💾 Archived View for gemini.bortzmeyer.org › rfc-mirror › rfc3482.txt captured on 2022-06-12 at 00:18:16.
⬅️ Previous capture (2021-11-30)
-=-=-=-=-=-=-
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:
- Interoperation between NP in GSTN and IP telephony
- NP implementation or emulation in IP telephony
- 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]