Function and principle DSL board ISDN 2B + D interface board, and each board provides eight independent user interface (U-mouth). DSL Digital Subscriber plate located in the user box compatible the slot with ordinary user board (ASL) board. CB03DSL board evolved in the CB02, CB01 board based on, and consistent with maximum. the main function (1) as ISDN 2B + D user interface board, each plate 8 Road "U" interface. (2) The board processing 8-channel ISDN LAPD (Q.921) protocol. (3) to support internal and external line test. (4) support program loading veneer. (5) the DSL board and switch motherboards interface signal compatible with the ordinary user board (ASL). (6) DSL board for CENTREX attendant console access.