Everything About ENUM

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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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