Patent Publication Number: US-2013243416-A1

Title: Optical network architecture for transporting wdm traffic

Description:
TECHNICAL FIELD 
     The present invention generally relates to a tiered optical network architecture for transporting wavelength division multiplexed (WDM) traffic, and particularly relates to a higher-tiered optical network composed of peer network nodes that have reduced complexity. 
     BACKGROUND 
     Increasing the flexibility with which an optical transport network can route wavelength division multiplexed (WDM) traffic has traditionally increased the efficiency of the network. Reconfigurable optical add/drop multiplexers (ROADMs) have greatly contributed to this increased routing flexibility by enabling traffic at the wavelength granularity to be selectively added or dropped at any node in the network. However, ROADMs employ fairly complex and expensive components to provide this flexible routing capability, meaning that ROAMDs prove cost-prohibitive in some contexts. 
     One such context relates to a network that efficiently transports the traffic of multiple services in a converged fashion. Rather than employing multiple different networks in parallel for transporting these different services (e.g., mobile, business, and residential services), a converged network transports those services together using the same network. A transport network that optically converges different services by transporting those services on different wavelengths would be advantageous, for a variety of reasons, but has heretofore been precluded by the high cost of the necessary hardware components (e.g., ROADMs). 
     Consequently, known transport networks converge different services using packet aggregation instead. While packet aggregation currently requires less hardware expense for converged transport, that expense will not scale equally with the significant traffic increases expected in the near future. Moreover, while packet aggregation suffices in many respects for realizing convergence, it proves inefficient in implementation. Indeed, converging multiple services at the packet level involves significant complexity in order to accommodate the different packet requirements associated with the different services. 
     SUMMARY 
     Embodiments herein advantageously reduce the complexity and accompanying cost of nodes in an optical network that transports WDM traffic, as compared to known networks. With reduced complexity and cost, the embodiments prove particularly useful for optically converging the traffic of multiple services. In fact, some embodiments exploit the increased traffic resulting from such convergence in order to eliminate or at least mitigate the complexity that known networks incur for flexibility in traffic routing. 
     More particularly, embodiments herein include a peer network node that is configured, in conjunction with other peer network nodes, to form a higher-tiered optical network that transports WDM traffic for multiple lower-tiered optical networks. These higher and lower tiered networks may respectively be a metro network and an access network, a regional network and a metro network, etc. Regardless, the peer network node comprises a plurality of dedicated bidirectional optical ports. These ports include two or more so-called lower-tiered ports and one or more so-called peer ports. Each lower-tiered port is dedicated for transporting WDM traffic to and from an individual lower-tiered network, while each peer port is dedicated for transporting WDM traffic to and from an individual peer network node. The bidirectional nature of each such ports advantageously enables WDM traffic to be transported between the peer network node and any given lower-tiered network or peer network node via a single optical fiber. 
     The peer network node further includes one or more so-called hub-side bidirectional optical ports. Each hub-side port is configured to transport WDM traffic to and from a hub node in the higher-tiered network. Further, at least one of the ports is a common port configured to transport WDM traffic aggregated across multiple lower-tiered ports. Similarly to the lower-tiered ports, the bidirectional nature of each hub-side port advantageously enables WDM traffic to be transported between the hub-side port and the hub node via a single optical fiber. 
     The peer network node finally includes a switching circuit. The switching circuit is configured to distribute WDM traffic received at the one or more hub-side ports to respective dedicated ports for dedicated transport to one or more of the lower-tiered networks and peer network nodes. Notably, the switching circuit is also configured to direct any WDM traffic received at the dedicated ports to the one or more hub-side ports for transport to the hub node, even if that traffic is actually destined for one of the lower-tiered networks to which a lower-tiered port is connected. This reduced routing flexibility, in conjunction with the bidirectional nature of the ports, advantageously reduces the complexity and cost of the peer network node, while at the same time satisfying the routing requirements for a wider range of applications. 
     In at least some embodiments, for example, the peer network node includes a single wavelength selective switch (WSS), which significantly reduces the complexity and cost of the node as compared to known approaches that employ a ROADM with at least two WSSs. In this and other embodiments, the node may also include a bypass path that bypasses traffic received at a peer port around any WSSs in the peer network node to a hub-side port, for transport to the hub node. Alternatively, the traffic received at the peer port may be input into a WSS, for aggregated transport to the hub with other traffic, e.g., lower-tiered network traffic. 
     One or more embodiments herein also provide for enhanced resiliency to faults in the network. In some embodiments, for instance, at least two different dedicated ports of the peer network node receive the same traffic. The switching circuit is configured to direct that traffic to a hub-side port by dynamically selecting from which dedicated port to acquire the traffic. This dynamic selection is based on a control signal associated with any faults in the higher or lower-tiered networks affecting those dedicated ports. 
     In other embodiments, the node alternatively employs redundancy for its connection to the hub node, in order to protect against faults in those connections or in any intermediate peer nodes. In this case, the node includes a redundant hub-side bidirectional optical port that is configured to transport the same traffic as another hub-side port, but to transport that traffic to and from a different, redundant hub node. 
     In still other embodiments, the node employs both redundancy for a lower-tiered network or a peer network node, and redundancy for its connection to the hub node. Such embodiments may utilize an additional WSS for realizing this enhanced resiliency. Some embodiments may further utilize an optical switch for protecting against a single simultaneous failure in both a connection to a lower-tiered network and a connection to a hub node. 
     Of course, the present invention is not limited to the above features and advantages. Indeed, those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a generic tiered architecture for optical transport networks configured to transport wavelength division multiplexed (WDM) traffic, according to one or more embodiments. 
         FIG. 2  is a block diagram of a peer network node configured to transport WDM traffic according to one or more embodiments. 
         FIG. 3  is a block diagram of a peer network node configured with a bypass path for bypassing WDM traffic around any wavelength selective switches (WSSs) in the node, according to one or more embodiments. 
         FIG. 4  is a block diagram of a peer network node configured with a single WSS and a bypass path according to some embodiments. 
         FIG. 5  is a block diagram of a peer network node configured with a single WSS, but no bypass path, according to other embodiments. 
         FIG. 6  is a block diagram of a peer network node configured with one or more redundant hub-side bidirectional optical ports according to one or more embodiments. 
         FIG. 7  is a block diagram of a peer network node configured with a splitter/combiner circuit for also distributing WDM traffic to one or more redundant hub-side ports, according to some embodiments. 
         FIG. 8  is a block diagram of a peer network node configured with at least two WSSs for WSS redundancy, according to one or more embodiments. 
         FIG. 9  is a block diagram of a peer network node configured with an optical switch for protecting against single simultaneous failure of both a connection to a lower-tiered network and a connection to a hub node, according to some embodiments. 
         FIG. 10  is a block diagram of the redundant interconnection of multiple peer network nodes according to one or more embodiments. 
         FIG. 11  is a block diagram of a hub node configured with dedicated ports and WSSs that are each dedicated to transporting traffic for an individual peer network node, according to one or more embodiments. 
         FIG. 12  is a logic flow diagram of a method implemented by a peer network node, according to one or more embodiments. 
         FIG. 13  is a logic flow diagram of a method implemented by a hub node, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a generic tiered architecture  10  for optical transport networks configured to transport wavelength division multiplexed (WDM) traffic. The lowest tier shown, tier  1 , includes a plurality of access networks that are each formed from a plurality of access nodes  12  interconnected via optical fiber  14  in a ring structure, a tree structure, a bus structure, a mesh structure, or any combination thereof. In general, each access network aggregates uplink WDM traffic from the network and places that aggregated traffic onto a higher-tiered network; namely, a metro network at tier  2 . The metro network is formed from a plurality of interconnected peer network nodes  16 , also referred to as central offices (COs), and transports WDM traffic for the plurality of access networks. In this regard, each peer network node  16  aggregates WDM traffic from one or more access networks to which it is connected and transports that aggregated traffic to a hub node  18  in the metro network. 
     The hub node  18  connects uplink WDM traffic from one or more network nodes  16  to a higher-tiered network called the regional network. More specifically, the hub node  18  routes uplink WDM traffic to an appropriate one of multiple edge nodes (not shown), e.g., a business services edge router, a residential services or mobile services broadband network gateway (BNG), a broadband remote access server (BRAS), etc. The edge node then performs subscriber management and routes the uplink traffic (typically at the packet level) towards an appropriate destination, such as to content servicers, back towards the access networks, to the Internet, etc. Such edge node routing may entail sending the uplink traffic to the regional network, which operates back at the optical layer. Thus, although omitted from  FIG. 1  for simplicity of illustration, the hub node  18  connects to multiple edge nodes and the edges nodes in turn connect to the regional transport network. 
     The regional network is also formed from a plurality of interconnected peer network nodes  16 , which hub WDM traffic to a hub node  18  in the regional network much the same as in the metro network. Traffic from the regional network is then placed onto a long haul network at tier  4 , for inter-regional transport. Downlink WDM traffic propagates through the networks in an analogous, but opposite, manner. 
     Known implementations of this tiered architecture  10  configure each peer network node  16  with significant routing flexibility. Each peer network node  16 , for example, includes a reconfigurable optical add/drop multiplexers (ROADM) that enables any WDM traffic to be selectively added or dropped from the node  16 . Equipped with such hardware, a peer network node  16  can immediately drop any uplink traffic that is received from another peer network node  16  if that traffic is destined for a connected lower-tiered network. However, because each ROADM requires at least two wavelength selective switches (WSSs) just to provide this flexible routing capability and may require additional WSSs to provide full flexibility in adding or dropping wavelengths, known implementations prove cost-prohibitive and/or operationally limited in some contexts. 
     Embodiments herein advantageously reduce the complexity and accompanying cost of peer network nodes  16 . With reduced complexity and cost, the embodiments prove useful in a wider range of applications, such as optically converging the traffic of multiple services. In fact, some embodiments exploit the increased traffic resulting from such convergence in order to eliminate or at least mitigate the complexity that known networks incur for flexibility in traffic routing. 
       FIG. 2  depicts one embodiment of a peer network node (PNN)  20  in this regard. In conjunction with other peer network nodes  16 , the peer network node  20  forms a higher-tiered optical network that transports WDM traffic for multiple lower-tiered optical networks. These higher and lower tiered networks may respectively be a metro network and an access network, a regional network and a metro network, etc. Regardless, the peer network node  20  comprises a plurality of dedicated bidirectional optical ports  22 . These ports  22  include two or more so-called lower-tiered ports  22 A and one or more so-called peer ports  22 B. Each lower-tiered port  22 A is dedicated for transporting WDM traffic to and from an individual lower-tiered network (LTN), while each peer port  22 B is dedicated for transporting WDM traffic to and from an individual peer network node  16 . The bidirectional nature of each such ports  22  advantageously enables WDM traffic to be transported between the peer network node  20  and any given lower-tiered network or peer network node  16  via a single optical fiber  14 . 
     The peer network node  20  further includes one or more so-called hub-side bidirectional optical ports  24 . Each hub-side port  24  is configured to transport WDM traffic to and from a hub node  18  in the higher-tiered network. Further, at least one of the ports  24  is a common port configured to transport WDM traffic aggregated across multiple lower-tiered ports  22 A. 
     Although as shown the hub-side ports of peer network node  20  transport traffic directly to and from a hub node  18 , rather than indirectly via one or more other peer network nodes  16 , this need not be the case. Indeed, the ports  24  are hub-side merely in the sense that traffic input into or output from such ports  24  at some point in time originates from or is destined for a hub node  18 . Similarly to the lower-tiered ports  22 , the bidirectional nature of each hub-side port  24  advantageously enables WDM traffic to be transported between the hub-side port  24  and the hub node  18  via a single optical fiber  14 . 
     The peer network node  20  finally includes a switching circuit  26 . The switching circuit  26  is configured to distribute WDM traffic received at the one or more hub-side ports  24  to respective dedicated ports  22 A,  22 B for dedicated transport to one or more of the lower-tiered networks and peer network nodes  16 . Notably, the switching circuit  26  is also configured to direct any WDM traffic received at the dedicated ports  22 A,  22 B to the one or more hub-side ports  24  for transport to the hub node  18 , even if that traffic is actually destined for one of the lower-tiered networks to which a lower-tiered port  22 A is connected. 
     By unconditionally transporting traffic to the hub node  18  in this way, the switching circuit  26  has reduced flexibility in routing WDM traffic. Indeed, traffic received at a dedicated port  22  that is destined for a lower-tiered port  22 A must first be routed to the hub node  18 , and then back to the peer network node  20  before finally being routed to that lower-tiered port  22 A. But as demonstrated in greater detail below, this reduced routing flexibility in conjunction with the bidirectional nature of the ports  22 ,  24 , advantageously reduces the complexity and cost of the peer network node  20 , while at the same time satisfying the routing requirements for a wider range of applications. 
     Consider, for example, the embodiment illustrated in  FIG. 3 . As shown in  FIG. 3 , the peer network node  20  includes two hub-side ports  24 A and  24 B. Hub-side port  24 A is the common port that is configured to transport WDM traffic aggregated across multiple lower-tiered ports  22 A. Hub-side port  24 A may, for instance, transport traffic aggregated by one or more wavelength selective switches (WSSs)  28  comprised in the switching circuit  26 . A WSS as used herein is configured to selectively switch or otherwise route each wavelength received at its common port to any one of its dedicated ports, independently of how other wavelengths are routed, and to aggregate wavelengths received at its dedicated ports for output from its common port. By contrast, hub-side port  24 B is dedicated for transporting traffic received at peer port  22 B to the hub node  18  and for transporting traffic received from the hub node  18  to peer port  22 B. 
     In this regard, the switching circuit  26  advantageously includes a bypass path  30 . The bypass path  30  is a circuit configured to bypass traffic received at hub-side port  24 B around the one or more WSSs  28  to peer port  22 B, for transport to an associated peer network node  16 . Likewise, the bypass path  30  bypasses traffic received at peer port  22 B around the one or more WSSs  28  to hub-side port  24 B, for transport to the hub node  18 . The bypass path  30  transports traffic to the hub node  18  in this way, even if that traffic is actually destined for a lower-tiered network to which a lower-tiered port  22 A is connected. In doing so, the bypass path  30  effectively eliminates any flexibility with which the peer network node  20  would otherwise be able to route the traffic received at ports  22 B,  24 B, to correspondingly reduce the complexity and cost of the node  20 . 
     Yet even with this reduced complexity and cost, the node  20  still proves useful in a wider range of applications. In one embodiment, for example, the node  20  is configured to transport the WDM traffic of multiple services (e.g., mobile, business, and residential services) in a converged fashion. This convergence may substantially increase traffic utilization of the optical fiber  14  connecting the node  20  to a peer network node  16  at the peer port  22 B, perhaps to near capacity. If so, little if any additional traffic should be added to that fiber  14  from the lower-tiered networks to which the node  20  is connected. The node  20  thereby exploits the filling of the fiber  14  to substantially full capacity as an opportunity to reduce the complexity and cost of the node, by bypassing the fiber  14  and the traffic it carries around the WSSs  28  that would otherwise add additional traffic to the fiber  14 . 
     Of course, if any of that bypassed traffic was actually destined for one of the lower-tiered networks, the traffic may be routed back from the hub node  18  to the peer network node&#39;s hub-side port  24 A. From port  24 A, the traffic is then distributed to the appropriate lower-tiered network. Because in many applications this additional round-trip transport is needed only very rarely, the complexity and cost reductions obtained from the bypass path  30  still prove advantageous on balance. 
     Indeed, as shown in  FIG. 4 , the complexity and cost reductions may be substantial, with the node  20  in at least some embodiments employing only a single WSS  27  (rather than multiple WSSs as in known approaches that employ a ROADM). In this case, the lower-tiered ports  22 A correspond to the dedicated ports of the WSS  27 , and hub-side port  24 A corresponds to the common port of the WSS  27 . As used herein, a port corresponds to another port if the two ports transport the same WDM traffic. The bypass path  30  may simply comprise a pass-through circuit  29  that includes one or more single-input, single-output fiber optic couplers or interconnects for connecting the fiber  14  at port  22 B to the fiber  14  at port  24 B. 
     Other embodiments herein, by contrast, contemplate that the reduction in complexity and cost results exclusively from the inclusion of a single WSS  27  in the node  20 , rather than also from the inclusion of a bypass path  30 .  FIG. 5  illustrates one such embodiment, which proves particularly useful in cases where the fiber  14  at peer port  22 B has not been substantially filled to capacity. As shown in  FIG. 5 , the peer network node  20  comprises a single WSS  27 . Rather than just the lower-tiered ports  22 A corresponding to the dedicated ports of the WSS  27 , as in  FIGS. 3 and 4 , both the lower-tiered ports  22 A and the peer port  22 B correspond to the WSS&#39;s dedicated ports. The WSS  27  aggregates WDM traffic from these ports  22 A,  22 B and outputs that traffic at its common port, which corresponds to a single hub-side port  24  of the node  20 . Conversely, the WSS  27  distributes WDM traffic received at the hub-side port  24  to respective dedicated ports  22 A,  22 B. 
     Regardless of the particular manner in which the complexity and cost of the node  20  are reduced, that reduction permits the node  20  to realize substantially the same meaningful functionality, with less expense. The reduction may additionally or alternatively permit the node  20  to realize new functionality, such as enhanced resiliency to faults in the network. 
     In one embodiment, for example, the node  20  employs redundancy for a lower-tiered network or a peer network node  16  to which it is connected, in order to protect against faults in the connection to that network or node  16 . In this case, at least two different dedicated ports  22  of the node  20  receive the same traffic. The switching circuit  26  is configured to direct that traffic to a hub-side port  24  by dynamically selecting from which dedicated port  22  to acquire the traffic. This dynamic selection is based on a control signal associated with any faults in the higher or lower-tiered networks affecting those dedicated ports  22 . 
     As one example of this,  FIG. 5  shows two dedicated ports  22  of the node  20  connected to the same lower-tiered network  32 . Where for instance the lower-tiered network  32  is a ring network, the dedicated ports  22  may connect to different arms/directions of the ring. Regardless, the two ports  22  receive the same traffic in a redundant manner. However, the WSS  27  is configured to dynamically select the traffic from only one of those ports  22 , for aggregating and sending to the hub node  18 . The WSS  27  dynamically configures this selection responsive to a control signal indicating a fault in one of the connections to the lower-tiered network  32 . The WSS  27  may receive this control signal from the hub node  18 , as described in greater detail below. 
     In other embodiments, the node  20  alternatively employs redundancy for its connection to the hub node  18 , in order to protect against faults in those connections or in any intermediate peer nodes  16 .  FIG. 6  illustrates one such embodiment. As shown in  FIG. 6 , the node  20  includes a redundant hub-side bidirectional optical port  24 C. This redundant port  24 C is configured to transport the same traffic as the previously-mentioned hub-side port  24 A. However, the redundant port  24 C does not transport the traffic to and from the same hub node  18  as hub-side port  24 A. Instead, hub-side port  24 A transports the traffic to and from hub node  18 A, and redundant hub-side port  24 C transports the traffic to and from a redundant hub node  18 C. The switching circuit  26  is correspondingly configured to direct any traffic received at the dedicated ports  22  also to the one or more redundant hub-side ports  24 C for transport to the redundant hub node  18 C. This way, if the connection to either of the hub nodes  18 A,  18 C fails, e.g., because of a fiber outage, an intermediate peer network node failure, or a hub node failure, traffic is still transported via the connection to the other hub node  18 C,  18 A. 
       FIG. 7  depicts one exemplary implementation of these embodiments in the context of a node  20  that includes a single WSS  27 , but no bypass path  30 . As shown in  FIG. 7 , the switching circuit  26  further includes a splitter/combiner circuit  34 . The splitter/combiner circuit  34  is configured to distribute the traffic output from the common port of the WSS  27  to both hub-side port  24 A and redundant hub-side port  24 C, for redundant transport of the same traffic to hub node  18 A and redundant hub node  18 C. Conversely, the splitter/combiner circuit  34  is configured to combine traffic received from hub-side port  24 A and redundant hub-side port  24 C, and output that combined traffic to the common port of the WSS  27 . 
     Of course, those skilled in the art will appreciate that  FIG. 7  can be extended to embodiments with a bypass path  30  as well. In such a case, the node  20  in  FIG. 4  may have a redundant hub-side port  24 C to provide redundancy for hub-side port  24 A, as well as a redundant hub-side port  24 D to provide redundancy for hub-side port  24 B. 
     In still other embodiments, the node  20  employs both redundancy for a lower-tiered network or a peer network node  16 , and redundancy for its connection to the hub node  18 . Such embodiments may utilize an additional WSS for realizing this enhanced resiliency. Although this increases complexity and cost, the embodiments still achieve greater functionality than that of known approaches with comparable complexity and cost. 
       FIG. 8  illustrates one embodiment of a node  20  with enhanced resiliency. As shown, the node  20  includes four lower-tiered ports  22 A- 1 ,  22 A- 2 ,  22 A- 3 , and  22 A- 4 . Lower-tiered ports  22 A- 1  and  22 A- 3  are both connected to lower-tiered network  36  and therefore both receive the same traffic. Likewise, lower-tiered ports  22 A- 2  and  22 A- 4  are both connected to lower-tiered network  38  and therefore both receive the same traffic. 
     The switching circuit  26  includes a first WSS  40  and a second WSS  42 . The first WSS  40  includes two dedicated ports that connect to or otherwise correspond to lower-tiered ports  22 A- 1  and  22 A- 2 . The second WSS  42  includes two dedicated ports that connect to or otherwise correspond to lower-tiered ports  22 A- 3  and  22 A- 4 . Configured in this way, the first WSS  40  is configured to direct traffic from lower-tiered network  36 , as received at lower-tiered port  22 A- 1 , to hub-side port  24 A, while the second WSS  42  is configured to direct traffic from that lower-tiered network  36 , as received at lower-tiered port  22 A- 3 , to redundant hub-side port  24 C. Similarly, the first WSS  40  is configured to direct traffic from lower-tiered network  38 , as received at lower-tiered port  22 A- 2 , to hub-side port  24 A, while the second WSS  42  is configured to direct traffic from that lower-tiered network  38 , as received at lower-tiered port  22 A- 4 , to redundant hub-side port  24 C. 
     While the example in  FIG. 8  illustrates redundancy in the context of lower-tiered networks, those skilled in the art will appreciate that the embodiment may be extended to redundancy of the peer port  22 B as well. In any event, such an arrangement not only provides the redundancy discussed above, but also provides redundancy to protect against WSS failure. Moreover, the multiple WSSs increase the number of possible lower-tiered connections to the node  20 . 
       FIG. 9  illustrates a variation of the above embodiment that provides even greater resiliency. More particularly, the variation protects against a simultaneous failure in both a connection to a lower-tiered network and a connection to a hub node  18 . For purposes of illustration, the variation is shown in  FIG. 9  in the context of a simplified example that involves only a single lower-tiered network  36 . Those skilled in the art will appreciate that the variation may be extended to multiple lower-tiered networks. 
     Nonetheless, as shown, the switching circuit  26  further comprises an optical switch  44 . The optical switch  44  includes a first port  44 A and a second port  44 B. The switch  44  is configured to dynamically switch the traffic from lower-tiered network  36 , as received at lower-tiered port  22 A- 1 , to either the first or second port  44 A,  44 B of the optical switch  44 . The switch  44  is further configured to dynamically switch the traffic from lower-tiered network  36 , as received at lower-tiered port  22 A- 3 , to either the first or second port  44 A,  44 B of the optical switch  44 . Such dynamic switching is performed responsive to a control signal associated with any faults in the higher or lower-tiered networks affecting those ports  22 A- 1 ,  22 A- 3 . 
     With the switch  44  configured in this way, the first WSS  40  is configured to direct the traffic received from the first port  44 A of the optical switch  44  to hub-side port  24 A. Meanwhile, the second WSS  42  is configured to direct the traffic received from the second port  44 B of the optical switch  44  to redundant hub-side port  24 C. 
     Such provides protection against a simultaneous failure in both a connection to lower-tiered network  36  and a connection to a hub node  18 . Consider an example where both the connection to lower-tiered network  36  at port  22 A- 1  fails and the connection to redundant hub node  18 C at port  24 C fails. In this case, the optical switch  44  is configured to dynamically switch the traffic from lower-tiered network  36 , as received at port  22 A- 3 , to the first port  44 A of the optical switch  44 . The first WSS  40  correspondingly directs this traffic to the hub-side port  24 A, for transport to hub node  18 A. 
     Those skilled in the art will of course appreciate that the redundant embodiments shown in  FIGS. 6-9  have been simplified in many respects for purposes of illustration. For example, the particular way in which the node  20  connects to the hub nodes  18 A,  18 C in a redundant fashion may vary, and those connections may be made via any number of intermediate peer network nodes  16 .  FIG. 10  illustrates this latter variation, for instance, in the context of several peer network nodes  16  configured according to  FIG. 7 . As shown in  FIG. 10 , a given node  16  may connect one of its hub-side ports  24 A,  24 C to the peer port  22 B of a different node  16  in order to realize a redundant connection to a different hub node  18 . 
     Those skilled in the art will further appreciate that while embodiments herein reduce the routing flexibility of a peer network node  16 , the embodiments actually provide more flexibility in terms of the wavelengths used to transport traffic. In this regard, the switching circuit  26  in one or more embodiments is configured to dynamically adapt the wavelengths used for transporting traffic to or from any given dedicated port  22 , responsive to a control signal received from a hub node  18 . This control signal may be sent, for instance, upon the introduction of additional traffic to the network, whereupon the switching circuit  26  dynamically allocates an additional wavelength for the traffic. Contrary to known ROADM architectures, dynamic allocation may entail re-assigning wavelengths across different dedicated ports  22 , as needed. 
     Turning now to additional details of a hub node  18 ,  FIG. 11  illustrates one or more embodiments of a hub node  18  configured for use with peer network nodes  16  from  FIG. 3  or  4 . As shown, the hub node  18  includes bidirectional optical ports  46 A,  46 B that are each dedicated for transporting traffic to and from an individual peer network node  16 A,  16 B. The hub node  18  also includes a switching circuit  50  that comprises WSSs  48 A,  48 B. WSSs  48 A and  48 B are each dedicated for selectively switching traffic transported by an individual bidirectional port  46 A,  48 B to and from a plurality of edge nodes (via connections  54 ). This architecture advantageously reduces the complexity and cost of a hub node  18 , as compared to known approaches, while providing simplistic yet sufficient routing flexibility for a wide range of applications. 
       FIG. 11  further illustrates that the hub node  18  may include a wavelength controller  52 . In some embodiments, this wavelength controller  52  is configured to generate a control signal used for protection switching according to various embodiments discussed above. In one embodiment, for example, the wavelength controller  52  is configured to monitor reports obtained regarding any faults in the higher or lower-tiered networks that affect the route via which the hub node  18  receives traffic. Based on this monitoring, the wavelength controller  52  generates a control signal that dynamically controls a peer network node&#39;s selection between which of multiple different dedicated ports  22  the peer network node  16  acquires the same traffic for directing to the hub node  18 . The wavelength controller  52  then sends the generated control signal to that peer network node  16 . In another embodiment, the wavelength controller  52  additionally or alternatively is configured to generate a control signal, based on monitoring fault reports, that dynamically controls to which of multiple wavelength selective switches  40 ,  42  in a peer network node  16  an optical switch  44  in that peer network node  16  directs traffic received at a given dedicated port  22 A- 1 ,  22 A- 3 . 
     In one or more other embodiments, the wavelength controller  52  is additionally or alternatively configured to generate and send a control signal that directs a peer network node  16  to dynamically adapt the wavelengths used for transporting traffic to or from any given dedicated port  22  of that peer network node  16 . Such generation may be performed responsive to determining that new traffic is to be introduced or otherwise transported by the network. 
     In view of the above modifications and variations, those skilled in the art will appreciate that a peer network node  16  herein generally performs the processing shown in  FIG. 12 . As shown in  FIG. 12 , processing includes receiving traffic at a plurality of dedicated bidirectional optical ports  22 , including two or more lower-tiered ports  22 A and one or more peer ports  22 B (Block  100 ). Processing further includes receiving traffic at one or more hub-side bidirectional optical ports  24 , with at least one of the hub-side ports  24  being a common port (Block  110 ). Processing then entails directing any traffic received at the dedicated ports  22  to the one or more hub-side ports  24  for transport to the hub node  18 , even if that traffic is actually destined for one of the lower-tiered networks to which a lower-tiered port  22 A is connected (Block  120 ). Finally, processing includes distributing the traffic received at the one or more hub-side ports  24  to respective dedicated ports  22  for dedicated transport to one or more of the lower-tiered networks and peer network nodes  16  (Block  130 ). 
     Likewise, those skilled in the art will appreciate that a hub node  18  herein generally performs the processing shown in  FIG. 13 . As shown in  FIG. 13 , such processing includes transporting, at each of multiple bidirectional optical ports  46 , traffic to and from an individual peer network node  16  for which the port  46  is dedicated (Block  140 ). Processing then entails selectively switching, at each of multiple wavelength selective switches  48 , traffic transported by an individual port  46  for which the switch  48  is dedicated to and from a plurality of edge nodes (Block  150 ). 
     Still further, those skilled in the art will understand that no particular type of WDM is required to practice the above embodiments. Thus, the embodiments may employ coarse WDM or dense WDM. The embodiments may even be used in the context of a WDM passive optical network (WDM-PON), with or without inverse return to zero/return to zero (IRZ/RZ) wavelength re-use. In one embodiment, for instance, the embodiments utilize 25 GHz channel spacing in both C and L bands, allowing for up to 400 wavelength channels per fiber. Another embodiment utilizes up to 100 GHz channel spacing. 
     Likewise, no particular type of technology is required to implement the one or more WSSs employed by the above embodiments. Indeed, WSSs herein may be realized using array waveguide gratings (AWGs), microelectromechnical systems (MEMs), liquid crystal on silicon (LCoS), or any other technology that may permit selective switching of optical signals on a per-wavelength basis. 
     Thus, those skilled in the art will recognize that the present invention may be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are thus to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.