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rfc:rfc3482

Network Working Group M. Foster Request for Comments: 3482 T. McGarry Category: Informational J. Yu

                                                         NeuStar, Inc.
                                                         February 2003
Number Portability in the Global Switched Telephone Network (GSTN):
                            An Overview

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

 This document provides an overview of E.164 telephone number
 portability (NP) in the Global Switched Telephone Network (GSTN).
 NP is a regulatory imperative seeking to liberalize local telephony
 service competition, by enabling end-users to retain telephone
 numbers while changing service providers.  NP changes the fundamental
 nature of a dialed E.164 number from a hierarchical physical routing
 address to a virtual address, thereby requiring the transparent
 translation of the later to the former.  In addition, there are
 various regulatory constraints that establish relevant parameters for
 NP implementation, most of which are not network technology specific.
 Consequently, the implementation of NP behavior 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.

Foster, et al. Informational [Page 1] RFC 3482 Number Portability in the GSTN: An Overview February 2003

Table of Contents

 1.  Introduction .................................................  2
 2.  Abbreviations and Acronyms ...................................  4
 3.  Types of Number Portability ..................................  6
 4.  Service Provider Number Portability Schemes ..................  7
     4.1   All Call Query (ACQ) ...................................  8
     4.2   Query on Release (QoR) .................................  9
     4.3   Call Dropback .......................................... 10
     4.4   Onward Routing (OR) .................................... 11
     4.5   Comparisons of the Four Schemes ........................ 11
 5.  Database Queries in the NP Environment ....................... 13
     5.1   U.S. and Canada ........................................ 13
     5.2   Europe ................................................. 14
 6.  Call Routing in the NP Environment ........................... 15
     6.1   U.S. and Canada ........................................ 16
     6.2   Europe ................................................. 17
 7.  NP Implementations for Geographic E.164 Numbers .............. 19
 8.  Number Conservation Method Enabled By NP ..................... 22
     8.1   Block Pooling .......................................... 22
     8.2   ITN Pooling ............................................ 23
 9.  Potential Implications ....................................... 23
 10. Security Considerations ...................................... 27
 11. IANA Considerations .......................................... 27
 12. Normative References ......................................... 27
 13. Informative References ....................................... 28
 14. Acknowledgement .............................................. 29
 15. Authors' Addresses ........................................... 29
 16. Full Copyright Statement ..................................... 30

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).

Foster, et al. Informational [Page 2] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 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

Foster, et al. Informational [Page 3] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 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.

2. Abbreviations and Acronyms

 ACQ     All Call Query
 AIN     Advanced Intelligent Network
 AMPS    Advanced Mobile Phone System
 ANSI    American National Standards Institute
 API     Application Programming Interface
 C7      Common Channel Signaling System Number 7
 CDMA    Code Division Multiple Access
 CdPA    Called Party Address
 CdPN    Called Party Number
 CH      Code Holder
 CIC     Carrier Identification Code
 CIDR    Classless Inter-Domain Routing
 CMIP    Common Management Information Protocol
 CO      Central Office
 CS1     Capability Set 1

Foster, et al. Informational [Page 4] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 CS2     Capability Set 2
 DN      Directory Number
 DNS     Domain Name System
 ENUM    Telephone Number Mapping
 ETSI    European Tecommunications Standards Institute
 FCI     Forward Call Indicator
 GAP     Generic Address Parameter
 GMSC    Gateway Mobile Services Switching Center or Gateway Mobile
         Switching Center
 GNP     Geographic Number Portability
 GSM     Global System for Mobile Communications
 GSTN    Global Switched Telephone Network
 GTT     Global Title Translation
 GW      Gateways
 HLR     Home Location Register
 IAM     Initial Address Message
 IETF    Internet Engineering Task Force
 ILNP    Interim LNP
 IN      Intelligent Network
 INAP    Intelligent Network Application Part
 INP     Interim NP
 IP      Internet Protocol
 IS-41   Interim Standards Number 41
 ISDN    Integrated Services Digital Network
 ISUP    ISDN User Part
 ITN     Individual Telephony Number
 ITU     International Telecommunication Union
 ITU-TS  ITU-Telecommunication Sector
 LDAP    Lightweight Directory Access Protocol
 LEC     Local Exchange Carrier
 LERG    Local Exchange Routing Guide
 LNP     Local Number Portability
 LRN     Location Routing Number
 MAP     Mobile Application Part
 MNP     Mobile Number Portability
 MSRN    Mobile Station Roaming Number
 MTP     Message Transfer Part
 NANP    North American Numbering Plan
 NGNP    Non-Geographic Number Portability
 NOA     Nature of Address
 NP      Number Portability
 NPA     Numbering Plan Area
 NPDB    Number Portability Database
 NRN     Network Routing Number
 OR      Onward Routing
 OSS     Operation Support System
 PCS     Personal Communication Services
 PNTI    Ported Number Translation Indicator

Foster, et al. Informational [Page 5] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 PODP    Public Office Dialing Plan
 PUC     Public Utility Commission
 QoR     Query on Release
 RN      Routing Number
 RTP     Return to Pivot
 SCCP    Signaling Connection Control Part
 SCP     Service Control Point
 SIP     Session Initiation Protocol
 SMR     Special Mobile Radio
 SPNP    Service Provider Number Portability
 SRF     Signaling Relaying Function
 SRI     Send Routing Information
 SS7     Signaling System Number 7
 STP     Signaling Transfer Point
 TCAP    Transaction Capabilities Application Part
 TDMA    Time Division Multiple Access
 TN      Telephone Number
 TRIP    Telephony Routing Information Protocol
 URL     Universal Resource Locator
 U.S.    United States

3. Types of Number Portability

 As there are several types of E.164 numbers (telephone numbers, or
 just TN) in the GSTN, there are correspondingly several types of
 E.164 NP in the GSTN.  First there are so-called non-geographic E.164
 numbers, commonly used for service-specific applications such as
 freephone (800 or 0800).  Portability of these numbers is called
 non-geographic number portability (NGNP).  NGNP, for example, was
 deployed in the U.S. in 1986-92.
 Geographic number portability (GNP), which includes traditional fixed
 or wireline numbers, as well as mobile numbers which are allocated
 out of geographic number range prefixes, is called NP or GNP, or in
 the U.S. local number portability (LNP).
 Number portability allows the telephony subscribers in the GSTN to
 keep their phone numbers when they change their service providers or
 subscribed services, or when they move to a new location.
 The ability to change the service provider while keeping the same
 phone number is called service provider portability (SPNP), also
 known as "operator portability."
 The ability to change the subscriber's fixed service location while
 keeping the same phone number is called location portability.
 The ability to change the subscribed services (e.g., from the plain

Foster, et al. Informational [Page 6] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 old telephone service to Integrated Services Digital Network (ISDN)
 services) while keeping the same phone number is called service
 portability.  Another aspect of service portability is to allow the
 subscribers to enjoy the subscribed services in the same way when
 they roam outside their home networks, as is supported by the
 cellular/wireless networks.
 In addition, mobile number portability (MNP) refers to specific NP
 implementation in mobile networks, either as part of a broader NP
 implementation in the GSTN or on a stand-alone basis.  Where
 interoperation of LNP and MNP is supported, service portability
 between fixed and mobile service types is possible.
 At present, SPNP has been the primary form of NP deployed due to its
 relevance in enabling local service competition.
 Also in use in the GSTN are the terms interim NP (INP) or Interim LNP
 (ILNP) and true NP.  Interim NP usually refers to the use of remote
 call forwarding-like measures to forward calls to ported numbers
 through the donor network to the new service network.  These are
 considered interim relative to true NP, which seeks to remove the
 donor network or old service provider from the call or signaling path
 altogether.  Often the distinction between interim and true NP is a
 national regulatory matter relative to the technical/operational
 requirements imposed on NP in that country.
 Implementations of true NP in certain countries (e.g., U.S., Canada,
 Spain, Belgium, Denmark) may pose specific requirements for IP
 telephony implementations as a result of regulatory and industry
 requirements for providing call routing and signaling independent of
 the donor network or last previous serving network.

4. Service Provider Number Portability Schemes

 Four schemes can be used to support service provider portability and
 are briefly described below.  But first, some further terms are
 introduced.
 The donor network is the network that first assigned a telephone
 number (e.g., TN +1-202-533-1234) to a subscriber, out of a number
 range administratively (e.g., +1 202-533) assigned to it.  The
 current service provider (new SP), or new serving network, is the
 network that currently serves the ported number.  The old serving
 network (or old SP) is the network that previously served the ported
 number before the number was ported to the new serving network.
 Since a TN can port a number of times, the old SP is not necessarily
 the same as the donor network, except for the first time the TN ports
 away, or when the TN ports back into the donor network and away

Foster, et al. Informational [Page 7] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 again.  While the new SP and old SP roles are transitory as a TN
 ports around, the donor network is always the same for any particular
 TN based on the service provider to whom the subtending number range
 was administratively assigned.  See the discussion below on number
 pooling, as this enhancement of NP further bifurcates the role of the
 donor network into two (the number range or code holder network, and
 the block holder network).
 To simplify the illustration, all the transit networks are ignored.
 The originating or donor network is the one that performs the
 database queries or call redirection, and the dialed directory number
 (TN) has previously been ported out of the donor network.
 It is assumed that the old serving network, the new serving network,
 and the donor network are different networks so as to show which
 networks are involved in call handling and routing and database
 queries in each of the four schemes.  Please note that the port of
 the number (process of moving it from one network to another)
 happened prior to the call setup and is not included in the call
 steps.  Information carried in the signaling messages to support each
 of the four schemes is not discussed to simplify the explanation.

4.1 All Call Query (ACQ)

 Figure 1 shows the call steps for the ACQ scheme.  Those call steps
 are as follows:
 1) The Originating Network receives a call from the caller and sends
    a query to a centrally administered Number Portability Database
    (NPDB), a copy of which is usually resident on a network element
    within its network or through a third party provider.
 2) The NPDB returns the routing number associated with the dialed
    directory number.  The routing number is discussed later in
    Section 6.
 3) The Originating Network uses the routing number to route the call
    to the new serving network.

Foster, et al. Informational [Page 8] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 +-------------+              +-----------+    Number   +-----------+
 | Centralized |              | New Serv. |    ported   | Old Serv. |
 |    NPDB     |    +-------->|  Network  |<------------|  Network  |
 +-------------+    |         +-----------+             +-----------+
     ^  |           |
     |  |           |
    1|  |         3.|
     |  | 2.        |
     |  |           |
     |  v           |
  +----------+      |         +----------+           +----------+
  |   Orig.  |------+         |   Donor  |           | Internal |
  |  Network |                |  Network |           |   NPDB   |
  +----------+                +----------+           +----------+
               Figure 1 - All Call Query (ACQ) Scheme.

4.2 Query on Release (QoR)

 Figure 2 shows the call steps for the QoR scheme.  Those call steps
 are as follows:
 +-------------+              +-----------+    Number   +-----------+
 | Centralized |              | New Serv. |    ported   | Old Serv. |
 |    NPDB     |              |  Network  |<------------|  Network  |
 +-------------+              +-----------+             +-----------+
     ^  |                          ^
     |  | 4.                       |
   3.|  |              5.          |
     |  |   +----------------------+
     |  |   |
     |  v   |
  +----------+      2.        +----------+           +----------+
  |   Orig.  |<---------------|   Donor  |           | Internal |
  |  Network |--------------->|  Network |           |   NPDB   |
  +----------+      1.        +----------+           +----------+
              Figure 2 - Query on Release (QoR) Scheme.
 1) The Originating Network receives a call from the caller and routes
    the call to the donor network.
 2) The donor network releases the call and indicates that the dialed
    directory number has been ported out of that switch.
 3) The Originating Network sends a query to its copy of the centrally
    administered NPDB.

Foster, et al. Informational [Page 9] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 4) The NPDB returns the routing number associated with the dialed
    directory number.
 5) The Originating Network uses the routing number to route the call
    to the new serving network.

4.3 Call Dropback

 Figure 3 shows the call steps for the Dropback scheme.  This scheme
 is also known as "Return to Pivot (RTP)."  Those call steps are as
 follows:
 1) The Originating Network receives a call from the caller and routes
    the call to the donor network.
 2) The donor network detects that the dialed directory number has
    been ported out of the donor switch and checks with an internal
    network-specific NPDB.
 3) The internal NPDB returns the routing number associated with the
    dialed directory number.
 4) The donor network releases the call by providing the routing
    number.
 5) The Originating Network uses the routing number to route the call
    to the new serving network.
 +-------------+              +-----------+    Number   +-----------+
 | Centralized |              | New Serv. |    porting  | Old Serv. |
 |    NPDB     |              |  Network  |<------------|  Network  |
 +-------------+              +-----------+             +-----------+
                                  /\
                                   |
                         5.        |
          +------------------------+
          |
          |
  +----------+       4.       +----------+     3.    +----------+
  |   Orig.  |<---------------|   Donor  |<----------| Internal |
  |  Network |--------------->|  Network |---------->|   NPDB   |
  +----------+      1.        +----------+    2.     +----------+
                     Figure 3 - Dropback Scheme.

Foster, et al. Informational [Page 10] RFC 3482 Number Portability in the GSTN: An Overview February 2003

4.4 Onward Routing (OR)

 Figure 4 shows the call steps for the OR scheme.  Those call steps
 are as follows:
 1) The Originating Network receives a call from the caller and routes
    the call to the donor network.
 2) The donor network detects that the dialed directory number has
    been ported out of the donor switch and checks with an internal
    network-specific NPDB.
 3) The internal NPDB returns the routing number associated with the
    dialed directory number.
 4) The donor network uses the routing number to route the call to the
    new serving network.
 +-------------+              +-----------+    Number   +-----------+
 | Centralized |              | New Serv. |    porting  | Old Serv. |
 |    NPDB     |              |  Network  |<------------|  Network  |
 +-------------+              +-----------+             +-----------+
                                  /\
                                   |
                                 4.|
                                   |
  +----------+                +----------+     3.    +----------+
  |   Orig.  |                |   Donor  |<----------| Internal |
  |  Network |--------------->|  Network |---------->|   NPDB   |
  +----------+      1.        +----------+    2.     +----------+
                Figure 4 - Onward Routing (OR) Scheme.

4.5 Comparisons of the Four Schemes

 Only the ACQ scheme does not involve the donor network when routing
 the call to the new serving network of the dialed ported number.  The
 other three schemes involve call setup to or signaling with the donor
 network.
 Only the OR scheme requires the setup of two physical call segments,
 one from the Originating Network to the donor network and the other
 from the donor network to the new serving network.  The OR scheme is
 the least efficient in terms of using the network transmission
 facilities.  The QoR and Dropback schemes set up calls to the donor
 network first but release the call back to the Originating Network
 that then initiates a new call to the Current Serving Network.  For
 the QoR and Dropback schemes, circuits are still reserved one by one

Foster, et al. Informational [Page 11] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 between the Originating Network and the donor network when the
 Originating Network sets up the call towards the donor network.
 Those circuits are released one by one when the call is released from
 the donor network back to the Originating Network.  The ACQ scheme is
 the most efficient in terms of using the switching and transmission
 facilities for the call.
 Both the ACQ and QoR schemes involve Centralized NPDBs for the
 Originating Network to retrieve the routing information.  Centralized
 NPDB means that the NPDB contains ported number information from
 multiple networks.  This is in contrast to the internal network-
 specific NPDB that is used for the Dropback and OR schemes.  The
 internal NPDB only contains information about the numbers that were
 ported out of the donor network.  The internal NPDB can be a stand-
 alone database that contains information about all or some ported-out
 numbers from the donor network.  It can also reside on the donor
 switch and only contain information about those numbers ported out of
 the donor switch.  In that case, no query to a stand-alone internal
 NPDB is required.  The donor switch for a particular phone number is
 the switch to which the number range is assigned from which that
 phone number was originally assigned.
 For example, number ranges in the North American Numbering Plan
 (NANP) are usually assigned in the form of central office codes (CO
 codes) comprising a six-digit prefix formatted as a NPA+NXX.  Thus a
 switch serving +1-202-533 would typically serve +1-202-533-0000
 through +1-202-533-9999.  In major cities, switches usually host
 several CO codes.  NPA stands for Numbering Plan Area, which is also
 known as the area code.  It is three-digits long and has the format
 of NXX where N is any digit from 2 to 9 and X is any digit from 0 to
 9.  NXX, in the NPA+NXX format, is known as the office code that has
 the same format as the NPA.  When a NPA+NXX code is set as "portable"
 in the Local Exchange Routing Guide (LERG), it becomes a "portable
 NPA+NXX" code.
 Similarly, in other national E.164 numbering plans, number ranges
 cover a contiguous range of numbers within that range.  Once a number
 within that range has ported away from the donor network, all numbers
 in that range are considered potentially ported and should be queried
 in the NPDB.
 The ACQ scheme has two versions.  One version is for the Originating
 Network to always query the NPDB when a call is received from the
 caller regardless of whether the dialed directory number belongs to
 any number range that is portable or has at least one number ported
 out.  The other version is to check whether the dialed directory
 number belongs to any number range that is portable or has at least
 one number ported out.  If yes, an NPDB query is sent.  If not, no

Foster, et al. Informational [Page 12] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 NPDB query is sent.  The former performs better when there are many
 portable number ranges.  The latter performs better when there are
 not too many portable number ranges at the expense of checking every
 call to see whether NPDB query is needed.  The latter ACQ scheme is
 similar to the QoR scheme, except that the QoR scheme uses call setup
 and relies on the donor network to indicate "number ported out"
 before launching the NPDB query.

5. Database Queries in the NP Environment

 As indicated earlier, the ACQ and QoR schemes require that a switch
 query the NPDB for routing information.  Various standards have been
 defined for the switch-to-NPDB interface.  Those interfaces with
 their protocol stacks are briefly described below.  The term "NPDB"
 is used for a stand-alone database that may support just one or some
 or all of the interfaces mentioned below.  The NPDB query contains
 the dialed directory number and the NPDB response contains the
 routing number.  There is certainly other information that is sent in
 the query and response.  The primary interest is to get the routing
 number from the NPDB to the switch for call routing.

5.1 U.S. and Canada

 One of the following five NPDB interfaces can be used to query an
 NPDB:
 a) Advanced Intelligent Network (AIN) using the American National
    Standards Institute (ANSI) version of the Intelligent Network
    Application Part (INAP) [ANSI SS] [ANSI DB].  The INAP is carried
    on top of the protocol stack that includes the (ANSI) Message
    Transfer Part (MTP) Levels 1 through 3, ANSI SCCP and ANSI TCAP.
    This interface can be used by the wireline or wireless switches,
    is specific to the NP implementation in North America, and is
    modeled on the Public Office Dialing Plan (PODP) trigger defined
    in the Advanced Intelligent Network (AIN) 0.1 call model.
 b) Intelligent Network (IN), which is similar to the one used for
    querying the 800 databases.  The IN protocol is carried on top of
    the protocol stack that includes the ANSI MTP Levels 1 through 3,
    ANSI SCCP, and ANSI TCAP.  This interface can be used by the
    wireline or wireless switches.
 c) ANSI IS-41 [IS41] [ISNP], which is carried on top of the protocol
    stack that includes the ANSI MTP Levels 1 through 3, ANSI SCCP,
    and ANSI TCAP.  This interface can be used by the IS-41 based
    cellular/Personal Communication Services (PCS) wireless switches
    (e.g., AMPS, TDMA and CDMA).  Cellular systems use spectrum at 800
    MHz range and PCS systems use spectrum at 1900 MHz range.

Foster, et al. Informational [Page 13] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 d) Global System for Mobile Communication Mobile Application Part
    (GSM MAP) [GSM], which is carried on top of the protocol stack
    that includes the ANSI MTP Levels 1 through 3, ANSI SCCP, and
    International Telecommunication Union - Telecommunication Sector
    (ITU-TS) TCAP.  It can be used by the PCS1900 wireless switches
    that are based on the GSM technologies.  GSM is a series of
    wireless standards defined by the European Telecommunications
    Standards Institute (ETSI).
 e) ISUP triggerless translation.  NP translations are performed
    transparently to the switching network by the signaling network
    (e.g., Signaling Transfer Points (STPs) or signaling gateways).
    ISUP IAM messages are examined to determine if the CdPN field has
    already been translated, and if not, an NPDB query is performed,
    and the appropriate parameters in the IAM message modified to
    reflect the results of the translation.  The modified IAM message
    is forwarded by the signaling node on to the designated DPC in a
    transparent manner to continue call setup.  The NPDB can be
    integrated with the signaling node or, accessed via an Application
    Programming Interface (API) locally, or by a query to a remote
    NPDB using  a proprietary protocol or the schemes described above.
 Wireline switches have the choice of using either (a), (b), or (e).
 IS-41 based wireless switches have the choice of using (a), (b), (c),
 or (e).  PCS1900 wireless switches have the choice of using (a), (b),
 (d), or (e).  In the United States, service provider portability will
 be supported by both the wireline and wireless systems, not only
 within the wireline or wireless domain but also across the
 wireline/wireless boundary.  However, this is not true in Europe
 where service provider portability is usually supported only within
 the wireline or wireless domain, not across the wireline/wireless
 boundary due to explicit use of service-specific number range
 prefixes.  The reason is to avoid caller confusion about the call
 charge.  GSM systems in Europe are assigned distinctive destination
 network codes, and the caller pays a higher charge when calling a GSM
 directory number.

5.2 Europe

 One of the following two interfaces can be used to query an NPDB:
 a) Capability Set 1 (CS1) of the ITU-TS INAP [CS1], which is carried
    on top of the protocol stack that includes the ITU-TS MTP Levels 1
    through 3, ITU-TS SCCP, and ITU-TS TCAP.
 b) Capability Set 2 (CS2) of the ITU-TS INAP [CS2], which is carried
    on top of the protocol stack that includes the ITU-TS MTP Levels 1
    through 3, ITU-TS SCCP, and ITU-TS TCAP.

Foster, et al. Informational [Page 14] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 Wireline switches have the choice of using either (a) or (b);
 however, all the implementations in Europe so far are based on CS1.
 As indicated earlier that number portability in Europe does not go
 across the wireline/wireless boundary.  The wireless switches can
 also use (a) or (b) to query the NPDBs if those NPDBs contains ported
 wireless directory numbers.  The term "Mobile Number Portability
 (MNP)" is used for the support of service provider portability by the
 GSM networks in Europe.
 In most, if not all, cases in Europe, the calls to the wireless
 directory numbers are routed to the wireless donor network first.
 Over there, an internal NPDB is queried to determine whether the
 dialed wireless directory number has been ported out or not.  In this
 case, the interface to the internal NPDB is not subject to
 standardization.
 MNP in Europe can also be supported via the MNP Signaling Relay
 Function (MNP-SRF).  Again, an internal NPDB or a database integrated
 at the MNP-SRF is used to modify the SCCP Called Party Address
 parameter in the GSM MAP messages so that they can be re-directed to
 the wireless serving network.  Call routing involving MNP will be
 explained in Section 6.2.

6. Call Routing in the NP Environment

 This section discusses the call routing after the routing information
 has been retrieved either through an NPDB query or an internal
 database lookup at the donor switch, or from the Integrated Services
 Digital Network User Part (ISUP) signaling message (e.g., for the
 Dropback scheme).  For the ACQ, QoR and Dropback schemes, it is the
 Originating Network that has the routing information and is ready to
 route the call.  For the OR scheme, it is the donor network that has
 the routing information and is ready to route the call.
 A number of triggering schemes may be employed that determine where
 in the call path the NPDB query is performed.  In the U.S. a "N-1"
 policy is used, which essentially says that for local calls, the
 originating local carriers performs the query.  Otherwise, the long
 distance carrier is expected to follow through with the query.  To
 ensure independence of the actual trigger policy employed in any one
 carrier, forward call signaling is used to flag that an NPDB query
 has already been performed and to therefore suppress any subsequent
 NP triggers that may be encountered in downstream switches, in
 downstream networks.  This allows the earliest able network in the
 call path to perform the query without introducing additional costs
 and call setup delays when redundant queries are performed
 downstream.

Foster, et al. Informational [Page 15] RFC 3482 Number Portability in the GSTN: An Overview February 2003

6.1 U.S. and Canada

 In the U.S. and Canada, a ten-digit North American Numbering Plan
 (NANP) number called Location Routing Number (LRN) is assigned to
 every switch involved in NP.  In the NANP, a switch is not reachable
 unless it has a unique number range (CO code) assigned to it.
 Consequently, the LRN for a switch is always assigned out of a CO
 code that is assigned to that switch.
 The LRN assigned to a switch currently serving a particular ported
 telephone number is returned as the network routing address in the
 NPDB response.  The service portability scheme that was adopted in
 the North America is very often referred to as the LRN scheme or
 method.
 LRN serves as a network address for terminating calls served off that
 switch using ported numbers.  The LRN is assigned by the switch
 operator using any of the unique CO codes (NPA+NXX) assigned to that
 switch.  The LRN is considered a non-dialable address, as the same
 10-digit number value may be assigned to a line on that switch.  A
 switch may have more than one LRN.
 During call routing/processing, a switch performs an NPDB query to
 obtain the LRN associated with the dialed directory number.  NPDB
 queries are performed for all the dialed directory numbers whose
 NPA+NXX codes are marked as portable NPA+NXX at that switch.  When
 formulating the ISUP Initial Address Message (IAM) to be sent to the
 next switch, the switch puts the ten-digit LRN in the ISUP Called
 Party Number (CdPN) parameter and the originally dialed directory
 number in the ISUP Generic Address parameter (GAP).  A new code in
 the GAP was defined to indicate that the address information in the
 GAP is the dialed directory number.  A new bit in the ISUP Forward
 Call Indicator (FCI) parameter, the Ported Number Translation
 Indicator (PNTI) bit, is set to imply that NPDB query has already
 been performed.  All the switches in the downstream will not perform
 the NPDB query if the PNTI bit is set.
 When the terminating switch receives the IAM and sees the PNTI bit in
 the FCI parameter set and its own LRN in the CdPN parameter, it
 retrieves the originally dialed directory number from the GAP and
 uses the dialed directory number to terminate the call.
 A dialed directory number with a portable NPA+NXX does not imply that
 a directory number has been ported.  The NPDBs currently do not store
 records for non-ported directory numbers.  In that case, the NPDB
 will return the same dialed directory number instead of the LRN.  The
 switch will then set the PNTI bit, but keep the dialed directory
 number in the CdPN parameter.

Foster, et al. Informational [Page 16] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 In the real world environment, the Originating Network is not always
 the one that performs the NPDB query.  For example, it is usually the
 long distance carriers that query the NPDBs for long distance calls.
 In that case, the Originating Network operated by the local exchange
 carrier (LEC) simply routes the call to the long distance carrier
 that is to handle that call.  A wireless network acting as the
 Originating Network can also route the call to the interconnected
 local exchange carrier network if it does not want to support the
 NPDB interface at its mobile switches.

6.2 Europe

 In some European countries, a routing number is prefixed to the
 dialed directory number.  The ISUP CdPN parameter in the IAM will
 contain the routing prefix and the dialed directory number.  For
 example, United Kingdom uses routing prefixes with the format of
 5XXXXX and Italy uses C600XXXXX as the routing prefix.  The networks
 use the information in the ISUP CdPN parameter to route the call to
 the New/Current Serving Network.
 The routing prefix can identify the Current Serving Network or the
 Current Serving Switch of a ported number.  For the former case,
 another query to the "internal" NPDB at the Current Serving Network
 is required to identify the Current Serving Switch before routing the
 call to that switch.  This shields the Current Serving Switch
 information for a ported number from the other networks at the
 expense of an additional NPDB query.  Another routing number, that be
 meaningful within the Current Serving Network, will replace the
 previously prefixed routing number in the ISUP CdPN parameter.  For
 the latter case, the call is routed to the Current Serving Switch
 without an additional NPDB query.
 When the terminating switch receives the IAM and sees its own routing
 prefix in the CdPN parameter, it retrieves the originally dialed
 directory number after the routing prefix, and uses the dialed
 directory number to terminate the call.
 The call routing example described above shows one of the three
 methods that can be used to transport the Directory Number (DN) and
 the Routing Number (RN) in the ISUP IAM message.  In addition, some
 other information may be added/modified as is listed in the ETSI 302
 097 document [ETSIISUP], which is based on the ITU-T Recommendation
 Q.769.1 [ITUISUP].  The three methods and the enhancements in ISUP to
 support number portability are briefly described below:
 a) Two separate parameters with the CdPN parameter containing the RN
    and a new Called Directory Number (CdDN) parameter containing the
    DN.  A new value for the Nature of Address (NOA) indicator in the

Foster, et al. Informational [Page 17] RFC 3482 Number Portability in the GSTN: An Overview February 2003

    CdPN parameter is defined to indicate that the RN is in the CdPN
    parameter.  The switches use the CdPN parameter to route the call
    as is done today.
 b) Two separate parameters with the CdPN parameter containing the DN
    and a new Network Routing Number (NRN) parameter containing the
    RN.  This method requires that the switches use the NRN parameter
    to route the call.
 c) Concatenated parameter with the CdPN parameter containing the RN
    plus the DN.  A new Nature of Address (NOA) indicator in the CdPN
    parameter is defined to indicate that the RN is concatenated with
    the DN in the CdPN parameter.  Some countries may not use new NOA
    value because the routing prefix does not overlap with the dialed
    directory numbers.  But if the routing prefix overlaps with the
    dialed directory numbers, a new NOA value must be assigned.  For
    example, Spain uses "XXXXXX" as the routing prefix to identify the
    new serving network and uses a new NOA value of 126.
 There is also a network option to add a new ISUP parameter called
 Number Portability Forwarding Information parameter.  This parameter
 has a four-bit Number Portability Status Indicator field that can
 provide an indication whether number portability query is done for
 the called directory number and whether the called directory number
 is ported or not if the number portability query is done.
 Please note that all of the NP enhancements for a ported number can
 only be used in the country that defined them.  This is because
 number portability is supported within a nation.  Within each nation,
 the telecommunications industry or the regulatory bodies can decide
 which method or methods to use.  Number portability related
 parameters and coding are usually not passed across the national
 boundaries unless the interconnection agreements allow it.  For
 example, a UK routing prefix can only be used in the UK, and would
 cause a routing problem if it appears outside the UK.
 As indicated earlier, an originating wireless network can query the
 NPDB and concatenate the RN with DN in the CdPN parameter and route
 the call directly to the Current Serving Network.
 If NPDBs do not contain information about the wireless directory
 numbers, the call, originated from either a wireline or a wireless
 network, will be routed to the Wireless donor network.  Over there,
 an internal NPDB is queried to retrieve the RN that then is
 concatenated with the DN in the CdPN parameter.
 There are several ways of realizing MNP.  If MNP-SRF is supported,
 the Gateway Mobile Services Switching Center (GMSC) at the wireless

Foster, et al. Informational [Page 18] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 donor network can send the GSM MAP Send Routing Information (SRI)
 message to the MNP-SRF when receiving a call from the wireline
 network.  The MNP-SRF interrogates an internal or integrated NPDB for
 the RN of the MNP-SRF of the wireless Current Serving Network and
 prefixes the RN to the dialed wireless directory number in the global
 title address information in the SCCP Called Party Address (CdPA)
 parameter.  This SRI message will be routed to the MNP-SRF of the
 wireless Current Serving Network, which then responds with an
 acknowledgement by providing the RN plus the dialed wireless
 directory number as the Mobile Station Roaming Number (MSRN).  The
 GMSC of the wireless donor network formulates the ISUP IAM with the
 RN plus the dialed wireless directory number in the CdPN parameter
 and routes the call to the wireless Current Serving Network.  A GMSC
 of the wireless Current Serving Network receives the call and sends
 an SRI message to the associated MNP-SRF where the global title
 address information of the SCCP CdPA parameter contains only the
 dialed wireless directory number.  The MNP-SRF then replaces the
 global title address information in the SCCP CdPA parameter with the
 address information associated with a Home Location Register (HLR)
 that hosts the dialed wireless directory number and forwards the
 message to that HLR after verifying that the dialed wireless
 directory number is a ported-in number.  The HLR then returns an
 acknowledgement by providing an MSRN for the GMSC to route the call
 to the MSC that currently serves the mobile station that is
 associated with the dialed wireless directory number.  Please see
 [MNP] for details and additional scenarios.

7. NP Implementations for Geographic E.164 Numbers

 This section shows the known SPNP implementations worldwide.
 +-------------+----------------------------------------------------+
 +   Country   +             SPNP Implementation                    +
 +-------------+----------------------------------------------------+
 +  Argentina  + Analyzing operative viability now. Will determine  +
 +             + whether portability should be made obligatory      +
 +             + after a technical solution has been determined.    +
 +-------------+----------------------------------------------------+
 +  Australia  + NP supported by wireline operators since 11/30/99. +
 +             + NP among wireless operators in March/April 2000,   +
 +             + but may be delayed to 1Q01. The access provider    +
 +             + or long distance provider has the obligation to    +
 +             + route the call to the correct destination. The     +
 +             + donor network is obligated to maintain and make    +
 +             + available a register of numbers ported away from   +
 +             + its network.  Telstra uses onward routing via an   +
 +             + on-switch solution.                                +
 +-------------+----------------------------------------------------+

Foster, et al. Informational [Page 19] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 +-------------+----------------------------------------------------+
 +   Country   +             SPNP Implementation                    +
 +-------------+----------------------------------------------------+
 +   Austria   + Uses onward routing at the donor network.  Routing +
 +             + prefix is "86xx" where "xx" identifies the         +
 +             + recipient network.                                 +
 +-------------+----------------------------------------------------+
 +  Belgium    + ACQ selected by the industry. Routing prefix is    +
 +             + "Cxxxx" where "xxxx" identifies the recipient      +
 +             + switch. Another routing prefix is "C00xx" with "xx"+
 +             + identifying the recipient network.  Plan to use NOA+
 +             + to identify concatenated numbers and abandon the   +
 +             + hexadecimal routing prefix.                        +
 +-------------+----------------------------------------------------+
 +  Brazil     + Considering NP for wireless users.                 +
 +-------------+----------------------------------------------------+
 +  Chile      + There has been discussions lately on NP.           +
 +-------------+----------------------------------------------------+
 +  Colombia   + There was an Article 3.1 on NP to support NP prior +
 +             + to December 31, 1999 when NP became technically    +
 +             + possible. Regulator has not yet issued regulations +
 +             + concerning this matter.                            +
 +-------------+----------------------------------------------------+
 +  Denmark    + Uses ACQ. Routing number not passed between        +
 +             + operators; however, NOA is set to "112" to         +
 +             + indicate "ported number."  QoR can be used based   +
 +             + on bilateral agreements.                           +
 +-------------+----------------------------------------------------+
 +  Finland    + Uses ACQ.  Routing prefix is "1Dxxy" where "xxy"   +
 +             + identifies the recipient network and service type. +
 +-------------+----------------------------------------------------+
 +  France     + Uses onward routing.  Routing prefix is "Z0xxx"    +
 +             + where "xxx" identifies the recipient switch.       +
 +-------------+----------------------------------------------------+
 +  Germany    + The originating network needs to do necessary      +
 +             + rerouting.  Operators decide their own solution(s).+
 +             + Deutsche Telekom uses ACQ.  Routing prefix is      +
 +             + "Dxxx" where "xxx" identifies the recipient        +
 +             + network.                                           +
 +-------------+----------------------------------------------------+
 +  Hong Kong  + Recipient network informs other networks about     +
 +             + ported-in numbers.  Routing prefix is "14x" where  +
 +             + "14x" identifies the recipient network, or a       +
 +             + routing number of "4x" plus 7 or 8 digits is used  +
 +             + where "4x" identifies the recipient network and    +
 +             + the rest of digits identify the called party.      +
 +-------------+----------------------------------------------------+

Foster, et al. Informational [Page 20] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 +-------------+----------------------------------------------------+
 +   Country   +             SPNP Implementation                    +
 +-------------+----------------------------------------------------+
 +  Ireland    + Operators choose their own solution but use onward +
 +             + routing now. Routing prefix is "1750" as the intra-+
 +             + network routing code (network-specific) and        +
 +             + "1752xxx" to "1759xxx" for GNP where "xxx"         +
 +             + identifies the recipient switch.                   +
 +-------------+----------------------------------------------------+
 +  Italy      + Uses onward routing. Routing prefix is "C600xxxxx" +
 +             + where "xxxxx" identifies the recipient switch.     +
 +             + Telecom Italia uses IN solution and other operators+
 +             + use on-switch solution.                            +
 +-------------+----------------------------------------------------+
 +  Japan      + Uses onward routing.  Donor switch uses IN to get  +
 +             + routing number.                                    +
 +-------------+----------------------------------------------------+
 +  Mexico     + NP is considered in the Telecom law; however, the  +
 +             + regulator (Cofetel) or the new local entrants have +
 +             + started no initiatives on this process.            +
 +-------------+----------------------------------------------------+
 + Netherlands + Operators decide NP scheme to use.  Operators have +
 +             + chosen ACQ or QoR.  KPN implemented IN solution    +
 +             + similar to U.S. solution.  Routing prefix is not   +
 +             + passed between operators.                          +
 +-------------+----------------------------------------------------+
 +  Norway     + OR for short-term and ACQ for long-term.  QoR is   +
 +             + optional. Routing prefix can be "xxx" with NOA=8,  +
 +             + or "142xx" with NOA=3 where "xxx" or "xx"          +
 +             + identifies the recipient network.                  +
 +------------ +----------------------------------------------------+
 +  Peru       + Wireline NP may be supported in 2001.              +
 +-------------+----------------------------------------------------+
 +  Portugal   + No NP today.                                       +
 +-------------+----------------------------------------------------+
 +  Spain      + Uses ACQ.  Telefonica uses QoR within its network. +
 +             + Routing prefix is  "xxyyzz" where "xxyyzz"         +
 +             + identifies the recipient network.  NOA is set to   +
 +             + 126.                                               +
 +-------------+----------------------------------------------------+
 +  Sweden     + Standardized the ACQ but OR for operators without  +
 +             + IN. Routing prefix is "xxx" with NOA=8 or "394xxx" +
 +             + with NOA=3 where "xxx" identifies the recipient    +
 +             + network. But operators decide NP scheme to use.    +
 +             + Telia uses onward routing between operators.       +
 +-------------+----------------------------------------------------+

Foster, et al. Informational [Page 21] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 +-------------+----------------------------------------------------+
 +   Country   +             SPNP Implementation                    +
 +-------------+----------------------------------------------------+
 + Switzerland + Uses OR now and QoR in 2001.  Routing prefix is    +
 +             + "980xxx" where "xxx" identifies the recipient      +
 +             + network.                                           +
 +-------------+----------------------------------------------------+
 +  UK         + Uses onward routing. Routing prefix is "5xxxxx"    +
 +             + where "xxxxx" identifies the recipient switch. NOA +
 +             + is 126. BT uses the dropback scheme in some parts  +
 +             + of its network.                                    +
 +-------------+----------------------------------------------------+
 +  US         + Uses ACQ.  "Location Routing Number (LRN)" is used +
 +             + in the Called Party Number parameter.  Called party+
 +             + number is carried in the Generic Address Parameter +
 +             + Use a PNTI indicator in the Forward Call Indicator +
 +             + parameter to indicate that NPDB dip has been       +
 +             + performed.                                         +
 +-------------+----------------------------------------------------+

8. Number Conservation Methods Enabled by NP

 In addition to porting numbers NP provides the ability for number
 administrators to assign numbering resources to operators in smaller
 increments.  Today it is common for numbering resources to be
 assigned to telephone operators in a large block of consecutive
 telephone numbers (TNs).  For example, in North America each of these
 blocks contains 10,000 TNs and is of the format NXX+0000 to NXX+9999.
 Operators are assigned a specific NXX, or block.  That operator is
 referred to as the block holder.  In that block there are 10,000 TNs
 with line numbers ranging from 0000 to 9999.
 Instead of assigning an entire block to the operator, NP allows the
 administrator to assign a sub-block or even an individual telephone
 number.  This is referred to as block pooling and individual
 telephone number (ITN) pooling, respectively.

8.1 Block Pooling

 Block Pooling refers to the process whereby the number administrator
 assigns a range of numbers defined by a logical sub-block of the
 existing block.  Using North America as an example, block pooling
 would allow the administrator to assign sub-blocks of 1,000 TNs to
 multiple operators.  That is, NXX+0000 to NXX+0999 can be assigned to
 operator A, NXX+1000 to NXX+1999 can be assigned to operator B, NXX-
 2000 to 2999 can be assigned to operator C, etc.  In this example,
 block pooling divides one block of 10,000 TNs into ten blocks of
 1,000 TNs.

Foster, et al. Informational [Page 22] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 Porting the sub-blocks from the block holder enables block pooling.
 Using the example above, operator A is the block holder, as well as
 the holder of the first sub-block, NXX+0000 to NXX+0999.  The second
 sub-block, NXX+1000 to NXX+1999, is ported from operator A to
 operator B.  The third sub-block, NXX+2000 to NXX+2999, is ported
 from operator A to operator C, and so on.  NP administrative
 processes and call processing will enable proper and efficient
 routing.
 From a number administration and NP administration perspective, block
 pooling introduces a new concept, that of the sub-block holder.
 Block pooling requires coordination between the number administrator,
 the NP administrator, the block holder, and the sub-block holder.
 Block pooling must be implemented in a manner that allows for NP
 within the sub-blocks.  Each TN can have a different serving
 operator, sub-block holder, and block holder.

8.2 ITN Pooling

 ITN pooling refers to the process whereby the number administrator
 assigns individual telephone numbers to operators.  Using the North
 American example, one block of 10,000 TNs can be divided into 10,000
 ITNs.  ITN is more commonly deployed in freephone services.
 In ITN the block is not assigned to an operator but to a central
 administrator.  The administrator then assigns ITNs to operators.  NP
 administrative processes and call processing will enable proper and
 efficient routing.

9. Potential Implications

 There are three general areas of impact to IP telephony works-in-
 progress with the IETF:
  1. Interoperation between NP in GSTN and IP telephony
  2. NP implementation or emulation in IP telephony
  3. Interconnection to NP administrative environment
 A good understanding of how number portability is supported in the
 GSTN is important when addressing the interworking issues between
 IP-based networks and the GSTN.  This is especially important when
 the IP-based network needs to route the calls to the GSTN.  As shown
 in Section 5, there are a variety of standards with various protocol
 stacks for the switch-to-NPDB interface.  Furthermore, the national
 variations of the protocol standards make it very complicated to deal
 with in a global environment.  If an entity in the IP-based network
 needs to query those existing NPDBs for routing number information to
 terminate the calls to the destination GSTN, it would be an

Foster, et al. Informational [Page 23] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 impractical, if not impossible, job for that entity to support all
 those interface standards to access the NPDBs in many countries.
 Several alternatives may address this particular problem.  One
 alternative is to use certain entities in the IP-based networks for
 dealing with NP query, similar to the International Switches that are
 used in the GSTN to interwork different national ISUP variations.
 This will force signaling information associated with the calls to
 certain NP-capable networks in the terminating GSTN to be routed to
 those IP entities that support the NP functions.  Those IP entities
 then query the NPDBs in the terminating country.  This will limit the
 number of NPDB interfaces that certain IP entities need to support.
 Another alternative can be to define a "common" interface to be
 supported by all the NPDBs so that all the IP entities use that
 standardized protocol to query them.  The existing NPDBs can support
 this additional interface, or new NPDBs that contain the same
 information but support the common IP interface can be deployed.  The
 candidates for such a common interface include ENUM (telephone number
 mapping) [ENUM], Lightweight Directory Access Protocol (LDAP) and SIP
 [SIP] (e.g., using the SIP redirection capability).  Certainly
 another possibility is to use an interworking function to convert
 from one protocol to another.
 IP-based networks can handle the domestic calls between two GSTNs.
 If the originating GSTN has performed NPDB query, SIP will need to
 transport and make use of some of the ISUP signaling information even
 if ISUP signaling may be encapsulated in SIP.  Also, IP-based
 networks may perform the NPDB queries, as the N-1 carrier.  In that
 case, SIP also needs to transport the NP related information while
 the call is being routed to the destination GSTN.  There are three
 pieces of NP related information that SIP needs to transport.  They
 are 1) the called directory number, 2) a routing number, and 3) a
 NPDB dip indicator.  The NPDB dip indicator is needed so that the
 terminating GSTN will not perform another NPDB dip.  The routing
 number is needed so that it is used to route the call to the
 destination network or switch in the destination GSTN.  The called
 directory number is needed so that the terminating GSTN switch can
 terminate the call.  When the routing number is present, the NPDB dip
 indicator may not be present because there are cases where the
 routing number is added for routing the call even if NP is not
 involved.  One issue is how to transport the NP related information
 via SIP.  The SIP Universal Resource Locator (URL) is one mechanism.
 Another better choice may be to add an extension to the "tel" URL
 [TEL] that is also supported by SIP.  Please see [TELNP] for the
 proposed extensions to the "tel" URL to support NP and freephone
 service.  Those extensions to the "tel" URL will be automatically
 supported by SIP because they can be carried as the optional
 parameters in the user portion of the "sip" URL.

Foster, et al. Informational [Page 24] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 For a called directory number that belongs to a country that supports
 NP, and if the IP-based network is expected to perform the NPDB
 query, the logical step is to perform the NPDB dip first to retrieve
 the routing number and use that routing number to select the correct
 IP telephony gateways that can reach the serving switch that serves
 the called directory number.  Therefore, if the "rn" parameter is
 present in the "tel" URL or sip URL in the SIP INVITE message, it,
 instead of the called directory number, should be used for making
 routing decisions assuming that no other higher priority routing-
 related parameters such as the "cic" (Carrier Identification Code)
 are present.  If "rn" (Routing Number) is not present, then the
 dialed directory number can be used as the routing number for making
 routing decisions.
 Telephony Routing Information Protocol (TRIP) [TRIP] is a policy
 driven inter-administrative domain protocol for advertising the
 reachability of telephony destinations between location servers, and
 for advertising attributes of the routes to those destinations.  With
 the NP in mind, it is very important to know, that if present, it is
 the routing number, not the called directory number, that should be
 used to check against the TRIP tables for making the routing
 decisions.
 Overlap signaling exists in the GSTN today.  For a call routing from
 the originating GSTN to the IP-based network that involves overlap
 signaling, NP will impact the call processing within the IP-based
 networks if they must deal with the overlap signaling.  The entities
 in the IP-based networks that are to retrieve the NP information
 (e.g., the routing number) must collect a complete called directory
 number information before retrieving the NP information for a ported
 number.  Otherwise, the information retrieval won't be successful.
 This is an issue for the IP-based networks if the originating GSTN
 does not handle the overlap signaling by collecting the complete
 called directory number.
 The IETF enum working group is defining the use of the Domain Name
 System (DNS) for identifying available services and/or Internet
 resources associated with a particular E.164 number.  [ENUMPO]
 outlines the principles for the operation of a telephone number
 service that resolves telephone numbers into Internet domain name
 addresses and service-specific directory discovery.  [ENUMPO]
 implements a three-level approach where the first level is the
 mapping of the telephone number delegation tree to the authority to
 which the number has been delegated, the second level is the
 provision of the requested DNS resource records from a service
 registrar, and the third level is the provision of service specific
 data from the service provider itself.  NP certainly must be
 considered at the first level because the telephony service providers

Foster, et al. Informational [Page 25] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 do not "own" or control the telephone numbers under the NP
 environment; therefore, they may not be the proper entities to have
 the authority for a given E.164 number.  Not only that, there is a
 regulatory requirement on NP in some countries that the donor network
 should not be relied on to reach the delegated authority during the
 DNS process.  The delegated authority for a given E.164 number is
 likely to be an entity designated by the end user that owns/controls
 a specific telephone number, or one that is designated by the service
 registrar.
 Since the telephony service providers may have the need to use ENUM
 for their network-related services (e.g., map an E.164 number to a
 HLR Identifier in the wireless networks), their ENUM records must be
 collocated with those of the telephony subscribers.  If that is the
 case, NP will impact ENUM when a telephony subscriber who has ENUM
 service changes the telephony service provider.  This is because that
 the ENUM records from the new telephony service provider must replace
 those from the old telephony service provider.  To avoid the NP
 impact on ENUM, it is recommended that the telephony service
 providers use a different domain tree for their network-related
 service.  For example, if e164.arpa is chosen for "end user" ENUM, a
 domain tree different from e164.arpa should be used for "carrier"
 ENUM.
 The IP-based networks also may need to support some forms of number
 portability in the future if E.164 numbers are assigned to the IP-
 based end users.  One method is to assign a GSTN routing number for
 each IP-based network domain or entity in a NP-capable country.  This
 may increase the number of digits in the routing number to
 incorporate the IP entities and impact the existing routing in the
 GSTN.  Another method is to associate each IP entity with a
 particular GSTN gateway.  At that particular GSTN gateway, the called
 directory number is then used to locate the IP-entity that serves
 that dialed directory number.  Yet, another method can be to assign a
 special routing number so that the call to an end user currently
 served by an IP entity is routed to the nearest GSTN gateway.  The
 called directory number then is used to locate the IP-entity that
 serves that dialed directory number.  A mechanism can be developed or
 used for the IP-based network to locate the IP entity that serves a
 particular dialed directory number.  Many other types of networks use
 E.164 numbers to identify the end users or terminals in those
 networks.  Number portability among GSTN, IP-based network, and those
 various types of networks may also need to be supported in the
 future.

Foster, et al. Informational [Page 26] RFC 3482 Number Portability in the GSTN: An Overview February 2003

10. Security Considerations

 In the PSTN, the NPDB queries are generated by the PSTN switches and
 carried over the SS7 networks to reach the NPDBs and back to the
 switches.  The SS7 networks are operated by telecommunications
 operators and signaling transport service providers in such a closed
 environment that make them difficult for the hackers to penetrate.
 However, when VoIP operators need the NP information and have to
 launch the NP queries from their softswitches, media gateway
 controllers or call managers, there would be security concerns if the
 NP queries and responses are transported over the Internet.  If the
 routing number or routing prefix in the response is altered during
 the message transport, the call will be routed to the wrong place.
 It is recommended that the NPDB queries be transported via a secure
 transport layer or with added security mechanisms to ensure the data
 integrity.

11. IANA Considerations

 This document introduces no new values for IANA registration.

12. Normative References

 [ANSI OSS] ANSI Technical Requirements No. 1, "Number Portability -
            Operator Services Switching Systems," April 1999.
 [ANSI SS]  ANSI Technical Requirements No. 2, "Number Portability -
            Switching Systems," April 1999.
 [ANSI DB]  ANSI Technical Requirements No. 3, "Number Portability
            Database and Global Title Translation," April 1999.
 [CS1]      ITU-T Q-series  Recommendations - Supplement 4, "Number
            portability Capability set 1 requirements for service
            provider portability (All call query and onward routing),"
            May 1998.
 [CS2]      ITU-T Q-series  Recommendations - Supplement 5, "Number
            portability -Capability set 2 requirements for service
            provider portability (Query on release and Dropback),"
            March 1999.
 [E164]     ITU-T Recommendation E.164, "The International Public
            Telecommunications Numbering Plan," 1997.
 [ENUM]     Falstrom, P., "E.164 number and DNS", RFC 2916, September
            2000.

Foster, et al. Informational [Page 27] RFC 3482 Number Portability in the GSTN: An Overview February 2003

 [ETSIISUP] ETSI EN 302 097 V.1.2.2, Integrated Services Digital
            Network (ISDN); Signalling System No.7 (SS7); ISDN User
            Part (ISUP); Enhancement for support of Number Portability
            (NP) [ITU-T Recommendation Q.769.1 (2000), modified]
 [GSM]      GSM 09.02: "Digital cellular telecommunications system
            (Phase 2+); Mobile Application Part (MAP) specification".
 [IS41]     TIA/EIA IS-756 Rev. A, "TIA/EIA-41-D Enhancements for
            Wireless Number Portability Phase II (December 1998),
            "Number Portability Network Support," April 1998.
 [ITUISUP]  ITU-T Recommendation Q.769.1, "Signaling System No. 7 -
            ISDN User Part Enhancements for the Support of Number
            Portability," December 1999.
 [MNP]      ETSI EN 301 716 (2000-10) European Standard
            (Telecommunications series) Digital cellular
            telecommunications system (Phase 2+); Support of Mobile
            Number Portability (MNP); Technical Realisation; Stage 2;
            (GSM 03.66 Version 7.2.0 Release 1998).
 [RFC]      Bradner, S., "The Internet Standards Process -- Revision
            3", BCP 9, RFC 2026, October 1996.

13. Informative References

 [ENUMPO]   Brown A. and G. Vaudreuil, "ENUM Service Specific
            Provisioning: Principles of Operations", Work in Progress.
 [SIP]      Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
            A., Peterson, J., Sparks, R., Handley, M. and E. Schooler,
            "SIP:  Session Initiation Protocol", RFC 3461, June 2002.
 [TEL]      Schulzrinne, H. and A. Vaha-Sipila, "URIs for Telephone
            Calls", Work in Progress.
 [TELNP]    Yu, J., "Extensions to the "tel" URL to support Number
            Portability and Freephone Service", Work in Progress.
 [TRIP]     Rosenberg, J., Salama, H. and M. Squire, "Telephony
            Routing Information Protocol (TRIP)", RFC 3219, January
            2002.

Foster, et al. Informational [Page 28] RFC 3482 Number Portability in the GSTN: An Overview February 2003

14. Acknowledgment

 The authors would like to thank Monika Muench for providing
 information on ISUP and MNP.

15. Authors' Addresses

 Mark D. Foster
 NeuStar, Inc.
 46000 Center Oak Plaza
 Sterling, VA 20166
 United States
 Phone: +1-571-434-5410
 Fax:   +1-571-434-5401
 EMail: mark.foster@neustar.biz
 Tom McGarry
 NeuStar, Inc.
 46000 Center Oak Plaza
 Sterling, VA 20166
 United States
 Phone: +1-571-434-5570
 Fax:   +1-571-434-5401
 EMail: tom.mcgarry@neustar.biz
 James Yu
 NeuStar, Inc.
 46000 Center Oak Plaza
 Sterling, VA 20166
 United States
 Phone: +1-571-434-5572
 Fax:   +1-571-434-5401
 EMail: james.yu@neustar.biz

Foster, et al. Informational [Page 29] RFC 3482 Number Portability in the GSTN: An Overview February 2003

16. Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Foster, et al. Informational [Page 30]

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