Having looked at the various codes used to describe ISDN equipment and references, now is a good time to get the remaining codes out of the way – those associated with ISDN protocols and standards. Different codes are used by the ITU-T to differentiate between standards and protocols that relate to the ISDN network, ISDN concepts, and ISDN switching and signaling. These include:
E-series. E-series ISDN standards relate to the telephone network. For example, standard E.164 describes international ISDN addressing.
I-series. I-series ISDN standards deal with ISDN concepts and interfaces. For example, standard I.112 outlines vocabulary terms of ISDN.
Q-series. Q-series ISDN standards relate to ISDN switching and signaling. For example, standard Q.921 describes the details of the Link Access Procedure on the ISDN D channel, LAPD.