Abstract: In an embodiment, a network service provider (NSP) operates a provider network to provide VPN services to its customers. A VPN links various customer sites allowing customers to send data between these sites over the NSP network. Each site network includes a customer edge router (CE) while the provider network includes a plurality of provider edge routers (PEs) to communicate with the CEs. The PEs include virtual routing address (VRFs), and the PEs and CEs include interfaces (IFs). A database stores information related to the relationships between the network components (e.g., VPNs, PEs, CEs, VRFs, IFs, etc.), and a management software package (MSP) has access to the database. When a fault occurs, the MSP, based on collected information and information in the database, determines the impacted network components. Other features include classifying the seriousness of the network's faults and representing different faults by a color scheme.

Abstract: A packet transmission diagnostic system transmits diagnostic packets during a communications session in which data packets are transmitted between a source and destination. The diagnostic packets elicit responses from routers along a path between the source and destination. The elicited responses, in the form of packets, are used by the diagnostic system to produce one or more packet-delivery metrics for one or more of the routers along a packet transmission path between the source and destination.

Abstract: A multicast transmission scheme is introduced. A multicast data stream is sent on a default tunnel and switched to a data tunnel during flooding conditions. The default tunnel is a point-to-multipoint tunnel from, for example, a source provider edge router to all other provider edge routers in a virtual private network. The data tunnel is a point-to-multipoint tunnel from, for example, a source provider edge router to other provider edge routers that have recipients. Flooding conditions can occur when sending the multicast data stream on the default tunnel, for example, when a specified bandwidth threshold has been exceeded on a source provider edge router. Additionally, the data tunnel is created by selecting a multicast distribution tunnel group from a configured pool of addresses, advertising the group, receiving data joins from, for example, provider edge routers with recipients, and waiting a specified send time.

Abstract: A bidirectional repeater and data multiplexer for serial data has A-side 12C port devices A1-A4 coupled to comparators 302-308 and pull-downs to ground 316-322. Comparator outputs are coupled responsive to select lines S1-S4 of N:1 Select 310 to terminal A1 of bidirectional control 210 to control pull-down to non-zero low voltage Vp 206 at B-side device B. An inverting comparator 208 coupled to terminal B1 of bidirectional control 210 responds to input threshold voltage Vt less than low voltage Vp, to prevent data lockup due to data flowback to devices A1-A4. Output data from comparator 208 is coupled responsive to select lines S1-S4 of 1:N Select 312 to control pull-downs 316-322. This selectively repeats routing of device A1-A4 data to device B. Data from device B is selectively routed to pull-downs of devices A1-A4.

Abstract: A monitor device includes a packet communication device, a report data transmitting device, a communication status determination device, and a transmission error history storage device. The report data transmitting device transmits report data for reporting an error of a broadcast radio wave retransmission system to an external monitoring center through the packet communication device. The communication status determination device determines whether or not transmission of the report data is successfully performed by the report data transmitting device. The transmission error history storage device stores an error in the report data transmission, when the communication status determination device determines that an error occurs in transmission of the report data, as a transmission error history in a storage device.

Abstract: A system and method using differential loop gain for fault analysis in line monitoring equipment. Differential loop gain data is calculated from loop gain data, and fault analysis is conducted using differential loop gain data, e.g. by comparing the differential loop gain data to predefined fault signatures.

Abstract: A wireless repeater configured for implementation in a wireless communication system is disclosed. The repeater can be configured to receive and rebroadcast communications using various types of airlinks in a wireless communication system. The wireless repeater can include a receiver configured to sense the type of airlink being supported by the repeater. The repeater can then selectively apply a repeater identifying watermark to a signal based on the type of airlink. The repeater may be configured to periodically scan an operating bandwidth for supported airlinks and can configure the watermarking applied to signals within the channels based on the detected airlinks.

Abstract: Data burst communication techniques for mobile communication devices operating in packet data sessions are disclosed. A mobile communication device is adapted to receive and decode packet data of a plurality of packet data bursts of a data block over a packet data channel during a packet data session. The mobile communication device is further adapted to receive and decode broadcast control data of a plurality of broadcast data bursts of the same or adjacent data block over a broadcast control channel contemporaneously with the packet data, where each broadcast data burst is interleaved with packet data bursts of the packet data session. When a time conflict between at least one of the packet data bursts and at least one of the broadcast data bursts exists, the mobile communication device is adapted to prioritize the receiving and decoding of the at least one packet data burst over the at least one broadcast data burst.

Abstract: A system, method and computer readable medium are provided for monitoring functionality of multiple network interfaces and providing fault recovery. The network interfaces are grouped into nodes and a monitoring address is assigned to each of the network interfaces such that the monitoring address for each of the network interfaces is on a different subnet than the monitoring addresses for all of the other network interfaces in the same node as that network interface. According to the method, each of the network interfaces is assigned one or more recovery addresses that each are different than its monitoring address. A monitoring message is periodically sent to each of the network interfaces via the monitoring addresses in order to determine the functionality of the network interfaces. If no monitoring message is received within a predetermined period, a recovery operation is performed for one of the network interfaces using one or more recovery addresses of the one network interface.

Abstract: A method of transmitting subpackets in a mobile communication system using at least one frequency carrier is disclosed. More specifically, the method comprises receiving a first broadcast subpacket from a base station (BS) at a first time slot on a first frequency carrier, and receiving at least one subsequent broadcast subpacket from the BS via at least one relay station (RS) at a second time slot on a second frequency carrier, wherein information of the first broadcast subpacket and the subsequent broadcast subpacket are the same.

Abstract: A method and apparatus for bus repeater tapering. The sizing of repeaters on a portion of one transmission line is chosen to propagate a signal transition at a particular rate. The sizing of repeaters on a substantially parallel portion of another transmission line, is chosen to propagate a second signal transition at a different rate. Thus, the worst-case capacitance coupling factor between the two transmission lines may be reduced.

Abstract: In embodiments of the present invention improved capabilities are described for sensing a change in a network topology using a discovery node in a mesh network; transmitting an explore frame, based at least in part on the sensed change, from the discovery node to a destination node using an intermediate node; receiving a response to the explore frame from the destination node; determining at least one functional network route from the discovery node to the destination node based at least in part on route information received in association with the response to the explore frame, wherein the route information includes a node path; and transmitting an instruction to the destination node from the discovery node based on a reverse node path.

Abstract: Redundant acknowledgment between agents performing a loopback test over bidirectional communications bus is described. In one example the acknowledgment is performed by initiating loopback communications from a first agent to a second agent, sending a packet including a redundant acknowledgment sequence from the first agent to the second agent, receiving the packet including the redundant acknowledgement sequence looped back from the second agent at the first agent, sending a test sequence from the first agent to the second agent, and receiving the test sequence looped back from the first agent.

Abstract: The present invention features embodiments of alleviating the impact to a system of stack switches, as well as to neighboring nodes communicating with such a system, when a primary master switch to secondary master switch failover occurs. The features of the present invention, generally enables a system of stack switches to retain, for a fixed or indefinite period of time, its stack address even when multiple primary master to secondary master failovers occur. This way recalculation of certain protocols—e.g., spanning trees and link aggregations—and updating of certain tables—e.g., address resolution protocol (ARP) and routing tables—are minimized.

Abstract: The invention relates to wireless repeater systems and methods. In embodiments, such systems and methods involve receiving a wireless transmission signal; and processing the wireless transmission signal using a digital signal processing facility (DSP); wherein the DSP is adapted to filter at least one sub-band of the wireless transmission signal using a digital bandpass filter.

Abstract: A transmission repeater has a function of recovery to normal out of the deadlock state of auto-negotiation sequence between switches. The switches exchange auto-negotiation configuration codes via first and second transmission repeaters for a state transition. The first transmission repeater monitors auto-negotiation configuration codes among data transmitted from the opposed switch, and when detecting more than a predetermined number of configuration codes of auto-negotiation having been transmitted during a given period of time, determines that the auto-negotiation sequence between the switches is at a deadlock, and reports it to the second transmission repeater, which then disconnects optical signals from the sending end of the second transmission repeater to the switch for more than twice the propagation time of signals between the switches, thus resolving the deadlock between the switches.

Abstract: A modulation device comprises for each carrier frequency: means of discrimination between a signal coming from a first channel of a communications link and a signal coming from a second channel of said link, so as to obtain differentiated first and second signals; first assigning means, enabling the assigning of the first differentiated signal to one of the two modulation arms, and the second differentiated signal to the other of the two modulation arms.

Abstract: A method of communicating in a mobile ad-hoc network is disclosed. It is determined whether a first directional transmission path between a transmitting node and a destination node is known to be free from obstruction. The transmitting node transmits a message to the destination node using the first directional transmission path. The message is retransmitted to the destination node using the first directional transmission path when a message receipt acknowledgement in an omnidirectional network transmission has not been received within a predetermined time. Additional transmissions via the first directional transmission path are blocked when the message receipt acknowledgement has not been received after more than a pre-set number of retransmissions. The message is transmitted to a proxy node using a second directional transmission path between the transmitting node and the proxy node, with instructions for the proxy node to directionally transmit the message to the destination node.

Abstract: Method, network apparatus and computer readable medium thereof for detecting the defect of the network are provided. The network apparatus comprises a controlling module and a relaying module. The controlling module comprises a generating module for generating a plurality of sub-detecting packets; and a first combining module for combining the plurality of sub-detecting module into a detecting packet; the relaying module is for receiving the detecting packet from the controlling module, the relaying module comprises a decomposing module for decomposing the detecting packet into the plurality of sub-detecting packets and transfer the sub-detecting packets to a target host; and a second combining module for receiving a plurality sub-result packets from the target host which are corresponding to the sub-detecting packets, combining the sub-result packets into a result packet and transferring the result packet to the controlling module to complete the detection.

Abstract: A sequence identification number is added to a header in each Generic Framing Procedure (GFP) frame. The byte sequence identification number identifies frames, preventing duplication or loss of exchanges, sequences, and frames. When this feature is enabled, a remote wavelength division multiplexing (WDM) node automatically checks the header to ensure that no duplication or loss occurs. The remote WDM node does not need to buffer data, instead the remote WDM node only needs to keep track of the sequence identification number information. A session identification number is also added to the header information, which allows the WDM node to keep track of which session was under way in the event that all connections between nodes across the WDM network are lost.

Abstract: Where each of switches opposed to each other detects optical-signal shutoff or optical-signal deterioration on the reception side of its port, the switch brings a link of the port down and outputs an auto-negotiation configuration code, as an LAN signal. Upon receiving the LAN signal transmitted from the switch, a repeating-transmission apparatus transmits the transmitted signal to an auto-negotiation monitor. The auto-negotiation monitor performs code analysis for the transmitted LAN signal. Where the auto-negotiation monitor detects that the transmitted LAN signal is the auto-negotiation configuration code, it notifies a monitor apparatus that “The switch detects a failure and the link is down.”, as an alarm.

Abstract: Specific bits of an incoming transmission are compared against a predetermined bit pattern. If the selected bits do not match the predetermined bit pattern, then the incoming transmission is rejected as a false packet. The predetermined bit pattern can include legal values for predetermined bits in a plurality of fields. Notably, these legal values are set by a networking standard. A parity check may check may be performed in addition to checking for predetermined bits in other fields. A user interface can be used to determine the predetermined bit pattern.

Abstract: A wireless local area network for data communications between a computer and mobile units is provided with at least two cell controllers connected to the computer and one or more rf ports each connected to a primary interface of one cell controller and to a backup interface of another cell controller. A control program in the cell controller monitors data communications between the other cell controller and the rf port by a backup interface. The program is arranged to cause the second cell controller to assume primary communications functions for the rf port in the event primary communications functions between a first cell controller and the rf port are inoperative.

Abstract: A data processing equipment having an input to receive an input stream of data frames. The data frames contain a repeating trace message, each instance of the trace message being divided in fragments distributed over a series of data frames. The data processing equipment further includes a trace message processor receiving the repeating trace message in the input stream. The trace message processor performs a first action when the input stream carries less than N consecutive anomalous instances of the trace message and a second action different from the first action when the input stream carries N or more consecutive anomalous instances of the trace message.

Abstract: A method for identifying a remote radio unit in a communication system comprises a plurality of remote radio units communicating with a central unit and pattern providing means for providing information about dedicated gain change patterns of the remote radio units. The method comprises receiving a signal from a station by a remote radio unit of the plurality of remote radio units. The method also includes changing the gain of at least one remote radio unit according to a dedicated gain change pattern of the at least one remote radio unit. The method also includes detecting a gain change pattern for the received signal and identifying the remote radio unit by comparing the detected gain change pattern with information about dedicated gain change patterns of the remote radio units provided by the pattern providing means. An example may be locating a station, such as a user equipment, in a distributed radio system.

Abstract: The communication system includes two communication channels for communication of protection and control information between two protective relays on a power line portion of an electric power system. A switch provided at the receiving relay connects one communication channel and provides a connecting link between the communication channel and the receiving relay. The transmitted data is applied on both communication channels and processed substantially identically. When the one communication channel is indicated to be faulty, the switch in the receiving relay is operated to connect the second communication channel, thereby substantially preventing delay in the receipt of the data from the transmitting relay.

Abstract: A remote test unit (RTU) and method of operation are provided for utilizes the ability of an access matrix ability to route signals. The RTU can emulate a central Digital Subscriber Line Modem (DSLM-C) for testing customer premises equipment containing a remote Digital Subscriber Line Modem (DSLM-R). The RTU can also emulate a DSLM-R for testing central offices equipment including a digital subscriber line access multiplexer (DSLAM) containing a DSLM-C. The RTU can also emulate a concentrator connected to the DSLAM, a router connected to the concentrator, an Internet service provider (ISP) connected to the router, and a web site connected to the ISP over the Internet. The RTU can further test, using emulation, ISO/OSI layers defined in the ISO/OSI reference model which are connected to the DSLAM.

Abstract: A multiplexing apparatus selectively performs cell discard processing in the case of congestion on the basis of a use state of the same connection formed by cells from the side of a switching unit and subscribers without installing UPC units, and the multiplexing apparatus, which is connected to the switching unit and each of the plural subscribers through communication lines.

Abstract: A repeater for HDSL transmission is presented. The repeater replaces the regenerator commonly used in HDSL by utilizing the activation/deactivation process in the HDSL specification. The repeater is adjusted to detect an activation/deactivation sequence, whereby a flip-flop in the repeater is alternated. A first state of the flip-flop allows transmission passing through the repeater to the terminating point, e.g. a network terminal, and a second state loops transmission back to the originating point, e.g. a line terminal. The looping may then be utilized for maintenance and error detection and recovery. By use of one of the free bit in the overhead channel in the HDSL transmission as an origin bit, wherein “1” is set in the upstream direction, and “0” is set in the downstream direction, it is possible to detect at the line terminal whether there is a loop in the repeater.

Abstract: An asynchronous transfer mode network, in accordance with the present invention, includes a plurality of remote terminals remotely disposed relative to a central office. The remote terminals are connected by a ring with two paths. The ring transfers signals thereon in opposite directions relative to each path. The central office feeds duplicate signals on each path. Each remote terminal includes a first multiplexer for routing signals transferred on the ring to and from an asynchronous feeder multiplexer. The asynchronous feeder multiplexer is adapted to route components of downstream signals on a first path of the ring to a destination and to replace the components of the signals on the first path with copies of components of signals running in an opposite direction on a second path of the ring such that at any location in the network both paths provide all signals.

Abstract: A transmission apparatus includes a reception line selecting unit, a communication confirming frame transmitting unit, a communication confirming table, and a reception line deciding unit. The reception line selecting unit selects one of several reception lines each formed by the synchronous network. The frame transmitting unit transmits a communication confirming frame to the synchronous network at least at a predetermined period. The communication confirming table stores each reception time of the communication confirming frame received through the synchronous network. The deciding unit decides occurrence of failure on the reception line corresponding to the reception time when a difference between a reception time stored in the table and a present time at the predetermined period exceeds a predetermined value, and transmits instructions for switching the reception line to the reception line selecting unit.

Abstract: A computer network employs a fault-tolerant or redundant switch architecture. The network includes redundant data paths coupling end nodes and switches. Fault-tolerant repeaters (FTRs) can be stand-alone devices or can be incorporated into the switches. Using error detection, the FTR checks to see if the data is good on all paths. If the data received on one path is “bad” and the data is “good” on another path, the FTR transmits the “good” data in place of the “bad” data. For any switch, a pair of incoming ports may be configured as redundant incoming ports and a pair of outgoing ports may be configured as redundant outgoing ports.

Abstract: Systems and methods are described for fault detection for subscriber loop repeaters. A method includes monitoring an upstream signal on a digital subscriber loop for a fault detection signal that is added to said upstream signal at a repeater; and determining if a fault has occurred. An apparatus includes a digital subscriber loop fault detection system including: a receiver circuit that monitors an upstream signal on said digital subscriber loop for a fault detection signal that is added to said upstream signal at a repeater; and a logic circuit that determines whether a fault has occurred.

Abstract: Disclosed is a WDM network which has: a lightwave path which connects between clients and each of which is provided with an overhead, and a sub-network which is defined by dividing the WDM network. In this WDM network, the sub-network has a partial lightwave path to go through the sub-network, the overhead has a partial lightwave path supervisory control information region which is terminated at both nodes of the partial lightwave path, and when a fault occurs on a lightwave path, the fault information of partial lightwave path including the position information of fault occurred is added to the partial lightwave path supervisory control information region of the overhead.

Abstract: Method and Apparatus for Network Problem Segment Isolation. The present invention provides a method for characterizing network segment quality for a segment transmitting messages from a first endpoint to a second endpoint on a network. The method includes the step of receiving messages transmitted between the first and second endpoints. The messages include a first set of quality metrics for message transmission from the first to second endpoint for both the first and second segments and a second set of quality metrics for message transmission from the first endpoint to the juncture. Furthermore, the method includes subtracting the second set of quality metrics from the corresponding quality metrics of the first set of quality metrics to calculate a third set of quality metrics representing the quality metrics of the second segment. Additionally, the method includes comparing the third set of quality metrics to a first set of segment qualities.

Abstract: A clock selection controller is shown which allows a user to assign a priority scheme to recovered clock signals obtained from communication facilities. The clock selection controller also receives a plurality of alarm signals corresponding to the communications facilities for each of the recovered clock signals. The clock selection controller then automatically selects, for output as a reference clock signal, the highest priority clock input available for which the corresponding alarm signals are inactive. In the absence of an available clock input in the priority scheme, the processor selects a free running clock input.

Abstract: A method and apparatus for the restart communication between network elements are described. In one embodiment, a method for restarting communication between a first network element and a second network element in a network comprises transmitting a modified hello packet to the second network element. The modified hello packet includes a time value that is greater than a time value of a hello packet that was transmitted to the second network element prior to International-System-International-System (IS-IS) network communications in the first network element becoming non-operational. The method also including updating routing information within the first network element through transmission of Complete Sequence Number Packets of the IS-IS standard to the second network element, independent of tearing down a communication link between the first network element and the second network element after IS-IS network communications in the first network element have become non-operational.

Abstract: In the case where two wireless communication systems are located close to each other and possibly interfere with each other, it is detected if broad band interference takes place, and, when broad band interference takes place, the use of frequencies is limited within such a frequency range in which the wireless communication systems are little affected by the interference. An interference detection system for detecting interference between one system and another system in a wireless communication making use of frequency division multiplexing is described.

Abstract: A method and apparatus for ensuring wireless transmission among respective communicating apparatuses when constructing a local area network with a plurality of wireless communicating apparatuses. Under the control of a control station (10), a wireless signal of a frame structure is transmitted between communicating stations (20) and (30). The wireless signal of a frame structure has a managing region and a data transmitting region. In the managing region, a test signal for checking quality of communication among a plurality of communicating stations and communication quality information representative of quality of communication among the plurality of communicating stations are transmitted. In the data transmitting region, isochronous data or asynchronous data is transmitted under the access control of the control station.

Abstract: An electronic filter has an input connector adapted for electrical connection to a complimentary output connector of an asymmetrical digital subscriber line. The input connector has an input voltage node for providing electrical input signals to the electronic filter. Similarly, the filter has an output connector adapted for electrical connection to a complimentary input connector of a data receiving device. The output connector has an output voltage node for providing electrical output signals from the electronic filter. The filter selectively passes frequency components of the electrical signals, frequency components below a predetermined corner frequency passing from the input connector to the output connector, frequency components above the predetermined corner frequency not passing from the input connector to the output connector.

Abstract: In a repeating unit testing system, a first tester includes first packet transmission means for transmitting a plurality of packets, whose transmission destination address is a second repeating unit, to a first repeating unit for a fixed period of time, and line connection-disconnection log information collection means for collecting log information regarding connection and disconnection of a line from the first or second repeating unit. The first repeating unit includes line connection-disconnection means for connecting a line to a second communication network or disconnecting the line if transmission of packets is not received for more than a fixed period of time, and second packet transmission means for receiving the packets and transmitting the packets to a third port.

Abstract: A DSL device performs a frame check sequence operation in a digital signal processor (DSP), i.e., layer one of the OSI seven layer model, heretofore performed only in layer two of the OSI seven layer model. Performing the layer two framing and calculating the frame check sequence in the DSP allows the DSP to use the layer two error detection results, and optionally, the mean squared error vector, to more accurately determine whether to update adaptive receiver parameters.

Abstract: A radio communication network comprising a plurality of communication terminals and a plurality of repeaters intermediate the communication terminals, with wayside communication paths being established between pairs of the repeaters via two radio channels carrying the same data. Protection switching of a wayside path is determined, in a preferred embodiment, by using one common wayside path to calculate the FEC (Forward Error Correction) and PEC (Parity Error Count). Switching to the better of said two channels is effected if FEC or PEC exceed predetermined thresholds. The common wayside path is defined as follows: (a) &Sgr; wayside A path+&Sgr; wayside B path −&Sgr; (wayside A ||wayside B), if waysides A and B are overlapped (||indicates overlapped portions of the two parameters.) (b) wayside A path OR wayside B path, if waysides A and B are not overlapped.

Abstract: In a connection-oriented network, a route table and a network resource table are provided in a source node which updates the network resource table in response to a topology state message which is flooded through the network whenever there is a change in the available network resources. The source node updates the network resource table in response to an early-arriving trouble report message from the network if such a change in the available network resources occurs when a connection is being progressively established or already established from the source node. In response to a connection setup message, the source node fetches a route from the route table and compares a user's requirement such as quality and/or bandwidth with the resource data of the fetched route stored in the network resource table. A connection is established via the fetched route if the resource data of the fetched route in the network resource table indicates that the fetched route can support the requirement.

Abstract: A high-speed sprchronous multiplexing apparatus has a low-speed device and a high-speed device and employs electrical signals to interface the devices with each other. The low-speed device is connected to a low-speed digital circuit. The high-speed device is connected to a high-speed synchronous multiplexing circuit. The apparatus also has a clock supplier for supplying a reference clock signal that defines a communication rate, to each of the low- and high-speed devices. The apparatus further has an alarm processor for separating faults in the low- and high-speed devices. The low- and high-speed devices are interfaced with each other with the use of the reference clock signal and electrical STM-0/STS-1 signals that employ a frame signal synchronized with the reference clock signal.

Abstract: This invention relates to the fast reconfiguration of tree switches. In particular, on-chip parallel control signals are time staggered so that they track the propagation of data or information bits. As a result, it is unnecessary to stop the flow of data bits from the data inputs to the data outputs during the reconfiguration of the tree switch.

Abstract: A system tests a repeating installation having transmitter-receivers (3, 4) and a line controller (2). The system includes the repeating installation to be tested, a first tester (10), a second tester (11), an input unit (12) for entering a first virtual network into the first tester (10), an input unit (13) for entering a second virtual network into the second tester (11), and a line control table which is provided for the transmitter-receiver (3) and into which the destination transmitter-receiver (4) is written according to address information added to the data input to the transmitter-receiver (4). Test data transmitted from the first tester to the virtual network of the second tester is received by the second tester, and the received test data is examined. This greatly reduces facility costs and installation space and realizes performance evaluation of a repeating installation under a large-scale network environment.

Abstract: A telecommunications switching system employing multi-protocol routing optimization which utilizes predetermined and measured parameters in accordance with a set of user priorities in determining the selection of a telecommunications path to be utilized for transmitting a data file to a remote destination. The switching system has a first memory for storing the data file to be transferred, a second memory for storing predetermined parameters such as cost data associated with each of the telecommunications paths, a third memory for storing a set of user priorities regarding the transmission of data files, and means for measuring the value of variable parameters such as file transfer speed associated with each of the telecommunications paths.

Abstract: Disclosed is an optical transmission system for transmitting data via an optical transmission line in accordance with a frame format having overhead which includes a K1 byte used for a switching request and a K2 byte used to respond to the K1 byte.

Abstract: In an optical transmission system including a plurality of stations, each of the stations includes a monitor/control circuit and a light signal output circuit. When the monitor/control circuit detects no arrival of a first light signal from the upstream-side station due to cutting of an optical cable or the like, a second light signal multiplexed with an alarm light signal of a predetermined wavelength is sent to the upstream-side station. The upstream-side station separates in wavelength the received multiplexed light signal so as to extract the alarm light signal therefrom and interrupts an optical output of the light signal output circuit toward the downstream-side station, i.e. a sender of the alarm light signal.