JP2002141929A - Lan connection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a LAN connection device having physical ports that can apply automatic exclusive connection switching to connections between networks. SOLUTION: The LAN connection device is provided with physical ports 206a-206d, a packet relay section 200 the relays a packet between channels connected to the physical ports, a master system changeover available port 206b and a sub system changeover available port 206a that share a channel 207a and are connected exclusively to the share channel 207a by a switch circuit 204, a carrier detection section 203 that detects a communication carrier from the sub system changeover available port 206a, and a switch control section 201 that connects the sub system changeover available port 206a to the share channel 207a at detection of a carrier and selects the main system changeover available port 206b when no switch changeover request comes from the carrier detection section 203 for a prescribed time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a LAN connection device having an automatic port switching function.

[0002]

2. Description of the Related Art In recent years, there has been an increase in remote maintenance in which a local area network (hereinafter, referred to as a LAN) of a remote maintenance center remotely accesses a LAN of a customer system via a wide area network to perform maintenance of the customer system. I have.

When a customer system includes a network that the user does not want to access directly from a maintenance center, such as an online LAN, an offline LA in the customer system is used.
N, etc., uploading programs and collecting logs. The exchange between the off-line LAN and the on-line LAN is performed only by local maintenance personnel, thereby ensuring the security of the customer system.

As means for prohibiting external access to the online LAN of the customer system, a firewall is installed between the wide area network, the online LAN and the offline LAN. As shown in FIG. 2, a LAN connection device having a port switching function is used as a firewall, and a wide area network and an offline LA are used.
When N is communicating, the connection between the two networks and the online LAN may be disconnected. With this configuration, even if the communication is falsified as if coming from the offline LAN or if the communication terminal of the offline LAN is deprived of the control right by an unauthorized user via the wide area network, the communication to the online LAN can be prevented. Access can be prevented. However, a conventional LAN connection device having a port switching function (for example, the LAN connection device 103 in FIG. 2) is configured by manual means such as a push button switch and a slide switch.

As a LAN connection device capable of automatically switching ports, a hub with a port priority selection function is disclosed in Japanese Patent Application Laid-Open No. H11-331233. Here, of the ports sharing the channel, the right to use the channel is given to the port where the link signal is detected, and the right to use the channel is given according to the priority when the link signal is detected at a plurality of ports. . In this example, port switching occurs only when the detection / non-detection state of the link signal changes. Since the link signal is a signal indicating whether or not a device is connected regardless of whether communication is performed or not, for example, if any failure occurs in a device connected to a port using a channel and the link signal is interrupted. Switching is performed.

[0006]

In a conventional automatic switching device for ports, switching can be performed according to the user's intention by inserting or removing a cable connected to the port or by powering a device connected to the end of the cable. It is necessary to perform a manual switching operation for turning on / off the power supply. However, manual switching has a risk of erroneous operation, and there is a strong demand for complete automation. Further, since the communication state of the switched port is not taken into consideration, there is also a problem that communication is not protected and secure switching is not performed.

[0007] An object of the present invention is to provide a LAN connection device capable of automatically and safely switching ports in view of the above-mentioned problems of the prior art.

[0008]

In order to achieve the above object, the present invention relates to a LAN connection device having a plurality of physical ports for connecting a local area network and relaying packets between the networks. A main unit switchable port and a sub system switchable port that share a predetermined channel with a packet relay unit that relays a packet between the connected channels and that are exclusively connected to the shared channel by a switch circuit; A carrier detection unit that detects a communication carrier from a sub system switchable port, and a switch that controls the switch circuit so that the sub system switchable port is connected to the shared channel when the carrier detection unit detects a carrier. A control unit is provided.

[0009] The switch control unit is configured to switch the connection of the shared channel from the sub system switchable port to the main system switchable port when there is no switch switching request from the carrier detection unit for a predetermined time. Control the circuit.

[0010] The present invention further comprises a lock control unit that issues a lock request to the switch control unit to prevent the connection of the switch circuit from being switched from the main system switchable port to the sub system switchable port. It is characterized by having.

When the lock control unit is in a call with a communication terminal connected to a LAN, the switch control unit holds the state of the switch circuit, and the lock control unit transmits a communication requesting a lock operation from the communication terminal. When receiving the lock request, the switch control unit executes a lock request by the lock control unit.

Further, according to the present invention, a communication terminal on a LAN connected to the physical port can transmit to and receive from the lock control unit regardless of the state of the switch circuit.

The present invention further includes a delay circuit provided between the sub system switchable port and the switch circuit for delaying transmission of a communication carrier from the sub system switchable port,
The delay time of the delay circuit is longer than the time from when the carrier detector detects a communication carrier to when the switch circuit completes the switching operation.

The operation of the LAN connection device according to the present invention will be described. In the initial state, the main system switchable port is connected to the shared channel. L connected to the sub system switchable port
When a packet is received from the AN, the communication carrier is detected, and the connection with the shared channel is switched to the sub system switchable port. While the switch circuit is performing the switching operation, the transmission of the packet is delayed by the delay circuit so that the packet is not transmitted to the switch circuit. After switching to the sub system, if the communication carrier at the sub system switchable port has not been detected for a certain period of time, the connection to the shared channel is switched to the main system switchable port. The undetected fixed time may be longer than the connection timeout time of the communication protocol to be used. As a result, it is possible to guarantee that the switching is performed after the logical disconnection of the communication between the sub system switchable port and another physical port.

Further, in order to protect communication between the main system switchable port and another port, the LAN connection device of the present invention comprises:
A lock control unit is provided for receiving an operation for suppressing switching from the main system switchable port to the sub system switchable port from the communication terminal and issuing a suppression request to the switch control unit. The lock control unit can communicate with communication terminals on the LAN connected to the main system switchable port, the sub system switchable port, and other physical ports, and can perform lock control operations from the communication terminals on the LAN. You. At the time of lock control operation from the sub system switchable port, the lock control unit holds the state of the switch circuit at the start of communication so as not to switch to the sub system switchable port due to carrier detection.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a LAN connection device according to the present invention will be described below. For example, it is an apparatus that can automatically perform exclusive connection switching for connection between networks of networks (10BASE-T, 100BASE-TX, etc.) standardized by IEEE802.3.

FIG. 1 shows a configuration of a LAN connection device with an automatic port switching function according to an embodiment. The LAN connection device has a packet relay unit 200 having a function of relaying packets between channels 207a to 207d, and a plurality of physical ports 206a to 206d for connecting LAN cables. Of these physical ports, the main system switchable port 206b and the sub system switchable port 206a share the channel 207a, and the connection is switched exclusively by the switch circuit 204. The switch circuit 204 is controlled by the switch control unit 201. The number of channels and the number of physical ports of the packet relay unit 200 can be expanded to any required number.

Further, the LAN connection device has a carrier detection unit 203 connected between the sub system switchable port 206a and the switch control unit 201 and detecting a communication carrier. The carrier detection unit 203 is connected to the switch control unit 201 by a switching request control line 208c, and when the carrier is detected, the switch control unit 201
To the sub-system switchable port 206. A delay circuit 205 is connected between the carrier detection unit 203 and the switch circuit 204 so that the packet carried by the communication carrier is not transmitted to the switch circuit 204 until the switching operation of the switch circuit 204 is completed. . A packet is a transmission unit of information composed of logical values of 0 and 1, and a communication carrier (carrier) is a physical entity.

Further, a signal line 207f branched from the channel of the main system switchable port 206b, a signal line 207e branched from the channel of the sub system switchable port 206a, and a lock connected to one channel 207d of the packet relay unit 200. Control unit 2
02. Signal line 207e from sub system switchable port 206b
Is branched from between the delay circuit 205a and the delay circuit 205b.

The lock control unit 202 has an address, and can communicate with a communication terminal on the LAN connected to the main system switchable port 206b, the sub system switchable port 206a, and the other physical ports 206c and 206d. When a lock control request is received from those communication terminals, a lock request is issued to the switch control unit 201 connected by the lock control request control line 208b.
Further, the lock control unit 202 issues a state holding request to the switch control unit 201 from the start of communication to the end of communication by using the state holding request control line 208a.

FIG. 3 shows a detailed configuration of the switch control unit 201. The switch control unit 201 has three registers 301a to 301c.
, And holds the input signals of the control lines 208a to 208c, respectively. Unlike the other registers 301a and 301b, the register 301c holds a positive integer value from "0" to "MAX", and
3. In combination with the clock generator 304, it operates as a counter register, and counts down the value held for each clock generated by the clock generator 304.

FIG. 5 shows a correspondence table between input signals from the control lines 208a to 208c and values held in the registers 301a to 301c. The values of the registers 301a to 301c are input to the sequential circuit 302 through inputs 305a to 305c, and the sequential circuit 302 controls the switch circuit 204 in accordance with the current internal state.

FIG. 6 shows a state transition table of the sequential circuit 302. The input from the registers 301a to 301c to the sequential circuit 302 is 3
For 01a and 301b, enter the holding value as is, and for 301c, the holding value is "
Enter "0" when the value is "0", and enter "1" when the held value is other than "0".

The state transition table will be described. The first stage is an initial state where there is no input, the current state Q = "0", the next state Qn.
= “0”, and the switch circuit 204 is the main system switchable port 2
Stay connected with 06b. The second stage is a sub system switchable port 2
In the switching from 06a to the main system switchable port 206b, if there is no input and Q = "1", Qn = "0". The third stage is reversed from the main system switchable port 206b to the sub system switchable port 206a.
Input, 305c = "1" (switch request), Q = "
If it is 0, then Qn = “1.” The fourth row is the continuation of the connection of the sub system switchable port 206a, and the input 305c = “1” (switch request),
If Q = "0", then Qn = "1".

The fifth to eighth stages are state transitions at the time of a lock request. If input 305b = "1" (lock request), input 30
Regardless of the value of 5c and Q, Qn becomes "0", and connection switching / maintenance to the main system switchable port 206b is forcibly performed. The ninth to sixteenth stages are state transitions during which the lock control unit 202 is communicating. Regardless of the values of the inputs 305b and 305c, Qn = Q and the current state is maintained.

FIG. 4 shows a detailed configuration of the lock control unit 202. The lock control unit 202 has an operation processing unit 400 including an operation unit 401, a storage unit 403, and an input / output control unit 402, and includes a channel and signal lines 207d to 207d branched from the channel.
It functions as a communication terminal that can communicate with the outside of the LAN connection device 103 via 7f.

Channel and signal line branched from channel
The packets received from 207d to 207f are
The destination address is checked in 6a to 406c, and if the destination address is addressed to the lock control unit, a status holding request is input from the status holding request unit 405 to the switch control unit 201, and the packet is transmitted via the input / output control unit 402. It is passed to the arithmetic processing unit 400. If the destination address is not for the lock control unit,
The packet is discarded.

The external communication terminal communicates with the communication program stored in the storage unit 403 of the arithmetic processing unit 400, and can request locking and unlocking of port switching. When a request for lock or unlock is given to the processing unit 400, the lock request unit
A lock or lock release request input is given from the switch 404 to the switch control unit 201. When the communication with the external communication terminal is completed, the state holding request unit 405 gives a state holding release input to the switch control unit 201.

The operation of the first embodiment configured as described above will be described. FIG. 9 illustrates a port switching operation, which will be described with reference to a state transition table of the switch circuit 204 (FIG. 6). In the initial state, the main system switchable port 206b
Is connected to the shared channel 207a, and the main system switchable port 206b can communicate with the ports 206c other than the sub system switchable port 206a as shown by 901. Sub system switchable port 20
For 6a, communication cannot be performed from any port as in 902.

Here, when communication is started from the sub system switchable port 206a to the port 206c and other ports as in 903, the switch circuit 204 connects the sub system switchable port 206a to the shared channel 207a. .

FIG. 7 shows an operation procedure when the sub system switchable port 206 receives a packet. In S102, the destination address of the packet received from the sub system switchable port 206a is checked by the address check unit 406e of the lock control unit 202. If the packet is not a packet addressed to the lock control unit 202, the process proceeds to S103, where a carrier is detected by the carrier detection unit 203, and a switch switching request is given to the switch control unit 201 in S104.

In response to the switch switching request, the register 301c
Is reset to “MAX”, and the switching request input is given to the sequential circuit 302. In the initial state, the state holding request input and the lock request are not given. Therefore, according to the state transition table of FIG.
Switch to 06a (S105). The time from when a packet is input until the switching operation of the switch circuit 204 is completed,
The transmission is delayed by the delay circuit 205a so that the packet is not transmitted to the switch circuit 204.

After switching of the switch circuit 204, reference numeral 903 in FIG.
As described above, the sub system switchable port 206a can communicate with the port 206c and other ports. However, as in 905 and 906, the main system switchable port 206b is separated from the shared channel 207a, so that it cannot communicate with any other ports.

When the communication of the sub system switchable port 206a is completed as in 904 and the carrier non-detection period 907 continues for a certain period of time, the register 301c counts down the held value as described above. Becomes "0" and the figure
The switch circuit 204 is switched again to the main system switchable port 206b according to the state transition table of FIG. It is desirable that the fixed time before switching is longer than the connection timeout of the protocol used for communication. Thereby, it is possible to guarantee that the switching is performed after the logical disconnection of the communication between the sub system switchable port 206a and the other port 206c.

Next, the operation of the lock control will be described. The lock control operation is performed by a communication terminal on the LAN connected to one of the physical ports 206a to 206d of the LAN connection device 103 from the arithmetic processing unit 4 of the lock control unit 202.
Communicates with 04 and executes. The communication terminal that performs the operation need not be on the LAN connected to the ports 206a to 206d, but may be any communication terminal on a LAN that can access those LANs.

In the lock control, the LAN of the communication terminal performing the operation is connected to the LA connected to the sub system switchable port 206a.
N or a LAN connected to another port.

Subsequently, the LA of the sub system switchable port 206a is
The operation from the communication terminal on N will be described. The communication terminal starts communication for lock control operation, and the sub system switchable port 20
When the packet is received at 6a, the packet is transferred to the lock control unit 202 through the signal line 207e branched from the channel. In S102 in FIG. 7, the destination address of the packet is checked by the address checking unit 406b of the lock control unit 202a. Since the communication is for lock control operation, the packet is addressed to the lock control unit. As a result, the process proceeds to S106, where the state holding request unit 40
5 immediately sends the state holding request input to the switch control unit 201, and sets the held value of the register 301a to "1". Since then
The switching operation of the switch circuit 204 is controlled according to the state transition table of FIG. 6 (S107).

Next, in step S108, the carrier detector 203 detects the carrier, and resets the value held in the register 301c to "MAX" in response to a switch switching request to the switch controller 201. However, the status hold request input
Since the value of 301a is set to "1", the switching of the switch circuit 204 is not performed according to the state transition table of FIG. 7 (S1).
Ten). Actually, since the processing of the state holding request input and the switching request input are performed in parallel, as shown in the flow diagram, the state holding request input is performed before the switching request input.
The delay circuit 205b is inserted as shown in FIG.

Proceeding to S111, the communication terminal and the lock control unit 202
When a lock request is made by communication of the arithmetic processing unit 400 (S112), a lock request input is given from the lock request unit 404 to the switch control unit 201 (S113), and the register 3
The held value of 01b is set to "1". Then, according to the state transition table of FIG. 6, the switch circuit is connected to the main system switchable port 206b.
204 is switched, and thereafter, the switch circuit 204 is locked and is not switched to the sub system switchable port 206a. And S1
Proceeding to 19, the state holding request input from the state holding request unit 405 is given to the switch control unit 201, the held value of the register 301a is reset to "0" in S120, and the state holding of the switch circuit 204 is released.

The unlocking operation is also performed from the communication terminal.
When a lock release request is made (S116), the lock request unit 404
, The lock request release input is given to the switch control unit 201 (S117). Release value changes the value of register 301b to "
It is set to "0", and switching to the sub system switchable port 206a is possible thereafter according to the state transition table of FIG.

Next, lock control from a communication terminal on the LAN connected to a port other than the sub system switchable port 206a will be described. Even if a packet is received by a port other than the sub system switchable port 206a, the control of the switch circuit 204 by carrier detection is not performed. Therefore, as shown in the flowchart of FIG. 8, the carrier detection S1 is obtained from the flowchart of FIG.
The same applies except that the processing from 03 to S105 is omitted.

When a lock control operation is performed from a communication terminal on the LAN connected to the main system switchable port 206b (or the sub system switchable port 206a), it is assumed that the switch circuit 204 is connected to a shared channel. , Lock control unit 202
Receives packets from both the channel 207d and the signal line 207f (or 207e) branched from the channel. In order to perform communication by only one of them, when the main system switchable port (or the sub system switchable port) is connected to the shared channel 207d, the arithmetic processing unit 400 of the lock control unit 202
Signal line 207f (or 207f) branched from the channel.
The operation of the address check unit 406c (or 406b) connected to e) may be suppressed.

When communication of a lock control operation is performed simultaneously from a plurality of communication terminals, one communication is performed by validating the operation performed last or by giving a priority to the communication terminals or ports. It is preferable to perform exclusive control so that only terminals can perform communication for lock control operation at the same time.

Next, an example in which the LAN connection device of this embodiment is applied to a relay between a customer system and a remote maintenance center will be described. Reference is made to the network connection diagram when the customer system shown in FIG. 2 is applied.

The customer system includes an online LAN 100c that needs to cut off access from outside the system, and an offline LAN 100c that receives access from outside the system.
It consists of two LANs, b. The remote maintenance center L is connected to the wide area network 101 by a LAN-WAN connection device such as a remote router 102, and is connected to a remote location.
Enable remote access from AN100a. Each LAN of the customer system is connected by the LAN connection device 103 of this embodiment, and the online system LAN 100c is connected to the main system switchable port 206b.
However, the remote router 102 is connected to the sub system switchable port 206a,
The off-line LAN 100b is connected to the other port 206c.

Regarding the operation procedure in the above configuration, as a first case, a case where a log file of a customer system is collected from a remote maintenance center will be described. FIG. 10 is a follow chart for downloading a file from an online LAN. First, a local maintenance worker performs a lock operation from the offline LAN 100b (S302), and downloads a file from the online LAN 100c (S303).
During this time, even if you try to access from outside the customer system,
Communication is not possible because the port to which the remote router 102 is connected is disconnected in the LAN connection device. When the file transfer is completed, the lock is released (S304).

When the maintenance staff of the remote maintenance center receives the notification from the local maintenance staff and performs remote access to the customer system via the wide area network 101 (S305), the port is switched because the lock has been released (S30).
6), communication with the off-line LAN 100b becomes possible.
During this time, contrary to the above, since the online LAN 100c is disconnected, the remote maintenance center 100a and the online
LAN 100c cannot communicate. The log file is collected from the remote maintenance center (S307). When the work is completed, the communication is terminated (S308), and the port is switched to the port to which the online LAN is connected again (S309).

As a second case, a case where a program file is uploaded from the remote maintenance center to the customer system will be described with reference to the follow chart in FIG.
When the maintenance staff at the remote maintenance center remotely accesses the customer system (S402), the port is switched to the port to which the remote router 202 is connected (S403), and communication with the offline LAN 100b becomes possible. During this time, the remote maintenance center cannot directly access the off-line LAN 100c.

During the transfer of the program file (S40)
5) When the maintenance staff must temporarily leave the communication terminal for some reason, there is a possibility that the communication terminal will be used by a third party and illegally accessed. In such a case, a lock operation is performed from the remote maintenance center side (S405), and the operation is interrupted. Further, if a function that requires user authentication using a password for the lock / unlock operation is provided, even if the communication terminal can be used by a third party, unauthorized access to the customer system can be prevented. .

The maintenance worker resumes the work by releasing the lock (S408) (S409). When the work is completed, the communication is terminated (S410) and the port is switched to the port to which the remote router 202 is connected (S411). The local maintenance staff receives the notification from the remote maintenance center maintenance staff, performs a lock operation (S412), and uploads the program file from the offline LAN 100b to the online LAN 100c (S413).

As described above, according to this application example, downloading a file from the online LAN of the customer system to the remote maintenance center or uploading a file from the remote maintenance center to the online LAN of the customer system is performed by the customer. This can be done automatically while ensuring system security.

[0052]

According to the present invention, since the port is automatically switched according to the state of the communication carrier of the sub system switchable port, communication is started from a communication terminal on the LAN connected to the sub system switchable port. Accordingly, the port can be switched to the sub system switchable port, and the port usable as the main system switchable port can be switched by stopping communication. Therefore, there is no need to insert or remove a cable or turn on / off the power of the device, and it is not necessary to manually set a push button switch, a slide switch, or the like, and thus complete automation can be realized.

Further, by the lock control for switching from the main system switchable port to the sub system switchable port, port switching occurs while the main system switchable port is communicating with another port, and communication is performed. Disconnection can be prevented, and automatic switching of secure ports can be performed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a LAN connection device according to an embodiment of the present invention.

FIG. 2 is a network connection diagram using a LAN connection device with a port switching function.

FIG. 3 is a detailed configuration diagram of a switch control unit.

FIG. 4 is a detailed configuration diagram of a lock control unit.

FIG. 5 is an explanatory diagram of a control line input and a held value of a register.

FIG. 6 is an explanatory diagram showing a state transition table of a sequential circuit of the switch control unit.

FIG. 7 is a flowchart showing the operation of the sub system switchable port when receiving a packet.

FIG. 8 is a flowchart showing the operation of another port when receiving a packet.

FIG. 9 is a state transition diagram of a switch circuit.

FIG. 10 is a flowchart showing an operation procedure of an application example.

FIG. 11 is a flowchart showing an operation procedure of an application example.

[Explanation of symbols]

100a: Remote maintenance center LAN, 100b: Offline LAN, 10
0c… Online LAN, 101… Wide area network, 102…
Remote router, 103 LAN connection device, 200 Packet relay unit, 201 Switch control unit, 202 Lock control unit, 203
... Carrier detector, 204 ... Switch circuit, 205a-205b ...
Delay circuit, 206a: sub system switchable port, 206b: main system switchable port, 206c to 206d: physical port, 207a: shared channel, 207b to 207d: channel, 207e to 207f: signal line branched from the channel, 208a: State hold request control line, 208b ...
Lock request control line, 208c switching request control line, 208d switch circuit control line, 301a to 301c register, 302 sequential circuit, 303 subtractor, 304 clock generator, 305a
State holding request input, 305b lock request input, 305c switching request input, 400 arithmetic processing unit, 401 arithmetic unit, 402
... I / O control unit, 403 ... Storage unit, 404 ... Lock request unit, 40
5: state holding request unit, 406a to 406c: address check unit.

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Yasuyuki Bito 5-2-1 Omika-cho, Hitachi City, Ibaraki Prefecture F-term (reference) 5K033 AA03 CC01 DA05 DB03 DB19 EB06 5K034 AA16 BB06 DD03 FF06 TT02

Claims (6)

[Claims] 1. A LAN connection device having a plurality of physical ports for connecting a local area network and relaying packets between the networks, a packet relay unit for relaying packets between channels connected to the physical ports. A main system switchable port and a sub system switchable port that share a predetermined channel and are exclusively connected to the shared channel by a switch circuit; and a carrier detection unit that detects a communication carrier from the sub system switchable port. When detecting the carrier of the carrier detection unit, controls the switch circuit so that the sub-system switchable port is connected to the shared channel, when there is no switch switching request from the carrier detection unit for a certain time, A switch for controlling the switch circuit to switch to the main system switchable port LAN connection device and providing a control unit. 2. A LAN connection device having a plurality of physical ports for connecting a local area network and relaying packets between the networks, a packet relay unit for relaying a packet between channels connected to the physical ports. A main system switchable port and a sub system switchable port that share a predetermined channel and are exclusively connected to the shared channel by a switch circuit; and a carrier detection unit that detects a communication carrier from the sub system switchable port. A switch control unit that controls the switch circuit so that the sub system switchable port is connected to the shared channel when a carrier is detected by the carrier detection unit; and the connection of the switch circuit is the main system switchable. A lock request to prevent switching from a port to the sub system switchable port. LAN connection device, characterized in that a lock control unit that performs the control unit. 3. The communication device according to claim 2, wherein, when the lock control unit is communicating with a communication terminal connected to a LAN, the switch control unit keeps a state of the switch circuit, and the lock control unit operates the communication terminal. Wherein the switch control unit executes a lock request from the lock control unit when receiving a communication requesting a lock operation from the LAN connection device. 4. The communication terminal according to claim 2, wherein the communication terminal on the LAN connected to the physical port can transmit and receive to and from the lock control unit regardless of a state of the switch circuit. LAN connection device. 5. The delay circuit according to claim 1, wherein a delay circuit for delaying transmission of a communication carrier from the sub-system switchable port is provided between the sub-system switchable port and the switch circuit. 2. The LAN connection device according to claim 1, wherein a delay time of the delay circuit is longer than a time from when the carrier detection unit detects a communication carrier to when the switching circuit completes the switching operation. 6. The remote maintenance center LAN according to claim 1, wherein:
An online LAN that is not desired to be accessed from the remote maintenance center is connected to the main system switchable port, and an online LAN of the customer system is disconnected from the shared channel during maintenance from the remote maintenance center. A LAN connecting device, wherein a LAN is connected to the shared channel.