faqs

If you have an ENUM-related question that is not addressed here, please submit it to
enum-questions@enum.org. If your question is deemed to be of merit to the wider ENUM community, it will be included on this page.

Neustar would like to recognize AusRegistry (www.enum.com.au) for their assistance and cooperation in the development of these FAQs.

What is ENUM?

ENUM is simply the convergence of Public Switched Telephone Networks (PSTN) to Internet Protocol (IP) Networks – in other words, the mapping of telephone numbers to domain names using a Domain Name System (DNS)-based architecture. ENUM helps to facilitate such services as Voice over IP (VoIP), and allows network elements to find services on the Internet using only a telephone number. ENUM is also the title of RFC 3761, the approved protocol document that discusses the use of DNS for the storage of E.164 numbers and the available services connected to an E.164 number.

It is also the title of RFC 2916, the approved protocol document that discusses the use of DNS for the storage of E.164 numbers and the available services connected to an E.164 number.

ENUM does not change the Numbering Plan and does not change telephony numbering or its administration in any way. ENUM will not drain already scarce numbering resources because it uses existing numbers.

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What is E.164?

E.164 is an existing global numbering system, and is therefore appropriate for use by ENUM.

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What is the ITU?

The International Telecommunications Union (ITU) is a global organization where governments and the private sector come together to coordinate global telecommunications networks and services. The ITU administers E.164 (described above) as the international country-level telephone numbering plan, the structure of which is described in the ITU document "E.164."

Country codes are assigned by the ITU; however, designated telecommunications regulatory bodies within each country administer their own telephone numbers. (An example of a fully qualified E.164 number includes that telephone number's country code, area code and the phone number. For example, the phone number 9777 1234 in Melbourne, Australia would be +61 3 9777 1234.)

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How many different types of ENUM are there?

There are three (3) forms of ENUM: Public User ENUM, Private Infrastructure ENUM and Public Infrastructure ENUM.

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What is Public User ENUM?

With Public User ENUM (also called "End User ENUM"), the end user can provision his or her records in the ENUM registry in the public domain e164.arpa. In other words, the end user is the registrant for the ENUM domain name within the domain e164.arpa; the implication is that the end user has "opted in" to the ENUM service.

"End User opt in" is generally considered a good policy if end user data is publicly available on the Internet. (The DNS is publicly available to any Internet user; it uses a domain available on the public Internet, and various regulatory, industry and standards bodies moderate the domain’s policies and practices.)

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What is Private Infrastructure ENUM?

Private ENUM uses the concepts of creating a domain name from a telephone number (TN) and resolving it to a URI. It does not, however, use the domain e164.arpa (or any other domain with regulatory oversight). Private ENUM is used when there is a closed user group that wants to use ENUM to exchange IP traffic.

An example of such a user group is a group of communications carriers that wants to exchange VoIP traffic. The user group that creates and uses the domain will also create the policies for the domain. There will be no regulatory oversight of this domain, because it is not intended for public use.

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What is Public Infrastructure ENUM?

National number administrators typically assign TNs to communications carriers, not to end users; carriers then assign the TNs to end users. Carriers have specific needs when it comes to provisioning routing information related to their customers’ TNs. Carriers map TNs to internal network addresses to enable call routing and features; these addresses are not publicly available. The carrier’s network and its associated addresses are highly secure, and access is strictly maintained and limited to other service providers. These characteristics have come to define the recent discussions of carrier ENUM.

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What does it mean to “provision” ENUM?

At the heart of the ENUM service is an Internet domain name registry.

Domain name registries have two core responsibilities: maintaining the authoritative database of domain name registrations, and publishing a "zone file" on the Internet that can be queried by any Internet user. The authoritative database has data about each domain name that is registered (who registered the name, contact information, the expiration date of the registration, etc.) The zone file contains information that will point the Internet user to the desired application, such as email or Web sites.

(For example, Neustar is the domain name registry for the .BIZ Internet domain. The Acme Company registers the domain name www.acme.biz with the .BIZ registry. As the registrant, Acme registers the name with the registry and is responsible for all adds, modifies and deletes related to this name. Further, when an Internet user types the address www.acme.biz into his or her Web browser, the browser will query the .BIZ zone file for a pointer to www.acme.biz, then it will query the Acme server to find the Acme Company Web page.)

In summary: To provision ENUM means to provision the administrative and pointer information for the ENUM domain name with the ENUM registry. For example, if someone's phone number was +1 202-555-1212, he or she would provision the domain name 2.1.2.1.5.5.5.2.0.2.1.e164.arpa with the registry that is responsible for the domain 2.0.2.1.e164.arpa. That would make him or her the registrant for that domain name.

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What are the main benefits of ENUM for subscribers?

The primary benefit of ENUM is allowing Internet users the ability to communicate with other people via a range of services when the caller knows only the telephone number of that person or only has access to a telephone keypad. Internet services and resources can be accessed by end users with IP telephones, ordinary telephones connected to the Internet and other numeric-input, Internet-connected devices. ENUM allows end users to choose how they wish to be contacted, giving them greater control.

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Who could use Public ENUM?

Aside from accredited Registrars, ENUM can be used by corporations, individuals, government agencies, military organizations and hosts of other non-individual users.

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How is ENUM going to work for the average end user?
What will it look like?

The mechanics of using ENUM when a call is made from one person to another are virtually invisible. For instance, if one person called another and both parties were using IP phones, the call would appear as if it were being made over the regular PSTN. The calling party would designate the services its number would use, and the called party would also have specified which ENUM service they wish to have used to be contacted.

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What kinds of applications could use ENUM?

The initial applications to which ENUM will be applied are Voice over IP (VoIP) and Voice Profile for Internet email (VPIM). The goal of the VoIP industry is to be able to make simple and high-quality voice calls using the internet, while the VPIM industry wants to develop voice mail systems that can exchange messages over IP networks. ENUM’s applications will most certainly reach beyond VoIP and VPIM to such services as Internet fax and Instant Messaging.

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How does ENUM work?

A telephone number, once entered, is translated into an Internet address. The following steps explain the process:

  • The number entered is translated into a full qualified E.164 number. (To quote an example from earlier, the number 9777 1234 in Melbourne, Australia would be +61 3 9777 1234.)
  • The number is reduced to digits only. (e.g., 61397771234)
  • The digits are re-ordered back to front. (e.g., 43217779316)
  • Dots are placed between each digit. (e.g., 4.3.2.1.7.7.7.9.3.1.6)
  • The domain e164.arpa is added to the end. (e.g., 4.3.2.1.7.7.7.9.3.0.1.6.e164.arpa)
  • A DNS query is then issued on this domain.

Once the authoritative name server is found, ENUM retrieves relevant NAPTR Resource records and will perform according to the user's registered services for that number.

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What is .arpa, and why is it the top level domain for ENUM?
Why not create a new top-level domain specifically for ENUM?

E164.arpa has been designated as the temporary DNS domain for use with ENUM. This designation may change as a result of ongoing discussions between the ITU, the IETF and other international organizations involved with ENUM. (In the event that the international community chooses a different ENUM domain, the structures discussed here and in IETF RFC 2916 will apply to that new designated domain.)

The .arpa domain has been designated for Internet infrastructure purposes. It is managed by the Internet Assigned Numbers Authority (IANA) in cooperation with the Internet technical community under the guidance of the Internet Architecture Board (IAB). A new top-level domain (e.g., .e164) was not created because ENUM is an infrastructure application appropriate for designation within the previously established .arpa domain. ENUM is considered appropriate as an infrastructure application because it provides a set of DNS-based resource directories, referenced by phone number, for use by various ENUM-enabled application clients (such as telephones, SIP servers and voice messaging systems).

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Why is the number reversed?

DNS, when reading a domain name, will first search for the top level domain (.arpa), then the second level domain (being e.164). It then searches for the country code, area code and phone number.

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Why are there dots between the numbers?

Each dot separates the number into administrative domains, or zones. This allows for delegation of authority at various points along the name, and eliminates the requirement for clients to know individual delegation schemes to know where to put the dots.

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Will ENUM telephone routing confuse the PSTN routing system?

ENUM will not affect PSTN application level functions such as call routing and signaling. ENUM facilitates the discovery of resources associated with a telephone number, and does not impact how applications operate.

It is important to note that ENUM will not change the existing right-to-use rules and principles for telephone numbers. ENUM is intended to facilitate applications using telephone numbers as subscriber names, not to change how telephone numbers are administered.

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How will the e164.arpa domain be organized?

One convenient way of doing this would be to delegate according to the ITU assigned country codes. It is important to understand, however, that delegation of a country code in DNS can occur at any digit or zone domain in DNS terms.

For example, within the root e164.arpa, there would be:

  • An NS listing for .1.e164.arpa - representing the country code (1) of the North American Numbering Plan (US, Canada, and several Caribbean countries).
  • An NS listing for .4.4.e164.arpa - representing the country code (44) of the UK.
  • An NS listing for .6.4.e164.arpa - representing the country code (46) of Sweden.
  • An NS listing for .1.8.e164.arpa - representing the country code (81) of Japan, and
  • An NS listing for .8.5.3.e164.arpa - representing the country code (358) of Finland.

At the national TN/NS level, further NS delegation [DNAME, CNAME, PTR] can occur with enterprises, TN/NS application service providers, carriers, and even individuals who have DNS servers in their homes.

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Why use DNS?

The beauty of DNS is that it exists already and is a global, efficient, open and scalable system. Using DNS for ENUM also provides a low-cost solution.

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What is the effect of e164.arpa deployment on the global DNS system?

This is going to require research, such as the effect of "wrong dials" on the root of e164. That is, caller specification of a wrong number can result in many additional queries to the e164.arpa root.

Additional work will be necessary in advising ENUM applications about such things as the level of data caching necessary in order to relieve stress; suppress escalating of poorly-formed queries; misdials; or cache misses on the root structure. For telephony applications, performance and load engineering is critical, as query volumes from small- to medium-sized cities alone can easily reach many thousands per second. Response times, as well as transaction loads, must be carefully considered. Conventional DNS caching is of significantly reduced value in ENUM due to the huge size of the name space and relatively even distribution of queries into the space over arbitrary time intervals. Unlike conventional DNS queries, call volumes are not highly concentrated into a popular small subset of the number space.

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How does ENUM affect the North American Numbering Plan (NANP)?

ENUM does not change the NANP. In fact, it does not change telephony numbering or its administration in any way. ENUM uses existing numbers, and thus will not drain already scarce numbering resources.

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What are SRV and NAPTR Records?

SRV and NAPTR are DNS Resource Records that contain information about resources, services and applications associated with a specific phone number.

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What happens if a user dials a number that cannot be resolved by DNS?

Where a number cannot be resolved by DNS, a 404-Not Found error message is returned to the device or program initiating the call. This error message is displayed in the case of a Web browser. With IP telephones, the call will be diverted to the PSTN and connected the traditional way.

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What happens if a user dials an emergency number?

Emergency numbers are not considered part of the E.164 and ENUM services, as they are locally designated by each country’s regulatory authority. A person dialing an emergency number with an IP phone will be connected via the PSTN.

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What protocol does ENUM use for Internet Telephony?

ENUM itself is "protocol-agnostic" because it is application-agnostic. It does not specify what applications a particular number is associated with, but instead provides a unified way of discovering resources associated with it. It can, for example, work with either H.323 or SIP.

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What can be said about the skepticism around VoIP?

VoIP will most certainly become a reality in the near future, where Internet telephony is fully integrated with traditional telephony on a global scale. Although in its early stages, VoIP is always evolving and improving. It should be remembered, however, that ENUM is not intended to facilitate VoIP solely; it is intended to work with a number of applications.

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What is SIP? How does ENUM relate to SIP?

Session Initiation Protocol (SIP) is the initiator of interactive communications sessions between users. It also terminates those sessions and modifies sessions. With ENUM, SIP can be used to initiate attempts to multiple locations to find the user receiving the call.

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Could ENUM be used to provide telephone number portability?

ENUM is not intended for this use, although it may have a place in countries that do not have a centralized database administration. There are implications and limitations in using ENUM in this way, because ENUM is a shared resource discovery service rather than an industry provisioning service. Countries that deploy number portability have telephone service providers that are generally required to comply with regulatory and industry processes and procedures regardless of the underlying technology they employ for telephony service delivery. It’s important to highlight that the ways in which ENUM will need to be deployed must be consistent with applicable national requirements; it does not create an alternate numbering system with its own set of rules and policies.

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How is the user of a number authenticated?

Users could be corporations, individuals, government agencies, military organizations and hosts of other non-individual users. Service providers typically assign large blocks of numbers to these entities; the telecom manager within these entities then assigns numbers to users, so even the service providers cannot identify the users for a large portion of the allocated numbers. This is an unresolved issue, but one that must be resolved prior to deploying a robust and secure ENUM service. It is likely that the service provider that allocated the number(s) to the user will be involved in the process of authentication.

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What about private numbering plans within a company?

The ENUM protocol can be used in private numbering plans the same way it can be used in the public E.164 numbering plan. The Internet Telephony gateway or proxy needs some intelligence to "decode" a particular dialing string and then decide how to look up resources for that particular number. Instead of looking for resources in e164.arpa, the gateway or proxy would look for SRV or NAPTR records for private numbers under some other structure, such as e164.bigcompany.com.

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Are users going to have to pay to have their telephone numbers ENUM-provisioned?

Probably, but it is likely that the costs will be indirectly recovered through the underlying prices for ENUM-enabled services that subscribers pay. This is a DNS-based system, and someone must pay to have a domain name registered in DNS. Listing telephone numbers will be no different. Whether the cost will be charged directly to the subscriber or will be an indirect charge as part of some larger services will depend on those offering the services.

It is important to remember that a user does not have to have ENUM list their phone number. ENUM would be a subscriber-controlled "opt-in" system to "announce," over the Internet, the availability of a particular telephone number to accept service sessions and how to manage those sessions as a result of having subscribed to an ENUM-enabled service. If a customer does not have an Internet telephony device or service, the associated phone number will likely not be listed. On the other hand, subscribers may not necessarily be aware that they have subscribed to such a service, and that they have had ENUM provisioned for that service by their service provider on their behalf.

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Are users going to have control over how this system is used with their phone numbers?

With Public ENUM, yes. To reiterate, the first principle in the creation and operation of a global ENUM service is that phone number subscribers or their designated representatives are the ultimate decision makers on how a DNS record for a phone number is to be provisioned.

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How will the rights of telephone number subscribers be protected?

A simple answer is by respecting existing regulatory and business rules regarding number administration, slamming, non-reliance and so forth. Only by replicating or re-implementing ENUM analogs to the existing "rules of the road" will we avoid a wide range of serious administrative, operational and political conflicts.

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How are you going to prevent "slamming" or "hijacking"?

Slamming, or the involuntary transfer of service providers, must be avoided in any ENUM system. It is a serious problem in the PSTN, and we must be careful not to expect more from Internet services than we are able to guarantee elsewhere.

Note that anti-slamming fundamentally requires a neutral third party solution. The U.S. industry is grappling with this issue with regard to long distance right now. It was solved on number portability from the outset.

Authenticated subscriber access is not a total solution, because if subscribers disconnect their telephony service, they lose rights to the phone number. Consequently, some combination of originator authentication as well as telephone number rights validation (using new and existing validation sources) can be used to solve the problem, depending on the level of standard required.

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Are there any examples of global namespace delegation that should be considered as models?

The closest technical equivalent is in-addr.arpa. That domain provides a reverse mapping from IP address to domain name. It is used as part of the Internet infrastructure operation to help authenticate an IP address and identify the operator associated with an IP address. It is not seen directly by users. The same is true for e164.arpa. It will be for operational infrastructure, rather than for direct access by end users. As with e164.arpa, in-addr.arpa, allocations are hierarchical according to the infrastructure administrative structure. For in-addr.arpa, the hierarchy uses the "CIDR" address allocation hierarchy. For e164.arpa, the hierarchy will be based on the ITU E.164 recommendation.

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