1. Introduction

   This document provides an overview of E.164 telephone number [E164]
   portability in the Global Switched Telephone Network (GSTN).  There
   are considered to be three types of number portability (NP): service
   provider number portability (SPNP), location portability (not to be
   confused with terminal mobility), and service portability.

   SPNP, the focus of the present document, is a regulatory imperative
   in many countries seeking to liberalize telephony service
   competition, especially local service.  Historically, local telephony
   service (as compared to long distance or international service) has
   been regulated as a utility-like form of service.  While a number of
   countries had begun liberalization (e.g., privatization, de-
   regulation, or re-regulation) some years ago, the advent of NP is
   relatively recent (since ~1995).

   E.164 numbers can be non-geographic and geographic numbers.  Non-
   geographic numbers do not reveal the location information of those
   numbers.  Geographic E.164 numbers were intentionally designed as
   hierarchical routing addresses which could systematically be digit-
   analyzed to ascertain the country, serving network provider, serving
   end-office switch, and specific line of the called party.  As such,
   without NP a subscriber wishing to change service providers would
   incur a number change as a consequence of being served off of a
   different end-office switch operated by the new service provider.
   The impact in cost and convenience to the subscriber of changing
   numbers is seen as a barrier to competition.  Hence NP has become
   associated with GSTN infrastructure enhancements associated with a
   competitive environment driven by regulatory directives.

   Forms of SPNP have been deployed or are being deployed widely in the
   GSTN in various parts of the world, including the U.S., Canada,
   Western Europe, Australia, and the Pacific Rim (e.g., Hong Kong).
   Other regions, such as South America (e.g., Brazil), are actively
   considering it.

   Implementation of NP within a national telephony infrastructure
   entails potentially significant changes to numbering administration,
   network element signaling, call routing and processing, billing,
   service management, and other functions.

   NP changes the fundamental nature of a dialed E.164 number from a
   hierarchical physical routing address to a virtual address.  NP
   implementations attempt to encapsulate the impact to the GSTN and
   make NP transparent to subscribers by incorporating a translation
   function to map a dialed, potentially ported E.164 address, into a
   network routing address (either a number prefix or another E.164
   address) which can be hierarchically routed.

   This is roughly analogous to the use of network address translation
   on IP is that enables IP address portability by containing the
   address change to the edge of the network and retain the use of
   Classless Inter-Domain Routing (CIDR) blocks in the core which can be
   route aggregated by the network service provider to the rest of the
   internet.

   NP bifurcates the historical role of a subscriber's E.164 address
   into two or more data elements (a dialed or virtual address, and a
   network routing address) that must be made available to network
   elements through an NP translation database, carried by forward call
   signaling, and recorded on call detail records.  Not only is call
   processing and routing affected, but also Signaling System Number 7
   (SS7)/Common Channel Signaling System Number 7 (C7) messaging.  A
   number of Transaction Capabilities Application Part (TCAP)-based SS7

   messaging sets utilize an E.164 address as an application-level
   network element address in the global title address (GTA) field of
   the Signaling Connection Control Part (SCCP) message header.
   Consequently, SS7/C7 signaling transfer points (STPs) and gateways
   need to be able to perform n-digit global title translation (GTT) to
   translate a dialed E.164 address into its network address counterpart
   via the NP database.

   In addition, there are various national regulatory constraints that
   establish relevant parameters for NP implementation, most of which
   are not network technology specific.  Consequently, implementations
   of NP behavior in IP telephony, consistent with applicable regulatory
   constraints, as well as the need for interoperation with the existing
   GSTN NP implementations, are relevant topics for numerous areas of IP
   telephony works-in-progress with the IETF.

   This document describes three types of number portability and the
   four schemes that have been standardized to support SPNP for
   geographic E.164 numbers specifically.  Following that, specific
   information regarding the call routing and database query
   implementations are described for several regions (North American and
   Europe) and industries (wireless vs. wireline).  The Number
   Portability Database (NPDB) interfaces and the call routing schemes
   that are used in North America and Europe are described to show the
   variety of standards that may be implemented worldwide.  A glance at
   the NP implementations worldwide is provided.  Number pooling is
   briefly discussed to show how NP is being enhanced in the U.S. to
   conserve North American area codes.  The conclusion briefly touches
   the potential impacts of NP on IP and Telecommunications
   Interoperability.