, line, and path overhead. These overhead bytes provide information for alarm sur- veillance and performance monitoring. A number of bytes in the SONET overhead are allocated for bit interleave parity 8 (BIP-8), a method of error monitoring. The BIP-8 is an even parity mechanism over a sequence of bits in the transmitting signal. Performance-monitoring statistics can be gener- ated with this machinery, helping to locate degraded conditions before they become serious. BIP-8 allows for the sectionalization of alarms and single- ended maintenance. Performance information is available for all levels down to the VT path levels (via a BIP-2 capability). With SONET capabili- ties, the time required to restore service can be minimized. Additionally, fewer maintenance actions are required to address an impairment issue because faults can be more easily sectionalized. An example of improved facility maintenance and surveillance appears in Figure 3-19. In case of a failure, an alarm indication signal (AIS) is sent to suppress downstream alarms. Because the terminal upstream at the left side (Terminal A) may be unaware that a failure has occurred, SONET pro- vides a yellow AIS at the path level from the far-end terminal to Terminal A. In addition, a Far-End Receive Failure (FERF) signal at the line-layer overhead is sent from the hub detecting the failure to instruct Terminal A that a failure has occurred downstream. Interexchange Carrier Network Provider Figure 3-19 Fault management in SONET. SONET SONET SONET SONET Terminal Terminal STS Path Alarm Indication Signal Detects Failure at Hub STS Path-Yellow Alarm Far-End Receive Failure Signal Traditional SONET Architectures Network survivability is imperative. Survivable rings and route diver- sity are crucial for the metropolitan environment. One of the key capabili- ties of SONET is service survivability. SONET supports both bidirectional line-switched rings (BLSRs) for internode networks and unidirectional path-switched rings (UPSRs) used in access networks. SONET-based BL