ENUM varieties

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.
• 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.

Parties having a direct interest in ENUM

Various parties are involved with ENUM. These include:

The registrant or subscriber

The registrant is the person or subscriber that makes his access information available to others through ENUM. The registrant or subscriber is thus the person whose information has been included in ENUM and must not be confused with the person who uses the Internet to find an address through ENUM.

The registrar

The registrar is the party who manages the registrant’s access information and ensures that it is publicly available on the Internet.

The registry

The registry is the manager of the National ENUM zone, or 4.3.e164.arpa . The registry forms, as it were, the top of the National ENUM pyramid and ensures that reference is made to the registrars’ servers on which the access information is located.

Because of the hierarchical structure of the DNS, there can only be one registry for every National ENUM zone. To prevent abuse of this position, requirements are laid down regarding the impartiality of the registry and the costs and quality of its service. In addition every registrant must have equal and open access.

The government or the regulator

Usually a governmental entity or a regulatory authority has control over the National zone of ENUM and will play a role in the appointment of the registry.

The number holder operator

Telephony services or telecommunication services operators have being assigned blocks of numbers by the regulator. They subsequently enable their users to use individual telephone numbers from those number blocks. Examples are the numbers for fixed telephony and mobile telephony. The number holder operator will be interconnected to other operators and will receive from them calls to his assigned range of numbers, for the calls to be terminated.

In ENUM the number holder operator will typically be the gateway operator or, alternatively, will have an arrangement with a gateway operator, to whom he will transit the calls.

But ENUM is a personal number, meant to be valid for the registrant life. Consequently in ENUM once the operator number holder assigns a number to a registrant, the number belongs to that registrant during his/her entire life.

Hence, if the registrant wishes to change his initial number holder operator (that might also coincide being his gateway operator) there have to be provisions for the ENUM number to be ported from the initial operator to other number holder operators.