d Tributaries Working Traffic Tributaries (up to 11 at each end of span) OC-48 Working Channel 11 (max.) STS-48 Protection Loops Mixed Mixed Tributaries Tributaries of pass-through traffic (see Figure 3-35). It offers cost savings and improved reliability in comparison to intermediate sites equipped with complex back- to-back terminal arrangements. As with point-to-point terminal applications, the survivability of linear add/drop routes can be enhanced through geographically diverse protection paths. Alternatively, a high degree of survivability may be achieved by using a hybrid linear ADM subtending ring configuration known as a path- protected linear ADM route. Subtending rings are covered in the following section. As an example, Nortel's S/DMS TransportNode's linear ADMs support mixed electrical/optical tributary types and flexible VT/STS time slot assignment technology. The latter enables any desired routing of any trib- utary VT/STS channel to any eastbound or westbound VT/STS channel-or direct pass-through from east to west-as desired for optimum use of fiber span capacity and tributary hardware. An Introduction to Self-Healing SONET Rings Self-healing ring architectures (see Figure 3-36) are the preferred solution for applications where survivability is of the utmost importance. They protect against cable Chapter 3 Point-to-Point Point-to-Point Figure 3-35 Terminal Linear Add/Drop Multiplexers Terminal Linear add/drop route example. Mixed Tributaries Network Element Figure 3-36 Configured as a UPSR Ring configuration or BLSR Node employed by all types of SONET self-healing rings. Self-Healing SONET Ring, UPSR, Two-Fiber BLSR, or Four-Fiber BLSR Mixed Tributaries cuts and node failures by providing duplicate, geographically diverse paths for each service. To ensure end-to-end survivability for services traversing multiple rings, adjacent rings may be interlocked using redundant matched-node inter-ring gateways. A SONET ring is generally of one of three types: a UPSR, two-fiber BLSR, or four-fiber BLSR. Alth