en no C2 bytes in five consecutive frames contain a payload label different than a label allowed by the receiver. PLM-P is not a path-level defect, but does cause the setting of one of the ERDI-P codes. In addition, this code is also used in ATM services to indicate a loss of cell delineation defect. Performance and Failure Alarm Monitoring Performance and fail- ure alarm monitoring requirements for the physical media, section, line, and path levels are specified in ANSI T1.231. Performance-monitoring functions in terms of collection, storage, valid- ity, register size, threshold and alerting, reporting, accuracy, and resolution are the same as for SONET path-that is, errored second path. 3.4 Reconfiguration Support This section focuses on the reconfiguration capabilities of SONET-based systems. The section is based on Nortel's materials, which are used with permission. 13 For illustrative purposes, some of the features of the product line are briefly highlighted. 3.4.1 SONET Networking Topologies Today's SONET transport networks typically employ a number of different topologies to satisfy important objectives for network simplicity, cost con- tainment, bandwidth efficiency, and survivability. For example, an optical hubbing configuration may be used to eliminate the need for a costly and complicated arrangement consisting of several back-to-back network ele- ments. Similarly, a self-healing ring can be deployed to ensure service sur- vivability through redundant, geographically diverse paths. This section defines each of the major SONET network element configurations and dis- cusses their key attributes. Point-to-Point Terminal The point-to-point terminal configuration is the classic transmission topology that terminates the entire SONET payload at each end of a fiber span or route. Point-to-point systems are Chapter 3 typically employed in a basic transport application calling for a single- system/single-route solution. Although not specifically designed to be com- pletely survivable, the reliability of a point