Electronic numbering

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Electronic numbering (ENUM), which is standardized in RFC 3761, defines the mechanisms to retrieve URIs from E.164 telephone numbers and vice versa. The ENUM records are housed in the lower tiers of the IP routing directories and allows URIs or telephone numbers to be used for VoIP calls between ITSPs. The ENUM system allows users to store address information as a function of the NAPTR (naming authority pointer) records. This information includes addresses related to fax, voice, email, etc devices. This supports many to one address mapping so that a single address or URI could be used to reach any type of device. The type of session would be used to directionalize the call once it reaches the destination(s).

The telephony address (E.164 PSTN telephone number to URI) information are contained in the E2U parameters in NAPTR records. The ENUM records and the traffic loads that are expected for resolution from these records could cause trouble for conventional DNS servers.

Electronic numbering (ENUM) is the convergence of the PSTN with IP networks because it maps a PSTN telephone number to Internet-based services. ENUM was developed to answer this question: How can users armed only with a telephone number or just a keypad, use Internet services?

ENUM is a protocol that translates a fully qualified telephone number into a fully qualified domain name address using the Internet’s DNS architecture. You can envision ENUM operating much like DNS where the email address paul@hill.com results in an MX-query in the DNS database, which returns the email server address at Hill Associates, pop.hill.com, to the user.

ENUM does not change the PSTN numbering plans or the PSTN administration in any way. It is based on the ITU-standard for telephone numbers known as E.164. A fully qualified E.164 number contains a country code, a city or area code, and a phone number. For example, Hill Associates’s fully qualified E.164 telephone number in Colchester, Vermont is +1-802-655-0940.

ENUM will be an integral part of the convergence of the PSTN with the IP world since the transition from E.164 telephone numbers to SIP URIs will clearly take several years. Moreover, this service allows a user with just a telephone key pad to access Internet-based services.

The operational sequence associated with ENUM is straightforward, if you are familiar with the domain name address structure and resolution process found in the Internet. The steps are:

  1. The telephone number is translated into a fully qualified E.164 number. In our example, 655 0940 is expanded to include the country code (1), the city/area code (802), and the “+” is pre-pended to denote this number as a fully qualified E.164 number. The non-numerical characters are removed; a dot is placed between each digit, the digits are reversed and finally the domain e164.arpa appended. This yields the following final address for our example:
  1. ENUM issues a DNS query on this domain name and expects to receive a naming authority pointer (NAPTR) resource record with the resolved domain name pointing to the SIP server that handles the telephone number.
  1. The NAPTR resource records create the rules for translating the fully qualified E.164 telephone number into a fully qualified domain name. In our case that would be sip:info@hill.com. To allow for queries that do not support SIP, a second record could exist that translates to mailto:info@hill.com.

This flexibility would allow a user to request a connection to a telephone number and connect with either a voice session or an e-mail session. The system can be expanded to support all Internet services.

ENUM has the potential of transforming how we communicate, click to dial, follow-me services, device compatibility services all are possible with this convergence of the PSTN numbering system with the Internet naming system.


<mp3>http://podcast.hill-vt.com/podsnacks/2007q3/enum.mp3%7Cdownload</mp3> | Electronic Numbering (ENUM)