Most people reading this weblog have heard of ENUM by now. Or at least the promises: "ENUM eliminates printing URLs on business cards! Your 10-digit phone number will identify your email address and webpage using the magic of DNS!" Essentially, the entire E.164 hierarchy will be mapped onto DNS under the e164.arpa zone. Phone numbers will be broken up by digit, so at each digit the tree can be delegated to an organization responsible for smaller and smaller geographic areas, until potentially the individual phone number can be delegated to the owner of the phone number. The number owner can populate the zone with NAPTR records containing an IM address, email address, SIP address, webpage, and whatever else they may want to store there.

Privacy arguments aside, at first glance this seems asinine. Why, in the age of DNS, would anybody in their right mind want to revert to using numeric strings as identifiers? Isn't something like jsmith@corporatesoft.com much more memorable and self-descriptive than 17123514225? If numbers were better, the DNS system would have never been invented in the first place and we'd all be typing user ID numbers and IP addresses into our email clients.

The only vaguely permissible excuse would be backward-compatibility with devices having only a numeric keypad; telephones being the obvious one. But hold on a tic; if it's so hard to enter a real email address or IM screen name into a telephone, wouldn't it be equally difficult to type the actual message that you want to send?

So anyway. Regardless of the marketing hype, ENUM excited me because it allows SIP addresses to be tied to phone numbers. This bootstraps VoIP from the phone system, in a sense, because VoIP customers and carriers can use ENUM to determine whether a call can be carried over the Internet rather than the PSTN. Presumably, a call made from one CLEC to another can be completed through the Internet without the need for a peering agreement. This ad-hoc, peer-to-peer architecture is the basis for the infrastructure of the Internet as we know it, and is the key differentiator between the Internet and old-style circuit-switched networks.

ENUM is starting to be deployed across Europe, and a rational mechanism is used to delegate responsibility for each portion of the e164.arpa tree. Generally, the country code is delegated to the federal government or a neutral national body, who delegates the top-level number assignments to the phone companies. Then the phone companies allow the customers to control the entries for their number, much like controlling how the number is listed in the phone book.

You'd think this framework would be fine for the North American country code. Apparently not. An international body, the ENUM Forum, was formed to come up with a plan. And it couldn't be simple: the current working revision paints a complex picture defining a system of registrars to manage the ENUM tree as an entirely separate world from the phone companies.

Here's how it works: a "Tier 1" registrar will be appointed who runs the DNS servers for the 1.e164.arpa zone. Then "Tier 2" registrars will take orders from customers to register their own phone numbers. Tier 2 tells Tier 1 to delegate the phone number's corresponding zone to the Tier 2's DNS servers, and Tier 2 manages the NAPTR records for the customer.

Huh??? What's going on here? The phone companies that own the phone numbers will have absolutely no control over the ENUM zones? The 1.e164.arpa zone will be a flat namespace instead of a tree? And worst, I'll have to pay to register a phone number that the phone company already assigned me?

So much for using ENUM as a carrier-to-carrier tool. If CLECs don't get to fill their own ENUM zones, the leap from PSTN to VoIP depends on customers to bridge from E.164 numbers to SIP by themselves. Slowly it will happen, as businesses and individuals become aware of the benefits of VoIP, but it will proliferate like fax machines—the customers will build the network from the edge, rather than having a guiding organization to do it from the inside. Ultimately, the marketplace will be best off with the customer in control of his telephony choices, but the telecom industry will have to migrate to VoIP without the benefits afforded by the world-wide cooperation that fostered the Internet of today.