ENUM based Number Portability in VoIP and IMS Networks Mario Iv ček Research & Development Centre ERICSSON Nikola Tesla d.d. Krapinska 45, 10002 Zagreb, Croatia Telephone: +385 1 365 4619 Fax: +385 1 365 3548 E-mail: [email protected] Abstract – Number portability is today implemented only in portability databases are updated with a routing number the Public Switched Telephony Networks (PSTN) by utilizing that directs call to a gateway. Intelligent Network (IN) functions for number mapping. At Limited number portability can be implemented in IP telephony network using redirect and proxy servers. Calls the time being, there is no specific solution for number to ported numbers are forwarded to a new destination by portability across Voice over IP (VoIP) and IP Multimedia the previous signaling server. However, this feature does Subsystem (IMS) networks. Limited portability can be more correspond to call forwarding than number implemented in IP telephony network using redirect and portability. The forwarding works with both calls to IP proxy servers that forward calls to gateways based on routing terminals and calls to PSTN destinations. number. This paper discuss possibility of using E.164 Number This paper, in chapter two, starts with a general Mapping (ENUM) for number portability in VoIP and IMS overview of number portability service. Chapter three networks, based on the most recent IETF enum working presents existing number portability between PSTN group achievements. networks in VoIP domain. Aspects of call routing in IP environment are described in chapter four. Chapter five aims at presenting problems encountered by number portability across network types. The ENUM, enriched I. I NTRODUCTION with newly registered ‘pstn’ enumservice, is considered as a potential solution to these problems, along with the NP Number portability [6] enables telephony subscribers to related information that is defined on SIP signaling level. keep their telephone numbers when they change service The ENUM based number portability applied to different provider (service provider portability), move to a new call scenarios is discussed in chapter six. Conclusions are location (location portability), or change the subscribed driven in chapter seven. services (service portability, e.g. from the plain old telephone service to Integrated Services Digital Network). Today, number portability is only implemented in the II. NUMBER PORTABILITY IN GENERAL Public Switched Telephony Networks (PSTN). The implementation of number portability differs in different countries. Common to all implementations, is that the Number portability (NP) impacts call signaling and Called Party Number dialed by the customer (also called routing. The former impact is connected with the need to directory number) is mapped to either a routing number or carry the NP-related information in the protocols (such as a routing prefix. ISUP, TUP) after a Number Portability Data Base (NPDB) A routing number is a hierarchical routing address, lookup has been performed. The later impact is for a which can be analyzed digit-by-digit to reach the correct signaling server to use the NP-related information in country, network provider, end-switch and subscriber line received ISUP message parameters to determine routing. A at the end. A routing prefix forms a routing number by routing number is associated with a telephone number that adding some digits in front of the directory number. The has been ported out from a donor network to a serving ISUP Called Party Number parameter (CdPN) in the IAM network. A donor network is the initial network where a will contain the routing prefix and dialed directory number. telephone number (e.g. +385 1 123 4567) was assigned to a This model is used in most European countries (Austria, subscriber before ever being ported. A "non-ported" Belgium, Finland, France, Germany, Italy, Norway, Spain, telephone number does not have any routing number Sweden, Switzerland, UK). associated with it because the first N digits of the number Most number portability solutions utilize Intelligent can be used for routing. The serving network is the network Network (IN) functions. In these solutions, the mapping that currently serves the ported number. database is stored in the Service Data Functions (SDF) elements. In this paper, we will discuss number portability in The introduction of IP telephony adds a new type of modern Next-Generation Network (NGN), which is number portability: between different network types [6]. At characterized by separated application and media control the time being, there is no specific solution for number layer. The NGN includes a telephony softswitch, media portability across the network types. A number that is gateway, signaling gateway and service management moved to the IP network can be handled in a normal way, platform. using number portability solutions on the PSTN. Number A telephony softswitch (TSS) is open-standard software portability point of view, the CdPN is referred to as that can perform distributed communications functions on Dialled Directory Number (DDN). an open computing platform and has the functionality of a 5. The NPDB returns a Routing Number (RN) that is traditional TDM telephone switch. A softswitch can associated with the requested DDN. The RN points at integrate voice, data and video, and it can execute protocol the ported-to subscriber in the recepient exchange. translations between separate networks such as PSTN and 6. The softswitch uses obtained RN to reroute the call IP. The media gateway communicates between networks onward towards the Recipient network. The IAM with different protocols, such as PSTN voice and IP data message to Recipient network caries complete networks. The signaling gateway translates telephone addressing information, i.e. both DDN and RN. signaling messages for transmission over packet data networks. The service management platform supports 7. Finaly, when terminating switch in the Recipient different management services, like intelligent network PSTN network finds its own routing information, by services for example. analysing the the supplied RN, it uses received DDN to terminate the call. III. NUMBER PORTABILITY IN VoIP NETWORK NPDB The four methods can be used to support number NP_Query (CdPN=DDN ) 4 5 NP_Response (RN ) portability across PSTNs in the VoIP environment: All Call TeS Query (ACQ), Query on Release (QoR), Call Dropback TSS ISUP and Onward Routing (OR) [6]. The Query on Release ISUP (RN, DDN) H.248 IAM method will be discussed further in this paper. H.248 6 ISUP MGM DDN When using the QoR method for number portability 7 -->CdPN MGM MGM G [10], the initiating exchange routes the call to a donor IAM (CdPN) REL 1 G G PSTN 2 3 PSTN exchange with an optional indication that a QoR capability IAM (CdPN) is possible. If the donor exchange is not a serving exchange, the call is released with appropriate release indication. On receipt of the release indication, an PSTNPSTN PSTNPSTN exchange in the upstream determines routing information ported by initiating query to an external database. A database response message contains a network Routing Number CdPN (RN). The following model depicts a typical PSTN network Figure 1: Number Portability in VoIP Network call scenario involving number portability based on QoR method [fig.1]. 1. The originating PSTN network receives a call from the IV. ROUTING IN IP NETWORK caller and routes the call to telephony softswitch, by sending the IAM message that containes a Called Party Number (CdPN) of the called subscriber. To establish a call to a terminal on an IP network, the 2. Based on analysis of CdPN, the softswith determines destination IP address must be known. Alternatively the that call is to be routed to terminating PSTN network, terminal can be identified by a host name, which is in which the called subscriber is supposed to be translated to an IP address by DNS. As terminals are resident. An IAM message with CdPN is sent towards equipped with an E.164 number, DNS service can be used terminating PSTN network. to map telephone numbers into IP addresses, by constructing a domain name from the E.164 number and 3. Upon receive an incoming call (IAM), a switch in the performing DNS NAPTR lookup. The result of NAPTR receiving PSTN network detects either that the called lookup is a Uniform Resource Identifier (URI), which number has been ported out to another network or, contains a signaling protocol and a host name [1]. optionally, that the number is just vacant in the network. After having determined (by looking at An additional DNS lookup is thus required to map the received signalling information) that one of the host name to an IP address. Note that for simplicity preceding networks has QoR capability, the receiving reasons, those additional lookups will not be considered in PSTN network returns a release message (REL) back this paper. to softswitch, with or without a special indication that ENUM uses the domain “e164.arpa” to store the the called number is ported out. From number mapping. Numbers are converted to domain names using portability point of view, this network is a refered to as the scheme defined in [3]. The E.164 number must be in its Donor network. full form, including country code. All characters and 4. The softswitch traps the release message and symbols are removed, only the digits remain. Dots are put determines that the preceding network has no QoR between the digits. The order of the digits is reversed and capability. In this scenario, the softswitch has access to the string “e164.arpa” is added to the end. This procedure an NPDB with the complete address to Recipient will map, for example, the number +385-1-1234567 into network for ported-out numbers. Based on received the host name “7.6.5.4.3.2.1.1.5.8.3.e164.arpa”. cause for release, the softswitch makes a database DNS stores information in different types of records.