Abstract: In a multi-channel, multi-site communications system, spans on working channels extending in an overlapping manner among the sites are afforded protection by a protection channel in accordance with a priority scheme. Enabling of protection switching for each span in the event of a signal failure on the span is dependent upon whether there is any protection higher priority overlapping span. This is determined individually for each span in response to each change in protection requests in the system.

Abstract: A DMA channel is shared by transmit and receive ports via which data packets are transmitted and received using a bit oriented protocol such as HDLC. For the transmission of each packet, an abort signal of the protocol is initially transmitted, and the packet is transmitted when the DMA channel is free after a delay equal to the packet transmission turnaround time. In response to receipt of the abort signal, the DMA channel is reserved for reception of a packet within a time-out period. The DMA channel adopts a receive state in response to the abort signal either immediately if it is free, or at the end of transmission if a packet is currently being transmitted.