ISDN protocols define how information is transferred from one device to another in the network.
The ITU-T has established three types of protocols to handle this information transfer:
Protocols beginning with the letter E specify ISDN on the existing telephone network.
Protocols beginning with the letter I specify concepts, terminology, and services.
Protocols beginning with the letter Q specify switching and signaling. Two Q standards of
interest are Q.921, which handles layer 2, and Q.931, which deals with layer 3 interfacing.
Spending some time reviewing the Q standards will help you use a couple of the IOS debug
commands covered later in this chapter. As just stated, the ITU-T recommendations Q.921 and
Q.931 handle switching and signaling. Q.921 uses Link Access Procedure, Data (LAPD) to communicate
with other ISDN devices across the D channel. LAPD’s primary purpose is to transport
signaling information.