primary rings. Mixed Tributaries Network Element Figure 3-40 Configured as a UPSR Nonroute diverse or BLSR Node folded ring. Mixed Tributaries Traditional SONET Architectures linear to a ring system today and then easily upgrade to a route diversity ring as additional fiber routes are constructed. Optical Hubbing Optical hubs consolidate traffic from multiple spur routes onto an optical channel extending to a remote site. The configura- tion eliminates the cost and complexity of multishelf arrangements that would otherwise be necessary to pass traffic from several lower-rate fibers to a single higher-rate system. The advantages of optical hubbing apply equally to point-to-point, linear add/drop, and ring systems (see Fig- ure 3-41). As an example, Nortel's S/DMS TransportNode OC3/12/48/192 network elements all support optical tributaries for hubbing applications. For added flexibility, mixed electrical/optical tributary interfaces may be combined on the same shelf as desired. OC-3 Express network elements offer the addi- tional advantage of tributary-to-tributary hairpinning for the convenient local termination of traffic entering from any spur route. (This feature was planned for introduction on OC-192 systems as well.) Regenerators A regenerator extends system reach by reconstituting the optical signal at an intermediate point between two service-terminating locations. If necessary, multiple cascaded regenerators can be deployed to reach hundreds of miles (see Figure 3-42). Unlike some optical amplifier solutions, regenerators interwork with the SONET overhead section bytes for greater flexibility in operations access and improved isolation of trou- bles to a specific section along a fiber route. Regenerators are generally of one of two types. A nonroute diverse regen- erator supports two bidirectional optical channels in each direction to Consolidated Traffic Consolidated Traffic Figure 3-41 Network elements configured as optical Optional Optional hubs. Electrical Electrical Tributaries Tributaries Spur Routes