Сопоставление телефонных номеров и интернет-адресация: ENUM и перспективы конвергенции сетей.

Telephone number mapping is a system of unifying the international telephone number system of the public switched telephone network with the Internet addressing and identification name spaces. Internationally, telephone numbers are systematically organized by the E.164 standard, while the Internet uses the Domain Name System (DNS) for linking domain names to IP addresses and other resource information. Telephone number mapping systems provide facilities to determine applicable Internet communications servers responsible for servicing a given telephone number using DNS queries. The most prominent facility for telephone number mapping is the E.164 number to URI mapping (ENUM) standard. It uses special DNS record types to translate a telephone number into a Uniform Resource Identifier (URI) or IP address that can be used in Internet communications.

Being able to dial telephone numbers the way customers have come to expect is considered crucial for the convergence of classic telephone service (PSTN) and Internet telephony (Voice over IP, VoIP), and for the development of new IP multimedia services. The problem of a single universal personal identifier for multiple communication services can be solved with different approaches. One simple approach is the Electronic Number Mapping System (ENUM), developed by the IETF, using existing E.164 telephone numbers, protocols and infrastructure to indirectly access different services available under a single personal identifier. ENUM also permits connecting the IP world to the telephone system in a seamless manner.

One way of doing call forwarding with ENUM is illustrated in the next figure. The caller uses the telephone to dial the number of another subscriber, which leads to an ENUM lookup (such as is provided by SIP Broker). The DNS responds to the caller by returning a list with NAPTR records for VoIP communication, telephone numbers and email addresses. Next, an attempt will be made, using the VoIP record from this list, to establish a connection with the subscriber. If the subscriber is not online, the next record selected will be that for a connection to a PSTN or mobile telephone. If this attempt fails too, a voice message will be sent to the subscriber via a listed email address. Subdomains of e164. arpa are delegated on a country code basis by the ITU. Each delegation is normally made to a regulatory body designated by the national government for the country code concerned. What happens at a country level is a National Matter. In general the conventional DNS registry registrar model is used. The national ENUM registry manages and operates the DNS infrastructure and related systems for country code. e164. arpa. It takes registration requests from registrars who are agents of the end users, the registrants. Registrars are typically VoIP providers and telcos who bundle an ENUM registration as part of a VoIP service package. People using an ENUM enabled VoIP service can dial the registrant's existing number and be connected to the registrant's VoIP telephone over the Internet instead of using the PSTN. When they call someone who does not use ENUM, calls complete over the Public Switched Telephone Network or PSTN in the usual manner. Support for e164. arpa varies widely between countries; many do not support it at all. Alternative ENUM like registries have also emerged. These services verify PSTN numbers and can be used in addition to or as an alternative to e164. arpa. However, if the registry in which a callee's number is not known by the caller, the choice between registries can create confusion and complexity. Multiple DNS lookups may be needed and it is far from simple to know which E.164 numbers are registered in which of these alternate ENUM like trees. It is also possible that if an E.164 number is registered in several of these trees, there can be inconsistencies in the information that is returned. Furthermore, the subscriber "owning" a particular E.164 number may not be aware that their number has been entered into one or more of these alternate ENUM like trees or what information these alternate trees are returning for their E.164 number.

ENUM can also be viewed as a called party facility. Basically, it is an indirect dialling service designed to work seamlessly on PSTN and VoIP that builds on the great value of the E.164 numbers: billions of people knowing how to dial using numbers. If the called person has opted to use ENUM she/he will have published the ENUM number and have entered (via ENUM NAPTR) his/her wishes for how the call should be terminated. This might be a single VoIP identifier, but most likely it will be a list of how the call should be forwarded to various fixed line, cellphones, secretarial or voice mail services, either at the IP or at the PSTN side of the network. It is the called party choice to opt in ENUM and also to decide to let the calling party know her/his wishes. Today when a user places a regular phone call, he has to begin deciding how to establish the call with the other party: via VoIP, Fixed line PSTN, cellphone, entering a URI or dialling a number. With ENUM indirect dialling it is the called party wishes that matter and solve that decision. Another benefit of indirect dialling is to free the user to change his phone telco, webpage, IMS, email or whatever telecom service he uses without having to tell all his contacts about that. A presence enhanced ENUM facility having various profiles could automatically change the called party wishes as a function of where he/she is available. This could be a mechanism to automatically switch between cellphone and VoIP to the most convenient (or the less costing) termination.

One potential source of confusion, when talking about ENUM, is the variety of ENUM implementations in place today. Quite often, people speaking of ENUM are really referring to only one of the following:
Public ENUM: The original vision of ENUM as a global, public directory like database, with subscriber opt in capabilities and delegation at the country code level in the e164. arpa domain. This is also referred to as user ENUM. Open Enum: An effort of mobile carriers and other parties involved in mobile numbering plans to generate complete, public database of all international numbering plan, available via public dns. Private ENUM: A carrier, VoIP operator or ISP may use ENUM techniques within its own networks, in the same way DNS is used internally to networks. Carrier ENUM: Groups of carriers or communication service providers agree to share subscriber information via ENUM in private peering relationships. The carriers themselves control subscriber information, not the individuals. Carrier ENUM is also referred to as infrastructure ENUM, and is being the subject of new IETF recommendations to support VoIP peering