ctor switches Inter-Ring Service "A" to secondary input if primary link fails. Vendor-Independent BLSR Inter-Ring Service "A" Vendor- Vendor- Independent STS-1, Independent BLSR Node OC-3, OC/ BLSR Node STS-12 Fiber Span Fiber Span Secondary Inter-Ring Gateways D&C = Drop & Continue Routing SS = Service Selector UPSR matched-node configurations employ standard UPSR path selec- tion techniques plus drop-and-continue routing, as shown in Figure 3-59. From the standpoint of an adjacent ring, this arrangement functions in exactly the same manner as BLSR matched-node inter-ring gateways. Chapter 3 To Path Selector at Service Figure 3-59 Termination Point Primary Inter-Ring Gateways UPSR matched-node configuration. UPSR Node Fiber Span Primary Inter-Ring Link From Service Origination Point Path Selector Drop & Continue Fiber Routing From Service UPSR Node Origination Point Secondary Inter-Ring Link Fiber Span Secondary Inter-Ring Gateways To Path Selector at Service PS = Path Selector Termination Point D&C = Drop & Continue Routing A network element provisioned as a primary gateway for one inter-ring service may be provisioned as a secondary gateway for another inter-ring service as needed for the most efficient use of the ring's available band- width. Also, a primary gateway on one ring may feed either a primary or secondary gateway on an adjacent ring as desired. Adjacent rings are physically interconnected via SONET links (such as STS-1, OC-3, or OC/STS-12 facilities) that serve as conduits for various inter-ring services (such as VT1.5/DS1 or STS-1/DS3 traffic). Routing and selection of inter-ring traffic are always performed at the service layer on both BLSRs and UPSRs. The service selector or path selector in a BLSR/UPSR matched-node con- figuration switches from the primary input signal to the secondary input based on standard performance parameters such as line and/or path AIS. In the case of Nortel's S/DMS TransportNode BLSR systems, both line- and path-layer analysis are provided to protect inter-ring service