[RFCs/IDs] [Plain Text] [From draft-ietf-enum-e164-gstn-np]
INFORMATIONAL
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.
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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).
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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
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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
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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
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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
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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
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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.
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+-------------+ +-----------+ 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.
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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.
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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
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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
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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.
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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.
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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.
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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.
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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
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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