Whats this ENUM business

The following is from The IP Multimedia Subsystem by Travis Russel:

In the SIP domain, subscription addresses use the form of a Universal Resource Identifier (URI). This is analogous with the Universal Resource Locators (URLs) we use to reach Web sites on the Internet, but they are assigned to subscriptions for reaching individual subscribers. It is the concept of the URI that makes communications models in the IMS unique. The ability to reach a subscriber based on these very personal identities rather than numbers, and to apply these addresses to all forms of communications, is the purposeof the SIP protocol within the IMS.

A URI can take two forms. A SIP URI uses the same form as an e-mail address, consisting of username@domain. The first part of the address is typically the username of the subscriber, while the last part is the domain name of the network provider where the subscription resides. The last part of the domain name (.com, .org, etc.) defines the type of organization according to Internet rules. SIP also supports addresses in the form of telephone numbers, referred to as TEL URIs. A TEL URI uses the same form as a SIP URI, substituting the telephone number for the user name. These are used most commonly when a call is originated in a non-SIP domain, or when a call is being placed to a non-SIP network (such as a call from the IMS to a wireline subscriber in the PSTN). The TEL URI is a good example of interoperability between legacy PSTN and IMS.Since legacy networks will continue to support the use of telephone numbers for some time to come, there remains a need to translate these telephone numbers into public identities for use within the IMS.

The actual conversion process is provided through a function known as ENUM. This function translates an E.164 telephone number into a SIP or TEL URI. The ENUM function does not translate the identity into an IP address, however. This remains the function of the Domain Name Server (DNS). We will talk more about these functions later. Eventually, TEL URIs may disappear completely as we become more and more accustomed to using SIP URIs for all communications. This is still a long way off, however, and something we may not see for decades.

For more information see the following book:

• ENUM Variants by Jim McEachern, Nortel
• ENUM will it pay? by Richard Shockey

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