1. Public Switch Telephone Network and Numbering Plan

The Public Switched Telephone Network (PSTN) refers to the network of the world's public circuit-switched telephone networks. It consists of transmission networks, switching systems, and facilities for maintenance equipment, billing systems, and databases. The original switches were manually configured by operators. Later, mechanical switches were developed, and Electronic Switching System (ESS) was introduced in the 1960s. The PSTN is now almost entirely based on digital telephone systems, which is also the base of mobile (cellular) networks.

The network termination point in the PSTN may be a telephone, a fax machine, a copy machine or other device. The PSTN uses a numbering scheme, which contains the information necessary to dial (identify)the intended endpoint for the telephone call. ITU E.164 specifies the global standard for dialing systems. It specifies the number structure and functionality for the four categories of numbers used for international public telecommunication: geographic area, global services, networks, and Groups of Countries (GoC). For each of the categories, it details the components of the numbering structure and the digit analysis requires to successfully route the calls. Figure 1-1 illustrates the international E.164-number structure for four categories.

Number portability allows customers to keep their telephone numbers when they change physical locations, service providers and types of service in the PSTN. E.164 Supplement 2 defines standard terminology for the different aspects of number portability within an E-164 numbering scheme. The ITU introduced the concept of Universal Personal Telecommunications (UPT) to allow global number portability regardless of geography or telecommunications carrier including those using new IP-based technologies in F.850. ITU E.168 specifies application of E.164 numbering plan for UPT. UPT uses a special segment of the international telephone number space, country code +878. ITU E.212 specifies International Mobile Subscriber Identity (IMSI). IMSI is a unique number that is associated with all GSM and Universal Mobile Telecommunications System (UMTS) network mobile phone users. The number is stored in the Subscriber Identity Module (SIM) in a mobile phone. It is maximum 15 digits composed of MCC (Mobile Country Code, 3 digits), MNC (Mobile Network Code, 2 to 3 digits) and MSIN (Mobile Subscriber Identification Number, maximum 10 digits).

With the advent of Voice over Internet Protocol (VoIP) such as ITU H.323 and IETF Session Initiation Protocol (SIP), Telephone Number Mapping (EUNM) is defined to provide inter-working between VoIP phones and PSTN phones are required. ENUM resolves E.164 telephone numbers into other resources or services on the Internet by using an indirect lookup method to obtain NAPTR records stored at a DNS database.

Numbering Plan Related Standards

ITU Recommendations:

E.164: The international public telecommunication numbering plan (Replaces former E.163 numbering plan). This Recommendation provides the number structure and functionality for the four categories of numbers used for international public telecommunication: geographic areas, global services, Networks and Groups of Countries (GoC). Annex A provides clarification and explanation to the structure and functionality of international E-164-numbers. Annex B describes the application of international E-164 numbers to ISDN numbering and addressing. E.164 also includes 4 supplementary recommendations.

E.164 Supplement 1: Alternatives for carrier selection and network identification

E.164 Supplement 2: Number Portability

E.164 Supplement 3: Operational and administrative issues associated with national implementations of the ENUM functions

E.164 Supplement 4: Operational and administrative issues associated with the implementation of ENUM for non-geographic country codes

E.168: Application of E.164 numbering plan for UPT. This Recommendation provides a framework of numbering for Universal Personal Telecommunications (UPT), which allows global number portability regardless of geography or telecommunications carrier

E.212: The international identification plan for mobile terminals and mobile users. This Recommendation describes an international identification plan for mobile terminals or mobile users of public networks enabling roaming capabilities

F.850: Principles of Universal Personal Telecommunication (UPT). In fixed telecommunication networks and some mobile telecommunication networks, users/subscribers are associated with a specific terminal. For the UPT service, the fixed association between terminal and user identification is removed by the means of an UPT Number. The UPT Users are personally associated with their own UPT Numbers, which are used as the basis for making and receiving calls independent of terminal.

IETF RFCs:

RFC 3761: The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application (ENUM). This document defines the use of the Domain Name System (DNS) for storage of E.164 numbers, specifically, how DNS can be used for identifying available services connected to each E.164 number.

RFC 3482: Number Portability in the Global Switched Telephone Network (GSTN): An Overview. This document provides an overview of E.164 telephone number portability (NP) in the Global Switched Telephone Network (GSTN).