Abstract:
A network and method of operating the network, the network comprising a transport layer that includes first and second disjoint but topologically substantially identical subnetworks A and B, each having a plurality of optical switch nodes and/or reconfigurable optical add-drop multiplexer (ROADM) nodes, and a routing layer wherein each router is coupled to at least one node in A and at least one node in B. The network is operable to provide a plurality of classes of services providing corresponding Quality of Service, and the routing layer or routing/transport interface can differentiate between traffic having different classes of service and schedule the traffic based on its service class. In the case either of the subnetworks becomes inoperable, all of the traffic is transferred to the other subnetwork, and the QoS of the classes of service is maintained by scheduling traffic to increase the delay experienced by delay-tolerant traffic.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application claims the benefit of U.S. Provisional Application 61/710,312 filed on Oct. 5, 2012. 
     
    
     BACKGROUND 
       [0002]    In fiber-optic communications, wavelength-division multiplexing (WDM) is used to multiplex a number of optical carrier signals onto a single optical fiber by using different wavelengths of laser light. A WDM system uses a multiplexer at the transmitter to join a plurality of signals together, and a demultiplexer at the receiver to split them apart. 
         [0003]    One benefit of implementing WDM in an optical system is that it allows the capacity of an optical communications network to be increased without laying more fiber. Thus, new technology can be implemented in an optical infrastructure without having to overhaul the backbone network. The capacity of a given link can be expanded simply by modifying or upgrading the multiplexers and demultiplexers at each end. 
         [0004]    Different WDM systems use different wavelength patterns to implement a plurality of communication channels in an optical fiber. The different systems are referred to either as Conventional, or Coarse, WDM (CWDM) and Dense WDM (DWDM). WDM systems that provide up to 8 channels per fiber are referred to as CWDM, and systems that provide more channels per fiber are referred to as DWDM. DWDM systems typically provide 40 channels or 80 channels per fiber. The number of channels that can be accommodated per fiber is determined by channel spacing. Wide spacing allows the use of less sophisticated, and therefore less expensive, transceivers. Herein, both CWDM and DWDM technologies are referred to collectively simply as WDM. 
         [0005]    Network systems in general can be described in terms of a networking model promulgated by the Open Systems Interconnection effort at the International 
         [0006]    Organization for Standardization, the so-called “OSI model”. The OSI model characterizes the functions of a communications system in terms of logical layers called abstraction layers. A layer serves the layer above it and is served by the layer below it, and various communication functions are grouped into the various layers. The OSI model is summarized in  FIG. 1 . As shown in the figure, layer 1 (L1) is called the Physical layer, and describes the physical media over which communication signals are propagated, as well as a description of the signal and binary transmission parameters. Layer 2 (L2) is called the Data Link layer, and describes datagram addressing. Layer 3 (L3) is called the Network layer, and its functions include path determination and logical addressing of messages in a communication channel. Collectively, L1, L2, and L3 are referred to as the media layers. 
         [0007]    In this context, a communications protocol is a system of digital message formats, and rules for exchanging the messages in or between devices in telecommunications. The protocols may include signaling, authentication, error detection, and correction capabilities. The Internet Protocol (IP) is a communications protocol used for relaying datagrams (network packets) across a network, and is responsible for routing packets across network boundaries in an internetwork. 
         [0008]    The herein disclosed systems and methods pertain to the operation of optical communication systems, and in particular systems that use IP over CWDM or IP over DWDM systems (collectively, IPoWDM). The network consists of an IP/MPLS 
         [0009]    (Multiprotocol Label Switching, a data-carrying mechanism) routing layer (L3) and an underlying WDM transport layer (L2/L1). The WDM layer consists of two identical disjoint sub-networks, referred to herein as WDM-A (A) and WDM-B (B). During normal operation A and B operate independently of each other, but at other times either one can act as a backup for the other. Any given router in L3 is connected to at least one node in A and at least one node in B. A and B can be described as mutual isomorphic and mutual disconnected sub-graphs within an IPoWDM graph. They are connected indirectly through vertices of an IP-sub-graph, such that each IP-vertex has at least one edge incident to A and at least one edge incident to B. Physically, a link connecting two nodes within A and the corresponding link connecting the corresponding two nodes in B are deployed in different ducts that are sufficiently separated from each other to minimize the possibility that an incident causing an outage in either duct, such as digging equipment that inadvertently cuts or otherwise damages a duct and the cable therein, would also cause an outage in the other. Each node thus includes three vertices in close proximity, each belonging to a different sub-network. This architecture can provide resilience by diverting traffic between A and B. For example, in the case of cable damage in A, the IP-WDM interface can divert traffic from A to B, thus maintaining the service during the incident. This capability of resilience relies on traffic management in the IP-layer, or more particularly, on the traffic management at the interface between IP-layer and WDM-layer. Different strategies of traffic management lead to different cost and performance. 
       SUMMARY 
       [0010]    A network and method of operating the network, the network comprising a transport layer that includes first and second disjoint but topologically substantially identical subnetworks, each having a plurality of optical switch nodes and/or reconfigurable optical add-drop multiplexer (ROADM) nodes, and a routing layer wherein each router is coupled to at least one node in A and at least one node in B. The network is operable to provide at least first class and second class services providing a high and a low Quality of Service, respectively. In normal operation, F traffic does not exceed half of the total instantaneous network traffic; all of the F traffic is carried on A and an amount of S traffic equal amount of F traffic is carried on B; and any remaining S traffic is split equally between A and B. In the case either of the subnetworks becomes inoperable, all of the traffic is transferred to the other subnetwork, the F traffic&#39;s QoS is maintained, and all of the S traffic is scheduled to share any remaining traffic carrying capacity of the operable subnetwork. 
         [0011]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate disclosed embodiments and/or aspects and, together with the description, serve to explain the principles of the invention, the scope of which is determined by the claims. 
           [0013]    In the drawings: 
           [0014]      FIG. 1  is a table summarizing the OSI model; 
           [0015]      FIG. 2  illustrates two different traffic management strategies, Scheme 1 (dotted boxes) and Scheme 2 (solid boxes), during normal operation; 
           [0016]      FIG. 3  illustrates the two different management strategies after an incident that renders A inoperable. 
           [0017]      FIG. 4  is a graph showing the relative performance of Scheme 1 versus Scheme 2 with regard to the quality of service provided by the network when A is inoperable. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    It is to be understood that the figures and descriptions provided herein may have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for the purpose of clarity, other elements found in typical optical signal detection systems and methods. Those of ordinary skill in the art may recognize that other elements and/or steps may be desirable and/or necessary to implement the devices, systems, and methods described herein. However, because such elements and steps are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps may not be provided herein. The present disclosure is deemed to inherently include all such elements, variations, and modifications to the disclosed elements and methods that would be known to those of ordinary skill in the pertinent art of optical network operation. 
       Resilience Schemes 
       [0019]    Some of the embodiments described hereinafter provide only two classes of service. However, as would be appreciated by one of ordinary skill in the art, the method disclosed can be extended to three or more service classes. Hereinafter, the abbreviations H and L will be used for “high class service” and “low class service”, respectively. The difference between the two classes is that H traffic is provided with a predetermined quality of service (e.g. QoS), while the L traffic is not. Consequently, the L traffic can tolerate more delay. 
         [0020]    Scheme 1 uses no more than half the traffic carrying capacity of network A and network B during normal operation. Thus, either one can accommodate all of the traffic from the other in case of an outage, such as an incident that damages an optical fiber cable. The network in which the outage occurs is referred to as the victim, and the other network is referred to as the backup. In this scheme, all traffic in the victim network is rerouted to the backup network. As such, the resilience of the system is simple and reliable. The drawback is the need for complete system redundancy, resulting in high cost for the network operator. 
         [0021]    In contrast, in Scheme 2 traffic at the IP-WDM interface can be discriminated by the router according to the service class of the traffic, and can be redirected to the backup network based on the traffic class. 
         [0022]    Exemplary scenarios are illustrated in  FIGS. 2 and 3 . In  FIGS. 2 and 3 , a network having a WDM layer comprising two disjoint subnetworks, WDM-A and WDM-B, is coupled to an IP layer. The IP layer is illustrated as containing two exemplary routers, and each router is communicatively coupled to a node in A and a node in B. It is understood that the IP layer would typically include a much greater number of routing elements, and each router can be coupled to a plurality of nodes in A and a corresponding plurality of nodes in B.  FIGS. 2 and 3  show two schemes of operation, Scheme 1 (dotted boxes on the left), and Scheme 2 (solid boxes on the right).  FIG. 2  shows each scheme during normal operation, and  FIG. 3  shows each scheme after an outage occurs in A and all of its traffic is transferred to B. In the figures, “t” signifies network traffic, and its subscript indicates the class of traffic (H or L), or the subnetwork on which the traffic flows (A or B). 
         [0023]    Using Scheme 1, during normal operation as shown in  FIG. 2 , the routers and the IP-WDM interface do not differentiate between H and L traffic, and the interface fills transport networks A and B equally. All network traffic, including both H traffic and L traffic, is split equally between A and B. In the case of an outage in A as shown in  FIG. 3 , all A traffic is diverted to B. As shown, the combined network traffic does not exceed the capacity of B and no traffic is dropped. However, if the combined traffic exceeds the capacity of B and the network is not able to differentiate between H and L traffic and schedule traffic, then some portion of the traffic will be dropped, including both H traffic and L traffic. 
         [0024]    In an exemplary scenario (not illustrated), assume that the IP-WDM interface is upgraded to differentiate between H and L traffic, and H traffic is carried entirely by A during normal operation. Since the traffic volume of H clients is normally less than that of L clients, and the two networks A and B have the same capacity due to symmetry, network A is typically less loaded than network B. Thus, some L traffic can be directed to A such that A and B are equally loaded during normal operation. Then, in case of an outage in A, B could drop all of its traffic, and all traffic on A can be diverted to B. Thereby, the QoS of all of the H traffic is maintained, and only a portion of the L traffic is dropped. 
         [0025]    However, in Scheme 2 the IP-WDM interfaces are further upgraded to have scheduling capability so that resources in B can be shared. As shown in  FIGS. 2 and 3 , during normal operation all of the H traffic is carried entirely by A, preferably together with a portion of the L traffic. Alternatively, both A and B can carry a portion of both the H traffic and the L traffic (not shown). In either case, in an outage of A, the A traffic can be diverted to B and the QoS of the H traffic can be maintained, and any remaining capacity on B can be shared by all of the L traffic. Thereby, the H traffic continues to receive the same QoS, while all of the L traffic continues being served, but with some additional delay. The amount of delay depends on the amount of H and L traffic in the total traffic volume, the capacity of the backup network, the restoration time in the WDM-layer, etc. This scheme takes advantage of the lower QoS and resulting tolerance for delay of the low class traffic to reduce the cost of capacity provisioning in the WDM-layer. 
         [0026]    In particular as shown in  FIGS. 2 and 3 , in Scheme 1, the IP-WDM interface does not differentiate between H and L traffic, and the interface fills transport networks A and B equally. In case of a fiber cut in A, the interface moves traffic in A (t A ) from A to B. To ensure traffic in B is not affected by this action, and since A and B are topologically equivalent, both A and B cannot be loaded to more than half of their capacity during normal operation. Therefore, as traffic volume grows, the capacity of both A and B must be increased when they are at only 50% capacity. 
         [0027]    In Scheme 2 however, the IP-WDM interface can differentiate between H and L traffic. In normal operation, network A carries all of the H traffic, and B is loaded with an amount of L traffic equal to the amount of H traffic on A. Because in general there is more L traffic than H traffic on a network, a portion of L traffic is not yet scheduled, equal to the total L minus the total H. In the illustrative embodiment shown in the figures, that L traffic can be split equally between A and B. Then, in the case of an outage in A, all A traffic, including all of the H traffic and a portion of the L traffic, is diverted to B. B is then operated so that the QoS of the H traffic is maintained, and all of the L traffic is managed to share any remaining capacity on B. When A is restored, all of the H traffic is directed back to A, and the L traffic is shared as before. In Scheme 2, as traffic volume grows, the capacity of A and B need not be increased until the H volume alone is sufficient to load either network to 100% of its capacity. This can result in a dramatic savings in capital expansion or leasing costs compared to Scheme 1. 
       Survivability 
       [0028]    A cable outage can be modeled as a stochastic process distributed on the edges of A and occasionally exceeds a threshold to cause a cable defect at an edge. Then, the service survivability is defined as the probability of a service that survives the event of cable cut somewhere in the WDM-layer. An event of cable cut in A will trigger the corresponding IP-WDM interface to divert all traffic from A to B. Let S A  and S B  be the event that a cable survives such an accident in A and in B, respectively. Then the probability of fiber outage in A and B are p A =1−Pr(S A ) and p B −1−Pr(S B ), respectively. 
         [0029]    In Scheme 1, all traffic shares the same fate, regardless of traffic class. Then in the event of an outage in a victim network, service survival has the probability 
         [0000]        P   I             Pr ( S   A   ∪S   B ))=1− p   A   p   B  
 
         [0000]    because S A  and S B  are mutually independent. 
         [0030]    In Scheme 2, the interface differentiates H and L traffic, places all H traffic on A and shares L traffic between A and B as previously described. Then, in an outage of a victim network, the respective survivability of the H clients and the L clients can be determined as follows. 
         [0031]    For the H traffic, the event of survival has the probability 
         [0000]        P   II ( H )           Pr[S   A ∪((S Ā   ∩S   B )]=1− p   A   p   B  
 
         [0032]    For the L traffic, resource sharing as previously described results in added delay due to scheduling, but all traffic is still served as long as the amount of H traffic is less than 100% of the backup network capacity. As a result, the probability of survival of L clients under this condition is 
         [0033]      P   II ( L,a &lt;½)           Pr[ ( S   A   ∩S   B )∪( S   Ā   ∩S   B ) ]=1−p   B    
         [0034]    On the other hand, if the amount of H traffic equals 100% of the backup network capacity, the probability of survival of L clients is 
         [0000]        P   II ( L,a =½)           Pr[ ( S   A   ∩S   B )]=(1− p   A )(1− p   B )
 
       Performance 
       [0035]    To evaluate the performance of Scheme II versus Scheme I, the following metrics may be used: 
         [0000]        C ( H ):=( P   II ( H ))/ P   I =(1− p   A   p   B )/(1− p   A   p   B )=1
 
         [0000]        C ( L,a &lt;½):=( P   II ( L,a &lt;½)) /P   I =( 1−p   B )/( 1−p   A   p   B )
 
         [0000]        C ( L,a =½):=( P   II ( L,a =½)) /P   i ( 1−p   A )(1− p   B ))/( 1−p   A   p   B )
 
         [0036]    A metric value less than one indicates performance degradation of Scheme 2 compared to Scheme 1. For p A ,p B  ε[0,1) we obtain the general relation 
         [0000]      1= C ( H ) ≦C ( L,a &lt;½) ≦C ( L,a =½)
 
         [0037]    Here, a value of one means there is no change in terms of survivability for H clients in Scheme 2. The service quality degradation for L clients in Scheme 2 is expected, because the service continuity for L clients is provided at the cost of additional delay. The delay is not, however, included in the above metric. The metric above is an indicator for the relative performance in terms of survivability ratio. As such, the magnitude of the service quality degradation is dictated by the cable outage in both A and B. As shown in  FIG. 4 , within a realistic range of values for p A  and p B , the magnitude of the metric is of the second order. 
         [0038]    This analysis shows that, while maintaining the same quality of service for H clients, Scheme 2 has a dramatic cost advantage over Scheme 1. This is because in Scheme 1, A and B can be loaded to only 50% of their capacity during normal operation but, both can be loaded to their full capacity in Scheme 2. Thus, the initial cost of building or leasing an optical network in accordance with Scheme 2 is only about half that of Scheme 1. Moreover, the cost of expanding an existing network is incurred in Scheme 1 when the network loading approaches only 50% of capacity. In contrast, the cost of expansion can be delayed in Scheme 2 until the network loading approaches 100% of capacity, a delay of perhaps many years. The time value of money can cause a long-delayed expenditure to have a present value that is a small fraction of the future cost. Accordingly, a relatively small cost incurred to upgrade the routers and/or the IP-WDM interface to discriminate between different traffic classes can result in a savings equal to the large majority of the cost of increasing the capacity of the network. 
         [0039]    It is noted that improvement in the QoS provided to L traffic can be achieved by increasing the speed of optical protection switching and restoration within the WDM-layer. 
         [0040]    Although the invention has been described and illustrated in exemplary forms with a certain degree of particularity, it is noted that the description and illustrations have been made by way of example only. Numerous changes in the details of construction and combination and arrangement of parts and steps may be made. Accordingly, such changes are intended to be included in the invention, the scope of which is defined by the claims.