Abstract:
A system of communication between distant regions is provided that solves the problems inherent in the present state of similar systems. The redundancy of the ADMs at each terminating site is eliminated, replacing the more costly ADM with a standard switching device. The switching devices are electrical, optical or wireless in nature, for example, a standard multiplexer or optical cross-connect switch. A second advantage is the elimination of the redundancy of connections between the terminating sites, as well as a reduction in cost and an increase in system-wide reliability.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    This application is related to, and claims the benefit of the earlier filing date of U.S. Provisional Patent Application No. 60/193,472, filed Mar. 31, 2000, entitled “Transoceanic Communication System and Method,” the entirety of which is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a system and method for communicating between distant regions, and more particularly, to a system and method for switching and routing communications data in transoceanic communication systems.  
           [0004]    2. Description of the Related Art  
           [0005]    One form of transoceanic communications involves laying cable, containing electrical conductors or optical fibers, along the ocean floor and terminating the cable at equipment sites on land at either end of the cable. The reliability of a transoceanic communications system is often improved by using two cables terminating, at both ends, at different points on land. This provides some spatial diversity so that a cable cut or equipment malfunction affecting one cable is unlikely to affect the other cable.  
           [0006]    [0006]FIG. 1 of the accompanying drawings illustrates a traditional transoceanic cable system comprising two separate cables. Optical fiber cables  170  and  172  are shown spanning across an ocean, but can span any region that presents economical or physical constraints in its construction and maintenance. A cable buried deep under the ocean is inaccessible, but nevertheless is subject to failure. In this context, it is impractical to erect, and provide power to, a network of equipment sites along the cable to permit, for example, a diversely routed mesh structure to be formed out at sea that would improve the reliability of the transoceanic span. A similar situation is foreseen where communications are attempted from one region to another region through intervening air or space, or spanning hostile environments or large undeveloped areas such as jungles, forests, mountains or deserts. The intervening area to be spanned may be in political unrest, such as a combat zone or an otherwise sensitive area, thus preventing even routine maintenance.  
           [0007]    The information conduits themselves may take the form of electrical or optical cables or may be a radio communication path. In all of these instances, reliable communications may be achieved through redundant but diversely routed spans to make up for the relative inaccessibility of the long spans. Ring networks are used in each region to provide landing site diversity and the interconnections between the rings are expressly provided for the purpose of spanning a lengthy inaccessible intervening region.  
           [0008]    Referring again to FIG. 1, the span provides communications between landmass  104  and landmass  106 . Upon failure of either cable  170  or  172  due to damage or equipment failure, the transoceanic connection is readily restored using the other cable to circumvent the failure through the use of protective switching schemes. The familiar self-healing ring design can be employed to facilitate this protective switching. This is accomplished by providing two additional fiber spans  174  and  176  between each pair of on-land terminating points of cable  170  and  172 , that is, between sites  144  and  146 , and  152  and  158 , respectively. Using an Add-Drop Multiplexer (ADM) at each terminating point, this arrangement forms a self-healing ring network structure, such as a bi-directional line switched ring network, the design and operation of which is well-known and understood among those of ordinary skill in the art.  
           [0009]    Furthermore, to provide some protection against terrestrial failures and to make terrestrial and submarine failures independent of one another, so-called “backhaul rings” are used at both terrestrial ends to couple traffic to the transoceanic ring. In FIG. 1 one such backhaul ring network is shown comprising sites  142 ,  144 ,  146 , and  148  as interconnected by a series of links or conduits. The links are cables, optical fibers, wireless systems, or the like. Thus, span  190 , comprised of two cables  162  and  174 , also referred to as an “interlink” span, traditionally comprises one link that is part of a transoceanic ring (e.g. cable  174 ) and one link that is part of the backhaul ring network (e.g. cable  162 ). Accordingly, the transoceanic ring is formed by cables  170  and  172 , sites  144 ,  152 ,  158 , and  146 , and interlink spans  190  and  192  (more particularly, cables  174  and  176 ) on landmasses  104  and  106 . The net result is a three-ring structure with two nodes of each backhaul ring network coupled to two nodes of the transoceanic ring network.  
           [0010]    The node of the system is a point along the ring where traffic may be added, dropped, or merely passed along, usually via an ADM. In some cases, the node may also comprise passive optical switches. The node has two or three input/output ports depending on its particular use in the ring structure. For example, as shown in FIG. 2, node  148  is a 2-port node; data enters into ADM  118  and is passed along to ADM  116  of node  146 . Node  142  is a 3-port node containing ADM  112 ; data enters into ADM  112  of node  142  via input ports  180 , and depending on the switch configuration of ADM  112 , the data can be transmitted to node  144  or node  148 .  
           [0011]    At each site where a terrestrial backhaul node adjoins a transoceanic node, the traffic is dropped from one ADM at a tributary rate and enters an adjoining node ADM at the tributary rate. The term “tributary” means that the data rate along a conduit is a fraction of the aggregate rate that is actually transmitted over the cable. For example, if an OC-192 optical signal transmitted at about 10 gigabits-per-second is received by ADM  114  the signal may be multiplexed into four tributary data streams of about 2.5 gigabits-per-second each transmitted across a connection of link  164 . As shown in FIG. 2, tributary connection  164  carries data extracted by ADM  114  from backhaul ring  110  and passes the extracted data to ADM  124  to be carried by transoceanic ring  120 .  
           [0012]    With reference to FIGS. 1 and 2, the following is an example of data communications under normal circumstances in the traditional three-ring network architecture. Information to be communicated is submitted along data inputs  180  and enters backhaul ring network  110  through ADM  112  of node  142 . The information proceeds to node  144 , wherein ADM  114  passes the data to ADM  124  over tributary connection(s)  164 . The data is sent along transoceanic cable  170  to reach ADM  122  of node  152 . At ADM  122  the information is “dropped” from transoceanic ring network  120  and coupled into backhaul ring network  130  via ADM  132 . The information travels through backhaul ring  130  via ADM  134  of node  154  and reaches its destination at ADM  136  of node  156  where it is delivered to output ports  182 . As shown in FIG. 2 and as described above, the dashed line throughout the figures depicts the routing path of the data. Also shown in FIG. 2 are ADMs  126 ,  128  and  138 , cables  162  and  174  (taken together referred to as interlink span  190 ), and cables  176  and  188  (taken together referred to as interlink span  192 ), and node  158 .  
           [0013]    Traditional three ring networks, such as shown in FIG. 2, include the pairing of ADMs (i.e.  114 / 124 ,  116 / 126 ,  122 / 132 , and  128 / 138 ) at a given terminating point site (i.e.  144 ,  146 ,  152  and  158 , respectively), as well as the duplication of cables or fibers (i.e.  162 / 174  and  176 / 188 ). This pairing of ADMs and duplication of cables or fibers greatly adds to the overall cost of the system and also adds additional elements that are prone to failure.  
           [0014]    This arrangement of ADMs to form adjoining rings are shown to be reliable against many site outages, tributary failures, terrestrial span outages, transoceanic span outages, and combinations thereof. Several terms are used throughout the industry to describe this common configuration, including, “matched-node configuration,” “dual ring interconnect,” and “dual junction.” There are also existing mechanisms and protocols, such as standardized Alarm Indication Signals (AIS) or Automatic Protect Switching (APS) schemes (e.g. K1/K2 bytes in SONET overhead), by which ADMs may be informed of failed connections by other ADMs.  
           [0015]    [0015]FIGS. 3 through 8 depict the traditional three-ring network architecture of FIGS. 1 and 2 under various failure conditions and indicate how traffic may be routed to maintain communications. Throughout the figures, similar references refer to similar elements.  
           [0016]    [0016]FIG. 3 depicts the three-ring network of FIG. 2 with a failure of cable  160 . When a failure similar to this occurs, ADM  114  sends an AIS throughout the system notifying it that ADM  114  is not receiving data. By utilizing an APS scheme, ADM  112  reroutes the data and transmits the data to ADM  118  via cable  161 . The system then routes the data along the path shown by the dashed line, i.e. along cable  171 , through ADM  116 , along cable  162 , through ADM  114 , thereby circumventing the failure, and eventually to data output ports  182 . The data is successfully rerouted.  
           [0017]    In FIG. 4 transoceanic cable  170  fails. Upon the failure of cable  170 , ADM  122  of node  152  detects no data and sends an AIS throughout the system. ADM  124  switches its data path through cable  174  under a preset APS scheme. The data travels to ADM  126  of node  146  where it is switched onto cable  172 . The data arrives at ADM  128  of node  158  where it is switched to cable  176 . The data arrives at ADM  122 , thus circumventing the failure, and sent along its normal path to data output ports  182 .  
           [0018]    In FIG. 5 tributary link  164  fails. Upon the failure of link  164 , ADM  124  of node  144  detects no data and sends an AIS to the system. ADM  114  switches its data path through cable  162  under a preset APS scheme. The data travels to ADM  116  of node  146  where it is passed along its tributary links to ADM  126 . ADM  126  switches the data onto cable  174 . The data arrives at ADM  124  of node  144 , thus circumventing the failure, and where it is switched onto cable  170 . The data arrives at ADM  122  of node  152  to be sent along its normal path to data output ports  182 .  
           [0019]    In FIG. 6 a complete node site failure of node  144  occurs. Upon the failure of node  144 , ADM  122  of node  152  detects no data and sends an AIS to the system. ADM  112  switches its data path through cable  161  under a preset APS scheme. The data travels to ADM  118  of node  148  where it is switched onto cable  171 . The data arrives at ADM  116  of node  146 . Normally, when data arrives at ADM  116 , it is switched onto cable  162 . However in this scenario since node  144  cannot receive data, ADM  122  will again send an AIS out to the system and upon reception of the AIS, ADM  116  will switch its data to be transmitted over its tributary links to ADM  126 . Similarly, ADM  126  will attempt to transmit its data to node  144 , this time over cable  174 . Again ADM  122  will receive no data and send an AIS out to the system and upon reception of the AIS, ADM  126  will switch its data to be transmitted over cable  172  to ADM  128  of node  158  where it is switched to cable  176 . The data arrives at ADM  122  of node  152 , thus circumventing the failure, and is sent along its normal path to data output ports  182 .  
           [0020]    While the scenarios shown in FIGS. 3 through 6 are readily restorable assuming the traditional ring network switching behavior of the ADMs, there are other failure scenarios that present costly and potentially catastrophic outages which are difficult to repair and to restore transmission. For example, FIGS. 7 and 8 show failure scenarios for which restoration is not physically possible unless additional switching logic is employed beyond the usual ring network switching logic.  
           [0021]    In FIG. 7 failures occur at cable  180  and cable pair  192 . When this type of failure occurs, ADM  134  of node  154  will send an AIS to the system to attempt a rerouting of the data. Since data can only flow in one direction over the tributary links due to the inherent design of an ADM, an ADM can only transmit data in one direction and to specific outputs, ADM  132  of node  152  cannot reroute the data and the system cannot therefore recover from the failure.  
           [0022]    In FIG. 8 failures occur at cable  170  and cable pair  190 . When this type of failure occurs, ADM  122  of node  152  will send an AIS to the system to attempt a rerouting of the data. Again, data can only flow in one direction over the tributary links since an ADM can only transmit data in one direction and to specific outputs, ADM  124  of node  144  cannot reroute the data and the system cannot therefore recover from the failure.  
           [0023]    Unless additional costly switching logic is employed beyond the usual ring switching logic, or unless bi-directional switching, advanced matched node software, or network protection equipment (NPE) is utilized, the failures in FIG. 7 and FIG. 8 cause an unrecoverable failure, also known as a data traffic outage. The failure scenarios depicted in FIGS. 3 through 8 are examples and are not meant to be inclusive of all possible failures.  
           [0024]    It is therefore desirable to reduce the initial installation costs and recurring operating costs of a transoceanic system. It is also desirable to reduce the possibilities of data traffic outages due to occasional failures of cables and equipment.  
         SUMMARY OF THE INVENTION  
         [0025]    According to a first embodiment of the present invention, paired ADMs at a matched node site are replaced with a single switching device, such as a modified ADM or simple multiplexer. Furthermore, where a prior art three-ring network structure uses two fibers to form the interlink span (one for the backhaul ring and one for the transoceanic ring), a single fiber is used. This practice is particularly applicable to the transoceanic three-ring structure because there is normally no working traffic provisioned between adjacent matched-node sites. Furthermore, there is no increase to system robustness or reliability by using two fibers because, in practice, they are usually not diversely routed anyway.  
           [0026]    A second embodiment of the present invention eliminates two of the terrestrial backhaul two-port nodes thus decreasing cost while increasing reliability and robustness. A two-port ADM contained in a two-port node does not add or drop any signals from the three ring system. The ADM at a two-port site merely passes data from one cable to another cable. The data stream can be routed directly from the previous node to the next node in the data path thus reducing the need for the additional ADM. In addition to the cost savings on the ADM, additional savings occurs because less cable is required to connect the two remaining nodes.  
           [0027]    A third embodiment of the present invention utilizes multi-node rings. It replaces the two port nodes with three port nodes. Thus data either enters or leaves from four data ports in the network instead of two data ports.  
           [0028]    According to a fourth embodiment of the present invention, the overall reliability of the system is increased to an even greater extent by replacing the single connection between the terrestrial sites with paired connections. Where the interlink span is desired to be particularly robust by virtue of diversely routed multiple cables, a 4-fiber bi-directional line switched ring (BLSR) network may be used for the terrestrial portions, and an ADM or optical cross-connect switch may be used to pass signals directly into the transoceanic links at a full aggregate rate rather than at a tributary rate.  
           [0029]    These features and advantages of the present invention will be more readily apparent from the accompanying drawings and detailed description that follows. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    The present invention is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings. In the accompanying drawings similar references indicate similar elements. The drawings are described as follows:  
         [0031]    [0031]FIG. 1 illustrates a traditional transoceanic cable system;  
         [0032]    [0032]FIG. 2 is a block diagram illustration of the traditional three-ring architecture depicted in FIG. 1;  
         [0033]    [0033]FIGS. 3 through 5 illustrate single point failures in the traditional transoceanic cable system;  
         [0034]    [0034]FIG. 6 illustrates a catastrophic site failure in the traditional transoceanic cable system;  
         [0035]    [0035]FIGS. 7 and 8 illustrate dual point failures in the traditional transoceanic cable system;  
         [0036]    [0036]FIG. 9 illustrates a first embodiment of the present invention;  
         [0037]    [0037]FIGS. 10 through 12 illustrate single point failures in the first embodiment of the present invention;  
         [0038]    [0038]FIG. 13 illustrates a catastrophic site failure in the first embodiment of the present invention;  
         [0039]    [0039]FIGS. 14 and 15 illustrate dual point failures in the first embodiment of the present invention;  
         [0040]    [0040]FIG. 16 illustrates a three-node ring communications system according to another embodiment of the present invention;  
         [0041]    [0041]FIG. 17 illustrates a bi-directional communications system according to a further embodiment of the present invention; and  
         [0042]    [0042]FIG. 18 illustrates a 4-fiber bi-directional line switched ring (BLSR) configuration according to yet another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0043]    Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.  
         [0044]    The present invention relates to a system for communicating between distant regions. The system utilizes a basic three ring network, wherein each ring network is comprised of at least three nodes. Each ring network, though connected to at least one node of another ring network, can be viewed as occupying a separate region from the other ring networks. The traditional three-ring architecture is depicted in FIG. 2, wherein three distinct rings are visible, i.e. backhaul rings  110  and  130 , and transoceanic ring  120 .  
         [0045]    [0045]FIG. 9 depicts a first embodiment of the present invention. Cables  174  and  176  shown in FIG. 2 are no longer required. As shown in FIG. 9, only cables  162  and  188  remain. For the sake of clarity, the system will still be described as having three ring networks, each of which is located in a distinct region: a first ring network  110  in a first region, a second ring network  120  in a second region, and a third ring network  130  in a third region. Each ring network is comprised of at least three nodes.  
         [0046]    [0046]FIG. 9 illustrates an extended transport dual-junction architecture in accordance with a preferred embodiment of the present invention. The system depicted in FIG. 9 is comprised of eight ADMs ( 112 ,  114 ,  116 ,  118 ,  132 ,  134 ,  136  and  138 ) and four multiplexers ( 910 ,  912 ,  914 , and  916 ). FIG. 9 depicts a four-node backhaul ring embodiment of the present invention. In contrast to the traditional three-ring architecture depicted in FIGS. 1 through 8, site  144  in FIG. 9 shows ADM  114  being coupled to a time-division multiplexer (TDM)  910  instead of a second ADM. Similarly, site  146  shows ADM  116  being coupled to TDM  914 . TDM  910  and TDM  914  serve to recombine (multiplex) tributary data streams from ADM  114  and ADM  116 , respectively, to yield an aggregate data stream to be transmitted along its respective transoceanic cable, i.e.  170  or  172 . Where cable  170  or cable  172  is a fiber optic cable, an optical transmitter (not shown) is used to couple a modulated optical carrier into the fiber optic cable. At the other end of each transoceanic cable, ADM  122  and ADM  128  are replaced by TDM  912  and TDM  916 , respectively. TDM  912  is used to adapt the received aggregate signal into the multiple tributaries expected by ADM  132 , and TDM  916  is used to adapt an aggregate signal it receives into the multiple tributaries expected by ADM  138 . TDM  910 , TDM  914 , TDM  912  and TDM  916  are depicted in FIG. 9 as separate elements for the purpose of parity with the traditional three-ring architecture of FIG. 1, but the multiplexing/demultiplexing functions can be accomplished with separate equipment, as shown in FIG. 9, or can be incorporated directly into the ADM switch element.  
         [0047]    Referring again to FIG. 9, under normal operating conditions data enters the system at data input ports  180  at node  142  wherein ADM  112  multiplexes the data and transmits the data through conduit  160  to node  144 . When the data arrives at node  144 , ADM  114  demultiplexes the data and transmits the demultiplexed data to TDM  910 . TDM  910  multiplexes the data and transmits the data through cable  170  to TDM  912  of node  152 . TDM  912  demultiplexes the data and transmits the data to ADM  132 , which in turn transmits the data to ADM  134  of node  154 . ADM  134  transmits the data to ADM  136  of node  156  which outputs the data at output ports  182  where it is routed to other networks of the system.  
         [0048]    One advantage of the embodiment of FIG. 9 is that existing installations and ADM equipment are readily convertible. Another notable difference between the embodiment shown in FIG. 9 and the prior art shown in FIGS. 1 and 2 is the elimination of interlink connection  174  between sites  144  and  146  and interlink connection  176  between sites  152  and  158  that were previously dedicated to the formation of the transoceanic ring. By eliminating the ADMs and the additional cables, the cost of the system is greatly reduced and the reliability of the system is increased. The cost reduction is due to the use of less ADMs and cable; the reliability is increased due to the fact that there are fewer components prone to failure, and more importantly, the system can recover from failures that the traditional three-ring structure could not as described below.  
         [0049]    [0049]FIGS. 10 through 15 depict the communications system of FIG. 9 under various failure scenarios.  
         [0050]    Shown in FIG. 10 is a failure of cable  160 . ADM  114  sends an AIS to the system and ADM  112  switches its data path to cable  161 . The data passes through ADM  118 , across cable  171  and to ADM  116 . ADM  116  switches the data to cable  162  and on to ADM  114 , thus circumventing the failure. The data is then routed along its normal data path to output ports  182 .  
         [0051]    [0051]FIG. 11 depicts a situation where one of the transoceanic cables fails. Referring to FIG. 11, transoceanic cable  170  experiences a failure. An AIS is sent through the system by ADM  132  informing ADM  114  that ADM  132  is not receiving data. ADM  114  switches its data route to cable  162 . When the data arrives at ADM  116 , it sends the data across tributary links to TDM  914 . TDM  914  multiplexes the data and routes the data across cable  172  to TDM  916 . TDM  916  demultiplexes the data and routes it to ADM  138 . The data is sent along cable  188  to ADM  132 , thus circumventing the failure, and where it is routed along its normal data path to output ports  182 .  
         [0052]    [0052]FIG. 12 depicts a tributary interconnect link failure. Link  164  experiences a failure. ADM  132  notifies the system that it is not receiving data. ADM  114  switches its data to output onto cable  162 . The data routes through ADM  116 , through its tributaries where it is multiplexed by TDM  914 . The data is routed along transoceanic cable  172  to TDM  916  where it is converted to tributary data for ADM  138 . ADM  138  switches the data to cable  188  to ADM  132 , thus circumventing the failure, and where it continues on its normal path to output ports  182 .  
         [0053]    [0053]FIG. 13 depicts a node site failure. Referring to FIG. 13, a failure occurs at node site  144 . An AIS is transmitted to the system by ADM  132  causing ADM  112  to switch its data path from cable  160  to cable  161 . The data passes from cable  161  through ADM  118  and onto cable  171 . Since data cannot pass along cable  162 , ADM  116  switches its data path from cable  162  to its tributary links along to TDM  914 . The aggregate data is transmitted along transoceanic cable  172  where it arrives at TDM  916 . TDM  916  demultiplexes the data and passes it along to ADM  138 . ADM  138  transmits the data onto cable  188 . ADM  132  receives the data, thus circumventing the failure, and where it then continues on its normal path to output ports  182 .  
         [0054]    The failures depicted in FIGS. 10 through 13 depict failures for which the conventional ring switching logic, AIS and APS schemes of the ADMs and system suffice to maintain communications. They are not intended to depict all possible failure scenarios.  
         [0055]    [0055]FIGS. 14 and 15 depict dual failures experienced by the communications system of FIG. 9. In the traditional three-ring architecture, these types of failures will result in a data traffic outage. By implementing the present invention, dual failure scenarios that traditionally result in data traffic outages are restorable by appropriate switching actions. The switching actions can be automatically implemented through an APS scheme, or through a manual control switching station.  
         [0056]    Referring to FIG. 14, when a dual failure of cable  160  and transoceanic cable  170  occurs data is routed along the path shown by the dashed line and rerouted to output ports  182 . ADM  132  communicates an AIS signal to the system indicating that the former is not receiving any data signals from any of the other nodes in the ring. ADMs  114  and  116  then coordinate to drop the signal at ADM  116  and transmit through cable  172  to ADM  138  where it may then reach its intended destination, output ports  182 . By removing ADM  124  from the system and replacing it with TDM  910 , the system can recover from the failure since the switching is now controlled only by ADM  114 . If ADM  124  were still in the system, it would be unable to reroute the data back to ADM  114  due to its inherent switching constraints.  
         [0057]    [0057]FIG. 15 depicts another dual failure scenario that traditionally results in traffic outage, but with the implementation of the present invention, even with complete faults to cables  162  and  170 , data traffic is still restorable by the appropriate switching actions. When a dual failure of cable  170  and cable  162  occurs, ADM  132  notifies the system of data loss. As depicted in FIG. 8, if ADM  124  were still present, the system would fail because the data can only flow one direction over the tributary links due to the design constraints of an ADM, and a data outage would occur. With the removal of ADM  124  and its replacement by TDM  910 , ADM  114  can now handle the required switchover back through cable  160  to ADM  112 . ADM  112  routes the data over cable  161  to ADM  118  where it is passed along onto cable  171 . ADM  116  receives the data and attempts a switch to cable  162 . If the attempt was made, an AIS would occur, and ADM  116  would then switch the data to its tributary links to TDM  914 . The data travels across cable  172  to TDM  916  where it is demultiplexed and forwarded to ADM  138 . ADM  138  switches the data to ADM  132  where it is routed along its normal data path to output ports  182 , circumventing the failure and avoiding a data outage.  
         [0058]    Another advantage of the present invention is that full aggregate data can be transmitted across the tributary links of link  164  and its counterparts contained in the other nodes. If one of the links fail the full aggregate data can easily be rerouted by an intranodal switch, rather than an internodal switch, to another tributary link, thus avoiding any further switching.  
         [0059]    In the three-node embodiment of the present invention depicted in FIG. 16, node  148  and ADM  118  are removed and a direct connection is made between node  142  and node  146 . Similarly, node  154  and ADM  134  are removed and a direct connection is made between node  152  and node  156 . Since, in the traditional configuration, ADM  118  and ADM  134  (depicted in the figure only for clarity, but not in ultimate design) merely serve to pass data along to ADM  116  and ADM  136 , respectively, ADM  118  and ADM  134  are unnecessary components in the overall system. By eliminating ADM  118  and ADM  134  their costs are eliminated. Also, the system is more reliable in that there are now two less components that may experience failure. Furthermore, by eliminating the two ADMs, the cable connecting ADM  112  to ADM  116  and the cable connecting ADM  132  to ADM  136  can be shorter thereby further decreasing the cost of the system.  
         [0060]    [0060]FIG. 17 depicts a multi-node ring configuration of the present invention. Even though a single direction of communications has been shown for clarity, those of ordinary skill in the relevant art will readily recognize that the present invention may achieve reliable bi-directional communications between two regions with little to no adaptation beyond what has already been taught herein. The system of FIG. 17 replaces the two port nodes (i.e. ADM  118  and ADM  134 ) with three port nodes (i.e. ADM  1718  and ADM  1734 ). Thus data either enters or leaves from four data ports in the network instead of two data ports adding further flexibility to the overall system. This system operates as described above.  
         [0061]    [0061]FIG. 18 depicts a fourth embodiment of the present invention. The overall reliability of the system is increased to an even greater extent by replacing the single connection between the terrestrial sites with paired connections. Where the interlink span is desired to be particularly robust by virtue of diversely routed multiple cables, a 4-fiber bi-directional line switched ring (BLSR) may be used for the terrestrial portions, and an ADM or optical cross-connect switch may be used to pass signals directly into the transoceanic links at a full aggregate rate rather than at a tributary rate. The overall system depicted in FIG. 18 operates as that shown in FIG. 9. Though the cost of the additional cables increases the overall system costs, the increase in system reliability balances any additional costs.  
         [0062]    While a preferred embodiment of the present invention has been shown and described in the context of a transoceanic cable, those of ordinary skill in the art will recognize that the present invention may be applied to achieving reliable communications through any form of information conduit across a span where the conduits are not readily accessible and it is impractical or impossible to employ intermediate sites to act upon the information traffic, thus resulting in improved robustness and reliability to the overall system.  
         [0063]    While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.