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The April 26, 2004 Issue Provided by System Dynamics Inc.
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ENUM: A Building Block for "End-to-end IP" Telephony

We've written before about IP telephony (VoIP) and the role of SIP. A protocol called ENUM is likely to play an important role in achieving the goal of "end-to-end IP" telephony.

While we were at the VON conference, we spent some time discussing ENUM with Richard Shockey of Neustar. Rich is the Co-Chair of the IETF ENUM Work Group, and volunteered to brief us on ENUM. While ENUM has been around for some time, it is just starting to move from trials to full-scale implementations.

Rich said that ENUM is trying to solve two problems on the path to "end to end IP":

  • How to use what look like ordinary phone numbers when calling to and from IP phones
  • How to avoid paying traditional telephony transaction fees for end-to-end IP calls that have no need to use any resources of the legacy telephone network (the "public switched telephone network" or PSTN)

A call made from an IP phone to a regular phone on the PSTN can't be free -- at some point, the call must be converted from IP to PSTN formats, and someone has to pay transaction fees to connect the call to the dialed number. Services like Vonage -- although based on IP telephony -- can't be free: while Vonage uses the Internet to carry calls around the world, it has to pay the traditional telephony companies to terminate the calls, and someone has to pay Vonage.

By contrast, any "end-to-end" IP call should be free. It is a standard Internet application like email and IM. IP calls made PC to PC are free with IM services from AOL, Yahoo and MSN.

But there's a problem when people use familiar-style phones to make end-to-end IP calls: IP phone numbers are difficult if not impossible to dial from standard keypads. My business card has both my cellphone number and my email address; if my email address is, I would probably want my SIP "phone number" to be so anyone who knows my email address would also know my sip address.

If someone wants to reach me from a PC, it's easy to enter "" on a PC keyboard. But if they want to use a phone to reach me, it's very hard to enter "" on a traditional numeric telephone keypad. It would be a lot easier if people could use my cellphone number, which identifies me uniquely the same way as my email address.

So it would really be convenient if someone who wants to call me could just dial 1-973-123-4567 and have it automatically (and invisibly) converted to "". That's what ENUM does.

ENUM translates standard phone numbers into SIP addresses using standard Internet mechanisms. It takes any telephone number including the country code, and converts it into a SIP address.

What makes ENUM rather unusual is that is "neither fish nor fowl": it bridges the new world of the Internet where standards are set by the IETF, and the old world of telephony where standards are set by the ITU. In the ITU world, almost everything to do with how phone numbers are assigned, formatted (for example, how many digits are in each phone number and how many of these are used for city codes or area codes), and maintained differs from country to country and therefore from country code to country code. Since ENUM is designed to be a universal mechanism for mapping phone numbers to SIP addresses, it needs to handle these differences.

The "international numbering plan" is defined in ITU-T Recommendation E.164. This describes how a unique phone number is composed of a country code (1 to 3 digits), an optional "national destination code" (area code or city code), and a subscriber number. ITU-T maintains a list of assigned country codes.

How ENUM Works

As described in RFC 2916, ENUM takes any telephone number and turns it into a unique domain name. This is accomplished by taking the phone number, removing all non-digit characters, reversing the digits, putting a "dot" between each digit, and appending the string "". Here are a few examples:

  • the North American phone number +1-571-434-5651 is converted to ""
  • the Swedish number +46-8-976123 is converted to ""

(There's a nice picture of this in this article from the Washington Post three years ago.)

This new domain name is then looked up through the standard DNS (domain name service) mechanism used throughout the Internet, and returns the user's SIP address. It can also return the user's email address, one or more phone numbers, domain name, and other information which that user wants to provide.

ENUM provides a bridge into the ITU world by processing country codes as subdomains, then delegating the further processing for these subdomains to an appropriate authority - typically the numbering plan administrator for that country code. Neustar currently plays this role for country code "1"; government agencies often play this role for other country codes.

For those interested in more details, RFC 2916 provides some examples.

ENUM and SS7

In the PSTN, number translation is one of the main services provided by SS7 ("signaling system 7"), the system by which traditional central office switches communicate with one another.

While SS7 is a closed world, ENUM provides an open mechanism for public number translation. The same ENUM mechanism can be used for private number translation -- say between PBX extensions at different offices of a company, or for direct company-to-company calls.

Thus ENUM, while a fairly simple application of existing DNS mechanisms, represents somewhat of a challenge to the private infrastructure of the long-established (and mostly not-so-long-ago monopoly) carriers.

ENUM Status

Many national ENUM trials are under way, including many European countries, Korea and Japan.

The ENUM Forum has been created to develop policy and steps to implement ENUM in the United States. The US government supports RFC 2916 and endorses moving forward with ENUM; see this press release from NTIA which supports "the ENUM Forumís consideration of a limited liability company (LLC) to select at least one Tier 1 provider."

Pending the establishment of a national ENUM administrator, Neustar is operating an ENUM Public Trial "for carriers, telephone service providers, and telecom equipment manufacturers who wish to test features and functionalities of products under RFC 2916."

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