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VoIP gateway IP400 version 5.01
4.2.9 The ENUM protocol
ENUM stands for a protocol, which has to do with mapping so-called E.164 num-
bers to Uniform Resource Identifiers (URI). It defines rules to uniquely map a tel-
ephone number to a domain. This domain can then be used, for example, to iden-
tify addresses for IP telephony.
ENUM uses the Domain Name System (DNS) to do that. One of DNS's jobs is to
create a logical connection between the addresses of computers attached to the
Internet (which are identified by pure numeric IP addresses) and domains, which
have the advantage of being easier to remember. Most Internet users probably
only know of domains in connection with E-mail addresses or web-sites. The DNS
infrastructure and the ENUM protocol enable telecommunications services to be
requested and addressed via domains. In contrast to web domains, users of ENUM
domains don't have a free choice, as there are fixed rules about mapping a tele-
phone number to a corresponding ENUM domain. The corresponding ENUM
domain can thus only be registered by the owner of the associated telephone
number. The ENUM protocol has to be supported by the Internet Service Provider
(ISP).
By linking telephone numbers and Internet resources, brand new services result.
A basic service is to find an Internet terminal capable of telephony, from a tradi-
tional telephone. ENUM can optionally, also supply details of additional communi-
cations options. If no Internet terminal is reachable, which is capable of tele-
phony, another means of communication can be selected from the list of
alternative applications.
An example should illustrate the principle: After entering a telephone number, for
which ENUM information is available, the call is first switched to a conventional
network connection. If no-one answers, the call is forwarded to the registered
mobile telephone number. If no connection can be made here, the message can
be recorded and sent to an E-mail address as an audio file. It is also conceivable
that a web site could be addressed, which could then supply information about
any other communications options.
This example makes the advantage of the new system clear: Instead of a multi-
tude of telephone numbers for the various applications, one number will do. The
entries in the ENUM name server deal with the allocation to the matching output
devices. An incoming fax is thus automatically directed to the correct piece of ter-
minal equipment.
The use of ENUM in modern communications scenarios offers a lot of options, with
a correspondingly large potential for use, in very different areas. For example,
when routing: here PSTN networks and VoIP networks can be linked.