Contact a Sales Representative

Head Office
Europe, Middle East & Africa
E-mail Head Office
Tel: +44 (0)1908 200022, #2

North America
E-mail North America Office
Tel: 1-703-574-5313

Asia-Pacific
E-mail Asia-Pacific Office
Tel: +61 755 245 575

Global Customer Support
E-mail Customer Support
Tel: +44 (0)1908 200022, #5
Fax: +44 (0)1908 325192

Register and validate your ENUM with Safenames, a leading global ENUM provider

Trust. Worldwide.

Frequently Asked Questions

  1. What is ENUM?
  2. What is E.164? Why use E.164 for ENUM?
  3. What are the main benefits that ENUM gives to subscribers?
  4. What kinds of customers could use ENUM?
  5. How is ENUM going to work for the average telephone subscriber?
  6. What kinds of applications could use ENUM?
  7. How does ENUM work?
  8. What is .arpa, and why is it the top level domain for ENUM? Why not create a new top-level domain (TLD) specifically for ENUM?
  9. Why is the number reversed?
  10. Why are there dots between the numbers?
  11. Will a user have to type in the dots and reversed numbers?
  12. Why use DNS?
  13. What are SRV and NAPTR Records?
  14. What happens if a user dials a number that cannot be resolved by DNS?
  15. What protocol does ENUM use for Internet Telephony?
  16. What is SIP? How does ENUM relate to SIP?
  17. What happens to the ENUM services when a subscriber cancels telephone services?
  18. How will the rights of existing telephone number subscribers be protected?

 

 

What is ENUM?

ENUM was developed as a solution to the question of how network resources can find services on the Internet using only a telephone number, and how telephones, can be used to access Internet services. ENUM at its most basic is the convergence of PSTN and IP networks; it is the mapping of a telephone number from the public switched telephone network to Internet services.

"ENUM" has a number of meanings. It is the name of a protocol that resolves fully qualified telephone numbers to fully qualified domain name addresses using a DNS-based architecture. It is the name of a chartered working group of the Internet Engineering Task Force (IETF) chartered to develop protocols that map telephone numbers to resources found on the Internet using the Domain Name System. 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.

 

What is E.164? Why use E.164 for ENUM?

E.164 is the name of the international telephone numbering plan administered by the International Telecommunication Union (ITU), which specifies the format, structure, and administrative hierarchy of telephone numbers. "E.164" refers to the ITU document that describes the structure of telephone numbers. The ITU issues country codes to sovereign nations, but administration of telephone numbers within each country is governed by that countries telecommunications regulatory agency. A fully qualified E.164 number is designated by a country code, an area or city code, and a phone number. For example, a fully qualified E.164 number for the phone number 200022 in Milton Keynes, (area code 1908) in the United Kingdom (country code 44) would be +44-1908-200022.

E.164 numbers are appropriate for use in ENUM because they are an existing system for global traceability. Further, because country codes and assignments change only in the event of the collapse or complete restructuring of governments, these numbers tend to be static.

 

What are the main benefits that ENUM gives to subscribers?

ENUM enables Internet-based users to make a selection from a range of services available for communicating with another person when the caller knows only a telephone number. ENUM allows users to access Internet-based services and resources from Internet-aware telephones, ordinary telephones connected to Internet gateways or proxy services.

ENUM enables users to specify their preferences for receiving incoming communications, and gives greater user control over communications. For example, a user can specify a preference for voice mail messages over live calls during certain times of day, or may indicate a destination for call forwarding.

 

What kinds of customers could use ENUM?

Although ENUM can be used by individual residential telephone customers, users could also be corporations, individuals, government agencies, military, and anybody who has an existing telephone number.

 

How is ENUM going to work for the average telephone subscriber?

If an average telephone customer were to make a telephone call using, for instance, an Internet-enabled phone to another Internet-enabled phone, all of the steps in between are invisible to the user. To the call initiator and call receiver, this phone call would appear the same as a call made over the PSTN.

 

What kinds of applications could use ENUM?

ENUM links a telephone number to a host or resources on the Internet that can connect the call, either end-to-end over IP networks or through a designated gateway to the PSTN. This is useful for connecting SIP- or H.323-compatible telephone systems together over the Internet.

The potential applications of ENUM are far-reaching; however, the principal applications for ENUM have centered on two areas. One is Voice over IP (VoIP) and the other centers on Voice Protocol for Internet Mail (VPIM). ENUM enables carrier and enterprise voice systems to find each other, interoperate, and exchange messages.

Although VoIP and VPIM are the most widely discussed applications, other application possibilities for ENUM, including but not limited to Internet Fax, follow me numbers and Instant Messaging, exist.

 

How does ENUM work?

Once a telephone number is entered, it is translated into an Internet address using the following steps:

1. The phone number is translated into a fully qualified E.164 number by adding the city (or area) and country code. Example: 200022 dialed in Milton Keynes, becomes +44-1908-200022, where the "44" represents the United Kingdom country code. The "+" indicates that the number is a fully qualified E.164 number.
2. All characters are removed except for the digits. Example: 441908200022
3. The order of the digits is reversed. Example: 220002809144
4. Dots are placed between each digit. Example: 2.2.0.0.0.2.8.0.9.1.4.4
5. The domain "e164.arpa" is appended to the end. Example: 2.2.0.0.0.2.8.0.9.1.4.4.e164.arpa
6. ENUM then issues a DNS query 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.

 

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

E164.arpa has been proposed as the DNS domain for use with ENUM. 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 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).

 

Why is the number reversed?

DNS names are structured from right to left. In the example used above (2.2.0.0.0.2.8.0.9.1.4.4.e164.arpa), DNS would first search for the top-level domain arpa, then search second-level domains for e164, then search the next level for the country code 44, etc.

 

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.

 

Will a user have to type in the dots and reversed numbers?

No, this will be done by the application (e.g., a web browser) or device (e.g., an Internet-enabled telephone) that supports ENUM. The user simply dials a telephone number in the traditional manner.

 

Why use DNS?

DNS has been selected for use with ENUM because it is a proven reliable and scaleable system. ENUM-enabled DNS provides a low-cost, shared infrastructure for IP services.

 

What are SRV and NAPTR Records?

These are DNS Resource Records that contain information about what resources, services, and applications are associated with a specific phone number. These services are determined by the subscriber.

 

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

In a similar manner to a 404-Not Found message in a web browser, an error message will be returned to the device or software initiating the call. In the case of a web browser, a 404-Not Found message will be displayed. In the case of a SIP telephone, it can open a gateway to the PSTN and connect in the traditional way.

What happens if a user dials an emergency number (e.g., 999 in the United Kingdom, 911 in the United States and 112 in Europe)?

Emergency numbers are generally considered "access codes" and are outside of E.164 and ENUM services. If the user dials an emergency number from a SIP phone, the phone will recognize that it cannot make a SIP connection and will open a gateway to the PSTN.

 

What protocol does ENUM use for Internet Telephony?

ENUM itself is not a communications protocol, but instead provides a unified way of discovering resources associated with it. It can, for example, work with either H.323 or SIP.

 

What is SIP? How does ENUM relate to SIP?

SIP, or Session Initiation Protocol, performs the initiation of interactive communications sessions between users, as well as termination of those communications and modifications to sessions. SIP is one protocol that may be used by ENUM to send out initiation attempts to multiple locations in order to find the user who is receiving a call.

 

What happens to the ENUM services when a subscriber cancels telephone services?

The Enum services associated with the telephone number must also be cancelled.

 

How will the rights of existing telephone number subscribers be protected?

Each applicant for an Enum registration must undergo a validation procedure to check entitlement to the number(s) being applied for. This ensures existing telephone number subscribers do not have there number assigned to someone else.