

                                                            Mark Foster 
Internet Draft                                              Tom McGarry 
Document: <draft-ietf-enum-e164-gstn-np-01.txt>                James Yu 
                                                          NeuStar, Inc. 
Category: Informational                                February 9, 2001 
 
 
              Number Portability in the GSTN: An Overview 
 
 
Status of this Memo 
 
   This document is an Internet-Draft and is in full conformance with 
   all provisions of Section 10 of RFC2026 [RFC].  
 
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1. Abstract 
    
   This document provides an overview of E.164 telephone number 
   portability (NP) in the Global Switched Telephone Network (GSTN).  
   There are three types of number portability: service provider number 
   portability (SPNP), location portability, and service portability.  
   Service provider portability, the focus of the present draft, is a 
   regulatory imperative in many countries seeking to liberalize local 
   telephony service competition, by enabling end-users to retain pre-
   existing telephone numbers while changing service providers.  
   Implementation of NP within national GSTN 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, thereby requiring the transparent translation of 
   the later to the former.  In addition, there are various regulatory 

  
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Number Portability in the GSTN: An Overview           February 9, 2000 
 
 
   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 work-in-progress 
   at IETF. 
 
    
2. Introduction 
    
   This document provides an overview of E.164 telephone number 
   portability in the Global Switched Telephone Network (GSTN).  There 
   are considered to be three types of number portability (NP): service 
   provider portability (SPNP), location portability (not to be 
   confused with terminal mobility), and service portability. 
    
   Service provider portability (SPNP), the focus of the present draft, 
   is a regulatory imperative in many countries seeking to liberalize 
   telephony service competition, especially local service.  
   Historically, local telephony service (as compared to long distance 
   or international service) has been regulated as a utility-like form 
   of service.  While a number of countries had begun liberalization 
   (e.g. privatization, de-regulation, or re-regulation) some years 
   ago, the advent of NP is relatively recent (since ~1995). 
    
   E.164 numbers can be non-geographic and geographic numbers.  Non-
   geographic numbers do not reveal the locations 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 cost and convenience impact to the subscriber of changing 
   numbers is seen as 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 
  
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   implementations attempt to encapsulate the impacts 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 addresses to enable IP address portability by containing the 
   impact of the address change to the edge of the network and retain 
   the use of 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 subscriberÆs 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 translations database, carried by forward 
   call signaling, and recorded on call detail records.  Not only is 
   call processing and routing affected, but also so is SS7/C7 
   messaging.  A number of TCAP-based SS7 messaging sets utilize an 
   E.164 address as an application-level network element address in the 
   global title address (GTA) field of the 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 work-in-progress at IETF. 

   This document describes three types of number portability and the 
   four schemes that have been standardized to support SPNP for 
   geographic E.164 numbersspecifically.  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 the North America and Europe are described to show 
   the variety of standards that may be implemented worldwide.  A 
   glance of 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 & Telecommunications 
   Interoperability.  Appendix A provides some specific technical and 
   regulatory information on NP in North America.  Appendix B describes 
   the number portability administration process that manages the 
   number portability database in North America. 
    
    
  
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3. Abbreviations and Acronyms 
    
   ACQ     All Call Query 
   AIN     Advanced Intelligent Network 
   AMPS    Advanced Mobile Phone System 
   ANSI    American National Standards Institute 
   CDMA    Code Division Multiple Access 
   CdPA    Called Party Address 
   CdPN    Called Party Number 
   CH      Code Holder 
   CMIP    Common Management Information Protocol 
   CS1     Capability Set 1 
   CS2     Capability Set 2 
   DN      Directory Number 
   DNS     Domain Name System 
   ETSI    European Technical Standards Institute 
   FCI     Forward Call Indicator 
   GAP     Generic Address Parameter 
   GMSC    Gateway Mobile Services Switching Center or Gateway Mobile 
           Switching Center 
   GSM     Global System for Mobile Communications 
   GSTN    Global Switched Telephone Network 
   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 
   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 
   NP      Number Portability 
   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 
   SMS     Service Management System 
   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 
    
    
4. 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-call 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, 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 Global 
   Switched Telephone Network (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 subscriberÆs 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 
   old telephone service to Integrated Services Digital Network (ISDN) 
   services) while keeping the same phone number is called service 
  
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   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. 
    
    
5. 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 if the TN ports back into the donor network and away 
   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 to NP further 

  
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   bifurcates the role of 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 been ported out of the donor network before.  
    
   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 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. 
    
    
5.1 All Call Query (ACQ) 
    
   Figure 1 shows the call steps for the ACQ scheme.  Those call steps 
   are as follows: 
    
    
   +-------------+              +-----------+    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. 
 
 
   (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 7. 
   (3) The Originating Network uses the routing number to route the 
       call to the new serving network. 
  
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5.2 Query on Release (QoR) 
 
  Figure 2 shows the call steps for the QoR 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 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. 
   (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. 
    
    
   +-------------+              +-----------+    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. 
    
    
5.3 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. 

  
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   (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. 
    
    
5.4 Onward Routing (OR) 
    
  Figure 4 shows the call steps for the OR scheme.  This scheme is also 
  called Remote Call Forwarding.  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. 
  
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5.5 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 resources.  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 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 contains 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 that is also 
   known as the area code.  It is three-digit 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 the first number out of an NPA+NXX 
   code is ported out to another switch, that NPA+NXX is called 
   "portable NPA+NXX." 
    
   Similarly, in other national E.164 numbering plans, number ranges 
   cover a contiguous range of numbers within that range.  Once a 
  
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   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 whether the dialed directory number is ported or 
   not. The other version is to check whether the dialed directory 
   number belongs to any portable number range.  If yes, an NPDB query 
   is sent. If not, no 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. 
    
    
6. 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 are 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. 
    
 
6.1 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 Signaling 
       Connection Control Part (SCCP), and ANSI Transaction 
       Capabilities Application Part (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 

  
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       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. 
    
   (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 be 
       accessed via an 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. 
    
      
6.2 Europe 
    
   One of the following three interfaces can be used to query an NPDB: 
  
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   (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 ITU-TS MTP Levels 1 through 3, ITU-TS SCCP, 
       and ITU-TS TCAP. 
    
   (c) ISUP triggerless translation.  NP translations are performed 
       transparently to the switching network by the signaling network 
       (e.g. 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. 
    
    
   Wireline switches have the choice of using either (a), (b), or (c); 
   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 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 7.2. 
    
    
7. 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 
  
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   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. an ôN-1ö 
   policy is used, which essentially says that for domestic calls, the 
   originating local carriers performs the query, otherwise, the long 
   distance carrier is expected to.  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 
   were redundant queries performed downstream. 
    
     
7.1 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. 
  
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   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 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. 
    
   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. 
    
    
7.2 Europe 
    
   In Europe, 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, may 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.  
    
   In addition to the addition of the routing prefix to the CdPN 
   parameter, some other information may be added/modified as is listed 
  
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Number Portability in the GSTN: An Overview           February 9, 2000 
 
 
   in the draft ITU-T Recommendation Q.769.1 [ISUP].   Those 
   enhancements in the ISUP to support number portability are briefly 
   described below.   
    
   Three methods can be used to transport the Directory Number (DN) and 
   the Routing Number (RN): 
    
   (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 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.  
      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 those enhancements are for national use.  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 never passed across 
   the national boundaries. 
    
   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. 
    
  
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   If MNP-SRF is supported, the Gateway Mobile Services Switching 
   Center (GMSC) at the wireless donor network, when receiving a call 
   from the wireline network, can send the GSM MAP Send Routing 
   Information (SRI) message to the MNP-SRF.  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. 
    
    
8. 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.                                + 
   +-------------+----------------------------------------------------+ 
  
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   +   Austria   + Uses onward routing at the donor network.  Routing + 
   +             + prefix is "86xx" where "xx" identifies the         + 
   +             + recipient switch.                                  + 
   +-------------+----------------------------------------------------+ 
   +  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 becomes 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.      + 
   +-------------+----------------------------------------------------+ 
   +  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.     + 

  
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   +             + 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.       + 
   +-------------+----------------------------------------------------+ 
   + 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.                                         + 
   +-------------+----------------------------------------------------+ 
    
    
  
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9. 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.  
     
    
9.1 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. 
      
    
9.2 ITN Pooling 
    
   ITN pooling refers to the process whereby the number administrator 
   assigns individual telephone numbers to operators.  Using the North 

  
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   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. 
    
 
10. Conclusion 
    
   There are three general areas of impact to IP telephony work-in-
   progress at IETF: 
    
   1. Interoperation between NP in GSTN and IP telephony 
   2. NP implementation or emulation in IP telephony 
   3. Interconnection to NP administrative environment 
    
   A good understanding of how number portability is supported in the 
   GSTN is important when addressing the interworking issues between IP 
   based networks and the GSTN.  This is especially important when the 
   IP based network needs to route the calls to the GSTN.  As shown in 
   Section 6, there are a variety of standards with various protocol 
   stacks for the switch-to-NPDB interface.  Not only that, the 
   national variations of the protocol standards make it very 
   complicate 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 impractical, if not an 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 
   can be deployed that contain the same information but support the 
   common IP interface. The candidates for such a common interface 
   include Lightweight Directory Access Protocol (LDAP) and SIP 
   [SIP](e.g., using the SIP redirection capability).  Certainly 
   another possibility is to use interworking function to convert from 
   one protocol to another. 
    
    
  
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   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 
   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.  If the called directory is +1-
   202-533-1234, and its associated routing number is +1-202-544-0000, 
   the "tel" URL may look like tel:+1-202-533-1234;rn=+1-202-544-
   0000;npdi=yes where "rn" stands for "routing number" and "npdi" 
   stands for "NPDB dip indicator."  "rn" and "npdi" will be two new 
   parameters to be added as extensions to the "tel" URL to support NP.  
   Since the "fax" URL is similar to the "tel" URL where NP can impact 
   the fax calls as well as the telephone calls, the same extensions to 
   the "tel" URL need to be applied to the "fax" URL as well.  Please 
   see [TELNP] for the proposed extensions to the "tel" URL to support 
   NP and freephone service.  Those extensions to the "tel" and "fax" 
   URLs 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 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 in the SIP INVITE message, it instead of 
   the called directory number should be used for making routing 
   decisions.  If "rn" 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 it is the 
   routing number, if present, not the called directory number that 

  
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Number Portability in the GSTN: An Overview           February 9, 2000 
 
 
   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 and collect the complete 
   called directory number. 
    
   The IETF enum working group is defining the use of Domain Name 
   System (DNS) for identifying available services associated with a 
   particular E.164 number [ENUM].  [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, the donor network should not be relied on to 
   reach the delegated authority during the DNS process because there 
   is a regulatory requirement on NP in some countries.  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 a third-party designated by the end-user or by the 
   industry.  
    
   The IP-based networks also may need to support some forms of number 
   portability in the future if E.164 numbers [E164] 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 then is 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.  Then a mechanism 
   is developed for the IP-based network to locate the IP entity that 
  
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Number Portability in the GSTN: An Overview           February 9, 2000 
 
 
   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. 
 
11. 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] P. Falstrom, "E.164 number and DNS," RFC 2916. 
    
   [ENUMPO] A. Brown and G. Vaudreuil, "ENUM Service Specific 
        Provisioning: Principles of Operations," April 27, 2000. 
     
   [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. 
    
   [ISUP] ITU-T COM 11-R 162-E, Draft Recommendation Q.769.1, 
        "Signaling System No. 7 - ISDN User Part Enhancements for the 
        Support of Number Portability," May 1999. 
    
   [MNP] Draft GSM 03.66 V7.2.0 (1999-11) 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). 
    


  
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Number Portability in the GSTN: An Overview           February 9, 2000 
 
 
   [RFC] Scott Bradner, RFC2026, "The Internet Standards Process -- 
        Revision 3," October 1996. 
    
   [TEL] A. Vaha-Sipila, RFC2806, "URLs for Telephone Calls," April 
        2000. 
    
   [TELNP] J. Yu, draft-yu-tel-url-02.txt, "Extensions to the "tel" and 
        "fax" URLs to support Number Portability and Freephone 
        Service," February 9, 2001. 
 
   [SIP] M. Handley, H. Schulzrinne, E. Schooler and J. Rosenberg, RFC 
        2543, "SIP: Session Initiation Protocl," March 1999. 
    
   [TRIP] J. Rosenberg, H. Salama and M. Squire, draft-ietf-iptel-trip-
        02.txt, "Telephony Routing Information Protocol (TRIP)," May 
        2000. 
    
    
12. Acknowledgments 
    
   The authors would like to thank Monika Muench for providing 
   reference information on ISUP and MNP. 
    
    
13. Author's Addresses 
    
   Mark D. Foster 
   NeuStar, Inc. 
   1120 Vermont Avenue, NW, 
   Suite 550 
   Washington, D.C. 20005 
   United States 
    
   Phone: +1-202-533-2800 
   Fax:   +1-202-533-2976 
   Email: mark.foster@neustar.com 
          
    
   Tom McGarry 
   NeuStar, Inc. 
   1120 Vermont Avenue, NW, 
   Suite 550 
   Washington, D.C. 20005 
   United States 
    
   Phone: +1-202-533-2810 
   Fax:   +1-202-533-2987 
    
    
   James Yu 
   NeuStar, Inc. 
   1120 Vermont Avenue, NW,  
   Suite 550 
  
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Number Portability in the GSTN: An Overview           February 9, 2000 
 
 
   Washington, D.C. 20005 
   United States 
    
   Phone: +1-202-533-2814 
   Fax:   +1-202-533-2987 
   Email: james.yu@neustar.com 
 
Full Copyright Statement 
 

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