As mentioned previously, traditional networks have been optimized for voice traffic, from both transport and protection levels. Many network topologies exist, from point-to-point, ring, and hub-and-spoke to fully meshed networks.

Meshed networks fall outside the common Telcordia specified protection schemes of Bidirectional Line-Switched Ring (BLSR) and Universal Path-Switched Ring (UPSR). As a result, legacy SONET equipment manufacturers have not offered viable solutions for meshed networks. With its path-protected meshed network (PPMN) capability, Cisco has extended the simple concept of path protection on a SONET ring to meshed networks, offering service providers a new degree of flexibility in designing their networks.

Meshed Networks

"Meshed networks" refers to any number of sites arbitrarily connected together with at least one loop. For this discussion, the connections between sites are SONET, at various line rates. Sites within the meshed network that can be reached from other sites through at least two distinct routes form the mesh, whereas the remaining sites are spurs off of this mesh. Meshed networks are often large rings with numerous sub-rings, as shown in Figure 1.

Figure 1. Sample Meshed Network

With PPMN, a network planner can design the mesh shown in Figure 1 with unprotected spans and various line rates. If a failure occurs on a route, connection is re-established through another path in the mesh within the well-known SONET restoration time of 50 milliseconds. By designing PPMN consistent with SONET standards, Cisco offers network planners flexibility they can use today.

Practical PPMN Networks

Good ideas are usually simple, and this one is no different. By using path protection, PPMN simply extends the UPSR beyond the basic ring topology to the meshed architecture. The software locates two diverse routes in the network between the source and destination of a circuit. These two routes form a logical ring for the path of that circuit, and they behave exactly as UPSR. The source bridges its traffic onto each of the diverse paths, and the destination selects between the two paths. With a failure on the active path, the destination simply switches to the standby path within 50 ms. Again, because of the strict adherence to SONET standards, PPMN applied to the logical ring is no different from the standard, Telcordia-specified UPSR.

The real benefit of PPPN, however, lies not in the development of PPMN itself, but in the user interface. Cisco's Java-based graphical user interface (GUI), the Cisco Transport Controller, makes provisioning within a meshed network as simple as clicking a mouse button. All the nodes on the network, as soon as they are turned up, begin the process of autodiscovery. Within minutes, each node has a full description and status of the other nodes and connections throughout the network. (This scenario is possible because Cisco uses Internet protocol [IP] and Open Shortest Path First [OSPF] for SONET Data Country Code [DCC] communications). Creating a circuit is then accomplished by simply specifying the source and destination, another Cisco innovation called A-Z Provisioning. Software then determines the shortest path through the network and establishes all the intermediate cross-connections. A check box determines whether the circuit is to be protected or not. When checked, PPMN is provisioned. A protect circuit is established on the second-shortest path through the network between the source and destination, and a second set of cross-connections is created. With this capability, turn-up and provisioning of circuits can be done in a matter of hours rather than days.

Cisco COMET Applications in Meshed Topologies

The following is an example of PPMN in the meshed network shown in Figure 1. Suppose a protected circuit is specified between nodes C and J. The PPMN software will determine that the shortest route between the two end nodes passes through node H and node G. Cross-connections at each of the four nodes (C, H, G, and J) are then automatically created, and working traffic is initially carried on this route. Concurrently, cross-connections are created for the protected traffic on the second-shortest unique route between nodes C and J, C – B – A – L – J. If a fiber is cut or other failure occurs on the primary route, node J immediately switches to the traffic coming in from node L (instead of node G), and service resumes. Figures 2 and 3 offer graphical descriptions of this scenario and also show how the ring formed by A – B – C – H – G – J – L is a UPSR ring for this circuit.

Figure 2. Working and Protecting Traffic Routed through a Meshed Network

Figure 3. Failure on Primary Path in Meshed Network Another application for PPMN in meshed Cisco COMET networks is building what is commonly called "virtual rings." Figure 4 shows nodes A, B, C, and D forming an existing OC–192 backbone ring. Nodes E, F, G, and H are then added with OC–48 links to the backbone. The ring formed by E – F – G – H, which uses some of the bandwidth on the OC–192 backbone, is termed a "virtual ring." Protecting circuits created in this network topology is no different from the aforementioned example. Furthermore, PPMN does not care if the OC–192 backbone is UPSR or BLSR, as long as there is protected path from source to destination.

Figure 4. Example of a Virtual Ring

Protection for Ethernet MANs

With Ethernet the accepted Local-Area Network (LAN) standard, many organizations are looking to extend Ethernet into the metropolitan-area network (MAN). This in turn provides numerous consequences with regard to how the network will handle this type of traffic from both QoS and protection levels. Although Ethernet provides a tremendous foundation on which to build this next-generation network, fully realizing this end-to-end solution requires an Ethernet with carrier-class robustness. New capabilities are necessary to provide comprehensive Operations, Administration, Maintenance, and Provisioning (OAM&P) in a unified Ethernet optical environment. Ethernet must provide optical performance monitoring to help carriers deliver quality services meeting committed service-level agreements (SLAs). If an optical failure occurs, Ethernet must provide alarm indications and failure-protection mechanisms and help with fiber-failure isolation.

QoS and class-of-service (CoS) capabilities are required to segregate and differentiate applications in a public services environment. Ethernet must continue to efficiently transport IP traffic while meeting the additional delay-sensitive requirements of certain applications. Optimizing this data traffic requires integrated routing and control for both IP and optical layers to build an optimal Cisco COMET network.