AT&T Technical Reference 41459 (June 1999)

TR 41459, June 1999

AT&T Network ISDN Primary Rate Interface and Special Application Specification User - Network Interface Description

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AT&T Technical Reference (TR) 41459 describes the service-related capabilities and features which will be supported by the AT&T Switched Network in its implementation of Integrated Services Digital Network (ISDN) Primary Rate Interface (PRI).

The ISDN Primary Rate Interface is a multi-purpose, high speed multiplexed digital interface based on International Telecommunication Union - Telecommunication Standardization Sector (ITU-T, formerly CCITT) ISDN Recommendations [1] . These Recommendations specify the use of a 1.544 Mbps (DS-1) digital interface, structured to contain bearer channels for the transport of end user information (e.g., voice, customer data, etc.) and a message-oriented, out-of-band, signaling channel used to control the bearer channels. The message-oriented signaling channel follows a layered protocol structure, based on the ITU-T I.441 (LAPD) and I.451 Recommendations for Layers 2 and 3, respectively [2] .

The key feature of the ISDN Primary Rate Interface is the use of out-of-band message-oriented signaling, carried in a separate digital channel on the access and/or egress facility. This feature enables AT&T to provide end-to-end services, over service-independent access channels, at 64 Kbps and certain multiples of 64 Kbps. It also allows for a wide variety of vertical features on existing and planned services.

The ISDN Primary Rate Interface supports the AT&T network features and services described in TR 41459 and serves as a foundation for many other special access services. Users with this ISDN Primary Rate Interface can communicate across the AT&T Network with those users who do not use ISDN Primary Rate Interface, subject to the limitations of their network interfaces and premises equipment.

Because of its multi-purpose nature, the ISDN Primary Rate Interface can be used in a range of applications, including:

  • The interconnection of Customer Premises Equipment (CPE) having ISDN Primary Rate Interfaces within a single building;
  • The interconnection of CPE having ISDN Primary Rate Interfaces using DS-1 private line transmission facilities, (such as ACCUNET® T1.5 Service), for geographically separated locations; and
  • The interconnection of CPE having an ISDN Primary Rate Interface to an ISDN Switched Common Carrier Network, such as the AT&T Communications Network or a Local Exchange Carrier Network.

Only the interconnection of CPE having an ISDN Primary Rate Interface to an ISDN Switched Common Carrier Network is discussed in TR 41459.

TR 41459 is published as a guide for the designers, manufacturers, consultants, and suppliers of systems and equipment which would meet the AT&T ISDN Primary Rate Interface. It contains a comprehensive specification of the ISDN Primary Rate Interface for the variety of applications suggested. However, since individual AT&T ISDN products and services may serve differing applications, specific aspects of the interface specification may not be applicable to a particular product or network interface. Additional AT&T documentation describing a particular AT&T product or network interface should be used in conjunction with TR 41459 to determine:

  • The applicability of the various protocol elements, procedures, and capabilities described in TR 41459 to a particular AT&T network interface;
  • The feature and service capabilities offered by that product or network; and
  • The product or network specific codings and procedures for those features or services.

Consistent with AT&T's intent to follow international standards, Technical Reference 41459 is based upon the ITU-T Recommendations and subsequent standards activities within both the ITU-T and the ANSI accredited T1S1 committee of the Exchange Carriers Standards Association (ECSA). Extensions and clarifications have been made as necessary to support additional capabilities and clarify areas where the existing standards are vague or undefined.

TR 41459 is updated to specify future additional AT&T network features and/or to reflect future changes resulting from ongoing standards activities, such as those in the ITU-T Standardization Sector and ANSI-Accredited Committee T1 - Telecommunications. TR 41459 does not cover Network Interconnect capabilities.

The AT&T Primary Rate Interface is compatible, subject to feature set differences, with other announced or future AT&T products as specified in other external AT&T publications.


Section I describes the physical layer of the ISDN Primary Rate Interface. It is based on the use of a standard DS-1 (1.544 Mbps) interface and ITU-T Recommendations I.211, I.412, and I.431. Requirements (or references to appropriate documents) are provided for electrical characteristics, channel structure, line coding, pulse density, clear and restricted channel operation, and the use of bit-inverted HDLC on restricted 64 Kbps channels subject to any HDLC processing by AT&T products or the AT&T Communications Network.

Section II describes the Link Access Procedure on the D-Channel (LAPD), for D-Channel signaling on the ISDN Primary Rate Interface. It is based on ITU-T Recommendations I.440 and I.441. LAPD provides for one or more logical data link connections over the 64 Kbps D-Channel. In addition, it provides a robust information transfer path - isolating the layer 3 protocol from transmission errors, supervisory frames and providing sequence and flow control, etc.

Section III specifies the message-oriented signaling procedures on the D-Channel for the control of circuit switched and packet connections. The Layer 3 signaling procedures use a set of functional messages to establish, maintain, and terminate connections between ISDN entities. The messages are composed of Information Elements that convey the information used in the signaling process. Besides basic call control, the Layer 3 messages can also convey additional information used for numerous applications, such as passing the identity of the calling party, passing information regarding terminal compatibility, allowing the redirection of calls, etc.

TR 41459 also contains four appendices:

Appendix 1 - Service Specific Capabilities

Appendix 2 - Alternate Destination Call Redirection

Appendix 3 - Vari-A-Bill (previously known as Flexible Billing)

Appendix 4 - Enhanced Service Provider-Credit Checking Application

ISDN Vendor testing program is currently suspended

  1. The ISDN Primary Rate Interface is one of two ISDN interfaces defined by the ITU-T Recommendations. The other interface is referred to as the ISDN Basic Rate Interface.
  2. 2. The Layers 2 and 3 Recommendations are also called Q.921 and Q.931, respectively. These Q-Series Recommendations are identical to the respective I-Series Recommendations, but have been numbered differently by ITU-T Study Group 11 that developed them. The I-Series Recommendations are from ITU-T Study Group 13 that oversees all of the ISDN standardization activities.