ough each type of ring features fully self- healing operation, differing characteristics may make one architecture preferable in certain situations. For example, a two-fiber BLSR is a good choice for networks with a highly distributed mesh traffic pattern. Table 3-3 shows ideal applications for each SONET ring architecture. Subtending Rings A subtending ring is an advanced dual-ring configu- ration where a node's tributary optics support a secondary ring (see Figure 3-37). This arrangement offers substantial capital savings and network simplification at a CO hub by allowing a single network element to serve in place of multiple collocated shelves. A subtending ring can employ either UPSR or BLSR technology. Subtending rings may be arranged for single homing (as in Figure 3-37) or, for greater geographic diversity, in a dual-homing architecture where the primary and subtending rings intersect at two points (see Figure 3-38). Subtending rings may also be deployed with ADM in a hybrid configuration to provide survivability for linear ADM routes. This is a path-protected lin- ear ADM route (see Figure 3-39). Traditional SONET Architectures Table 3-3 Ring Architecture Best Used In How It Works Ring Architecture Comparison Access networks where most See Section 3.4.2. traffic terminates in a CO hub Two-fiber BLSR Access and interoffice networks with See Section 3.4.3. a highly distributed traffic pattern Four-fiber BLSR Applications requiring extra- See Section 3.4.4. high capacity and/or protection against multiple concurrent faults Mixed Tributaries Figure 3-37 Subtending ring with single homing. Self-Healing SONET Ring, UPSR or BLSR Line-Side Optics Network Element Configured as a UPSR or BLSR Node Tributary Optics Subtending Ring with Single Homing As an example, Nortel's S/DMS TransportNode OC-3 Express network elements currently support subtending UPSRs with either single or dual homing. In addition, a subtending BLSR feature is planned for future intro- duction on S/DMS TransportNode OC-192 systems. Folded Rings