Telephone Number Mapping (ENUM or Enum, from E.164 NUmber Mapping) is a suite of protocols to unify the telephone numbering system E.164 with the Internet addressing system DNS by using an indirect lookup method, to obtain NAPTR records. The records are stored at a DNS database.
Although it facilitates calling VoIP users from IP and PSTN networks, ENUM is not a VoIP function and should not be confused with common VoIP routing based on SIP, H.323 or IAX protocols with a Uniform Resource Identifier (URI).
Being able to dial telephone calls the way customers have come to expect is considered crucial for the convergence of classic telephone service (PSTN) and Internet telephony (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 (also known as Telephone Number Mapping), 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.
For an ENUM subscriber to be able to activate and use the ENUM service it needs to obtain three elements from a Registrar:
This works as follows: (1) the Registrar provides the Subscriber (or Registrant) with a domain name, the URI, that will be used to access a DNS server to fetch a NAPTR record, (2) a personal E.164 telephone number (the ENUM number). The URI domain name of (1) is biunivocally associated (one-to-one mapped) to the subscriber E.164 ENUM number of (2). Finally (3) the NAPTR record corresponding to the subscriber URI contains the subscriber call forwarding/termination preferences.
Therefore, if a calling party being at the PSTN network dials a called party ENUM number by touch typing the E.164 called party number, the number will be translated at the ENUM gateway into the corresponding URI. This URI will be used to look-up and fetch the NAPTR record obtaining the called party wishes about how the call should be forwarded or terminated (either on IP or on PSTN terminations) – the so-called access information – which the registrant (the called party) has specified by writing his/her choice at the ‘NAPTR record’, "Naming Authority Pointer Resource Records" as defined in RFC 2915, such as e-mail addresses, a fax number, a personal website, a VoIP number, mobile telephone numbers, voice mail systems, IP-telephony addresses, web pages, GPS coordinates, call diversions or instant messaging. Alternately, when the calling party is at the IP side, the User Agent (UA) piece of software of the dialler will allow to dial a E.164 number, but the dialler UA will convert it into a URI, to be used to look-up at the ENUM gateway DNS and fetch the NAPTR record obtaining the called party wishes about how the call should be forwarded or terminated (again, either on IP or on PSTN terminations).
Calling by using a new personal E.164 number (the ENUM number) to look-up at a database is therefore an indirect calling support service.
The ITU ENUM allocates a specific zone, namely "e164.arpa" for use with ENUM E.164 numbers on the IP side of the network. RFC 3761 define how any ENUM number, such as +1 555 42 42 can be transformed into a URI, by reversing the numbers, separating them with dots and adding the e164.arpa suffix thus: 188.8.131.52.184.108.40.206.e164.arpa
The URI can then be used to look up at the DNS the Internet addresses for services such as SIP VoIP telephony. NAPTR records are used to set the subscriber call forwarding/termination preferences. Therefore the whole system can 'translate' E.164 addresses to SIP addresses. An example NAPTR record is:
IN NAPTR 100 10 "u" "E2U+sip" "!^.*$!sip:email@example.com!" .
IN NAPTR 102 10 "u" "E2U+mailto" "!^.*$!mailto:firstname.lastname@example.org!" .
This example specifies that if you want to use the "E2U+sip" service, you should use sip:email@example.com as the address. The regular expression can be used by a telephone company to easily assign addresses to all of its clients. For example, if your number is +15554242, your SIP address is sip:firstname.lastname@example.org; if your number is +15551234, your SIP address is sip:email@example.com.
The following Figure illustrates how ENUM works by giving an example: Subscriber A sets out to call Subscriber B.
The ENUM user does not notice anything of this reversal and the DNS database look-up, as this is done automatically behind the scene using a user agent software in his PC or terminal, or at the PABX or Gateway. For instance, when the user types the telephone number in his web browser ENUM enabled agent and indicates what item of information he is looking for (email address, telephone number, web address, etc.) in the PC or terminal the number is converted to a domain name. This is sent to ENUM servers on the Internet, which send back the NAPTR records associated with the name. The access information and any priority indicated for them are stored in these. The user gets the requested address back on his PC or terminal. ENUM therefore in fact functions as a mechanism for translating a telephone number into a domain name with the requested address or number associated with it, but without the user viewing how this is done, just as he is currently unaware that he is using the DNS when he makes a connection with the Internet or what is going on at the telephone switch when he makes a call.
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 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.
Alternative ENUM-like services such as E164.org have also emerged. These services might verify PSTN numbers and can be used in addition to e164.arpa. However this leads to 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.
Various parties are involved with ENUM. These include: The registrant or subscriber