After some months in the wilderness, ENUM (electronic number mapping), the protocol that maps telephone numbers to IP addresses, is once again a hot topic as VOIP providers look for ways to charge customers for new services and keep their calls off the PSTN.

ENUM, or e164.arpa, uses a lookup system for routing that’s similar to the domain name system (DNS) for Web addresses. It can allocate a single number that can then be used for multiple IP services, such as VOIP, email, instant messaging, and so on.

ENUM works by reversing a phone number's digits, putting a dot between each digit, and adding the string ".e164.arpa" to the end. So, for example, the telephone number +44-0207-863-3142 becomes 2.4.1.3.3.6.8.7.0.2.0.4.4.e.arpa.

Then, if you enter a phone number into your cellphone or PDA, the ENUM software delivers information about all the possible ways to contact the owner of that number (call, email, instant message, and so on). The user would then choose one of those connections and be charged appropriately.

The possibility of having so much information for each person tied to a single number not only presents carriers with some new revenue opportunities, but also has exposed some limitations of the DNS system, which wasn't developed to handle such complicated records.

But now that carriers have identified ENUM's issues, and are working around them, they'll soon be implementing solutions that make ENUM an integral part of VOIP.

Indeed, while the International Telecommunication Union (ITU), which controls the standard worldwide, drags its feet on launching a centralized ENUM directory, VOIP providers have begun rolling out their own ENUM exchanges.

For example, in recent weeks: e164.info has launched an ENUM-based international peering network of VOIP providers; XConnect Global Networks Ltd. has launched a VOIP interconnection service with an ENUM exchange; and Stealth Communications Inc. has been expanding its Voice Peering Fabric (VPF) ENUM Registry (see iBasis, XConnect Launches in North America, Telx, Stealth Launch VOIP Peering, and RCN, Stealth Partner to Expand VPF).

The ENUM standard has been around for several years, with various groups “tinkering around with it in garages,” but “nobody with a checkbook was talking about it,” says Albert Gouyet, vice president of marketing at Nominum Inc., provider of DNS and ENUM server software.

But in the past few months, Gouyet says, carriers such as BT Group plc (NYSE: BT; London: BTA) and Tele2 AB (Nasdaq: TLTO) have started to act on their plans for rollout.

A similar buzz was heard last year, when operators were engaging in trials and one provider, Sentiro, launched a commercial service (see Carrier ENUM Gains Ground and Sentiro Launches Global ENUM Service).

What's changed? As VOIP has taken off, providers are increasingly looking for peering partners to keep calls off the PSTN, and ENUM provides a way to do that.

There are three different ways to use ENUM, Gouyet says: public ENUM, which functions as an open database on the Internet, mapping a single telephone number to multiple user services; private ENUM, which works as a database inside a carrier’s network to connect calls; and something in the middle called carrier ENUM, which allows VOIP providers to look up the destination number on another IP network directly and bypass the PSTN.

Although public ENUM has been getting all the publicity for its potential to offer converged services, as well as raising concerns over privacy, carrier ENUM is where the money’s to be found.

Nominum, which released ENUM benchmark results earlier this month, has seen demand for its software jump as carriers that have been conducting trials get ready to roll out ENUM early next year, says Gouyet (see Nominum Claims DNS Record). The company also received its fourth round of venture funding last week (see Nominum Raises Another $16M).

Twenty-four countries have been delegated country codes for ENUM and 11 have started public trials: Australia, Austria, China, Finland, France, Japan, the Netherlands, Poland, Sweden, the U.K., and the U.S. A trial is also set to begin in New Zealand.

“What people don’t understand about ENUM is that it’s going to completely change lots of different things at the same time,” says Gouyet.

— Nicole Willing, Reporter, Light Reading