ENUM or "Electronic Numbering" could revolutionize communications (not to mention simplify your business card). But many non-technical obstacles remain.
by Richard Grigonis
Originally called "Electronic Numbering," ENUM is a technology that bridges the PSTN (Public Switched Telephone Network) and the Internet.
A bit of background: when you type a website name, such as http://www.vonmag.com, into your web browser, you're actually dealing with a URL (Uniform Resource Locator) which serves as a user-friendly address to resources on the web. The first part of the address indicates what kind of protocol the browser should use (HTTP or FTP) and the rest of the address specifies the domain name where the resource (web page) is located. The DNS (Domain Name System) does a database lookup and translates www.vonmag.com into the IP address, 216.247.187.235.
ENUM, which originated with Cisco Engineer Patrik Faltstrom and the Internet Engineering Task Force's (IETF's) Telephone Number Mapping working group, is a DNS-based service that maps a standard telephone number to a list of contact URLs. With this service, ENUM-enabled applications can use a single phone number to contact a person's fax machine, email, voicemail, instant messaging client, or other resource. In theory, you could send someone an email by sending it to their telephone number.
The phone numbers used are "e.164 telephone numbers" (the e.164 standard is the international public telecommunication numbering plan, i.e. the structure of the telephone numbers commonly used on the PSTN as recommended by the International Telecommunication Union's Telecommunication Standardization Sector, or ITU-T). Despite the Internet's tremendous popularity and widespread use of its addressing and naming schemes, the e.164 numbering system, known as Recommendation e.164, is still the most used and widespread addressing and naming scheme and the only one supported by millions of telephonic and other communications devices. (All US telephone numbers conform to the e.164 standard syntax, for example: +1-212-555-1212.) It seems appropriate that both e.164 and Internet domain names should interoperate, necessitating the use of ENUM or some functional equivalent. (People working in large organizations have their own dialing plans and don't necessarily use e.164 numbers, but dialing plan numbers can also be resolved by use of the same technology as ENUM.)
The DNS is essentially just a big, distributed database. Normally, when you're surfing the Internet, the DNS just takes a domain name and looks up the IP address of a website. But if that same database could contain other information, then you could add other kinds of address information, such as other public phone numbers, your SIP phone address, your fax number, your instant messaging address, your credit card number, your "follow me" number to your vacation home, etc. ENUM, acting as an overlay to DNS, aggregates this data into the DNS.
A caller's ENUM query returns a series of DNS Naming Authority Pointer Records (NAPTRs) which can be used to contact a resource associated with that number, such as a URI (Uniform Resource Identifier). A URI is a sequence of characters which makes it possible to identify resources such as a document, image, file, database, email address or other resource or service presenting a common format. The most wellknown form of URI is the URL which, as we've seen, is used to locate resources using the World Wide Web. Thus, the ENUM protocol is a big address book that makes it possible to convert e.164 telephone numbers into Internet domain names and then to associate them with services for communication through the associated URI.
With one number, then, you could reach someone at either their PSTN phone or VoIP phone.
The DNS database is indexed by domain names. There is a single common ENUM domain, which is e164.arpa. An organization administering a domain, such as the top-most e164.arpa domain, not only can partition it into subdomains, it can "delegate" each of these subdomains to another organization such that the delegated-to organization is now responsible for maintaining all data in the subdomain. Such a delegated subdomain is called a "zone." Zones such as country codes, area codes, or primary delegated blocks of numbers can be delegated as well as individual numbers.
In the DNS architecture, "name servers" are computers, often running Berkeley Internet Name domain (BIND) DNS software, that store information about the domain name space, as well as NAPTR resource and service records, and they maintain information about a zone (or multiple zones), which they load from a file or from another name server. The name servers have "authority" for that zone(s) and are referred to as "authoritative name servers." For example, the name server for the e164.arpa zone contains only pointers to the name servers with the data for the zones with information on subdomains of e164.arpa.
The top-level zone for the e164.arpa domain is called ENUM Tier-0, hosted by the RIPE NCC (Network Coordination Center) one of five Regional Internet Registries (RIRs) providing Internet resource allocations, registration services and co-ordination activities that support the operation of the Internet globally.
Public and Private ENUM
Tom McGarry, VP of Strategic Technical Initiatives at NeuStar, itself a registrar, says: "There are actually several 'ENUMs.' The first is ENUM the standard, or Public ENUM. This is the open, global ENUM, which dictates that the domain names are in e164.arpa, that there's a regulatory oversight of it, that the ITU is involved, etc. You could also call it e164.arpa ENUM. Then there's Private ENUM, encompassing implementations not in e164.arpa; they could be in anything, such as 'enum.net'. But it functions the same way as Public ENUM, just on a smaller scale. A network element, such a VoIP switch, queries it with a telephone number using an ENUM-like domain name, and gets a response back containing the address of another switch or user to whom a phone call can be sent."
"Private ENUM implementations are proliferating," says McGarry. "We at NeuStar have what amounts to a Private ENUM implementation that's been in place for a couple of years now. It helps the US wireless industry route MMS [Multimedia Messaging Service] messages. When number portability was mandated in 2003, the wireless industry didn't have a solution for how to route their MMS messages for ported numbers. They came to us and we developed an ENUM solution for them, because MMS is an entire IP network, it's not a PSTN type of network at all. It doesn't use any of the PSTN-type of addressing schemes, so we had to figure out another way to get this information. So we built what's essentially a Private ENUM solution to route MMS messages for a few wireless carriers. When someone sends an MMS message from one carrier to another, they'll dip into our ENUM database to find the correct MMS controller and carrier to send that message to."
"Other private ENUM implementations exist," says McGarry, "which amount to a group of carriers wanting to exchange traffic with each other, and they have a third party in the middle who provides the routing service amongst them. NeuStar has been trialing such a service-we call it IP Traffic Exchange, where a carrier can query our database and determine if they can terminate a call directly over IP to another carrier which must be part of the group."
"RIPE NCC hosts e164.arpa, so they delegate the country codes to countries," says McGarry. "For example, they would delegate country code 44 to the UK. That's where the ITU got involved, with the delegations, since the ITU administers telephone number country codes among sovereign nations. Country code 1 is shared by the US and multiple countries including Canada and the Caribbean islands. It's a little more complex figuring out how to delegate country code 1, but the industry has collectively gone to those countries' regulators and they've developed an acceptable way to delegate it. Now they must determine how to put an ENUM registry in, for example, the US, which has hundreds of millions of active phone numbers. Theoretically, they could be accessing a registry for that many domain names. After all, ENUM is, at the top level, just like a top level domain such as '.com' or '.net'. It's a bunch of phone numbers that look like domain names."
"The industry collectively is developing requirements for the registry, requirements for registrars, business practices and policy issues related to how you do and don't provision the numbers," says McGarry. "They've come quite far, but they're not at the point where they're going to issue an RFP. They have a trial document out, so they can actually ask for a trial delegation to start trialing ENUM here in the US under country code 1."
"When I say 'the industry,' I mean the ENUM Forum [www.enumf.org] and it's members, such as NeuStar," says McGarry. "The ENUM Forum has created an LLC [a limited liability corporation] that theoretically could manage the selection of Tier-1 providers for ENUM deployment here in the US. It's a big milestone and they've managed to do it internally."
Carrier ENUM
"We're also attempting to figure out a way to modify the current ENUM as it's defined, to include something that called 'Carrier ENUM' or 'infrastructure ENUM'," says McGarry. "ENUM was originally envisioned as a consumer service. I, Tom McGarry, can register my phone number with the ENUM registry and I can point those records to my SIP server on my PC in my basement that's connected to my DSL line. Therefore, I could be my own VoIP service provider. That's basically the concept. Typically, things that come out of the IETF have a 'grass roots' aspect to them. But carriers such as SBC, Verizon, AT&T and MCI have a need to provision ENUM information to facilitate the routing of VoIP calls, when they finally roll out VoIP. These carriers are trying to set up ENUM in such a way so that there's a space within ENUM where they control the records. Today, the consumer controls the records. It has always been expected that the majority of the ENUM records would be controlled by carriers as agents of the consumer."
"But carriers want more control over the situation," says McGarry. "They want to control access to the records, because they don't want their network node addresses available on the public Internet. So they've been working on a way they can modify ENUM to include something called Carrier ENUM, which gives the carriers-the companies that were assigned telephone numbers by the number administrator-some greater control over the records in this section of ENUM. They're still in the middle of trying to figure out how to get that done."
Today Austria, Next Year the US?
Richard Stastny is a major figure in the ENUM field. He is co-senior strategic analyst in OeFEG (?sterreichische Fernmeldetechnische Entwicklungsund F?rderungsgesellschaft), a 100% subsidiary of Telekom Austria, the incumbent telecom operator in Austria. OeFEG is responsible for the national speci- fications related to telecommunications (ISDN, SS7, GSM, IN; OAM, etc), performing acceptance tests for digital switching and OAM equipment. In its role as a national standards body, OeFEG is an ETSI member. It also serves as the hosting platform of various national bodies; e.g. the Intercarrier platform (AK-TK) and Austrian ENUM platform.
"Austria led with ENUM; we've had it commercially available since the end of 2004," says Stastny. "In one or two weeks, we will open up the number 780 domain, which is for ENUM only, where you can call ENUM numbers from the PSTN via a gateway."
"But ENUM is not yet a global phenomenon. Currently you can only reach numbers in Austria, Germany and a few other places; the US is lagging at least a year behind. We definitely need to have numbers in the US."
"The second problem we have with ENUM in general is that VoIP providers must supply their residential customers with a SIP URI [Session Initiation Protocol Uniform Resource Identifier] along with their VoIP account," says Stastny.
"Customers get a SIP URI with Free World Dialup and SipGate in Germany, but they don't get one with Vonage's service, or BT's, or Verizon's. This means you can do VoIP only via the PSTN. It's is a major problem, since everybody is doing SIP, but if your device has a SIP client, and you're using something like Vonage, you can only enter a phone number. You can't enter a SIP URI. Therefore, you can't call from your Vonage account directly to someone's SIP URI on the Free World Dialup. Actually, sometimes you can, but only under very specific circumstances where the service providers have an agreement."
"As long as you can't enter a SIP URI on your business card and say 'call me', ENUM cannot be used to its fullest extent. As SIP expert Henry Sinnreich has said at VON and various conferences: 'If you can dial a SIP URI and you can be called by a SIP URI then you truly have access to Voice over IP, otherwise you're just using an emulation of the classic PSTN. It's not endclient SIP'."
So, ENUM isn't quite ready for world conquest as yet. Let's see what next year brings.
Richard Grigonis is Editor-in-Chief of VON Magazine.
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