Abstract: An internal signal within a SONET element has a transport format having overhead and payload mapped in a manner similar to the Synchronous Optical Network standard mapping, except having selected overhead bytes defined differently, including a byte used for communicating odd parity calculated over an odd number of bytes of a frame of the transport format to determine correct or incorrect parity, selected bytes used for inter-module automatic protection switching, and a pointer having a selected fixed value, along with an adjusted virtual tributary pointer in a virtual tributary mode.

Abstract: A receive SONET line interface includes an elastic store which receives and stores incoming signals from a pointer tracking circuit and retrieves stored signals for providing to a pointer generating circuit wherein, for both the pointer tracking and pointer generating circuits, separate state memories are provided for keeping track of the state of previous state pointer tracking and generating signals in time slots of repetitive frames of an incoming SONET signal.

Abstract: Redundant facility payload signals (12, 14) received by redundant interface modules (16, 18) are redundantly provided (28, 30; 32, 34) to redundant downstream modules (24, 26) in an equipment (10). A non-symmetrical facility protection algorithm is resident in the B interface module (18). Symmetrical equipment protection algorithms are independently resident and operative in each module of pairs of identical A and B modules at each stage of the data stream. Module or data failures result in switchover only of the disabled data path or module so that the ability to transport payload is maintained and other modules or data paths are unaffected. Facility switchover is effected by detecting a working line failure and transmitting commands to switch the protection line to the A path modules. Equipment switchover is effected by checking for a working or protection module failure independently in each path and independently communicating independent switchover decisions to the other side and downstream.