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 International Telecommunication Union Telecommunication Standardization Sector (ITU-T), formerly known as the Consultative Committee for International Telegraph and Telephone (CCITT). The ISDN standards are a set of protocols that encompass digital telephony and data communications. The ITU-T groups and organizes the ISDN protocols according to the following general topic areas:

• E Protocols – Recommend telephone network standards for ISDN. For example, the E.164 protocol describes international addressing for ISDN.
• I Protocols – Deal with concepts, terminology, and general methods. The I.100 series includes general ISDN concepts and the structure of other I-series recommendations. I.200 deals with service aspects of ISDN. I.300 describes network aspects. I.400 describes how the UNI is provided.
• Q Protocols – Cover how switching and signaling should operate. The term signaling in this context means the process of establishing an ISDN call.

ISDN standards define two main channel types, each with a different transmission rate. The bearer channel, or B channel, is defined as a clear digital path of 64 kbps. It is said to be clear because it can be used to transmit any type of digitized data in full-duplex mode. For example, a digitized voice call can be transmitted on a single B channel. The second channel type is called a delta channel, or D channel. There can either be 16 kbps for the Basic Rate Interface (BRI) or 64 kbps for the Primary Rate Interface (PRI). The D channel is used to carry control information for the B channel.

When a TCP connection is established, there is an exchange of information called the connection setup. This information is exchanged over the path on which the data will eventually be transmitted. Both the control information and the data share the same pathway. This is called in-band signaling. ISDN however, uses a separate channel for control information, the D channel. This is called out-of-band signaling.

ISDN specifies two standard access methods, BRI and PRI. A single BRI or PRI interface provides a multiplexed bundle of B and D channels.

BRI uses two 64 kbps B channels plus one 16kbps D channel. BRI operates with many Cisco routers. Because it uses two B channels and one D channel, BRI is sometimes referred to as 2B+D.

The B channels can be used for digitized speech transmission. In this case, specialized methods are used for the voice encoding. Also, the B channels can be used for relatively high-speed data transport. In this mode, the information is carried in frame format, using either high-level data link control (HDLC) or PPP as the Layer 2 protocol. PPP is more robust than HDLC because it provides a mechanism for authentication and negotiation of compatible link and protocol configuration.

ISDN is considered a circuit-switched connection. The B channel is the elemental circuit-switching unit.

The D channel carries signaling messages, such as call setup and teardown, to control calls on B channels. Traffic over the D channel employs the Link Access Procedure on the D Channel (LAPD) protocol. LAPD is a data link layer protocol based on HDLC.

In North America and Japan, PRI offers twenty-three 64 kbps B channels and one 64 kbps D channel. A PRI offers the same service as a T1 or DS1 connection. In Europe and much of the rest of the world, PRI offers 30 B channels and one D channel in order to offer the same level of service as an E1 circuit. PRI uses a Data Service Unit/Channel Service Unit (DSU/CSU) for T1/E1 connections.

Web Links Integrated Services Digital Network (ISDN) http://www.cisco.com/univercd/cc/ td/doc/ cisintwk/ito_doc/ isdn.htms