ISDN Layer 3 does not impose the use of any network layer protocol for the B channels. The use of the D
channel is defined in Q.931 and specified in ITU I.451 and Q.931 + Q.932. Q.931 is used between the TE
and the local ISDN switch. Inside the ISDN network, the Signaling System 7 (SS7) Internal Signaling
Utility Protocol (ISUP) is used. Link Access Procedure on the D channel (LAPD) is the ISDN data link
layer protocol for the D channel. The data link protocol for the B channel, however, can be any of the
available protocols because the information can be passed transparently to the remote party. HDLC, PPP, or
Frame Relay encapsulations can be used to pass data over the B channel. LAPD is the data link layer
protocol for the D channel. It defines the framing characteristics for payload transmission.