An add/drop multiplexer, wherein the add/drop multiplexer comprises: a first line board and a tributary board, wherein the first line board comprises at least a first interface and a second interface, the first interface is disposed between the tributary board and the first line board, and the second interface is disposed on a network side of the first line board; the first line board is configured to receive a first signal from a first link, and output the first signal through the second interface; the first line board is further configured to receive a second signal from the first link, and output the second signal to the tributary board through the first interface; and the first line board is further configured to receive a third signal from the tributary board through the first interface, and input the third signal into the first link, wherein the add/drop multiplexer further comprises: a second line board, wherein the second line board comprises at least a third interface and a fourth interface, the third interface is disposed between the tributary board and the second line board, and the fourth interface is disposed on a network side of the second line board; the second line board is configured to receive a fourth signal from a second link, and output the fourth signal through the fourth interface; the second line board is further configured to receive a fifth signal from the second link, and output the fifth signal to the tributary board through the third interface; and the second line board is further configured to receive a sixth signal from the tributary board through the third interface, and input the sixth signal into the second link, wherein the tributary board further comprises a switch as a combiner/selector and a switch as a replicator/distributor; the tributary board is configured to receive the second signal from the first line board, generate a downlink service by using the combiner/selector, and send the downlink service to a client device; and the tributary board is further configured to receive an uplink service from the client device, generate the third signal by using the replicator/distributor, and input the third signal into the first line board a first line board and a tributary board, wherein the first line board comprises at least a first interface and a second interface, the first interface is disposed between the tributary board and the first line board, and the second interface is disposed on a network side of the first line board; the first line board is configured to receive a first signal from a first link, and output the first signal through the second interface; the first line board is further configured to receive a second signal from the first link, and output the second signal to the tributary board through the first interface; and the first line board is further configured to receive a third signal from the tributary board through the first interface, and input the third signal into the first link, wherein the add/drop multiplexer further comprises: a second line board, wherein the second line board comprises at least a third interface and a fourth interface, the third interface is disposed between the tributary board and the second line board, and the fourth interface is disposed on a network side of the second line board; the second line board is configured to receive a fourth signal from a second link, and output the fourth signal through the fourth interface; the second line board is further configured to receive a fifth signal from the second link, and output the fifth signal to the tributary board through the third interface; and the second line board is further configured to receive a sixth signal from the tributary board through the third interface, and input the sixth signal into the second link, wherein the tributary board further comprises a switch as a combiner/selector and a switch as a replicator/distributor; the tributary board is configured to receive the second signal from the first line board, generate a downlink service by using the combiner/selector, and send the downlink service to a client device; and the tributary board is further configured to receive an uplink service from the client device, generate the third signal by using the replicator/distributor, and input the third signal into the first line board a second line board, wherein the second line board comprises at least a third interface and a fourth interface, the third interface is disposed between the tributary board and the second line board, and the fourth interface is disposed on a network side of the second line board; the second line board is configured to receive a fourth signal from a second link, and output the fourth signal through the fourth interface; the second line board is further configured to receive a fifth signal from the second link, and output the fifth signal to the tributary board through the third interface; and the second line board is further configured to receive a sixth signal from the tributary board through the third interface, and input the sixth signal into the second link, wherein the tributary board further comprises a switch as a combiner/selector and a switch as a replicator/distributor; the tributary board is configured to receive the second signal from the first line board, generate a downlink service by using the combiner/selector, and send the downlink service to a client device; and the tributary board is further configured to receive an uplink service from the client device, generate the third signal by using the replicator/distributor, and input the third signal into the first line board