|  | 5.1 | ISDN
| 5.1.5 |
Differentiate between E, I, and Q ISDN protocols

Work on standards for ISDN began in the late 1960s. A comprehensive set of ISDN recommendations was published in 1984 and is continuously updated by the Consultative Committee for International Telegraph and Telephone (CCITT), now the International Telecommunication Union Telecommunication Standardization Sector (ITU-T). ITU-T groups and organizes the ISDN protocols as described in the table. Q.921 recommends the data-link process on the ISDN D Channel. Q.931 governs the network layer functionality between the terminal endpoint and the local ISDN switch. This protocol does not impose an end-to-end recommendation. The various ISDN providers and switch types can and do use various implementations of Q.931. Other switches were developed before the standards groups finalized this standard. Because switch types are not standard, when configuring the router, you need to specify the ISDN switch you are connecting to. In addition, Cisco routers have debug commands to monitor Q.931 and Q.921 processes when an ISDN call is initiated or being terminated.
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