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November 2006, Volume 9/ Number 11

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The Long View on ENUM

By Chris Risley


VoIP offers tremendous opportunities that cable operators, telcos and others are rushing to address. As you build out VoIP offerings, it is important to keep the broader context of next-generation communication services in mind � because VoIP is just the beginning. Once you free voice traffic from the PSTN telephone lines and integrate it with IP-based traffic, the possibilities for converged communication services are enormous. As you make decisions about VoIP infrastructure, keep an eye on the long-term objectives.

ENUM (from TElephone NUmber Mapping, but also called �Electronic NUMbering�) is a prime example of a technology that has both immediate applications and long-term potential in carrier networks. Many carriers today are making ENUM decisions based on immediate needs for VoIP peering � interconnecting VoIP calls between networks over IP networks. Initially defined by the IETF as enabling a global, publicly-available database of phone numbers, ENUM is now being widely adopted as a mechanism to facilitate VoIP peering. While public ENUM efforts are still underway around the globe, carrier ENUM (or private ENUM) implementations are solving immediate business needs. As with many technologies, the potential uses of ENUM exceed the originating vision.

ENUM connects E164 phone numbers to URIs (and hence IP addresses), enabling efficient routing of voice traffic over IP networks. Today carriers are deploying ENUM to route calls between VoIP networks, enabling end-to-end IP connectivity for VoIP calls. VoIP peering is essential for carriers that want to maintain the cost benefits of VoIP (bypassing the PSTN) and offer new, next-generation services, such as automatically switching into video conferencing mode when both endpoints have the capability to do so.

ENUM�s potential is much greater than simply directing VoIP calls. ENUM can connect all kinds of nextgeneration services to a phone number � allowing subscribers to use a single number for a wide range of communication services. ENUM directories can contain preference ordering, optimal routing information, and can serve as broader IP-application Routing Directories (IPRDs).

What this means for providers deploying VoIP services today is as follows:

� There isn�t one right way to deploy ENUM; there are many ways to derive value from the technology.

� You can phase-in ENUM and VoIP peering, starting with a limited scope to address an immediate problem and adding subscribers and levels of information over time.

� Whatever approach you take, consider future scalability and service integration as you implement ENUM directories.

We�ll briefly discuss several areas that ENUM can address today.

Softswitch Interconnectivity

For networks employing softswitch equipment from different vendors, ENUM can be used to centralize routing information, rather than maintaining and updating routing tables on all softswitches. In this function, ENUM simplifies the management of in-network routing data. In a sense, ENUM becomes a standard for sharing interconnectivity information between softswitches.

Intracarrier Peering

A single carrier environment may have multiple VoIP �islands� within their network � for example, VoIP offerings for different regional subscriber groups, or for business/consumer users. Often in such situations there is no mechanism for exchanging routing information across the islands and so calls between the VoIP islands use the PSTN.

Routing VoIP-to-VoIP calls over the PSTN degrades call quality (with voice transcoding required at the gateways between IP and PSTN), and imposes telecommunications costs. Additionally, extra features that rely on IP networks will not have corollary features in the PSTN. ENUM is a great solution to this problem because it creates a standards- based repository of call routing data. In this �intracarrier peering� situation, the data within the ENUM directory is the carrier�s own subscriber data. The carrier can use this information to route the data entirely over IP networks for optimal efficiency and functionality.

Inter carrier Peering, or Connecting VoIP Calls between Carriers

Carriers offering VoIP services face similar issues when routing VoIP calls out of their networks to other carriers� subscribers. The advantages of end-toend IP connectivity include reduced costs, consistent quality of service without transcoding, and the ability to support new, IP-based features. Many carriers are entering into VoIP peering agreements with other carriers to exchange subscriber information, supporting direct IP connections with partner VoIP networks.

In addition to the obvious requirements for scalability and performance, reliable and secure provisioning of the ENUM directory is a key consideration. The directory may accept information and updates from multiple sources, including internal subscriber data, partner data, and possibly service bureau data. Provisioning interfaces and performance during updates are both critical factors to consider for an ENUM directory in this role.

Least-Cost Routing

The ENUM protocol is currently being extended to reduce call placement costs through �enumservices� definitions that provide PSTN routing information traditionally available on telephony networks. Using data in these service-specific fields, carriers can avoid unnecessary SS7 dips and reduce the cost of call placement.

For example, for routing calls to a destination, it is possible to directly host the number portability data locally in the ENUM database rather than require the services from an SCP. Using the enumservices type E2U+pstn:tel in the ENUM record, the querying switch can receive the associated routing number that should be used for the call. This allows the call to be routed directly to the destination central office without requiring an external number portability dip, or routing the call to the PSTN for resolution.

Other cost-saving features of ENUM include encoding the closest gateway to the destination (�far end hop off �) or the least-expensive market to reduce the cost of the PSTN �hop� for the call. In other words, the ENUM directory may provide the information to optimize the call routing through a combination of IP and PSTN networks.

Optimal routing information changes frequently. Over time, the ENUM directory will become a dynamic repository for routing information, offering arbitrage between routes as the cost and availability of routes change continuously. The ENUM server will eventually include logic that allows the call to take the most efficient route, depending on time of day, network congestion, business rules, etc.

Beyond Voice: IP-Application Routing Directories

At its core, ENUM directories offer routing information for services connected with a phone number. These services may easily extend beyond traditional voice traffic to include fax, video conferencing, gaming, and a large range of next-generation services. At Nominum (News - Alert), we refer to this role as an IP-application Routing Directory (IPRD). In this capacity, ENUM plays an essential role in next-generation networks.

Why use ENUM for this role? The very attributes that make it a good choice for the applications described above also contribute to its broader utility as an IPRD. As a standards-based technology, ENUM provides interoperability in heterogeneous networks and between diverse networks. It leverages the proven, highly scalable Domain Name System (DNS) protocol, which has served to direct traffic on the Internet for decades. It is inherently extensible to support a wide variety of services and types of information. Functioning in this capacity, ENUM directories will be integrated with a wide variety of applications and network elements.

High performance interfaces for querying, provisioning, and administration will be essential. As converged networks are deployed, ENUM-based directories will expand with local in-network copies of many types of session routing information, such as number portability information or CNAM data replicated from SS7 databases or service bureaus. This data will be combined with powerful routing logic to provide a critical function in these environments, connecting multimedia sessions in the most efficient way.

Steps to Take Today

If you are using or deploying ENUM for any of the issues described above, keep the long-term benefits in mind. In summary:

1. Treat the ENUM directory as the essential network element it is � with the reliability and availability necessary for telephony-grade traffic.

2. Think big � beyond your initial trial to the larger potential uses of ENUM. Some providers start their ENUM trials using open-source DNS software, either to save money or because it is what they are familiar with. These general-purpose servers were not designed with the scalable performance or capacity to handle the large (and different) data volumes of ENUM. Most of these trials quickly encounter the limits of the open-source software.

3. For long-term cost of ownership, insist on adherence to industry standards and carrier-grade manageability. The ENUM directory server should fit smoothly into existing element and network management processes rather than adding to management complexity.

4. Work with providers who share the long-term vision for ENUM, with a roadmap for future applications and close partnerships with softswitch vendors, to support the full potential benefits of ENUM. IT

Chris Risley is CEO of Nominum. For more information, please visit the company online at

If you are interested in purchasing reprints of this article (in either print or PDF format), please visit Reprint Management Services online at or contact a representative via e-mail at or by phone at 800-290-5460.


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