Answer: D - LAPB is used for out-of-band signaling.
Key Takeaway: LAPD, or Link Access Procedure Balanced, implements the data link layer as defined in the X.25 protocol suite. It is a bit-oriented protocol derived from HDLC that ensures that frames are error-free and in the right sequence. It is used to manage communication and packet framing between data terminal equipment and the data circuit-terminating equipment devices in the X.25 protocol stack. LAPB sessions can be established by either the DTE or DCE. The station initiating the call is determined to be the primary and the responding station is the secondary.
