Add/drop multiplexer and method, and Bi-directional line switcher ring featuring such multiplexers

A multiplexing method in a multiplexer comprising:

• providing a plurality of I/O ports to external services each having an address;
• providing at least one set of groups of tributaries, the set of groups of tributaries comprising:
• a) a first group of input working channel tributaries on a first physical link, a first group of output working channel tributaries on a second physical link;
• b) a first group of input protection channel tributaries on the second physical link and a first group of output protection channel tributaries on the first physical link;
• c) a second group of input working channel tributaries on a third physical link, a second group of output working channel tributaries on a fourth physical link;
• d) a second group of input protection channel tributaries on the fourth physical link and a second group of output protection channel tributaries on the third physical link;
• the second group of output protection channel tributaries being associated with the first group of input working channel tributaries, and the first group of output protection channel tributaries being associated with the second group of input working channel tributaries;
• configuring for each of said I/O ports a respective one of said groups of output working channel ports;
• for each set of groups:
• a) for traffic received on the first group of input working channel tributaries, dropping traffic addressed to one of said I/O ports, outputting traffic not addressed to any of said I/O ports on the first group of output working channel tributaries unless the second physical link is down in which case outputting the traffic on the second group of output protection channel tributaries;
• b) for traffic received on the second group of input working channel tributaries, dropping traffic addressed to one of said I/O ports, outputting traffic not addressed to any of said I/O ports on the second group of output working channel tributaries unless the fourth physical link is down in which case outputting the traffic on the first group of output protection channel tributaries;
• c) for traffic received on the first group of input protection channel tributaries, dropping traffic addressed to one of said I/O ports, outputting traffic not addressed to any of said I/O ports on the first group of output protection channel tributaries unless the second physical link is down in which case terminating the traffic;
• d) for traffic received on the second group of protection working channel tributaries, dropping traffic addressed to one of said I/O ports, outputting traffic not addressed to any of said I/O ports on the second group of output protection channel tributaries unless the fourth physical link is down in which case terminating the traffic;
• for each traffic unit received from the I/O ports, outputting it on a respective configured one of said groups of output working channel tributaries unless the physical link for the configured group is down in which case outputting the traffic on the group of protection channel tributaries associated with the configured group.

An asynchronous transfer mode (ATM) based add/drop multiplexer (ADM) comprising:

• a plurality of service terminating units (STUs) each having an associated ATM identifier operable to receive information units from external services and to convert these into ATM cells containing said associated ATM identifier, and to receive ATM cells having the associated ATM identifier and combine them into information units and output them to the external services;
• an ATM switch having a first set of dedicated receive ports operable to receive working channel traffic, a first set of dedicated transmit ports operable to transmit working channel traffic, a second set of dedicated receive ports operable to receive protection channel traffic, a second set of dedicated transmit ports operable to transmit channel traffic, and an STU port for each STU operable to receive cells from the STU and to send cells to the STU;
• wherein cells received by the ATM switch from the STUs are output through the first set of dedicated transmit ports unless a protection event has occurred effecting the first set of dedicated transmit ports in which case the cells arc output through the second set of dedicated transmit ports;
• and
• wherein the ATM identifier of each cell received by the ATM switch from the first set of dedicated receive ports is examined by the ATM switch, and if the ATM identifier is associated with one of said STUs, the cell is passed to the one of said STUs, and otherwise the cell is output through the first set of dedicated transmit ports unless a protection event has occurred effecting the first set of dedicated transmit ports in which case the cells are output through the second set of dedicated transmit ports.