+ + "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.
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
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.
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
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.
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", RFC2916, September
2000.
[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, RFC2026, 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", RFC3461, 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)", RFC3219, January
2002.
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
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