Intact versus fractional switching in SONET/SDH cross - connects

Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) cross connect switching structures may be made non-blocking by providing sufficient hardware resources. For example, in a three-stage Clos switching structure this is accomplished by providing a sufficient number of middle stage switches to allow connections to be made without blocking. Nonblocking SONET/SDH cross-connects are more expensive than architectures which reduce the hardware complexity in their implementations at the expense of increased blocking probability. Under certain circumstances, initially unsuccessful new connections may still be connected if rearrangement of existing connections is possible. Factors which make rearranging connections difficult in a SONET/SDH signal environment is the multirate and multicast nature of the traffic, the requirement that any rearrangements must not interrupt existing sessions, and subnetwork connection protection. Cross-connects with this subnetwork connection protection capability must provide hardware and software functionality that selects between two independent copies of a signal and passes the error-free copy through the matrix. We explain how subnetwork protection introduces a requirement for intact switching of SONET/SDH signals. We also present a comparison of intact and fractional SONET/SDH connection and rearrangement algorithms in the context of subnetwork connection protection requirements. We demonstrate the blocking probability impact of introducing subnetwork protection connections into cross-connects.