Abstract: An object of the invention is to provide a diagnosis method for standby systems in an optical ring network having a redundant configuration, which can confirm normal states of standby systems in all optical transmission apparatuses on the network, even during operations of active systems. To this end, in the present diagnosis method for standby systems, when a diagnosis start command is given to an arbitrary optical transmission apparatus on the network, the optical transmission apparatus provides a false signal to a standby system unit to diagnoses an operation state thereof, and transmits the result to the optical transmission apparatus on the downstream side by utilizing overhead information in the active system optical channel.

Abstract: An optical protection switch and a method for optical protection switching are provided. The optical protection switch includes a loop mirror-based optical switch with two circulators and a direction-dependent phase shifter in the loop mirror. The direction-dependent phase shifter introduces phase shifts in counter-propagating optical signals in the loop mirror such that either one of a first optical signal and a second optical signal are switched as an output optical signal. The direction-dependent phase shifter is controlled by a controller which initiates switching from the first optical signal to the second optical signal if a drop in power level is detected in the first optical signal and a corresponding drop in power level is not detected in the second optical signal and vice versa.

Abstract: An optical access network employs a ring topology in which connections between the network units and between the network units and the optical line terminal are via a protection switch. The protection switch monitors connections from the optical network units to detect a loss of signal and, on detecting a loss of signal from an optical network unit, switches the respective optical network unit out of the ring, thus maintaining continuity of the ring topology.

Abstract: A transmission apparatus that receives an optical signal by selecting any one of a plurality of provided optical signal transmission paths through protection control is configured to include a plurality of optical signal outputting sections that output the optical signals transmitted through said optical signal transmission paths respectively as optical signals having wavelengths that are different from each other, a wavelength selective optical switch capable of selectively outputting light of a wavelength corresponding to any one of the optical signals coming from the optical signal outputting sections on the basis of the frequency of a controlling frequency signal, and an optical switch controlling section that supplies said controlling frequency signal to the wavelength selective optical switch so as to output the optical signal coming from the optical signal transmission path side that is selected by said protection control among the optical signals coming from the optical signal outputting sections.

Abstract: A network is maintained functional in the presence of a wide range of faults, including physical fiber cuts, transmission impairments creating signal quality degradation, and failure of equipment modules, for example, transceivers, using a common control and backup channel, either alone or in combination with fiber loopback protection or wavelength loopback protection, or fiber loopback protection or wavelength loopback protection alone. Systems constructed according to the invention require only a single transceiver per wavelength at each node that accesses that wavelength. The invention is equally applicable to type 1 nodes, which drop and add a single, predetermined wavelength, and to type 2 nodes which have the capability to drop and add any subset of the wavelengths employed in the network.

Abstract: A transmission apparatus including: a plurality of asynchronous network interface units each having an asynchronous network termination unit for inputting/outputting a packet from/to an asynchronous networks and a mapping unit for converting the packet into a first synchronous frame and vice versa, a plurality of synchronous network interface units each used for inputting/outputting a second synchronous frame from/to a synchronous networks, a buffer provided on at least a first of the asynchronous network interface units, and a control unit for storing a packet output by the asynchronous network termination unit for accommodating a specific asynchronous networks and a packet output by the asynchronous network termination unit of the first asynchronous network interface unit into the buffer by adding identifiers for identifying the asynchronous network termination units in the event of a line failure occurring in a synchronous networks on a route related to the specific asynchronous network.

Abstract: The present invention provides for a method for reserving spare bandwidth for a link in a communication network including a plurality of links. The method provides for monitoring the volume of traffic routed through each link of the communication network. A single link failure for each link is then simulated and the volume of traffic which would be rerouted through each link for maintaining communication and the volume of traffic removed from each link are determined for each simulated single link failure. The difference between the volume of traffic which would need to be rerouted through each link and the corresponding volume of traffic removed from each link is then computed, and a maximum difference value is determined for each link for all simulated single link failures. An amount of spare bandwidth equivalent to the determined maximum difference is then reserved for each link.

Abstract: A system and method for communicating between serially connected electrical devices of a network is provided. The network includes a series of electrical devices, and fiber optic connectors between electrical devices of the series of electrical devices forming a closed communication ring in which output of each electrical device is communicatively connected to input of a subsequent electrical device of the series of electrical devices.

Abstract: An Ethernet passive optical network (EPON) ring for providing protection against fiber failures. The optical network unit (ONU) is coupled to the ring fiber by a three-port passive optical splitting module that has three two-way optical passages. By the three two-way optical passages, the OUN receives/transmits data from/to the two ends of the optical line termination (OLT) to provide protection while the fiber failure. Moreover, it provides better authorization of users and simpler collision detection by the two-way transmission of the three-port passive optical splitting module to prevent hackers from invading and to reduce collisions.

Abstract: The signal relay apparatus has a plurality of photoelectric converters which convert an optical signal input from an electronic equipment (A-C) into an electric signal and outputs the same to the photoelectric converter (21-24), and which converts an electric signal input from another photoelectric converter and outputs the same to the corresponding electronic equipment; a plurality of switch circuits (31-34) which are provided at input and output ends of the electric signal of the photoelectric converter (21-23) and which are opened and closed to bypass the electric signal; and a power source supply section (41-43) which supplies a power source to the electronic equipment connected to the photoelectric converter.

Abstract: Optical bypass node upgrade configurations are disclosed: (1) a configuration where optical taps are pre-positioned in wavelength division multiplex (WDM) line systems terminating at optical-electrical-optical (OEO) core switching nodes to allow for future upgrade of the nodes to degree-two or higher optical bypass; (2) a configuration where the taps are pre-positioned in a degree-two optical bypass node to allow for future upgrade to a degree-N optical bypass node; and (3) a configuration and procedure for upgrading OEO core switching nodes to optical bypass when the taps have not been pre-positioned in the WDM line systems. These configurations do not introduce bit errors for non-upgraded optical paths.

Abstract: A method and system provide capacity-efficient restoration within an optical fiber communication system. The system includes a plurality of nodes each interconnected by optical fibers. Each optical fiber connection between nodes includes at least three channel groups with different priority levels for restoration switching in response to a connection failure. The system maintains and restores full-capacity communication services by switching at least a portion of the channel groups from a first optical fiber connection to a second optical fiber connection system based on the priority levels assigned to the channel groups. Service reliability is effectively maintained without incurring additional costs for dedicated spare optical fiber equipment by improving idle capacity utilization.

Abstract: An optical WDM ring network includes at least two add/drop nodes connected in a ring path. The ring path comprises only a single optical fiber arranged for bidirectional traffic between the nodes. A 2×2 switch is used in one of the add/drop nodes for switching signals forwarded from the node to the other node to travel either on a first segment of the ring path or on a second, complementary segment of the ring path when required for protection purposes. Signals from the other node to the first node are issued to travel on both segments but the switch selects the actual segment from which the signals are received in the first node. The signals switches by the switch can be high priority signals used in protected channels communicated between the node and another node. On the segment from which the high priority signals are not received, low priority signals in non-protected channels can be communicated between the nodes, these channels also passing the switch.

Abstract: A communication system couples communication devices using optical switches and links. The optical switches provide OSI Layer 1 restoration in response to a fault. The communication devices provide OSI layer 2/3 restoration in response to the fault after OSI Layer 1 protection is complete.

Abstract: A medium-scale IP telecommunications network is configured in a low-cost optical network with good reliability and expandability. A physical configuration example has a center node 2-1 and eight local nodes 2-11 through 2-18 connected in one OADM ring 2-21/2-22. The logical configuration is a star configuration with the central node 2-1 at its origin with all traffic passing through the center node 2-1. The local nodes 2-11 through 2-18 are connected to the central node 2-1 by wavelength-unit optical channels or optical paths ?1 through ?8. Channels are added as required. Initially, for example, the center node 2-1 and the local node 2-5 are connected by ?5, but ?13 can added when the need arises. Since the logical star network is limited to approximately two add/drop optical channels at local nodes, costs are reduced by using inexpensive filters (e.g., dielectric interference film filters or fiber Bragg reflectors) that are capable of extracting only the specific wavelength of the optical channel.

Abstract: A communication system includes nodes that exchange communications over optical fibers. In the event of a fault, the nodes implement ring protection for a first set of the communications and implement mesh protection for a second set of the communications. Ring protection may be used for two degree nodes and mesh protection may be used for higher degree nodes, or a customer may select between ring protection and mesh protection.

Abstract: An ring type optical LAN device includes a master node and a plurality of slave nodes that are interconnected by an optical fiber cable. A plurality of optical bypass transmission lines are provided in correspondence with each one of the slave nodes. Each of the optical bypass transmission lines bypasses the corresponding one of the slave nodes. Each slave node includes an E/O converter and an optical cutoff circuit. Each of the E/O converters is controlled to flash for generating an optical signal, which is transmitted to a network. When any one of the slave nodes fails such that the corresponding E/O converter is maintained in a turned on state, the associated optical cutoff circuit forcibly switches the E/O converter to a turned off state. This suppresses a network crash caused by the failure maintaining the E/O converter in the turned on state.

Abstract: An optical network includes an optical ring and at least three subnets. Each subnet includes a plurality of add/drop nodes coupled to the optical ring. The add/drop nodes are operable to passively add a first traffic stream in a first direction on the optical ring and a second traffic stream in a second direction on the optical ring. The first traffic stream comprises different content than the second traffic stream, and the first traffic stream and the second traffic stream are transmitted on the same wavelength. The network also includes a plurality of gateway nodes. The gateway nodes are each coupled to the optical ring at a boundary between neighboring subnets and are operable to selectively pass and terminate wavelengths between subnets to allow wavelength reuse in the subnets.

Abstract: An optical network includes a plurality of subnets. The subnets each include a plurality of add/drop nodes coupled to the optical ring and operable to passively add and drop traffic to and from the optical ring. The network further includes a plurality of gateway nodes. The gateway nodes are each coupled to the optical ring at a boundary between neighboring subnets and operable to selectively pass and terminate wavelengths between subnets to allow wavelength reuse in the subnets and to provide protection switching.

Abstract: A method for assigning a predetermined wavelength between two different nodes in a wavelength division multiplexing (WDM) ring communication network that has an N number of nodes and at least one pair of optical fibers sequentially connecting the N number of nodes is disclosed. A matrix is formed by an algorithm representing optical-path configuration and wavelength assignment for nodes representing three cases: (1) when the number of nodes is an even number; (2) when the number of nodes is increased; and (3) for recovery from network-cut failures.

Abstract: A method for performing protection switching in an optical network does so in an optimal manner by performing the protection switching actions in the various nodes of a protection path in parallel rather than in sequence. To do so, a channel failure message is transmitted from an end node A in the optical network upon detecting a failure in a link in the optical network that includes the end node A. If a failure can be detected at both end nodes, the channel failure messages are transmitted from both end nodes the moment they detect failure without any further waiting. Upon receiving the first channel failure message regardless its originating end node, switching actions in intermediate nodes are initiated. The channel failure messages are forwarded from the intermediate nodes to their corresponding next nodes before completing switching actions in the intermediate nodes.

Abstract: Ongoing growth in transport demand is served while deferring or eliminating expenditure for additional capacity by reclaiming the protection capacity and inefficiently used working capacity in existing multi-ring network. Reclamation is through re-design of the routing and restoration in the network using mesh principles within the pre-existing ring capacities. The installed working and protection capacity of existing rings is viewed as a sunk investment, an existing resource, to be “mined” and incorporated into a mesh-operated network that serves both existing and ongoing growth. A complete double or even tripling of demand could be supported with little or no additional capacity investment through the period of ring-to mesh conversion by ring-mining. An existing ring set may be converted to a target architecture of “p-cycles” instead of a span-restorable mesh, through placement of straddling span interface units to convert ring ADMs to p-cycle nodal elements.

Abstract: A network component (3d) for an optical network (1) is described, including a coupling device (2; 4, 5) for optical coupling of the network component (3d) to the optical network (1), the coupling device (2; 4, 5) having a receiving module (4) and a transmitting module (5); a first data processing device (8, 9) which is unidirectionally connected to the receiving module (4); a second data processing device (10, 11) which is unidirectionally connected to the transmitting module (5); the first data processing device (8, 9) being unidirectionally connected to the second data processing device (10, 11) for transmitting data to the second data processing device; a detection device (4) for detecting the network status; and a two-way switch (23) for switching of the input of the first data processing device (8, 9) between the receiving module (4) and the output (19) of the second data processing device (10, 11) as a function of the network status detected.

Abstract: A protection system protects paths of internetwork communication between WDM ring networks, and includes a plurality of optical wavelength multiplexing networks having a plurality of nodes and a network management system for monitoring conditions of the plurality of nodes. The nodes include a first node having add/drop functions of adding/dropping wavelength-multiplexed optical signals; a second node having a signal transfer function as well as the add/drop functions; and a third node having an internetwork connection function between the networks. Each first, second and third nodes further includes optical path cross-connect switches and a table for indicating conditions of the optical path cross-connect switches and a detected node fault condition. The network management system controls to set the optical path cross-connect switches in the first, second and third nodes so that the optimal optical path connection may be obtained according to the detected node fault condition indicated in the node tables.

Abstract: A network is protected against interruption of service while one or more faulty switches or optical fiber transmission lines are repaired or replaced, by an interconnecting configuration of small N×N optical input/output switches, where N is 2 or greater than 2. The switches are configured among protection and working transmission lines. The small number of fibers for each switch improves repair and installation connection reliability and permits configurations that flexibly meet differing requirements. Also the fault is monitored with a fault check signal.

Abstract: A method and system for providing tandem protection in a communication system. Path protection is provided using at least two redundant communication paths and selecting the communication having higher signal quality. Interface protection is provided through a protection transceiver thus implementing M:N equipment protection and 1+1 optical protection.

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: Described is a method for managing the traffic protection in OMS-SPRING networks and allowing the wavelength allocation change in the traffic transit nodes. The method of managing the traffic protection in an OMS-SPRING network should a span of the path fail is characterized in that it comprises the steps of providing a wavelength interchange mechanism; performing a ring Span Switch action by the OMS-SP mechanism; and re-routing the wavelength interchanged path on the protecting wavelength corresponding to the working wavelength of the span affected by a failure.

Abstract: A protection arrangement for an optical switching system includes protection switching for switch planes in the optical core and for ports in the tributary cards. For a multiple layer switch core protection is also provided between layers. A first layer is for switching optical channels. The protection switches used may be 1×N, 2×N or 3×N MEMS optical switches. The 1×N MEMS provides for protection switching. The 2×N MEMS provides protection switching and testing capability. The 3×N MEMS provides for protection switching, testing and backup of the protection switching. Application of these protection switches is shown in lambda plane switch cores, multiple layer switch cores and combined switch cores.

Abstract: A method and apparatus for variable rate pipes is described. In one embodiment of the invention, a computer implemented method comprises allocating a pipe from part of a working channel and at least part of a protecting channel of a span of a bi-directional line switched ring (BLSR), the pipe having a bandwidth, transmitting a set of layer 2/3 traffic in the pipe, and reducing the pipe's bandwidth when a failure occurs in the ring.

Abstract: A first optical fiber transmission path is used to transmit continuous-wave light for upstream signal light from the center node to each of the remote nodes, while a second optical fiber transmission path is used to transmit downstream signal light from the center node to each remote node and to transmit upstream signal light (obtained by modulating continuous-wave light) from each remote node to the center node. In this network, specific wavelength bandwidths are allocated to each remote node for the continuous-wave lights (for upstream signal lights) and the downstream signal lights, and the wavelength bandwidths for the continuous-wave lights (for upstream signal lights) and the downstream signal lights are alternately set.

Abstract: Optical transmission systems of the present invention include optical protection systems, apparatuses, and methods that provide increased reliability in the system. The optical system generally includes an optical switch having distinct, cooperating dedicated and shared protection sections. The shared protection sections in a plurality of nodes can be configured to provide a protection net, in which a plurality of traffic demands, or working channels, are commonly protected as protection group using one protection channels supported by the protection net.

Abstract: According to the invention, in a transmission apparatus, there are provided an equipment supervision unit detecting an obstacle in the equipment and a switching control unit controlling the switching operation of transmission lines. When the equipment supervision unit detects condition in which obstacles have occurred in more than one groups of the cross connect unit or the clock unit in which paths provided are disconnected, the same K-bytes as in the case when SF failure is detected are outputted to all the fibers to be inputted to the equipment. Alternatively, an FS-R command is executed to both sides, or Line-AIS is inserted in all the outputted transmission lines, or an output is disconnected, so that the equipment is isolated and the paths provided through a node is relieved.

Abstract: An optical network includes an optical ring and a plurality of add/drop nodes coupled to the optical ring. Each of the add/drop nodes is operable to passively add and drop one or more traffic streams to and from the optical ring, and each traffic stream comprises a channel. A hub node also coupled to the optical ring is operable to selectively pass and terminate individual traffic streams.

Abstract: An apparatus and a method for forwarding data packets through a fiber optic ring network includes forwarding data packets on a packet by packet basis. A node on the fiber optic ring network decides on a packet by packet basis whether to transmit on the working or protection path. Because this decision is being made on a packet level, reservation of throughput resources no longer is made at a one to one ratio. Rather, protection path resources are reserved at a ratio significantly less than one to one. In one embodiment, no resources are reserved for path restoration or protection path routing. Rather, quality of service provisioning is used to resolve interference situations wherein instantaneous demand exceeds capacity. A node evaluates ring conditions in relation to the demand of traffic resources and the relative quality of service ratings to determine whether and how to forward a packet.

Abstract: In a ring having first and second adjacent nodes, and two fibers carrying information between the first and second nodes, the first fiber carries information in one direction, while the second fiber carries information in another direction. Each fiber includes wavelength capacity allocated to working and protection traffic. The working and protection wavelength capacities in the first fiber are respectively assigned to first and second disjoint sets of wavelengths, while the working and protection wavelength capacities in the second fiber are respectively assigned to the second and first disjoint sets of wavelengths.

Abstract: A communication network is arranged in a ring configuration of network elements (12) coupled by working spans (16) associated protection spans (18), with multiple rings sharing network elements (12a and 12b). A shared protection span (18ab) is coupled between pairs of shared network elements (12a and 12b). A matrix 28 in a shared network element (12a or 12b) can couple any channel from one of the incoming working spans or protection spans to any channel of the shared protection span (18ab).

Abstract: A communication network using a ring structure incorporates shared protection channels (22ab, 22bc, 22bd) to reduce costs in implementing protection spans. The shared protection network elements (12a, 12b, 12c, 12d) use a protocol of conventional messaging to integrate with traditional fully redundant network elements.

Abstract: A link based network protection path calculation mechanism wherein a protection route is calculated that is guaranteed not to traverse the link it is intended to protect. The mechanism takes advantage of the fact that the same color cannot pass twice through the same optical fiber. The protection path is determined by eliminating all colors from the logical topology of the network except for the color corresponding to the link to be protected before executing the search algorithm. This serves to guarantee that the protection path calculated will not traverse the same physical fiber as the link to be protected. Virtual colors can be assigned to the links running through fiber bundles such that they do not pass through the same fiber/bundle twice.

Abstract: A bidirectional WDM self-healing ring network is provided and includes an outer ring network and an inner network for processing N units of optical signals and for performing a protection switching by using the outer ring network when the inner ring network suffers a link failure.

Abstract: A WDM bidirectional self-healing optical ring network is provided in which an outer ring network and an inner ring network can both handle N optical signals. The network includes a node; an optical add/drop multiplexer having a 1×N demultiplexer and a 1×N multiplexer; a pair of optical switching devices each connected to an optical fiber link intervening between the optical add/drop multiplexer and another node on the both ends of the optical add/drop multiplexer; and, a pair of circulators each connected between the optical add/drop multiplexer and a particular optical switching device in the optical switching device pair, each circulator having three ports, two of the three ports being connected to a corresponding optical switching device and remaining one port being connected to the optical add/drop multiplexer.

Abstract: An all optical network for optical signal traffic has at least a first ring with at least one transmitter and one receiver. The first ring includes a plurality of network nodes. At least a first add/drop broadband coupler is coupled to the first ring. The broadband coupler includes an add port and a drop port to add and drop wavelengths to and or from the first ring, a pass-through direction and an add/drop direction. The first add/drop broadband coupler is configured to minimize a pass-through loss in the first ring and is positioned on the first ring.

Abstract: An optical transmission system includes an optical wavelength branching unit that shifts a wavelength transmission band in the short-wavelength or long-wavelength directions based on a branching filter operation temperature, and performs a branching operation on a wavelength-multiplexed signal. A reception transponder performs decoding on an error correction code. An error correction monitoring unit gathers an error correction amount upon which a branching filter temperature control unit sets the branching filter operation temperature. A transmission transponder performs encoding on an error correction code. An optical wavelength combining unit shifts the wavelength transmission band in the opposite direction from the shifting direction based on a combining filter operation temperature, to perform a combining operation on optical signals and output a wavelength-multiplexed optical signal.

Abstract: A system and method for forwarding fault information in an optical network is disclosed. Responsive to a fault being detected at a node, the fault information is forwarded to the destination node in the form of a loss-of-signal condition by turning off laser transmitters used to propagate the signal towards its destination. Thus, a node or other network element not capable of generating an alarm signal consistent with an established alarm signaling protocol, such as a SONET alarm indication signal (AIS) or an SDH multiplexer section alarm indication signal MS-AIS, may still forward the fault condition to a downstream element.

Abstract: A system and method of reserving node resources along a desired path through a communications network that can be used to reserve resources across a fault such as an unconnected node interface or a disconnected link. The resource reservation technique is performed on a communications system including a plurality of nodes interconnected by at least one path to form a ring communications network. A communications link is identified within the path such that at least one node is disposed on a first side of the link and at least one node is disposed on a second side of the link. The identified link may have a fault. Resources are then reserved on the nodes disposed on the first side of the link by accessing the nodes over the path in a first direction around the ring network.

Abstract: A communication network includes first and second terminals, nodes, and links. The first node is coupled to the first terminal through the first link, and coupled to the second terminal through the second link and the second node. The first node preferably includes communication paths, each coupled to one corresponding first link and to the second link, through a multiplexing device, and routing signals between the links. The first node also preferably includes an alternate communication path coupled through the multiplexing device to the second link, a switch coupled to the alternate path, and a detector detecting failure of a communication path. A controller is responsive to the detector detecting a failure in a communication path and controls the switch to couple the alternate path to a corresponding first link, thereby enabling a signal to be routed between that first and second links through the alternate path.

Abstract: Disclosed herein is a bidirectional wavelength division multiplexed self-healing ring network. The ring network includes a central office and a plurality of remote nodes. Two optical fibers each connect the central office and the remote nodes in a ring form to allow optical signals to be bidirectionally received and transmitted between the central office and the remote nodes. One of the two optical fibers is a drop fiber for transmitting optical signals from the central office to the remote nodes, while the other is an add fiber for transmitting optical signals from the remote nodes to the central office.

Abstract: An autoprotected optical communication ring network includes a first and a second optical carrier having opposite transmission directions and a plurality of optically reconfigurable nodes optically connected along the first and the second optical carrier and adapted to communicate in pairs on links susceptible to failure, the ring network having a normal operative condition in which the nodes of each pair are optically configured so as to exchange optical signals on a working arc path at a respective first wavelength (?x) on the first carrier and at a respective second wavelength (?y) different from the first wavelength (?x) on the second carrier, the working path having a complementary arc path defining a protection arc path in which the first wavelength (?x) on the first carrier and the second wavelength (?y) on the second carrier can be used for further links.

Abstract: A 1:N protection scheme that provides both equipment and line protection against a single point of failure for a network hub situated in an optical ring network. The invention is suitable for use in DWDM based optical ring networks, and in particular, in networks constructed having a logical star over physical ring topology. The hub is modified to comprise N+1 transceivers, assuming an N channel DWDM network wherein each of the N transmitters operates using a different wavelength. Protection against line failures is achieved by optically splitting the transmit signal into two paths and transmitting each separately in opposite directions over the ring (i.e. 1+1 protection). In the event of a line failure, the other signal should still arrive at the destination. Protection against equipment failures is achieved through the use of a spare transceiver having a tunable wavelength of operation (i.e. 1:N protection).

Abstract: Systems and methods for link discovery and verification technique that minimize the need for line termination resources that generate and interpret packets. Of two nodes verifying a link to one another, only one node need have any line termination capability. The node lacking line termination capability simply loops back packets generated by the other node thus verifying the link. Thus, an optical cross-connect can verify links to a wide variety of node types by employing a single line termination unit capable of terminating any suitable packet type. Alternatively, a router can verify connectivity to an optical cross-connect even when the optical cross-connect lacks any line termination capability at all. This saves greatly on implementation costs for optical networks.