Errorless line protection switching in asynchronous transfer mode (ATM) communications systems

摘要

Reliable communication of information is accomplished in a communications system using an asynchronous transfer mode protocol involving a plurality of packets or cells, each containing a header and data, by providing an active line (18) which normally transports user data cells along one path between one location and another location in the communications system and a redundant standby line (20) which also transports the same user cells between the two locations but on a physically different path. Due to the different paths taken by the user cells on the active line and the standby line, the user cells in one of the active or standby lines lead those in the other of the active and standby lines. The user cells on the active and standby lines are aligned by first determining which line carries leading user cells and then determining by how much the user cells on that line lead the user cells on the other line. A switching element (30) then is commanded to connect the line containing the leading user cells to one input of a selection element (48) after passing the user cells through a variable time delay unit (34). The variable delay unit (34) is capable of providing a maximum amount of time delay to the user cells passing through it equal to the maximum amount the user cells in one of the active and standby lines is expected to lead the user cells in the other of the active and standby lines. The switching element (30) is also commanded to connect the line containing lagging user cells either directly to another input of the selection element (48) or through an element (36) which is capable of providing a maximum time delay to the lagging user cells which is less than the maximum amount of time delay able to be provided by the variable time delay unit (34). The time delay provided by the variable time delay unit (34) is controlled such that the lead or lag between the user cells of the active and standby lines is reduced to zero and the user cells on those lines are aligned at the inputs of the selection element (48). The selection element (48) normally directs the user cells which came on the active line to an output (26) of the selection element. The selection element may be commanded to disconnect the active line (18) from the output of the selection element and to connect the standby line (20) to the output of the selection element. This change over to the standby line is accomplished errorlessly and with a reduced amount of data storage for accomplishing alignment of the user cells from the active and standby lines. A control circuit (42) calculates the amount of lead or lag between the active and standby lines. The control circuit controls the switching element (30) to direct the leading cell stream to a lead channel (31) containing the variable time delay unit (34) between the switching element and the selection element (48). The control circuit directs the lagging cell stream to a lag channel (33) between the switching element and the selection element. The control circuit controls the amount of delay provided by the variable delay element to the leading cell stream. The control circuit also monitors the aligned cell streams to confirm continued alignment and to detect loss of alignment.