Abstract: An add-drop multiplexer includes first and second cross-connect systems. At least one pair of first and second protection switches and at least one pair of third and fourth protection switches are provided, the first protection switch being connected to an input port of the first cross-connect system. One of the second, third and fourth protection switches is connected to an input port of the second cross-connect system. An output port of the second cross-connect system is connected to the second, third and fourth protection switches through a node. A pair of fifth and sixth protection switches is connected to output ports of the first and second cross-connect systems. Signals from the fifth protection switch are multiplexed into an uplink high-speed signal and a downlink high-speed signal is demultiplexed into component signals which are supplied to input ports of the first and second cross-connect systems.

Abstract: A data communication apparatus and a storage medium storing therein a program enable error connection of cable to be detected in the case where a plurality of low speed SHELVes are connected to a high speed SHELF through cables respectively. A collation code from a low speed SHELF to be a second communication device is received by a high speed SHELF to be a first communication device, before judging whether it is the collation code from an interface unit 102 or it is the collation code from an interface 103. When the collation code is outputted from the interface 103, it is judged of the error connection of the cable. Further a collation code from a high speed SHELF is received by the low speed SHELF, when established SHELF number does not agree with the collation code, it is judged as the error connection of the cable.