ENUM -- It's All In The
SCHROEDER & RICHARD SHOCKEY
Todayï¿½s telephony market has some shining stars with Voice
over Internet Protocol (VoIP) being one of the brightest. In a
Telecommunications Industry Association (TIA) benchmarking report released
in March (www.tiaonline.org/marketdev/benchmarking.cfm),
the migration to VoIP by the reseller community is well documented. The
convergence of voice and data has even resulted in the emergence of a new
class of reseller, the Voice VAR. Many Voice VARs have migrated from the
traditional interconnect business by incorporating customized software
applications into their offerings. While maintaining a strong focus on the
sale of voice products (more than 50 percent of revenue) they incorporated
data as a supplemental business focus. Many of these resellers prefer to
ï¿½fly below the radarï¿½ to keep competitors from realizing the potential of
Standards bodies and equipment suppliers have made significant
improvements to the quality of VoIP. It is clear now that voice
communications made over the Internet sound as good if not better than a
traditional circuit switched call.
There are 400 million telephone numbers and over 130 million Internet
customers in the United States. This alone is creating an unstoppable trend
towards convergence of which VoIP is now in the center. Customer focus on
the convergence of voice and data is a key to the growth of VoIP. TIA
calculates a total voice/data market size of $296.8 billion for 2001. This
represents a rise of 12 percent from 2000. The move to VoIP is driven by
both the desire of IT departments to merge voice and data, and by financial
managers to take advantage of the lower phone charges. For multi-site
companies, the migration to VoIP allows for multiple locations to connect
via IP thus dramatically lowering phone charges.
One of the most vexing problems of VoIP is inter-domain call routing based
on a telephone number. This is an issue that has plagued both H.323 and SIP
for some time. As it stands now, most VoIP is used intra-domain or for
calling within an enterprise to various remote locations. Vendors have all
developed proprietary routing tables in their gateways or proxies that
translate the dial string to a host name or URL necessary to set up a call.
This is where ENUM enters the picture. ENUM was developed by the Internet
Engineering Task Force as a solution to the question of how VoIP systems can
find each other on the Internet using only a telephone number, and how
telephones, which have an input mechanism limited to 12 keys on a keypad,
can be used to access new and innovative Internet services. ENUM at its most
basic is the convergence of PSTN and IP networks.
ENUM has a number of meanings but the most basic is that it is a protocol (RFC
2916) that resolves a complete international telephone number (i.e., +1 202
123 1234) to a series of URLs using a Domain Name System (DNS)-based
architecture. In other words, your personal phone number can also become
your personal VoIP phone address. ENUM preserves the convenience,
simplicity, and investment that consumers and businesses have made in
How does it actually work? Once a telephone number is entered into a VoIP
terminal or other Internet enabled device, it is translated into an Internet
address using the following steps:
1. The number is first stored as +1-202-555-1234. ï¿½1ï¿½ is the country code
for the United States, Canada, and the seventeen other countries that make
up the North American Numbering Plan (NANP). The ï¿½+ï¿½ indicates that the
number is a complete, international E.164 telephone number. E.164 is the
name of the international telephone numbering plan administered by the.
2. All characters are removed except for the digits. Example: 12025551234
3. The order of the digits is reversed. Example: 43215552021. The telephone
number is reversed because DNS reads addresses from right to left, from the
most significant to the least significant character. Dots are placed between
each digit. Example: 18.104.22.168.22.214.171.124.0.2.1. This is very important since, in
DNS terms, each digit becomes a ï¿½zone.ï¿½ This means that authority can be
delegated to any point within the number.
4. The domain ï¿½e164.arpaï¿½ is appended to the end of the numbers in order to
create a Fully Qualified Domain Name. For example:
5. The Domain name is queried for the Resource Records that define URLs
necessary to access services such as SIP- or H.323-based VoIP.
Once the authoritative name server for that domain name is found, ENUM
retrieves relevant records and will use that data to actually complete the
call or service. So 12025551234 would ultimately return back SIP:email@example.com,
VoIP is generally considered the most important application for ENUM. The
workings of ENUM are designed to be invisible to both the end user and the
subscriber. The IP network will be accessible either by the use of a
Internet enabled telephone, or a standard phone that has access to an IP
enabled PBX or even a classic carrier class 5 switch that has IP enabling
software added to it.
It is a core principle in the design of the system that ENUM will not change
the existing right-to-use rules and principles for telephone numbers. ENUM
is not intended to change how telephone numbers are administered, but
instead facilitate a wide range of applications using phone numbers as
subscriber names. ENUM also will not interfere with existing PSTN functions
and technology, such as circuit switching, SS7 (ISUP or TCAP), or
Through the use of ENUM additional applications can be linked to telephone
numbers, such as Internet fax or Internet enabled voice mail. More
importantly, ENUM can be used to facilitate interoperability of services
between the PSTN and the Internet, such that SMS messages on wireless phones
could be linked to desktop Instant Messaging and vice versa.
What is now important for the industry to consider is that there will be a
variety of policies and procedures necessary to make sure that the ENUM
system here in the United States and around the world maintains the security
and integrity of phone numbers.
No one has to use ENUM. It is not a requirement for Internet telephony or
any other service; rather, ENUM is a technology that consumers or
enterprises may choose to use in order to facilitate communications over IP
There will have to be a process by which a user can register a telephone
number in the ENUM system and be assured that the information being stored
is accurate and complete. The process of ENUM registration may be part of a
traditional sale of a PBX system or hosted IP telephony service from a
carrier. That company would inform you that it would, on your behalf and
with your consent, register your telephone number within the ENUM system.
First, the service provider or reseller, will need to authenticate that you
actually have the right to the telephone number you are trying to register,
in order to prevent others from hijacking your number. Second, that reseller
or service provider will create NAPTR records for your number in an
appropriate and secure DNS server. Finally, it will need to inform the
appropriate national ENUM registry of where those servers are located.
ENUM trials and discussions are now going on around the world as governments
have recognized that ENUM can play an important role in the convergence of
the PSTN and the Internet and become a platform for the development of new
and innovative services.
Here in the United States, the U.S. ENUM Forum has been established by a
variety of companies in the telecommunications and Internet related
industries to discuss and make recommendations on how ENUM should deploy.
And the U.S. government has issued a clear statement of direction that it
believes the United States should participate in ENUM and it is actively
soliciting comments and input from industry and interested parties.
Max Schroeder is Chairman of TIAï¿½s VAR Working Group and Vice President,
Channel Operations, Telephony@Work,
Richard Shockey is Senior Manager, Strategic Technology Initiatives,
TIA is a leading trade association serving the communications and
information technology industry, with proven strengths in market
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