Directory Number

Description

A Directory Number (DN) is a fundamental identifier in telecommunications networks, defined by 3GPP as a string consisting of one or more characters from the set {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, *, #, a, b, c}. This string is associated with a nature of address indicator (NAI) and a number plan indicator (NPI), which provide context for interpreting the number, such as whether it is an international, national, or subscriber number, and which numbering plan it belongs to (e.g., ISDN/telephony, data, telex). The DN is used to identify the called or calling party in a telecommunication service and is essential for routing calls and messages through the network.

In circuit-switched networks like GSM and UMTS, the DN is typically the Mobile Station International Subscriber Directory Number (MSISDN), which is the phone number associated with a SIM card. The MSISDN follows the E.164 numbering plan and consists of a country code, national destination code, and subscriber number. The DN can also represent supplementary service codes, such as *21* for call forwarding, where the * and # characters are used to invoke services. However, when using the public Man-Machine Interface (MMI) for controlling supplementary services, the * and # characters cannot be part of any Service Code (SC) or Supplementary Service Information (SI) field, ensuring unambiguous parsing of service activation strings.

In the IP Multimedia Subsystem (IMS) and VoLTE/VoNR, the DN is represented as a SIP URI or Tel URI, which may include the MSISDN or a alphanumeric identifier. The DN is used in the IMS to identify the public user identity, which is registered and used for session establishment. The nature of address indicator and number plan indicator are mapped to parameters in the SIP signalling, such as the 'phone-context' parameter in a Tel URI, to ensure correct interpretation.

The DN is also used in emergency services, where specific numbers (e.g., 112, 911) are recognized as emergency DNs, triggering special handling in the network. In 5G, the DN concept persists for voice and supplementary services, although the underlying protocols are IP-based. The 3GPP specifications 21.905 (Vocabulary), 22.030 (Man-Machine Interface), and 22.066 (Support of Mobile Number Portability) provide the definition and usage of DN.

Overall, the DN is a versatile identifier that enables users to reach services and other users across various network technologies, ensuring interoperability and correct routing. Its definition has remained stable across releases from Rel-4 to Rel-19, reflecting its foundational role in telecommunications.

Purpose & Motivation

The Directory Number exists to provide a human-readable and dialable identifier for subscribers and services in telecommunications networks. Before standardized DNs, networks used various proprietary numbering schemes, which hindered interoperability and roaming. The DN, with its associated nature of address and number plan indicators, allows a called party to be uniquely identified and reached regardless of the originating network or country. It solves the problem of routing calls across interconnected networks by providing a common language for numbering.

Moreover, the DN enables supplementary services through the use of special characters like * and #, allowing users to activate, deactivate, and control services via the Man-Machine Interface. This flexibility is crucial for user-friendly service management. The DN also supports number portability, allowing subscribers to retain their number when changing operators, as specified in 3GPP TS 22.066. Without a standardized DN, such features would be difficult to implement consistently.

In the evolution from circuit-switched to packet-switched networks, the DN concept has been adapted to SIP URIs, ensuring continuity of service. The DN remains a key identifier in 5G for voice and emergency services, demonstrating its enduring importance.