We will describe here the essence of the schema for user registration in
ENUM, though various business interests and the resulting political pressures
may lead to further evolution of ENUM registration. The ENUM functional
architecture discussed here is also know under the name “Public ENUM,” and
there are other schemas as well, such as “Private ENUM” that may apply to
consortiums of telephone companies (where the relevant DNS data is visible
only to consortium members).
We will illustrate the public ENUM architecture with an example for North
America, as designed by the North American ENUM Corporation with Lim-
ited Liability (ENUM LLC). This design has been developed for trials of
ENUM in North America [14]. Similar work is being performed in Europe by
RIPE in the ENUM WG [15]. ENUM work in the Asia-Pacific region is
described in reference [16].
The ENUM functional architecture as seen by the ENUM LCC during the
2005 trial stage is shown in Figure 4.4.
The elements of the ENUM functional architecture are:
■■ The DNS root “.” and the first leaf “.arpa” that is also shown in
Figure 4.4.
■■ The ENUM Tier 0 for the domain “e164.arpa”. Tier 0 contains the del-
egations for the telephony country codes and is, therefore, international
in scope.
■■ ENUM Tier 1 that contains the delegations for North America: Canada,
the United States, and the Caribbean countries. Tier 1 is split into two
layers: Tier 1A for the country code 1 for North America and second
layers, such as Tier 1B, which is the trial Tier for the United States. Tier
1 contains the registries for the DNS name servers with ENUM RR for
the countries with country code 1. The DNS registry is populated with
ENUM RR by the DNS registrar that, in turn, takes its data from the
registrants in Tier 2.
■■ ENUM Tier 2 contains the service providers that, interestingly enough,
are called in the ENUM trial documents the application service
provider (ASP). The ASP provide applications directly to end users,
such as telephony based on SIP, although other applications are also
envisioned, as shown in Table 4.1. Tier 2 service providers manage the
ENUM domain names associated with an E.164 telephone number or
block of numbers, and act as the registrars for the NAPTR RR.70 Chapter 4