality determination is performed based on the line information stored in the Port 77. However, the present invention is not limited to this. Out of the histories registered in the ID-SUM history 61, a predetermined number of times, that is, the number of times that a line can be regarded as having an abnormality. It may be determined that the line is abnormal.

In the above-described embodiment, the history information storage unit 16 corresponds to the history storage means, and corresponds to step S in FIG.
21, S22, S25 and S26 correspond to search means,
The processing in step S13 in FIG.
Steps S23, S27 and S29 correspond to history management means, and the processing of FIG. 8 corresponds to abnormality detection means.

[0044]

According to the dual network system of the first aspect of the present invention, even if the receiving device receives the same data from both of the duplexed lines, the data already notified to the upper layer is not transmitted to the upper layer. , It can be easily avoided that redundant data is notified to an upper layer, and the history of the history storage unit is deleted when the redundant data is received. Therefore, the history management of the history storage means can be easily performed without a significant increase in load.

According to the dual network system of the present invention, line information indicating which of the two lines the data is received from is added to the identification information to store the history. Means, the line abnormality can be easily determined based on the line information added to the identification information corresponding to the data received from the same transmission source device among the identification information stored in the history storage means. Can be detected.

According to the redundant network system of the third aspect, when the number of lines specified by the line information is equal to or more than a predetermined number, the other line is determined to be abnormal. Therefore, a line abnormality can be easily detected. Further, according to the redundant network system of the fourth aspect, since the history storage means is formed by a ring buffer for each transmission source device, management of identification information can be easily performed.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a redundant network system to which the present invention is applied.

FIG. 2 is an example of a frame.

FIG. 3 is an explanatory diagram illustrating a configuration of a history information storage unit 16;

FIG. 4 is a flowchart illustrating an example of a processing procedure of a process when transmitting a frame.

FIG. 5 is a flowchart illustrating an example of a processing procedure of a process when receiving a frame.

FIG. 6 is a flowchart illustrating an example of a processing procedure of a filtering process.

FIG. 7 is an explanatory diagram illustrating an example of an update status of an ID-SUM history.

FIG. 8 is a flowchart illustrating an example of a processing procedure of an abnormality detection process.

[Explanation of symbols]

 1, 2 communication device 12 transmission / reception processing unit 14 upper layer 16 history information storage unit 18 line abnormality detection unit

Claims (4)

[Claims] In a duplex network system in which a line between devices is duplicated and the same data is transmitted from a transmission side device to each line, the transmission side device specifies a transmission side device for data to be transmitted. History information storage means for storing identification information including the transmission source information of the received data and information for identifying the received data; and A search means for searching whether or not identification information corresponding to the received data is stored in the history storage means when received; as a result of the search by the search means, the identification information corresponding to the received data is stored in the history. A relay unit that relays the received data to an upper layer only when the data is not stored in the storage unit; and a search result corresponding to the received data as a result of the search by the search unit. When the identification information is not stored in the history storage means, the identification information of the received data is stored in the history storage means, and when the identification information corresponding to the received data is stored in the history storage means, the history storage is performed. History management means for deleting all the identification information corresponding to the data received from the transmission source apparatus of the data before receiving the data among the identification information stored in the means. Network system. 2. The history management unit adds line information indicating from which of the lines the data is received to the identification information, stores the line information in the history storage unit, and stores the line information in the history storage unit. The apparatus includes an abnormality detection unit configured to perform abnormality detection of the line based on the line information added to identification information corresponding to data received from the same device and stored in the history storage unit. The duplex network system according to claim 1, wherein 3. The apparatus according to claim 1, wherein the abnormality detecting means determines that a line different from the line is abnormal when the number of lines specified by the line information is equal to or more than a predetermined number. The duplex network system according to claim 2. 4. The duplex network system according to claim 1, wherein said history storage means is formed of a ring buffer for each transmission source device.