Abstract:
An improved optical communication circuit is disclosed which is capable of overcoming a damaged link and maintaining communication. The circuit is in the form of a ring with links connecting between nodes, and the circuit includes the ability to detect a failure and to re-route a message around a break by reconfiguring the message at a modified wavelength and forwarding on a different path.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the field of communication by transmission of optical signals and more particularly to a method and apparatus for delivery of a message that initially failed because of a discontinuous optical circuit. In a more general embodiment, the present invention relates to a flexible and configurable optical switching and routing apparatus.  
         BACKGROUND OF THE INVENTION  
         [0002]    Optical signals form the basis of a reliable and efficient communication system. Communication, particularly digital optical signal communication is efficient and flexible. Optical signals have greater bandwidth available, thus greater communication capacity, than electrical signals for communication purposes. Therefore, optical signal transmission is quickly and strongly surpassing electrical signal transmission for communication systems.  
           [0003]    Optical digital communication signals are typically generated by lasers which project a beam into an optical fiber that has a cladding with a relatively high index of refraction. Many optical signals, propagated at different wavelengths, may be transmitted through a single cable simultaneously, and each wavelength can transmit a distinctive message.  
           [0004]    However, a major drawback of optical cables has been that the fibers, made of fine fiberglass, tend to be damaged easily, causing a disruption in communication. While protection switching paths can be provided, the provision of protection switching requires a tradeoff because to build a protection or backup path for each active path would require that half of the capacity of the network is wasted during normal operation.  
           [0005]    It is an object of the invention to create an optical switching and routing arrangement that recovers from faults rapidly and efficiently without having to keep excessive extra fibers.  
           [0006]    It is another object of the invention to provide an optical switching and routing arrangement that can operate without an excessive number of additional fibers.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides an apparatus and method for the rerouting of a message transmitted via an optical circuit communication link through an alternate continuous link such that any interruption of communication is kept to a minimum time.  
           [0008]    The invention provides an optical communication circuit that is adapted for diverting to an alternate route a message that has become blocked because of a discontinuous fiber cable link. The circuit includes a controller which is programmed to determine a communication discontinuity by non-receipt of a delivery confirmation signal, followed by identification of an available wavelength in an alternate link, converted to the revised wavelength by a tunable transponder, and then re-routed to its intended destination via the alternate link.  
           [0009]    In a more general embodiment of the invention, the arrangement implements the following method: First, it is determined that a break in communications has occurred. Then, incoming data previously destined for the now faulty optical fiber is switched to a new wavelength, while substantially simultaneously switching the physical path on which the new wavelength is output to a different fiber. The combination of switching the physical output path and switching the wavelength allows a backup path to be utilized.  
           [0010]    The wavelength may be switched by using a tunable transponder, or in other embodiments the wavelength may be switched by using a different transponder fixedly tuned to a different wavelength.  
           [0011]    The foregoing and other advantages of the present invention will become apparent from examining the following description of the drawings and preferred embodiments of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1A is a schematic depiction of an optical communication circuit according to the present invention in which four communication nodes are connected by eight cable links.  
         [0013]    [0013]FIG. 1B is a schematic depiction of the communication circuit of FIG. 1A and wherein two of the cable links are damaged.  
         [0014]    [0014]FIG. 2 is an enlarged detailed diagrammatic representation of a node of the circuit of FIG. 1A. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    As shown in FIG. 1A, a communication circuit is presented in which nodes  10 ,  20 ,  30  and  40  are represented as stations for transmission of messages in optical form. Although illustrated as being substantially uniformly spaced, in actual practice nodes are typically separated from each other by varying distances. Cable link  24  connects node  10  to node  20  so as to convey messages thereto, and cable link  22  transmits messages from node  20  to node  10 . Similarly, links  12  and  14  transmit messages between node  10  and node  40 ; links  32  and  34  transmit messages between node  20  and node  30 ; and links  42  and  44  transmit messages between node  30  and node  40 . The cable links are formed of transparent glass fibers and are capable of transmitting electromagnetic radiation in many wavelengths.  
         [0016]    Thus, by selecting the links, the communication circuit illustrated in FIG. 1A is able to transmit communications between any of the four nodes  10 ,  20 ,  30  and  40  in a clockwise or counterclockwise direction as depicted. If a message is intended to be transmitted, e.g., from node  20  to node  30 , the message is sent along link  34  as a shortest available path. If the message is to go from node  20  to node  40 , the message may be sent clockwise along links  34  and  44 , or, alternately counterclockwise along links  22  and  12 . If the message is being transmitted from node  20  to node  10 , it is sent along link  22 . A communication circuit having a greater number of nodes would work similarly, with additional permutations of link routing possible for different origin and destination nodes. A provisioning computer (not shown) is in communication with each of nodes  10 ,  20 ,  30  and  40  to manage message transmission and routing through control of the nodes, as will be described below.  
         [0017]    Referring now to FIG. 1B, the communication circuit of FIG. 1A is illustrated as having sustained a break  38 , causing link  32   a  and link  34   a  to be discontinuous. Pursuant to the occurrence of break  38 , messages cannot be transmitted directly between node  20  and node  30 , causing a message being sent in either direction between node  20  and node  30  to be undelivered. However, according to the invention disclosed herein, an alternate route is available for delivering a message from one node to another, e.g., from node  20  through nodes  10  and  40 , to node  30 .  
         [0018]    Under normal conditions each node  10 - 40  sends a signal to the provisioning computer that a message has been generated (or has arrived from a different node) and has been forwarded to a next sequential node. Each message or packet contains a distinctive header segment that is used for identification. If the controller receives a signal, e.g. from node  20 , that a specific identified message has been received and forwarded to node  30 , but the computer does not subsequently receive a message that node  30  has received the sent message, the computer is alerted that link  32 - 34  is open. As will be understood by those skilled in the art, the lack of confirmation of receipt could result from a break only in link  34   a , but it is more likely that both links  32   a  and  34   a  would be simultaneously damaged, which is fatal to delivery of a message in either direction.  
         [0019]    Referring now to FIG. 2, a typical node  20  of the invention is illustrated in enlarged schematic view. According to the illustration, node  20  is connected to communication links  22  and  24  as a first cable pair connected to node  10 , and to links  32  and  34  as a second cable pair connected to node  30 . In addition, node  20  is connected to receive messages from and send messages to an external network, e.g. a wide area network (WAN), via connectors  82 ,  84 ,  86  and  88 . Links  22  and  32  are designated to transmit messages in a counterclockwise direction, as illustrated in FIG. 1A, and links  24  and  34  transmit in a clockwise direction. In practice, a link can be used in either direction, but a single link only operates to transmit messages in a single direction at any time.  
         [0020]    Node  20  is an assembly of components connected in four optical series circuits for the transmission of optical signals. A typical set of serial components, e.g. those in communication with link  24 , is made up of demultiplexer  52  (for an incoming message) and transceiver  62  connected through switching matrix  50  along path a to transponder  72 . The use of the multiplexers and demultiplexers places plural wavelengths onto a single fiber and then separates those wavelengths for processing after transmission. This technique of transmissioner, known as wavelength division multiplexing (WDM) is well known in the art. Multiplexers  56  and  58  (for outgoing messages) serve to integrate signals at varied wavelengths into a common cable link, and demultiplexers  52  and  54  serve to divide the signals as they exit a fiber cable transmission link.  
         [0021]    Transceivers  62 - 68 , according to the embodiment of the invention, maintain the messages in their separate paths between multiplexers/demultiplexers  52 - 58  and switching matrix  50 . It is noted that single arrows  22 ,  24 ,  26  and  28  are used to represent that the message transmission along respective links between nodes is contained in a single cable carrying a single beam of electromagnetic energy, although comprised of multiple distinct wavelengths. As divided into its multiple wavelengths, four message lines, identified as wavelengths λ 1 , λ 2 , λ 3  and λ 4 , are shown entering transceiver  62  from demultiplexer  52 . Signals λ 1 -λ 4  arrive on link  24  as a single WDM signal. Demultiplexer  52  separates the WDM signal into its component wavelengths, and sends the four separate component wavelengths, λ 1 -λ 4 , to transceiver  62 . Transciever  62  regenerates the four signals represented by λ 1 -λ 4  onto four preselected wavelengths for input to switching matrix  50 . The preselected wavelengths may optionally be the same wavelengths as λ 1 -λ 4 .  
         [0022]    One or more of the four exemplary wavelengths is reserved for spare capacity or emergency use. In an actual system where larger numbers of paths are used, a larger number of wavelengths are reserved for spare use, for example up to  50 % of the possible wavelengths. For purposes of simplicity, a single line a, representing four wavelengths, connects from the four transmission wavelengths entering switching matrix  50  to connect to transponder  72 . Switching matrix  50  contains apparatus for directing a message to a specified output port. Transceiver  62  operates to receive an optical signal on any of four exemplary wavelengths, λ 1 , λ 2 , λ 3  and λ 4 , convert the signals from optical to electrical form, to amplify the electrical signal by known electrical means, and to convert the signal back to optical energy. Transceiver  62  will also rectify the form of the incoming signal to minimize any acquired distortion or noise before sending the message onward.  
         [0023]    The wavelength conversion by transceiver  62  enables a message received by node  20  through link  24  on first wavelength λ 1  to be re-sent through a different link, e.g. link  34 , at a second wavelength, e.g. λ 5 . The message is transmitted from transceiver  62  through line a to transponder  72 . If the message is set to go to the WAN, it connects through line  82 . If the message is to go to another node, e.g. node  30 , it goes through line i to transceiver  66  and out through multiplexer  56  and to link  34 . Transponder  72  is also connected via lines b, c and d to each of the other transceivers  64 ,  66  and  68 . Each transponder  72 ,  74 ,  76  and  78  is able to connect to each transceiver  62 ,  64 ,  66  and  68  via lines a-p, as shown, to enable any signal to be diverted to any equipment component.  
         [0024]    Assuming that a message arrives at node  20  through link  24  on wavelength λ 1 , upon being amplified and passed through transceiver  62  and received by transponder  72 , the message signal will be transferred through transceiver  66  to multiplexer  56  to exit through cable  34  and then to be transmitted to node  30  (see FIG. 1). If the message so transmitted is received at node  30 , a receipt-confirming signal is sent from node  30  to the controller (not shown) to so indicate. However, if the cable is damaged as illustrated in FIG. 1B, the message sent from node  20  does not reach node  30 , and no confirming signal goes to the provisioning computer. The computer thus determines that the cable has a break  38  and immediately re-routes the message to travel from node  20  to node  30  circuitously by way of nodes  10  and  40 .  
         [0025]    The incoming message on line  24  through demultiplexer  52  and transceiver  62  to transponder  72  is transmitted onward according to instructions from the provisioning computer. The computer sends the message at node  20  back through link  22  to node  10 , through link  12  to node  40 , and through link  42  to node  30 . In order for the message thus re-routed not to be confused with other messages on any of cables  22 ,  32  or  42 , a further instruction is sent to modify the wavelength of the re-routed message to an available wavelength after amplifying and upon re-sending the message. For this reason, transponder  72  has the capacity of being tunable to a selected wavelength. Such a tunable transponder is commercially available from such sources as Nortel Networks, Agility Communications, and Bandwidth9.com. Transponder  72  will, for example, send the message received at wavelength λ 1  to be transmitted outward at wavelength λ 2  over path m to transceiver  68  and multiplexer  58  to link  22 .  
         [0026]    Therefore, as described above, the present invention provides an optical communication circuit that is capable of maintaining communication between nodes by re-routing a message when a break in a cable link occurs.  
         [0027]    The transceivers  62 ,  64 ,  66 , and  68  are available off the shelf using standard technology. Thus, when an input arrives at transceiver  68 , for example, the particular port on which the input signal is transmitted is determinative of the wavelength transmitted out of the transceiver to WDM  58 . The wavelength arriving therefore, on line m for example, must be on the right port and adjusted to the correct wavelength.  
         [0028]    In a first embodiment, transponders  72 ,  74 ,  76 , or  78  are of the fixed wavelength type. In such an embodiment, the switching of an output signal from a primary to a backup path requires two switching operations. Using an exemplary switching path for purposes of explanation, a primary path might enter matrix  50  from transceiver  62  on line a, be transmitted through transponder  72  and then back out line m and through transceiver  68 . Upon detection of a fault however, the input a from transceiver  62  is switched by matrix  50  to a new transponder, say  76 , since transponder  76  is fixedly tuned to the desired backup wavelength. Additionally, and preferably substantially simultaneously, a second switching operation switches the output of transponder  76  to a backup transceiver, say  66 , so that the signal affected by the fault is switched to both a new wavelength and a new physical line substantially simultaneously.  
         [0029]    A second technique involves utilizing tunable transponders  72 ,  74 ,  76 , and  78 . Upon detection of a fault, the transponder through which the undelivered signal is traveling is switched to the new designated wavelength that has been preassigned for backup use. Substantially simultaneously therewith, matrix  50  switches to a new physical path to put the backup wavelength onto the correct port. Note that in such an embodiment, the transceivers may not even be needed, since the wavelength desired to be input into the WDM mulitplexer (e.g.  58 ) may be transmitted by the tunable transponder.  
         [0030]    The technique of switching to the backup wavelength and port may be accomplished therefore by either switching two physical paths through the matrix  50 , as in the first embodiment, or by switching one physical path and retuning a transponder. Additionally, these techniques may be combined to suit particular systems, and the user would be permitted through a system interface to provision the switch by choosing which switching operations and retuning operations are required in order to deliver the message. It is also possible that certain faults will be backed up through two switching operations, and other faults will be backed up through a single switching operation and a retuning of a transponder to an alternate wavelength.  
         [0031]    While the present invention is described with respect to specific embodiments thereof, it is recognized that various modifications and variations thereof may be made without departing from the scope and spirit of the invention, which is more clearly understood by reference to the claims appended hereto.