Abstract: A transmission unit of this application includes a transmission unit, operating in one of modes including a normal communication mode and a connection confirmation mode, wherein the transmission unit transmits, in the normal communication mode, an OSC (Optical Supervisory Channel) signal with time average power beyond predetermined upper limit power; and wherein the transmission unit transmits, in the connection confirmation mode, the OSC signal with time average power lower than the upper limit power and with a transmission rate lower than a transmission rate in the normal communication mode.

Abstract: A flexible open ring optical network includes a plurality of nodes connected by twin or other suitable optical rings. Each node is operable to passively add and passively drop traffic from the rings. The nodes may include a transport element for each ring. The transport elements include an optical splitter element and an optical combiner element. The optical splitter element is operable to passively combine an add signal including local add traffic and a first transport signal including ingress traffic from a coupled optical ring to generate a second transport signal including egress traffic for transmission on the coupled optical ring. The optical combiner element is coupled to the optical splitter element and is operable to passively split a third transport signal including the ingress traffic to generate a drop signal including local drop traffic and a fourth transport signal including the ingress traffic.

Abstract: A method and apparatus for controlling the power level of an optical signal includes detecting the loss of a supervisory signal counter-propagating in an optical fiber.

Abstract: Systems and methods for monitoring a data transmission link, especially an optical, bidirectional data transmission link, in which a digital transmit signal is transmitted on a first transmission path from a local end of the data transmission link toward a remote end of the data transmission link. A portion of the power of the transmit signal sent at the local end is transmitted, delayed by a non-zero delay time on a second transmission path as a control signal toward the remote end of the data transmission link. Both signals are received at the remote end and are tested for the presence of events of a predetermined type. A conclusion can be reached on the quality of the transmission link as a function of a time correlation and frequency of the appearance of events in the received transmit signal and in the received control signal.

Abstract: Apparatus and method to measure the quality of burst signals and to perform optical line diagnostics in and optical passive optical network (PON). Statistical information about phase noise (jitter), signal distortion, clock distortions, and any other effects present in burst signals is generated. The statistics are based on phase and bit-length distortions, direction and length of the effect as detected by a phase error detector integrated in a burst mode clock and data recovery (BCDR) circuit. The invention can be further adapted to perform optical line diagnostics to detect the root cause performance degradation and failures in the PON, thereby providing an optical layer supervision tool for monitoring the PON. The statistical information can be used to estimate the quality of service (QoS) per customer connected to the PON. In addition, the generated statistic information can be used to calibrate transmission parameters of optical network unit (ONU) transmitters.

Abstract: The present invention provides a mechanism and a method for indirectly controlling a packet handling device interface from an optical management system in an packet-optical network. A mechanism is provided for controlling a packet handling device, such as a router, interface from a management system indirectly, by using optical equipment as a proxy and communicating between the optical gear and router via a peer-to-peer signaling protocol. The present invention provides a management method that allows separate management systems for the optical layer and the packet network layer and a method for managing the network across the domains.

Abstract: The invention relates to a method for detecting a check-back signal in a transmission system for optical signals. According to said method, a constant proportion of the output in a defined frequency range of the check-back signal is concentrated in a narrow-band spectral range and is determined after a transmission phase by means of a narrow-band detection of the concentrated energy around the spectral range. If no signal is identified during the narrow-band detection, a line interruption is determined and no pump source is switched on for safety reasons. The narrow-band detection of the check-back signal also allows the transmission attenuation of the transmission system to be measured.

Abstract: A method for localizing an optical network termination in an optical access network including an optical line termination and a number of optical links.

Abstract: A link connectivity verification message to recognize a physical link connection state is transmitted from the WDM to the PXC on a C-plane. The PXC transmits a link connectivity verification ACK message including information representing a physical link connection state, and the WDM transmits a cross-connect instruction message to cross-connect a transmission side and a reception side of a port (port 1) of the PXC. Probe light transmitted from the port (port a) of the WDM is turned back by a cross-connect of the port (port 1) of the PXC. The WDM receives the probe light to verify a link connectivity.

Abstract: Methods and apparatuses are provided for performing jitter measurements in a transceiver module. Accordingly, there is no need to use expensive test equipment that must be inserted into and removed from the network in order to obtain these measurements. In addition, because the measurements can be obtained at any time without any interruption in communications over the network, jitter performance can be monitored more closely and more frequently to facilitate better and earlier diagnosis of problems that can lead to failures in the network.

Abstract: A chromatic dispersion compensation system for an optical transmission system incorporates circuitry which determines the length of an optical fiber extending between an output amplifier and an input amplifier. Based on fiber type, the total chromatic dispersion on the fiber is determined. Compensation can then be automatically implemented.

Abstract: The present invention relates to an optical cross-connect apparatus supporting plural directions, and provides a method of effectively performing an alarm indication by a node supervisory unit of the optical cross-connect apparatus. The node supervisory unit includes an alarm indication information storage unit that manages a management table for each of the plural directions. The management table manages not only received alarm indication information but also “internal WSS input/output port optical fiber connection state”, “WSS optical path setting state on DROP/THROUGH selection side”, “transponder destination location”, and “destination direction”, with respect to each direction. Then, a process flow is added to match received FDI information with the management information stored in the management table for each direction, and to specify an alarm destination.

Abstract: A system and method for implementing an automatic discovery function in a DWDM network are provided, wherein the system includes: an optical supervisory channel (OSC) being a bi-directional physical channel, configured to transmit information through multiplexing/de-multiplexing a wavelength independent of a master optical channel with the master optical channel; a first ASON control unit, being located in a first node, configured to interact with a second ASON control unit in an adjacent second node through the OSC channel so as to obtain the information of the second ASON control unit; and the second ASON control unit, being located in the second node, configured to interact with the first ASON control unit in the adjacent first node through the OSC channel so as to obtain the information of the first ASON control unit. Thereby, using the method and system of the present invention, the automatic discovery function can be implemented through the OSC channel transmission mechanism.

Abstract: The subassembly includes a laser for emitting signals towards fibers to be monitored, a passive alignment carrier, a photodetector for monitoring reflected laser signals from the fibers and for monitoring laser output power, and an optical fiber. The laser is disposed within the passive alignment carrier. The optical fiber is embedded in the passive alignment carrier, and has an angled fiber facet. The laser emits signals toward and through the angled fiber facet, whereby a portion of the laser signal illuminates the photodetector, and another portion illuminates the fibers that are being monitored and reflects back to the photodetector such that faults on the fibers can be detected.

Abstract: A method and apparatus for transmitting signals from a plurality of input channels over a TDM optical network, where each of the input channels contains an optical data signal and an electrical control signal containing control information relating to the optical data signal. In accordance with the invention, respective optical receivers convert the optical data signals to respective electrical data signals, which a TDM data multiplexer time-multiplexes to generate a multiplexed data signal. A TDM control signal multiplexer time-multiplexes the electrical control signals to generate a multiplexed control signal that is combined with said multiplexed data signal to generate a composite electrical signal. An optical transmitter generates a composite optical signal from the composite electrical signal that is transmitted over the network, optionally after WDM multiplexing it with other composite optical signals.

Abstract: The Fibre Channel Credit Extender (FCCE) (600) is a network device that is disposed between and connected to an end node (210) and an optical repeater (220). The FCCE (600) contains as many buffer credits as necessary, to solve bandwidth problems in a network. In a situation where maximum bandwidth is required in both directions of a link, the FCCE (600) breaks a single logical link into three physically separated “linklets.” The short-distance linklets attain maximum bandwidth by use of the existing buffer credits of the end nodes. The long-distance linklet attains maximum bandwidth by use of very high receive buffer credits in the FCCEs (600). In this way, only those links that need maximum bandwidth over distances not covered by end-node credit counts need be attached to an FCCE (600). The FCCE (600) contains the optical repeater to gain distance on that link, and contains high credit count receive buffers to gain bandwidth on the link.

Abstract: A method for integrating an Optical Service Channel (OSC) with a Quantum Key Distribution (QKD) channel across a DWDM network having a single mode optical fiber is provided. An optical signal is received. An OSC is coupled with the optical signal. A QKD channel is integrated with the OSC on the single mode optical fiber.

Abstract: The network operating system includes an embedded platform for controlling operation of an agile optical network at the physical layer level. At the module embedded level, each module (card-pack) is provided with an embedded controller EC that monitors and control operation of the optical modules. At the next level, each shelf is provided with a shelf processor SP that monitors and control operation of the ECs over a backplane network. All optical modules are connected over an optical trace channel to send/receive trace messages that can then be used to determine network connectivity. At the next, link management level, a network services controller NSC controls the SPs in a negotiated span of control, over a link network. The control is address-based; each NSC receives ranges of addresses for the entities in its control, and distributes these addresses to the SPs, which in turn distribute addresses to the ECs in their control.

Abstract: Out-of-band data communication of diagnostic and/or configuration data is performed using transceivers in a data or communication network. A light beam or other carrier is modulated with high-speed data and out-of-band diagnostic and/or configuration data to create a double modulated data signal. A physical layer signal is created that includes modulations of the double modulated signal. The physical layer signal is transmitted onto a physical link. The diagnostic and/or configuration data can be transmitted in the out-of-band signal without substantially reducing or otherwise interfering with the transmission rate of the high-speed data.

Abstract: A method and apparatus for initializing an end-to-end link in a fiber optic communications system in which a pair of nodes interconnect a pair of end devices. A first node, upon initializing a device link segment with an end device to which the node is coupled, sends a signal to the other node over a network link segment indicating that the sending node has initialized its device link segment. The first node completes initialization of the end-to-end link upon receiving a signal from the other node over the network link segment indicating that the other node has initialized its device link segment. In an alternative initialization scheme, a node momentarily operates its data channel in a loopback mode to allow its end device to initialize the device link segment in accordance with a predetermined protocol before returning to a transparent mode.

Abstract: Identification of optical channels in wavelength division multiplex (WDM) optical networks may be confounded by unwanted noise tones interfering with pilot/dither tones used for identifying optical channels. The invention describes a method of selecting pilot/dither tones that are selectively restricted to avoid allocating dither/pilot tone frequencies that appear as noise tones along the path of an optical channel in the optical network.

Abstract: An apparatus and methods for testing a passive optical network with regard to fiber connectivity and attenuation losses, and with regard to the proper operation of packet-based communication protocols thereon.

Abstract: During initial start-up of an optical communication system, an ASE reference span loss is calculated based on transmitting power and received power of ASE light generated by an optical amplifier, and an OSC reference span loss is calculated based on the transmitting power and the received power of OSC light. During normal operation of the optical communication system, a span loss is calculated using the OSC light, and an amount of change in the span loss representing a difference between the span loss and the OSC reference span loss is calculated. A current span loss between a transmitting station and a receiving station is calculated by adding the amount of change in the span loss to the ASE reference span loss.

Abstract: An optical signal connection apparatus comprising a first node and a second node connects first and second optical signal transmission lines to respective first and second routers. The first node includes a first port coupled to the first router via the first optical signal transmission line, and is configured to handle a first type of optical signals associated with the first router. The second node includes a second port coupled to the second router via the second optical signal transmission line, and is configured to handle a second type of optical signals associated with the second router. The information regarding the first type of optical signal is compared with information regarding the second type of optical signal to determine whether or not the first and second optical signal transmission lines are compatible to each other.

Abstract: A method of optical communication includes providing a plurality of optical signals each associated with a distinct channel of a composite optical signal. A unique pilot tone signature is superimposed on each channel. The spectral composition of each signature includes a plurality of frequencies. In one embodiment, binary frequency shift keying at a frequency fm is used to shift between instantaneous frequencies f1 and f2 wherein f2+?f=f1??f=fc. Choosing ? ? ? f f m ? 1.1602 produces a spectrum having a plurality of dominant components at least some of which have substantially the same amplitude. A method of detecting the presence of a selected pilot tone signature having a plurality n of dominant spectral components F1 . . . Fn includes generating a power spectrum P0(f). Sk(f) is calculated as a product of a plurality of frequency shifted versions of P0(f), where k?{1 . . . n}. The existence of a dominant spectral component at Fk indicates the presence of the selected pilot tone signature.

Abstract: The network operating system includes an embedded platform for controlling operation of an agile optical network at the physical layer level. At the module embedded level, each module (card-pack) is provided with an embedded controller EC that monitors and control operation of the optical modules. At the next level, each shelf is provided with a shelf processor SP that monitors and control operation of the ECs over a backplane network. All optical modules are connected over an optical trace channel to send/receive trace messages that can then be used to determine network connectivity. At the next, link management level, a network services controller NSC controls the SPs in a negotiated span of control, over a link network. The control is address-based; each NSC receives ranges of addresses for the entities in its control, and distributes these addresses to the SPs, which in turn distribute addresses to the ECs in their control.

Abstract: In an optical transmission system for transmission of data along an optical fiber link, a receiver for an optical supervisory channel is capable of detecting data at a lower rate at which a Raman pump is enabled, and at a higher rate which is commenced when the higher power of a Raman amplified signal has been received.

Abstract: The invention provides an optical shutter (400) for a communication system of a type comprising first (B1, B2) and second (A1, A2) communication paths along which information-bearing radiation propagates in opposite directions. The shutter (400) comprises: an optical tap (440) and a power monitor (430) for monitoring power of information-bearing radiation propagating along the first path (B1, B2) and for generating a corresponding radiation power indicative signal; a control unit (420) for comparing the indicative signal with a threshold value to generate a control signal (control); and shutter switch (410) for selectively substantially transmitting or blocking radiation propagating along the second path (A1, A2) in response to the control signal.

Abstract: The invention provides for optical circulators which redirect light from port to port sequentially in one direction used to separate traffic in a bidirectional optical fiber transmission system. The invention provides for using two optical circulators in each span of bidirectional fiber so that the OSC channel can be transmitted in one direction opposite to the WDM channels. The invention also provides for a gigabit Ethernet path between chassis which is utilized for control traffic and customer traffic. The invention is placed in a non-critical region of the optical spectrum and is independent of all other chassis equipment. The invention also provides the advantage in alternate embodiments of providing the option of a second counter propagating WDM channel being transmitted along with the OSC to provide additional system capacity. The invention also provides the advantage in an alternate embodiment of allowing the OSC to be amplified through a raman source without the need of complete system retrofit.

Abstract: A WDM system includes a plurality of P to P WDM systems connected for wavelength-multiplexing optical signals of a plurality of wavelengths, transmitting them to a transmission path, transmitting wavelength-division multiplexed light while performing amplification by using optical AMP in the transmission path, and subjecting the wavelength-division multiplexed light to wavelength isolation at the reception side. An OSC which is a monitoring control signal such as a normal optical signal is used closely within one P to P WDM system. However, by transmitting and receiving the OSC between a plurality of P to P WDM systems, it is possible to solve a problem at the system rise. Moreover, since accumulative OSNR data is transmitted up to the P to P WDM system of the final stage, it is possible to detect an accurate OSNR value and a leak ASE light quantity at the end side.

Abstract: One apparatus embodiment includes a first light source, a second light source, and a receiver having a photodetector. The first light source emits a first signal at a nonvisible wavelength for data transmission. The second light source emits a second signal at a visible wavelength for fault detection when combined with the first signal for transmission over a fiber optic path. The receiver converts the first signal from the nonvisible wavelength to an electrical signal.

Abstract: A node for managing an optical signal includes a first system optics card for providing channels to be transported over a first optical transport link and receives channels from a second optical transport link. Channels received over the second optical transport link are provided to an optical converter card for transport to a client device, for feedback onto the first optical transport link, or pass through to a second system optics card of the node. The first system optics card is capable of dropping network channels from the second transport link to associated client devices through optical converter cards and add client channels received from optical converter cards to the first transport link. The first system optics card may include one or more express input and output ports to couple with one or more other system optics cards in order to provide multiple degrees of communication capability.

Abstract: A communication network comprising at least one first terminal, at least one second terminal, a plurality of links, and at least first and second nodes. The first node is bidirectionally coupled to the first terminal through at least a first one of the links, and also is bidirectionally coupled to the second terminal through at least a second link and the second node. Preferably, the first node comprises a plurality of communication paths, each of which is coupled at a first end thereof to at least one corresponding first link. Second ends of the communication paths are all coupled to the second link, through a multiplexing device, and route signals between the first and second links. The first node also preferably comprises at least one alternate communication path having a first end coupled through the multiplexing device to the second link, at least one switch that is coupled to the alternate communication path, and a detector for detecting a failure in at least one of the communication paths.

Abstract: An eye mask is provided that is defined at least partially in terms of absolute, or non-relative, optical power level values. In essence, the eye mask of the invention is a hybrid of the traditional eye mask in that the eye mask of the invention includes power level values on the optical power axis that are based on the minimum OMA set forth in the applicable standard or data sheet specification rather than on measured power level values obtained from the part being tested. Using the hybrid eye mask of the invention obviates the need to perform at least some of the tests often used to measure transmitter attributes. In addition, using the hybrid eye mask of the invention reduces the possibility that a transmitter may fail the eye mask test even though the transmitter operates satisfactorily.

Abstract: There is provided an optical signal transmission apparatus having a stable dispersion compensation function without unnecessarily controlling a compensation value even when a main signal quality is deteriorated due to a factor other than dispersion or in the case of a transmission failure. When it is determined that a signal quality is deteriorated due to dispersion of a fiber by determining a control mode of a variable dispersion compensator by means of optical noise information and received power information in addition to bit error information of a received signal, a compensation value of the variable dispersion compensator is varied and a compensation value other than the dispersion of the optical fiber is held to an existing set value.

Abstract: A method and system for high bit rate fiber-optic communications utilize a wavelength tunable transmitter controlled by a feedback signal from an error analyzer associated with a received optical data stream.

Abstract: A system and method that utilizes digital coding techniques to combine a high-rate data stream with a lower rate side data stream to produce a combined data stream in such a way that the side data can be extracted by an inexpensive, low-bandwidth receiver. The method generally comprises the steps of: combining at least one payload data stream with at least one side data stream into a composite electrical data stream; applying the composite data stream to an optical transmitter to produce an optical signal; detecting the optical signal with an optical receiver having a maximum frequency of operation less than one-half of the rate of the composite data stream; and recovering the side data stream from the electrical output of the optical receiver.

Abstract: An optical transmission device includes: an attenuator that attenuates an optical signal from an adjacent optical transmission device; an optical element that is arranged downstream of the attenuator; a detector that detects a change in a characteristic of a transmission path; and a controller that adjusts, when the change is detected, an attenuation of the attenuator to keep the level of the optical signal input to the optical element at a predetermined level.

Abstract: A dual structure for a multiplexing section extended to an OSU is obtained without adding a dynamic function, such as an optical switch, to a W-MULDEM. The W-MULDEM of an optical wavelength division multiplexing access system divides, among ports corresponding to the individual ONUs, downstream optical signals having wavelengths ?d1 to ?dn, which are received along a current-use optical fiber, or downstream optical signals having wavelengths ?d1+?? to ?dn+??, which are received along a redundant optical fiber. The W-MULDEM also multiplexes, for the port that corresponds to the current-use optical fiber or the redundant optical fiber, upstream optical signals having wavelengths ?u1 to ?un or wavelengths ?u1+?? to ?un+??, which are received along optical fibers corresponding to the ONUs. A wavelength difference between the downstream optical signal and the upstream optical signal that are consonant with each ONU is defined as an integer times the FSR of an AWG.

Abstract: A method and apparatus configured to transmit module data between optic modules over a primary communication channel, such as an optic fiber configured to carry network data or outgoing data. A control center may send the module data, such as any type of DDMI data, over the optic fiber to control one or more aspects of the optic module system. The optic module may also be configured to send module data regarding any aspect of module status or operation, to a control center, via the optic fiber. Use of the primary communication channel, normally reserved for only network data, allow for module to module communication or module to control center communication without need of cumbersome two wire interface or supplement channels. Optic module data communication may occur concurrent with network data transmission. Optic module data back-up may occur via the optic channel to provide rapid re-load of important optic module data.

Abstract: The invention relates to a method for conveying management information in a WDM system from a number of wavelength converters to a central management unit, wherein a management information signal is superimposed on the WDM signal from the respective wavelength converter. A fraction of the optical signal in the common optical transmission line is tapped off to a detector and the different management information signals are recovered by a receiver unit which is connected to the detector. The invention also relates to a WDM system and a pluggable WDM wavelength converter.

Abstract: A method and apparatus for providing a supervisory channel in a wavelength division multiplexing (WDM) fiber-optic communication system uses a controlled optical attenuator disposed in an optical path between a demultiplexer (DMUX) and a multiplexer (MUX) of an Optical Add-Drop Multiplexer (OADM.

Abstract: Apparatus and method for transmitting a supervisory signal for optical network applications. The apparatus includes a subcarrier transmission system configured to receive a first supervisory signal and output a second supervisory signal, and an electrical-to-optical conversion system configured to receive the second supervisory signal and a data signal and output an optical signal. The second supervisory signal is associated with a subcarrier frequency. The data signal is associated with a data bandwidth, and the data bandwidth includes a data frequency. At the data frequency, a power density of the data signal is substantially equal to zero. A ratio of the subcarrier frequency to the data frequency ranges from 0.8 to 1, and the subcarrier frequency ranges from 2.4 GHz to 2.483 GHz.

Abstract: A transceiver test module and method for testing an optical transceiver. An optical wrap interconnects the optical transmitter and optical receiver of an optical transceiver. A processor system reads information from an optical transceiver; provides a signal to the optical transceiver to operate the optical transceiver to transmit a signal at the optical transmitter, which is received via the optical wrap; detects diagnostic information from the optical transceiver for errors of the optical transceiver; and determines the rated speed of the optical transceiver from the read information. Operation of the optical transceiver at the rated speed is verified by the diagnostic information.

Abstract: The disclosed technology provides a dynamic interconnection system which allows to couple a pair of optical beams carrying modulation information. In accordance with the disclosed technology, two optical beams emanate from transceivers at two different locations. Each beam may not see the other beam point of origin (non-line-of-sight link), but both beams can see a third platform that contains the system of the disclosed technology. Each beam incident on the interconnection system is directed into the reverse direction of the other, so that each transceiver will detect the beam which emanated from the other transceiver. The system dynamically compensates for propagation distortions preferably using closed-loop optical devices, while preserving the information encoded on each beam.

Abstract: Disclosed is a device and method for optical supervisory channel framing in an optical transport network system including at least one of an optical transmission section, an optical multiplex section, and an optical channel section. A maintenance signal is generated according to a first LOS (Loss OF Signal) signal generated from at least one of the optical transmission section, the optical multiplex section, and the optical channel section. The maintenance signal and a frame-related channel such as a message communication channel are multiplexed to generate an optical supervisory channel frame. The optical supervisory channel frame is output to another device including a repeater and a terminal system through a single supervisory channel.

Abstract: Provided are an optical transponder that processes a G.709 frame that includes an overhead for operation, administration, and maintenance of an optical channel and an overhead for forward error correction, and a method of detecting and treating errors in optical-channel sublayers of the same. The method includes detecting an error signal or a maintenance signal from a G.709 frame that includes an overhead for operation, administration, and maintenance of an optical channel and an overhead for forward error correction; treating errors in a predetermined layer of a plurality of layers that requires error treatment when the error signal or the maintenance signal is detected or canceled; and investigating a reason for the errors in the predetermined layer.

Abstract: A sub signal modulation apparatus, with the intention of stabilized transmission of a sub signal superimposed on main signal light irrespective of the intensity of the main signal light when used in a general-purpose main signal transmitter, including a sub signal outputting unit; a variable optical attenuator for variably attenuating main signal light, into which the main signal has been modulated, at a response speed lower than ½ of a speed of the main signal; an attenuation determining unit for determining, in accordance with the sub signal output from the sub signal outputting unit, an attenuation of the variable optical attenuator for the main signal light; and a stabilizing controller for controlling the attenuation determining unit such that an amplitude of the attenuation of the variable optical attenuator is stabilized to have a constant ratio to a mean intensity of the main signal light.

Abstract: A system and method for a transparent WDM metro ring architecture in which optics enables simultaneous provisioning of dedicated wavelengths for high-end user terminals, while low-end user terminals share wavelengths on “virtual rings”. All wavelengths are sourced by the network and remotely modulated at customer “End Stations” by low cost semiconductor optical amplifiers, which also serve as transmission amplifiers. The transparent WDM metro ring architecture permits the communication of information and comprises a fiber optical feeder ring, at least one fiber optical distribution ring, a network node (NN), at least one access node (AN) said network node and said at least one access node connected via said fiber optical feeder ring and at least one end station (ES) connected via said fiber optical distribution ring to said at least one access node, wherein said user is attached to said at least one end station.

Abstract: The control of the transmission of useful optical signals on different line paths of an optical transmission device is accomplished via at least one of the following features: using signal sources and signal sinks, the useful optical signals are coupled into the line paths, or are coupled out of them; at least one portion of the optical line paths is configured as normal line paths having coupling nodes via which a switchover to an alternative line path can be undertaken if a normal line path is disturbed; in addition to the useful optical signals, test signals, whose evaluation is used for the switchover between the line paths, are transmitted bidirectionally section-by-section; at least two types of test signals can be transmitted, of which a first type is used as an indicator for an intact line path and a second type as an indicator for a disturbed line path; and any switchover to an alternative line path is only undertaken if, before the detection of the disturbance, a test signal of the first type has be