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As a standard blueprint for next-generation networks, the IP Multimedia Subsystem (News - Alert) (IMS) promises to help us handle the converging communications world efficiently by providing a common network infrastructure for fixed mobile convergence, VoIP peering, and potentially hundreds of applications.

 

One challenge that I see looming ahead, however, is the question of interoperability of applications across networks. If ten different carriers all build out their IMS networks, how do they route application services between them efficiently and effectively? How do they handle E.164-based numbering traffic without resorting to the PSTN? Interoperability between carrier networks is essential to delivering the promises of IMS.

 

VoIP networks, the precursors of full-fledged IMS networks, today rely a great deal on softswitches to connect and route VoIP calls. But this approach has limitations, such as incompatibility between softswitches. It also does not address the broader challenges of intercarrier application routing in IMS networks. 

 

The IMS architecture requires specialized components that route all kinds of IP application traffic between networks. These components, the IP-application Routing Directories (IPRD) will provide routing information based on source and destination end points identified by E.164 numbers or Uniform Resource Identifiers (URIs). They must be able to integrate data from Local Number Portability (LNP) databases and peering partners, enabling the efficient routing of services between different carriers’ IMS networks.

 

We don’t have to wait for IMS to see the benefits of IP-application Routing Directories. They solve problems we face today in the converging communications environment. For example, to route VoIP-to-VoIP calls over all-IP networks requires a way to connect E.164 phone numbers to IP addresses.

 

Applications such as packet-based Push-To-Talk, presence, and multimedia conferencing also require the ability to route IP traffic using standard E.164 telephone numbers. Fixed/mobile convergence will require end-to-end multi-vendor interoperability across the fixed/mobile boundary and within IP networks. 

 

ENUM is the obvious choice to power the IP-address Routing Directory, for several reasons:

 

1. The ENUM standard maps E.164 numbers with IP addresses – merging the traditional way of dialing numbers on a ten-digit telephone keypad with the Internet’s Domain Name System (DNS).

 

Using ENUM, a phone number is automatically converted to a domain name. For example, +1.650.381.6000 becomes either of the following:

 

0.0.0.6.1.8.3.0.5.6.1.e164.arpa 
0.0.0.6.1.8.3.0.5.6.1.carrierdomain.com

 

depending on whether public ENUM is used (see below). The addressing information belonging to that domain name (such as the IP address of the SIP proxy handling that subscriber number) is then retrieved with a simple DNS-like query to dedicated routing directories containing that information.

 

2. Like IMS itself, ENUM leverages core IP network technologies – in this case, DNS technology that is standard and pervasive. ENUM builds on a global infrastructure already in place.

 

3. ENUM is already being deployed in carrier networks today. For example, mobile operators use ENUM for peering MMS traffic, while cable operators are deploying ENUM for VoIP peering.

 

This last point deserves a brief clarification. ENUM was originally envisioned as a way of creating a global and public directory of E.164 phone numbers. This vision is in various stages of deployment around the globe, and is referred to as public ENUM.

 

But, seeing other possibilities for the technology, carriers and other are deploying ENUM-based servers to route voice traffic within their own global networks or between carriers (referred to as private or carrier ENUM.) The directories used to route traffic belong to the carrier, and contain information from the carrier’s partners and possibly other data sources, such as LNP databases. It is in this private or carrier configuration that service providers are deploying ENUM today.

 

In practice, IMS implementations require capabilities and performance levels from the IP infrastructure that are not currently present in most data-centric networks. These requirements center on the need for robust IP-application Routing Directories that include: 

 

Extremely low latency: IPRDs sit in the middle of a call set-up path, playing a critical function in call completion. Telephony users have very little tolerance for delay, and IP network elements must depend on a very fast response for an ENUM look-up.

 

Massive scalability: ENUM-based IPRDs will need to store hundreds of millions of records, with several provisioning sources creating a constant stream of updates. 

 

Availability and manageability: The IPRD must integrate into carrier networks, with telephony-grade availability and manageability.

 

In other words, the ENUM protocols may be standardized, but their real-world implementation is not. 

 

Nominum has combined years of research and work with leading carriers, as well as in-depth knowledge and expertise in Internet naming and addressing, to create an ENUM-based IP-application Routing Directory that meets these requirements. Nominum Navitas implements the ENUM standards while addressing the performance, scalability, provisioning and management requirements of production ENUM environments. By doing so, it will help carriers realize interoperable IMS networks through inter-network routing.

           

IMS will achieve mainstream adoption not in one forklift upgrade but in the evolutionary, push-pull fashion that often characterizes technological advancements. And ENUM is likely to play a critical role in this evolution. The need for efficient VoIP-to-VoIP call placement is driving the adoption of more powerful and scalable ENUM-based directories in the short term. 

 

If designed with adequate flexibility and scalability, these directories are able to fill a broad IP application routing function. This takes us one step closer to fulfilling the promise of IMS by supporting interoperability between existing communication networks and sustaining the increased demands of converging communications.