Abstract:
An apparatus and method that extend the automatic protection switching protocol to address at least 256 network nodes. By using overhead bytes as extended APS node IDs, large single ring SONET/SDH systems can be avoided. This means APS messages that force every node into a single ring can be avoided and recovery performance from a break in the ring or a node fault can be improved. The protocol for the extended automatic protection switching channels takes multiple extended APS node IDs from tributary lines and merges those extended APS ID&#39;s into a single SONET/SDH stream on another line. Placement of the extended APS node ID&#39;s in the overhead bytes of SONET/SDH frames allows easy relay around each SONET/SDH ring.

Description:
FIELD OF THE INVENTION 
     The present invention relates to telecommunications in general, and, more particularly, to telecommunications networks that use an automatic protection switching (APS) protocol. 
     BACKGROUND OF THE INVENTION 
     SONET/SDH systems have been built and used for a number of years. Although differences exist between SONET and SDH, those differences are mostly in terminology. In most respects, the two standards are the same and 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 ). An STS- 1  frame is an 810-byte data packet comprising transport overhead (the information required to maintain communication) and payload (the data itself). 
     SONET/SDH systems come in many different configurations, but frequently form a network of SONET/SDH nodes connected by links in a closed loop, known in the art as a “ring.” In a ring network, there are two paths between any pair of nodes, one transporting communications signals clockwise and the other counterclockwise. 
     To protect communications signals from link failures, SONET/SDH rings use one of these two paths as the service, or working, connection and bandwidth is reserved along the other path as a backup (known in the art as a “protection channel”). When a break or fault occurs in a link, a message is sent out requesting protection switching around the break or fault to maintain communications. The request message is communicated by means of an automatic protection switching (APS) channel which uses a two-byte field, in which the two bytes are referred to as K 1  and K 2 , located within the transport overhead of a frame. This ability to rapidly respond with automatic protection switching around breaks and/or faults has made SONET/SDH systems very popular. 
     Traditional SONET/SDH systems use the STS- 1  line overhead to communicate bytes K 1  and K 2 , which are used for indicating a requested source node address and a selected adjacent destination node address as two ends of the protection switching path that is used to bypass any breaks or faults in a corresponding ‘working-ring-segment’ between those two nodes. The request message is usually followed by a command to switch the data traffic to a protection switching path that is predetermined as per BellCORE generic requirement, GR-1230, which is hereby incorporated by reference. BellCORE GR-1230 concerns SONET/SDH systems that have bi-directional line switched rings (BLSR) and defines the use of bytes K 1  and K 2  of STS- 1  section/line overhead for identifying a protection switching path. GR-1230 requires that the source node and the destination node of a protection switching path around a break or a fault be identified in bytes K 1  and K 2  in an APS message in BLSR SONET/SDH systems. 
     The GR-1230 APS channel format for bytes K 1  and K 2  is as follows: 
     K 1  Byte:
         Bits  1 – 4 : Type of request (e.g., Lock out of Protection, Forced Switch—Span, Forced Switch—Ring, Signal Fail—Span, Signal Fail—Ring, Signal Degrade—Protection, Signal Degrade—Span, Signal Degrade—Ring, Manual Switch—Span, Manual Switch—Ring, etc.)   Bits  5 – 8 : Destination Node ID (Address)       

     K 2  Byte:
         Bits  1 – 4 : Source Node ID (Address)   Bit  5 : Indication of architecture (Short Path or Long Path)   Bits  6 – 8 : Mode of operation (Line AIS-L, Line RDI-L, Extra Traffic, Bridged and Switched Status, Bridged Status and Idle).       

     Bytes K 1  and K 2  are used for APS channel signaling between line terminating entities for bi-directional protection switching and for detecting alarm indication signal (AIS-L) and Remote Defect Indication (RDI) signals. 
     In a BLSR SONET/SDH system, bytes K 1  and K 2  provide the only APS signaling channel. Bits  5 – 8  of byte K 1  indicate destination node ID and bits  1 – 4  of byte K 2  indicate source node ID. As can be readily appreciated, with only four bits available for specifying a node ID, a typical SONET/SDH system can only support up to sixteen nodes for a ring using standard protocol. 
     Furthermore, bytes K 1  and K 2  do not support more complex networks than rings (e.g., rings within rings, virtual rings, etc.). This shortcoming exists because the one APS channel and the sixteen nodes maximum are closely tied to the physical working ring and to the physical nodes of the ring. 
     Thus, it is desirable to provide a SONET/SDH system comprising multiple APS channels. It is also desirable to have a SONET/SDH system that is not limited to a maximum of sixteen nodes. It yet further desirable to provide a SONET/SDH system participating in multiple rings, whether the rings are real or virtual, with multiple APS channels and having a capability to address more than sixteen nodes per ring. 
     SUMMARY OF THE INVENTION 
     The aforementioned shortcomings in the art are addressed and a technological advance is achieved by providing a SONET/SDH system in which overhead bytes other than K 1  and K 2  carry additional protection switching (“APS”) node IDs. These additional APS node IDs are used to provide multiple multiplexed APS channels instead of the single unmultiplexed APS channel of the generic standard. The multiple multiplexed APS channels support more complex network arrangements, such as virtual rings and also rings within rings, which were previously not available through the use of the generic standard. The additional APS node IDs also help support more complex network arrangements. 
     The aforementioned shortcomings in the art are addressed and a technological advance is achieved by providing a SONET/SDH APS message that includes: a first group of bits designating one of a first group of APS nodes as a source node for fault condition operation of a first ring, and a second group of bits designating one of the first group of APS nodes as a destination node of the first ring for fault condition operation. The first group of bits and the second group of bits are located within a common line overhead of one SONET/SDH frame. Also included in the APS message are a third group of bits designating one of a second group of APS nodes as a source node of a second ring for fault condition operation, and a fourth group of bits designating one of the second group of APS nodes as a destination node of the second ring for fault condition operation. The third group of bits and the fourth group of bits are located in line overhead bytes of the same SONET/SDH frame at distinct bit locations than the first group of bits and the second group of bits. 
     The aforementioned shortcomings in the art are addressed and a technological advance is achieved by providing an apparatus for automatic protection switching in a SONET/SDH system having at least one ring, including a network element of the ring time-multiplexing frames between a slower downstream link and a faster upstream link by terminating the downstream frames and generating upstream frames with corresponding information for transmission on the faster link. The network element addresses nodes of the ring by four address bits as defined by GR-1230 and a group of extended address bits for representing automatic protection switching node addresses in support of protection switching on the slower downstream link. The network element also addresses nodes of the ring by four address bits as defined by GR-1230, and by a group of extended address bits for representing automatic protection switching node addresses on the faster upstream link. Thus, extended APS node signaling is communicated from a lower speed link to a higher speed link by the network element and the extended APS addresses are carried in line overhead bytes of a SONET/SDH frame. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a simplified block diagram of two entities that are connected by representative networks; 
         FIG. 2  illustrates how multiple APS channels are distributed while multiplexing lower speed Optical Channel (OC) ring nodes to a higher speed link supporting multiple rings simultaneously; and 
         FIG. 3  illustrates how multiple APS channels are distributed while multiplexing lower speed OC ring nodes to intermediate speed OC ring nodes, intermediate speed links carrying multiple rings simultaneously and also lower and intermediate speed OC ring nodes to a higher speed OC link carrying multiple rings simultaneously. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , telecommunications system  10  comprises network element  12  that is connected to another network element, network element  14 , by link  16 . Two rings are shown, and two more are partially shown. In this way, there are four rings and link  16  is part of each ring. In  FIG. 1 , link  16  is optical and has a data rate of 40 gigabits-per-second (i.e., OC- 768 ). Arrows in  FIG. 1  indicate one direction around the ring, the clockwise direction, provides a normal flow of data, and the counterclockwise direction provides protection around a fault. However, since the invention envisions an extension of a BLSR SONET/SDH system, the links can be bidirectional. For example, if link  19  breaks, any traffic on link  19  is rerouted on links  21 ,  23 , and  16 . A feature of the present invention is that the same link  16  will also be part of the reroute if say link  29  were to break. Thus, link  16  and its operation will be an important part of this description. 
     Node  12  and  14  are SONET/SDH-like nodes connected to link  16  at high speed nodes  13  and  15 . Node  12  and  14  are referred to as SONET/SDH-like because they use a different type of APS protocol than the GR-1230 APS protocol. GR-1230 uses the line overhead of STS- 1  specified by the SONET/SDH standards. Node  12  and  14  are operating with a new APS protocol, according to an exemplary embodiment of this invention, that is a superset of the GR-1230 requirements. Also connected to node  12  and  14  are rings OC-m- 1 , OC-m- 1 – 2 , through OC-m- 4 . Ring OC-m- 1  is shown connected with network element  12 , optical line  19 , network element  20 , optical line  21 , network element  22 , optical line  23  and network element  14 . Ring OC-m- 2  is shown connected with network element  12 , optical line  29 , network element  30 , optical line  31 , network element  32 , optical line  33  and network element  14 . Ring OC-m- 3  is shown, in part, connected with network element  12 , optical line  39 , optical line  43 , and network element  14 . OC-m- 4  is shown, in part, connected with network element  12 , optical line  49 , optical line  53 , and network element  14 . The intervening topology for OC-m- 3  and OC-m- 4  is inconsequential, as long as the total node count is less than the maximum addressing capacity of the new APS protocol or the total bandwidth (including OC-m- 1  and OC-m- 2 ) is not more than that of link  16  (i.e., 40 gigabits per second if link  16  is OC- 768  or equivalent). Link  16  is part of the four rings OC-m- 1  to OC-m- 4 . It is worth noting that if a ring has more than 16 nodes, then all the node in that ring must be extended SONET/SDH elements. On the other hand, for a ring that has 16 or fewer nodes, only node  12  and  14  need to be extended SONET/SDH. 
     Telecommunications system  10  shown in  FIG. 1  has one high-speed, wide bandwidth link  16  carrying data from multiple tributary rings.  FIG. 1 , among other things, illustrates one way to upgrade a portion of a SONET/SDH system that has reached either fiber or bandwidth exhaustion. The upgrading of two node  12  and  14 , and of link  16  leads to an interconnecting of slower data rate lines and node, thereby forming structures known as meshes. Without the multiple APS channels provided by the present invention, multiple rings could not be handled as individual entities; rather, all of the system rings would have to be folded into one large “ring”. With such a large single ring, any fault on any one of the links or node would cause a serious slowdown in APS channel traffic performance, which could affect telecommunications system  10  entirely. Such a slowdown could increase the recovery time of telecommunications system  10 . The ability to work in large networks without forcing the operation of the APS channel as if it belonged to a single large ring is one of the advantages of the present invention. As will be seen, that ability comes in part by extending BLSR SONET/SDH to include multiple APS channels. 
     The implementation and operation of additional APS nodes is provided by extending the GR-1230 SONET/SDH APS protocol according to the multiple APS channel protocol of a preferred embodiment of the present invention. That protocol for byte  1 , byte  2  and byte  3  within each of extended APS channel  2 , extended APS channel  3  and extended APS channel  4  signaling is given immediately below. 
     Byte  1 :
         Each of byte D 4 , byte D 7 , and byte D 10  is byte  1  of extended APS channel  2 , extended APS channel  3  and extended APS channel  4 , respectively.   Bits  5 – 8  provide extended destination node identifications (IDs), and bits  1 – 4  provide extended source node IDs.       

     Byte  2 :
         Each of byte D 5 , byte D 8  and byte D 11  is byte  2  of extended APS channel  2 , extended APS channel  3  and extended APS channel  4 , respectively.   Bits  1 – 8  provide the same bit coding as the coding of byte K 1  of the standard APS channel  1 .       

     Byte  3 :
         Each of byte D 6 , byte D 9  and byte D 12  is byte  3  of extended APS channel  2 , extended APS channel  3  and extended APS channel  4 , respectively.   Bits  1 – 8  provide the same bit coding as the coding of byte K 2  of standard APS channel  1 .       

     Per SONET/SDH standards, the nine bytes D 4  through D 12  are allocated for line data communications. Typically, these bytes are used for alarms, maintenance, control, monitoring, administration and other communication needs between line terminating entities. 
     APS channel  1  has the same signaling protocol as the standard GR-1230 K 1 /K 2  coding. APS channel  2 , APS channel  3  and APS channel  4  use extended APS channel protocol. Because the extended APS channels use and extend the K 1 /K 2  coding of GR-1230, that extended coding is hereinafter referred to as “K 1 /K 2 /K 3 .” K 1 /K 2 /K 3  refers to the coding of the bits, not the positions of the K 1  and K 2  bytes in a frame. Since Byte  2  corresponds to K 1  coding and Byte  3  corresponds to K 2  coding, Byte  1  logically corresponds to K 3 . 
     An alternative embodiment of the present invention uses bytes Z 1 -Z 2 -E 2  of the line overhead, instead of bytes D 4  through D 12  or in addition to bytes D 4  through D 12 . Bytes Z 1 -Z 2 -E 2  are also identified in SONET/SDH standards. The addition of using bytes Z 1 -Z 2 -E 2  would provide for an extended APS channel  5 . The four-bit (i.e., bits  5 – 8 ) extended destination node ID along with the four request bits coding of K 1  (i.e., K 1  bits  1 – 4 ) form an extended destination node ID. Similarly, the four-bit (i.e., bits  1 – 4 ) extended source node ID along with the four select bits coding of K 2  (i.e., K 2  bits  5 – 8 ) form an extended source node ID. Together, the extended APS IDs provide a possible 256-node capability in the preferred embodiment. The 256-node ID capability is a needed extension to the way standard GR-1230 uses source node IDs and destination node IDs. 
     Referring now to  FIG. 2 , one embodiment of a left-hand portion of  FIG. 1  is illustrated. Network element  12  is shown connected over link  16 , which in this embodiment is an OC- 768 . Network element  12  is also connected over lower speed links  19 ,  29 ,  39  and  49 , which in the embodiment of  FIG. 2  are OC- 48  links. Network element  12  manages the connections to links  19 ,  29 ,  39  and  49 , as well as the connection to link  16 , as “ring” connections as far as the extended APS channel protocol is concerned. Data from link  19  travels into and out of network element  12  and into and out of link  16 . The last portion of data associated with link  19  is located as indicated by the arrowhead associated with link  19 . Data from link  29  travels into and out of network element  12  and into and out of link  16 . The last portion of the data associated with link  29  is located as indicated by the arrowhead associated with link  29 . Data from link  39  travels into and out of network element  12  and into and out of link  16 . The last portion of the data associated with link  39  is located as indicated by the arrowhead associated with link  39 . Data from link  49  travels into and out of network element  12  and into and out of link  16 . The last portion of data associated with link  49  is located as indicated by the arrowhead associated with link  49 . As mentioned previously, data is organized as frames having 90-byte columns by 9 rows. The frames include an overhead, 87 bytes of which are moved as shown and 3 bytes of which are terminated by network element  12 . Also, for the purposes of simplifying the illustration and description,  FIG. 2  does not show any data traffic that is being added or dropped at the node of network element  12 . 
     The traffic on link  16  from links  19 ,  29 ,  39  and  49  are respectively represented by lower left to upper right hatching; heavily hatched hatching; light cross hatching, and upper left to lower right hatching. As shown in  FIG. 2  by the dashed lines and by arrows to bit maps of portions of the line overhead, the extended APS channels are provided by STS- 2  line overhead bytes D 4 -D 5 -D 6 ; D 7 -D 8 -D 9 ; and D 10 -D 11 -D 12  and by STS- 3  line overhead bytes D 4 -D 5 -D 6 . Thus, in this embodiment, four extended APS channels are provided so four rings can have extended APS channels, with each ring having up to 256 node IDs. Further, the extended APS channel protocol multiplexes the four extended APS channels from links  19 ,  29 ,  39  and  49  into a single SONET/SDH data stream on link  16 . This multiplexing is provided by network element  12  by terminating the K 1 /K 2 /K 3  bytes coming in on link  19 , for example, and regenerating the information from K 1 /K 2 /K 3  to bytes D 10 , D 11  and D 12  of link  16 . It is important to note that link  19  and the portion of link  16  carrying the K 1 /K 2 /K 3  data on bytes D 10 , D 11  and D 12  are parts of the same ring. 
     Because there are three extended APS channels per line overhead and four incoming data streams, the fourth extended APS channel is moved to D 4 -D 5 -D 6  of STS- 3 . If another three bytes in each line overhead could be used for APS channels, such as Z 1 -Z 2 -E 2 , then only one overhead of one SONET/SDH frame rather than two would be required to manage four extended APS channels. If more rings need APS channels, the number of extended APS channels can be easily extended further by using more D 4  through D 12  bytes on other STSs within the extended APS channel protocol. 
     Referring now to  FIG. 3 , another embodiment of the present invention is illustrated. Network element  12  is connected to OC- 48  links  49 ,  39  and  29 . Network element  12  is also connected to OC- 192  link  19 A and OC- 768  link  16 . Link  49  and link  39  are multiplexed in network element  12  onto link  16  into a single SONET/SDH stream on an OC- 768 . Extended APS channels are used to provide the automatic protection switching features available in this extension of SONET/SDH APS protocol. In  FIG. 3 , three extended APS channels are represented by line overhead bytes D 4 -D 5 -D 6 ; D 7 -D 8 -D 9 ; and D 10 -D 11 -D 12  of STS- 2  on the link  16 . These extended APS channels provide automatic protection switching for SONET/SDH rings. These rings include a first ring containing link  49  and parts of link  16 , second ring containing link  39  and parts of link  16 , and a third ring containing link  19 A and parts link of  16 . As shown in  FIG. 3 , only part of the payload of link  19 A is part of the third ring. Other extended APS channels are represented by line overhead bytes D 4 -D 5 -D 6  and D 7 -D 8 -D 9  of STS- 2  on link  19 A. An extended APS channel for the ring containing link  29  is provided by D 4 -D 5 -D 6  line overhead bytes of STS- 2  of link  19 A. An extended APS channel of link  19 A is provided by D 7 -D 8 -D 9  line overhead bytes of STS- 2  of link  19 A and by D 10 -D 11 -D 12  line overhead bytes of STS- 2  of link  16 , at least the portion that is part of the same ring as link  19 A. In this way, a fourth ring is represented containing links  29  and part of  19 A, extended APS channels for which are provided by D 4 -D 5 -D 6  overhead bytes of STS- 2  on link  29  and D 7 -D 8 -D 9  overhead bytes of STS- 2  on link  19 A. For every ring (or part thereof) defined on network element  12 , there exist two extended APS channels defined on two different links. 
     As illustrated in  FIG. 3 , the extended APS channel protocol provides multiple APS channels on link  16  and puts them in a single SONET/SDH data stream. Also, as in  FIG. 2 , the extended APS channel protocol can address 256 node IDs. 
     Network element  12  dynamically conserves bandwidth on the high speed link by removing frames that have already reached their desired nodes. Such frames are dropped completely without any placeholder frames being sent over link  16 . This dynamic conservation helps make room on link  16  for any extra SONET/SDH frames sent along link  16  for extended APS channel signaling. 
     It is worth noting that for a single point of presence unit having multiple node with OC- 48  and/or OC- 192  rings similar to  FIG. 3  in capability, those multiple node could be replaced with a single high speed network element. In such a case, each of the previous rings will behave the same and enjoy the same protection scheme as existed previously. Further, backhaul to digital cross connect systems (DCSs) that were necessary for previous APS channel protocols to share APS messages among the rings are now unnecessary. Such backhaul DCSs are avoided by the extended APS channel protocol according to the present invention. Thus, the resulting network has a lot less equipment, a lot less floor space and power required, and no need to route traffic to DCSs. 
     It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. For example, the bytes D 4  through D 12  could be from a line overhead of any SONET/SDH frame instead of STS- 2  and STS- 3  as described above. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.