Abstract:
A telecommunications network node architecture is disclosed that enables a telecommunications network that uses automatic protection switching to be expanded to include more nodes than its standard protocol provides for without modifying the standard protocol or the existing nodes in the network. Although the illustrative embodiment is depicted as using the SONET/SDH protocol, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention that use automatic protection switching with another protocol. The illustrative embodiment comprises: an automatic protection switching channel that defines an address space in the telecommunications network; a node that is uniquely identified by an address in the address space; and a node that is not uniquely identified by an address in the address space.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of telecommunications, and, more specifically, to a means to increase the number of nodes in a telecommunications network (e.g., a SONET/SDH network, a dense wavelength division multiplexing network, etc.) that has automatic protection switching beyond current limitations without changing the telecommunications network&#39;s protocol or the existing nodes in the network. 
     BACKGROUND OF THE INVENTION 
     Today, optical fiber systems are in widespread use in both public and private telephone and data networks. In the early stages of optical fiber networks, however, deployment was limited to high-revenue-generating applications. This limited deployment was due to communications-equipment manufacturers making network components using unique, proprietary architectures. The result of which, of course, was that the network components from one manufacturer did not work with other manufacturers&#39; network components. An operating company implementing an early optical fiber network had to purchase most, if not all, of its network components from one manufacturer. 
     In order to provide inter-operability among components from the various manufacturers (and thus lower costs to the operating companies), Bellcore established a standard for connecting one optical fiber component or system to another. That standard is officially named the “Synchronous Optical Network,” but is more commonly called “SONET.” The international version of the standard is officially named the “Synchronous Digital Hierarchy,” but it is more commonly called “SDH.” 
     Although differences exist between SONET and SDH, those differences are mostly in terminology. In most respects, the two standards are the same and, therefore, virtually all equipment that complies with either the SONET standard or the SDH standard also complies with the other. Therefore, for the purposes of this specification, the SONET standard and the SDH standard shall be considered interchangeable and the acronym/initialism “SONET/SDH” shall be defined as either the Synchronous Optical Network standard or the Synchronous Digital Hierarchy standard, or both. 
     The basic SONET/SDH signal is defined as a Synchronous Transport Signal level 1 (STS-1) frame. An STS-1 frame is an 810-byte data packet that comprises transport overhead (the information required to maintain communication) and payload (the data itself). For the purposes of this specification, a “STS-N” is defined to comprise N STS-1s. For example, an STS-768 comprises the data from 768 STS-1s plus the overhead of the STS-768. Furthermore, for the purposes of this specification, an “STS-N frame” is defined to comprise N STS-1 frames of data and the overhead of the STS-N frame. For example, an STS-768 frame comprises 768 STS-1 frames. 
     Also, for the purposes of this specification, a “SONET/SDH network” is defined as two or more nodes and transmission facilities (e.g., optical fibers, repeaters, etc.) that connect the nodes.  FIG. 1  illustrates a block diagram of a SONET/SDH network  10  in the form of a “ring,” as is well known in the art. In this elementary example, there are three nodes  12 ,  14  and  16  connected in a closed loop by a pair of optical-transmission facilities  18  and  20 . 
     For the purposes of this specification, a “node” is defined as a network element in a telecommunications network that;
         i. originates and/or terminates digital signals, or   ii. that digitally cross-connects digital signals, or both i and ii.       

     In this example, each of nodes  12 ,  14 , and  16  is connected to a plurality of sources and/or destinations for data traffic, which are well known in the art as “tributaries.” Node  12  originates/terminates traffic between network  10  and tributaries  32 , node  14  originates/terminates traffic between network  10  and tributaries  34  and node  16  originates/terminates traffic between network  10  and tributaries  36 . Each of nodes  12 ,  14 , and  16  receives data from one or more of its respective tributaries  32 ,  34 , and  36  at an STS-N rate, multiplexes the data to the data rate of the ring (which is, by definition, higher than the data rate of the tributaries), and transmits the data around SONET/SDH ring  10 . Simultaneously, each of nodes  12 ,  14 , and  16  receives data from the SONET/SDH ring  10 , demultiplexes the data to the data rate of the destination tributary and sends the data on the tributary. 
     As stated above, each of nodes  12 ,  14 , and  16  in SONET/SDH ring  10  is connected to the next node by a pair of optical transmission facilities  18  and  20 . In normal operation, each node transmits STS-N frames around ring  10  either counterclockwise on optical transmission facilities  18  or clockwise on optical transmission facilities  20 . 
     When a discontinuity or failure occurs in a SONET/SDH ring, the affected traffic is re-routed around the discontinuity in accordance with a procedure called “automatic protection switching.” In order to implement automatic protection switching, each SONET/SDH ring defines a distinct address space and a unique address (or “Node ID”) that uniquely identifies each node within the network. The current SONET/SDH standard specifies that addresses in the address space of a SONET/SDH ring are carried in the K 1  and K 2  bytes in the line overhead of an STS-N frame. 
     The K 1  and K 2  bytes comprise: 
     
       
         
               
               
               
             
           
               
                   
               
             
             
               
                 K 1  Byte: 
                 Bits 1–4: 
                 Type of automatic protection switch request 
               
               
                   
                   
                 (lock out of automatic protection switching, 
               
               
                   
                   
                 forced switch, signal failure, signal 
               
               
                   
                   
                 degradation, manual switch, etc.). 
               
               
                   
                 Bits 5–8: 
                 The destination Node ID of the automatic 
               
               
                   
                   
                 protection switch message. 
               
               
                 K 2  Byte: 
                 Bits 1–4: 
                 Source Node ID of the automatic protection 
               
               
                   
                   
                 switch message. 
               
               
                   
                 Bit 5: 
                 Indication of automatic protection switching 
               
               
                   
                   
                 (short or long path). 
               
               
                   
                 Bits 6–8: 
                 Mode of operation (Line alarm indication 
               
               
                   
                   
                 signal, line remote defect indication, etc.). 
               
               
                   
               
             
          
         
       
     
     In the example of  FIG. 1 , nodes  12 ,  14 , and  16  have Node ID&#39;s according to Table  1 : 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Node Addresses for SONET/SDH Ring 10 
               
             
          
           
               
                   
                 Node 
                 SONET/SDH Ring 10 Node ID 
               
               
                   
                   
               
               
                   
                 Node 12 
                 0 
               
               
                   
                 Node 14 
                 1 
               
               
                   
                 Node 16 
                 2 
               
               
                   
                   
               
             
          
         
       
     
     For purposes of understanding automatic protection switching in the prior art, assume that node  12  is receiving traffic on one or more tributaries  32  destined for node  14 &#39;s tributaries  34 . Furthermore, assume that node  14  detects a fault or failure on the optical transmission facility  18  between node  12  and node  14 . Node  14  notifies both node  12  and node  16 . To this end, node  14  populates an STS-N frame overhead K 1  and K 2  bytes for node  12  as follows: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 K 1 : 
                 bits 1–4 
                 automatic protection switch request. 
               
               
                   
                   
                 bits 5–8: 
                 the Node ID of node 12 (“0” in this example). 
               
               
                   
                 K 2 : 
                 bits 1–4: 
                 its own Node ID (“1” in this example). 
               
               
                   
                   
                 bit 5: 
                 short path. 
               
               
                   
                   
                 bits 6–8: 
                 the remote defect indication (“RDI”). 
               
               
                   
                   
               
             
          
         
       
     
     Node  14  sends the STS-N frame in the clockwise  20  direction. 
     Node  14  notifies node  16  by populating an STS-N frame overhead K 1  and K 2  bytes as follows: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 K 1 : 
                 bits 1–4 
                 automatic protection switch request. 
               
               
                   
                   
                 bits 5–8: 
                 the Node ID of node 12 (“0” in this example). 
               
               
                   
                 K 2 : 
                 bits 1–4: 
                 its own Node ID (“1” in this example). 
               
               
                   
                   
                 bit 5: 
                 long path. 
               
               
                   
                   
                 bits 6–8: 
                 the bridged and switched state. 
               
               
                   
                   
               
             
          
         
       
     
     Node  14  sends the STS-N frame in the counterclockwise  18  direction. 
     Node  12  receives the K 1  and K 2  bytes from the STS-N frame on clockwise optical transmission facility  20 . Node  12  reacts to the K 1  and K 2  bytes by discontinuing transmission on optical transmission facility  18 , and switching to clockwise optical transmission facility  20 . In the counterclockwise direction, node  16  reads the K 1  and K 2  bytes, notes that its own Node ID, “2,” is not in the K 2  byte, and does not change the K 1  and K 2  bytes (“pass through mode”). 
     For the purposes of this specification, the term “short path” is defined as the path between the two nodes adjacent to the failed span that includes the failed span, and the term “long path” is defined as the path between the two nodes adjacent to the failed span that does not include the failed span. Therefore, when a discontinuity or failure occurs in a SONET/SDH ring, the affected traffic is re-routed from the short path to the long path. 
     The example of  FIG. 1  illustrates only three nodes in network  10 . More nodes are usually present, as is well known in the art. A problem in the art exists, however, because the SONET/SDH standard limits the maximum number of source and destination nodes in it definition of K 1  and K 2  bytes to four bits, or 16, thus limiting the size and flexibility of SONET/SDH rings. As demand for data traffic increases, this limitation on the number of nodes in a ring requires that, after 16 nodes are equipped in a network, an entire new network must be added at considerable expense. 
     Furthermore, the number of tributaries in one location along the ring can require more tributaries that one node can support. In the prior art, this scenario requires a new node to be defined in the address space of the network. Therefore, it is an object of this invention to provide a means to increase the number of nodes in a SONET/SDH network beyond current limitations without changing the SONET/SDH standard or modifying the existing nodes in the ring. 
     SUMMARY OF THE INVENTION 
     The present invention provides a telecommunications network node architecture that avoids some of the costs and disadvantages of telecommunications network node architectures in the prior art. In particular, the present invention enables a telecommunications network that uses automatic protection switching to be expanded to include more nodes than its standard protocol provides for without modifying the standard protocol or the existing nodes in the network. Although the illustrative embodiment is depicted as using the SONET/SDH protocol, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention that use automatic protection switching with another protocol (e.g., dense wavelength division multiplexing, etc.). 
     In particular, the illustrative embodiment of the present invention enables a first node in a SONET/SDH ring to affect the operation of a second node in the ring, wherein the second node does not have an address within the address space of the SONET/SDH network, wherein the address space of the SONET/SDH network is defined by bits  5 – 8  of the K 1  byte and bits  1 – 4  of the K 2  byte of the line overhead. 
     In accordance with the illustrative embodiment, a node in a SONET/SDH network that does not have an address in the address space of the network, as defined by the K 1  and K 2  bytes of the line overhead, is referred to as an “invisible” node. Furthermore, an “invisible” node can originate, terminate, and/or switch STS-N channels, and can modify the section, line, and path overhead, as necessary or appropriate, except that an “invisible” node does not modify the K 1  byte of the line overhead. In other words, the “invisible” node monitors the K 1  and K 2  bytes of the line overhead and modifies its operation based on the K 1  and K 2  bytes of the line overhead, but an “invisible” node does not modify the K 1  byte of the line overhead. 
     In contrast, a node in a SONET/SDH network that has an address in the address space of the network, as defined by the K 1  and K 2  bytes of the line overhead, is herein referred to as a “visible” node. In accordance with the illustrative embodiment, a “visible” node can originate, terminate, and/or switch STS-N channels, and can modify the section, line, and path overhead, including the K 1  and K 2  bytes of the line overhead. 
     When an invisible node receives an STS-N frame that indicates the need for automatic protection switching, the invisible node performs the automatic protection switching as necessary and forwards the STS-N frame on to the next node without modifying the K 1  and K 2  bytes of the line overhead. 
     The illustrative embodiment comprises: an automatic protection switching channel that defines an address space in the telecommunications network; a node that is uniquely identified by an address in the address space; and a node that is not uniquely identified by an address in the address space. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the invention can be obtained from a consideration of the specification in conjunction with the drawings, in which: 
         FIG. 1  depicts a schematic diagram of a SONET/SDH ring in accordance with the prior art; 
         FIG. 2  depicts a schematic diagram of a SONET/SDH ring with an invisible node in accordance with an illustrative embodiment of this invention; 
         FIG. 3  depicts a schematic diagram of the SONET/SDH ring of  FIG. 2  wherein a first example of automatic protection switching is illustrated in accordance with an illustrative embodiment of this invention; 
         FIG. 4  depicts a schematic diagram of the SONET/SDH ring of  FIG. 2  wherein a second example of automatic protection switching is illustrated in accordance with an illustrative embodiment of this invention; 
         FIG. 5  depicts a schematic diagram of the SONET/SDH ring of  FIG. 2  wherein a third example of automatic protection switching is illustrated in accordance with an illustrative embodiment of this invention; 
         FIG. 6  depicts a block diagram of the salient components of an invisible node in accordance with the illustrative embodiment of this invention; 
         FIG. 7  depicts a flowchart of the operation of the invisible node of  FIG. 6  in accordance with the illustrative embodiment of this invention; and 
         FIG. 8  depicts a schematic diagram of a SONET/SDH network ring with multiple invisible nodes in accordance with another illustrative embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  represents a block diagram of SONET/SDH ring  100  in accordance with the illustrative embodiment of the present invention. SONET/SDH ring  100 , which is similar to the SONET/SDH network  10  of  FIG. 1 , comprises a plurality of nodes, represented by nodes  102 ,  104 ,  106 ,  108 ,  110 , and  112 . In accordance with the illustrative embodiment, nodes  102 ,  104 ,  106 ,  108 ,  110 , and  112  are assigned the following addresses in the address space of SONET/SDH ring  100 : 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Node Addresses for SONET/SDH Ring 100 
               
             
          
           
               
                   
                 Node 
                 SONET/SDH Network 100 Address 
               
               
                   
                   
               
               
                   
                 Node 102 
                 0 
               
               
                   
                 Node 104 
                 Undefined (“NULL”) 
               
               
                   
                 Node 106 
                 1 
               
               
                   
                 Node 108 
                 2 
               
               
                   
                 Node 110 
                 3 
               
               
                   
                 Node 112 
                 4 
               
               
                   
                   
               
             
          
         
       
     
     In accordance with the illustrative embodiment, node  104  is not assigned an address and is, therefore, invisible to the other nodes in ring  100 . 
     Each of nodes  102 ,  104 ,  106 ,  108 ,  110 , and  112  supports a plurality of tributaries  130 ,  132 ,  134 ,  136 ,  138 , and  140 , respectively, which originate and terminate traffic, as is well known in the art. A pair of fiber optic transmission facilities  118  and  120  interconnects nodes  102 ,  104 ,  106 ,  108 ,  110 , and  112 . Data is transmitted on fiber optic transmission facility  118  in a counterclockwise direction and on fiber optic transmission facility  120  in a clockwise direction. 
     Node  104  and node  106  together compose a virtual node, virtual node  114 . Within virtual node  114 , node  106  is a “master” to “slave” invisible node  104 , because it reacts to the K 1  and K 2  bytes of the line overhead that are addressed to/from master node  106 . Furthermore, a communications link  142  between master node  106  and slave node  104  provides out-of-band communication between master node  106  and slave node  104 . Communications link  142  can be a dedicated communication channel, an Ethernet connection, etc., as is known in the art. 
     Turning now to  FIG. 3 , the SONET/SDH ring  100  of  FIG. 2  is illustrated with discontinuity  300  in fiber optical facility  118  (the counterclockwise ring) between nodes  102  and  112 . For purposes of this example, node  112  transmits data to invisible node  104 . Thus, the “short path” is nodes  112 → 102  and the “long path” is nodes  112 → 110 → 108 → 106 → 104 → 102 . Node  102  detects the discontinuity in a manner that is well known in the art. Node  102  takes corrective action by reconfiguring its switching node and sending a protection switching alarm in both directions around ring  100 . 
     Invisible node  104  learns of discontinuity  300  from node  102  in two ways. 
     First, node  102  sends the next STS-N frame overhead in the clockwise direction  120  (towards node  112 ) with the K 1  and K 2  bytes populated as follows: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 K 1 : 
                 bits 1–4: 
                 automatic protection switch request. 
               
               
                   
                   
                 bits 5–8: 
                 the Node ID of node 112 (“4” in this example). 
               
               
                   
                 K 2 : 
                 bits 1–4: 
                 its own Node ID (“0” in this example). 
               
               
                   
                   
                 bit 5: 
                 short path. 
               
               
                   
                   
                 bits 6–8: 
                 RDI. 
               
               
                   
                   
               
             
          
         
       
     
     And second, node  102  sends the next STS-N frame overhead in the counterclockwise  118  direction (towards node  104 ) with the K 1  and K 2  bytes populated as follows: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 K 1 : 
                 bits 1–4: 
                 automatic protection switch request. 
               
               
                   
                   
                 bits 5–8: 
                 the Node ID of node 112 (“4” in this example). 
               
               
                   
                 K 2 : 
                 bits 1–4: 
                 its own Node ID (“0” in this example). 
               
               
                   
                   
                 bit 5: 
                 long path. 
               
               
                   
                   
                 bits 6–8: 
                 Bridged and switched State. 
               
               
                   
                   
               
             
          
         
       
     
     Invisible node  104  receives the STS-N frame from the short path before node  106  receives it. Invisible node reads the STS-N header information, including the K 1  and K 2  bytes, and executes automatic protection switching (in this example, node  104  starts receiving the STS-N data that it is expecting from node  112  from the long path instead). Invisible node  104  then sends an STS-N frame to node  106  (the “master” node) but does not alter the K 1  and K 2  bytes. Master node  106  performs protection switching and any other functions and also does not modify the K 1  and K 2  bytes (“pass through mode”), as is well known in the art and, therefore, is not discussed further. 
       FIG. 4  illustrates the SONET/SDH ring  100  of  FIG. 2  with discontinuity  400  in fiber optical facility  120  (the clockwise ring) between nodes  106  and  108 . For purposes of this example, node  108  is sending data to node  104 . Thus the short path is node  108 → 106 → 104  and the long path is  108 → 110 → 112 → 102 → 104 . Node  106  detects the discontinuity in a manner well known in the art. Node  106  takes corrective action by reconfiguring its switching node and sending a protection switching message in both directions. 
     Node  106  alerts node  108  of discontinuity  300  in two ways. 
     First, node  106  alerts node  108  of discontinuity  400  by sending the next STS-N frame overhead in the counterclockwise direction (towards node  110 ) with the K 1  and K 2  bytes populated as follows: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 K 1 : 
                 bits 1–4: 
                 automatic protection switch request. 
               
               
                   
                   
                 bits 5–8: 
                 the Node ID of node 108 (“2” in this example). 
               
               
                   
                 K 2 : 
                 bits 1–4: 
                 its own Node ID (“1” in this example). 
               
               
                   
                   
                 bit 5: 
                 short path. 
               
               
                   
                   
                 bits 6–8: 
                 RDI. 
               
               
                   
                   
               
             
          
         
       
     
     And second, node  106  alerts node  108  of discontinuity  400  by sending the next STS-N frame overhead in the clockwise direction (towards node  104 ) with the K 1  and K 2  bytes populated as follows: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 K 1 : 
                 bits 1–4: 
                 automatic protection switch request. 
               
               
                   
                   
                 bits 5–8: 
                 the Node ID of node 108 (“2” in this example). 
               
               
                   
                 K 2 : 
                 bits 1–4: 
                 its own Node ID (“1” in this example). 
               
               
                   
                   
                 bit 5: 
                 long path. 
               
               
                   
                   
                 bits 6–8: 
                 Bridged and Switched State. 
               
               
                   
                   
               
             
          
         
       
     
     Invisible node  104  monitors the K 1  and K 2  bytes in the line overhead of the STS-N frame transmitted in the counterclockwise direction on optical fiber  118  and the K 1  and K 2  bytes in the line overhead of the STS-N frame transmitted in the clockwise direction on optical fiber  120  and performs automatic protection switching by looking for the data from node  108  on the long path. 
     In accordance with the illustrative embodiment of the present invention, there is a situation in which an invisible node does in fact modify the K 2  byte of the line overhead. In particular, the invisible node modifies the K 2  byte of the line overhead when it detects a discontinuity between itself and another node (either master or slave) within its virtual node. This is because the invisible node within the virtual node must inform one or more other nodes of the discontinuity. 
     For example, in  FIG. 5 , SONET/SDH ring  100  is illustrated with a discontinuity in fiber optical facility  120  between invisible node  104  and master node  106 . If invisible node  104  did not exist, then node  106  would inform node  102  of the discontinuity in well-known fashion. But because invisible node  104  does exist and blocks node  102 &#39;s ability to directly observe the discontinuity, node  104  must “pretend” to be node  106  and must inform node  102  of the discontinuity as if it were node  106 . 
     Therefore, because the invisible node detects the discontinuity between itself and master node  106 , invisible node  104  notifies node  102  of the situation by populating bits  6 - 8  of the K 2  byte with the next STS-N frame header with the Line Alarm Indication Signal (AIS-L) status. In this manner, the discontinuity is communicated to node  102  as if it were between node  102  and node  106 . Once node  102  learns of the discontinuity, it performs automatic protection switching in well-known fashion. 
       FIG. 6  depicts a block diagram of the salient components of invisible node  104 , which comprises add/drop multiplexer-digital cross-connect system (“ADM/DCS”)  601 , input ports  611 - 1  through  611 -j, and output ports  612 - 1  through  612 -k, wherein j and k are positive integers and wherein j+k&gt;2. 
     Each of input ports  611 - 1  through  611 -j receives a signal (e.g., a low-rate tributary, a STS-N, etc.) from an optical fiber or other transmission facility (e.g., metallic wireline, microwave channel, etc.) and passes the signal to ADM/DCS  601 , in well-known fashion. Each of output ports  612 - 1  through  612 -k receives a signal from ADM/DCS  601  and transmits the signal via an optical fiber or other transmission facility, in well-known fashion. When invisible node  104  receives a signal from one or more tributaries, ADM/DCS  601  enables invisible node  104  to add the tributaries into one or more STS-Ns. When invisible node  104  transits a signal via one or more tributaries, ADM/DCS  601  enables invisible node  104  to drop the tributaries from one or STS-Ns. 
     For purposes of describing the illustrative embodiments of this invention, ADM/DCS  601  can be the same as or similar to the ADM/DCS described in U.S. patent application No. Ser. No. 09/974,448, filed Nov. 10, 2001, which is assigned to the present assignee and is incorporated herein by reference. ADM/DCS  601  can, however, be similar to any ADM/DCS heretofore known or used in the art. 
     In  FIG. 7 , the operation of an invisible node, such as invisible node  104  and ADM/DCS  601  is described. Operation starts in circle  700  and moves to action box  702 , where the invisible node receives one or more signals that are associated with a SONET/SDH network. Processing continues in action box  704 , where the invisible node terminates the lines as provisioned, including performing add/drop multiplexing, as known in the art. 
     Processing moves to decision diamond  706 , where a determination is made whether the K 1  and K 2  bytes indicate protection-switching action should be taken. If protection-switching action is required, the invisible node takes action appropriate for protection switching (as provisioned) in action box  708 . In contradiction to the prior art, the invisible node modifies the STS-N frame overhead information as appropriate, but does NOT modify the K 1  and K 2  bytes. Processing from action box  708  and the “no” leg of decision diamond  706  continues to action box  710 , where the invisible node transmits a data signal via an optical fiber to the subsequent nodes in its ring. The next node in the ring, if it is not an invisible node, terminates the K 1  and K 2  bytes and takes appropriate action, including modify the K 1  and K 2  bytes. Processing loops back to  702 . 
     There can be more than one invisible node in a network.  FIG. 8  represents a block diagram of a further SONET/SDH network  800  in accordance with another aspect of this invention. According to this illustrative embodiment of this invention, there are a plurality of nodes connected in a ring, represented by nodes  802 ,  804 ,  806 ,  808 ,  810  and  812 . The nodes are connected in a ring structure by optical transmission facilities  818  and  820 . Each node  802 ,  804 ,  806 ,  808 ,  810  and  812  supports a plurality of tributaries  830 ,  832 ,  834 ,  836 ,  838  and  840 , respectively, as is known in the art. 
     Nodes  802  and  806  are invisible nodes as defined above, and node  804  is the master node, thus forming virtual node  814 . Each invisible node  802  and  806  communicates with master node  804  via communication links  842 . 
     In accordance with this illustrative embodiment, node  802 ,  804 ,  806 ,  808 ,  810  and  812  are assigned the following addresses in the address space of SONET/SDH ring  800 : 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Node Addresses for SONET/SDH Network 800 
               
             
          
           
               
                   
                 Node 
                 SONET/SDH Ring 800 Address 
               
               
                   
                   
               
               
                   
                 Node 802 
                 NULL 
               
               
                   
                 Node 804 
                 0 
               
               
                   
                 Node 806 
                 NULL 
               
               
                   
                 Node 808 
                 1 
               
               
                   
                 Node 810 
                 2 
               
               
                   
                 Node 812 
                 3 
               
               
                   
                   
               
             
          
         
       
     
     Node  802  and node  806  are illustrated in Table 3 as having NULL addresses and are thus invisible nodes according to the exemplary embodiment of this invention. 
     Plural invisible nodes do not have to be in a particular order, such as the order illustrated in  FIG. 8 . Two (or more) invisible nodes can be connected to one side or the other side of the master node. Furthermore, there can be more than one virtual node in a ring. One skilled in the art will appreciate the variety of architectures available when using invisible nodes in a ring structure, after reviewing this specification. 
     It is to be understood that the above-described embodiment is merely illustrative of the present invention and that many variations of the above-described embodiment can be devised by one skilled in the art without departing from the scope of the invention. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.