Telephone Number Mapping (ENUM or Enum, from TElephone NUmber Mapping) is a suite of protocols to unify the telephone system with the Internet by using E.164 addresses with DDDS and DNS. ENUM also refers to "E164 NUmber Mapping".

Although it facilitates VoIP, the protocol used for voice communication across the Internet, ENUM is not a VoIP requirement and should not be confused with common VoIP routing based on SIP and H.323 protocols with a Uniform Resource Identifier (URI). VoIP service providers assign a URI to a customer in order to complete calls over the internet. ENUM is a DNS-based protocol that is best suited to offer services that not only help facilitate, but expand on ways to complete calls over VoIP networks as described below. It provides a user with a domain name on an E.164 server in order to associate a common international telephone number with a URI and provide other DNS-related services. The server is maintained by the Service Provider and is expected to become standard since it can successfully address locating URIs with nothing more than a common international telephone number.

The ITU ENUM allocates a specific zone, namely "e164.arpa" for use with E.164 numbers. Any phone number, such as +1 555 42 42 can be transformed into a hostname by reversing the numbers, separating them with dots and adding the e164.arpa suffix thus: 2.4.2.4.5.5.5.1.e164.arpa

DNS can then be used to look up Internet addresses for services such as SIP VoIP telephony. NAPTR records are used to 'translate' E.164 addresses to SIP addresses for example. An example NAPTR record is: $ORIGIN 2.4.2.4.5.5.5.1.e164.arpa. IN NAPTR 100 10 "u" "E2U+sip" "!^.*$!sip:phoneme@example.net!" IN NAPTR 102 10 "u" "E2U+mailto" "!^.*$!mailto:myemail@example.com!"

This example specifies that if you want to use the "E2U+sip" service, you should use sip:phoneme@example.net 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:4242@555telco.example.net; if your number is +15551234, your SIP address is sip:1234@555telco.example.net.

Alternative ENUM lookup protocols such as sbXML have also emerged. These services often provide a faster and easier mechanism to translate E.164 numbers into SIP addresses.

ITU e164.arpa subdomains are first delegated to ("registered by") regulatory bodies designated by the national government of the country code concerned, which further delegates zones to telecommunications providers. Your telephone company is therefore in charge of the NAPTR records, usually. Some countries are proposing to let end-users register their own telephone numbers via an intermediary, which need not be their own telco. This is considered a good idea as VoIP is a major Enum use. People who use an Enum-enabled VoIP service can dial your existing number and be connected not to your existing phone line, but to your own VoIP telephone directly via the Internet, bypassing the telephone system entirely. When they call someone who does not use Enum, calls complete over the Public Switched Telephone Network or PSTN.

Alternative free public and private ENUM services such as E164.org and VPF ENUM Registry have also emerged. These services often verify PSTN numbers and can be used in addition to e164.arpa. Competing ENUM zones may be seen as beneficial, to keep prices low in the VoIP market.

References

• RFC 2915 - The Naming Authority Pointer (NAPTR) DNS Resource Record
• RFC 3761 - The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application (ENUM)
• RFC 2168 - Resolution of Uniform Resource Identifiers using the Domain Name System
• http://www.enum.org/
• http://www.enumllc.com/ Country Code 1 ENUM LLC
• http://www.itu.int/osg/spu/enum/
• http://www.ietf.org/html.charters/enum-charter.html
• Distributed Universal Number Discovery
• ENUM - The bridge between telephony and internet
• ENUM - It's All in the Numbers
• Public ENUM database e164.org
• Voice Peering Fabric ENUM Registry

External links

• CircleID: ENUM Convergence
• ENUM: Mapping the E.164 Number Space into the DNS
• CC1 ENUM LLC
• ENUM Forum
• Voice Peering Forum

Telephone numbering plans

Telephone Number Mapping | ENUM | ENUM | ENUM | Enum