Number Portability in the IMS World

By Ronald Gruia

This year’s edition of Futurecom (LatAm’s largest telco show, held in São Paulo, Brazil) provided yet again a good perspective on how that dynamic marketplace is evolving as it transitions to the NGN. One of the hot topics discussed at the show was NP (Number Portability), or the mechanism that allows fixed and mobile subscribers to preserve their telephone numbers when changing service providers, physical locations or types of service.

In Brazil, the first stage of NP (as mandated by the local regulator Anatel) went into effect at the end of August 2008. Brazil represents the second largest implementation after the US, benefitting more than 170 million Brazilian fixed and mobile subscribers. From a Latin American standpoint, other countries such as Chile, Colombia, Ecuador and Peru will soon follow suit, as NP gets mandated within the next couple of years.

Traditionally speaking, number portability was based on IN nodes. For mobile portability, the Mobile Subscriber ISDN (MSISDN) number is checked in the NPDB (Number Portability Data Base), and then a prefix is added (the RN or Routing Number, which is associated with a geographical or telephone number that has been ported out from a donor carrier to another carrier). Any node in core wireless network is responsible to perform the MSISDN check in the NPDB if it has not been done yet. The NPDB is typically populated based on regulator requirements.In the IMS domain, number portability can be viewed from a different perspective, moving from a traditional service-specific numbering to a technology-neutral “identity” management system; i.e., ENUM. (For more on ENUM, visit www.tmcnet.com/2591.1) A common numbering solution such as ENUM can facilitate the porting of numbers between fixed and mobile networks, albeit it does not address other issues such as tariff transparency.

In the future, with IMS, it is conceivable that users will be dialing with the tel URI for telephone numbers (as specified in IETF RFC 3966), which is translated by the CSCF to a SIP URI. The SIP URI would encompass the domain name, which can then direct the call to a specific operator’s I-CSCF. So at least in theory, a futuristic IMS-only implementation of NP would simply involve a change in the respective entry in the DNS/ENUM system. However, the actual process is more involved, as ENUM requires other aspects: it has to know how to translate the number (number @domain) and also to work with the DNS configuration in order to determine what that means in terms of IP address and which type of domain it is routed to. For the case of legacy PSTN users, the process would be more involved, as it would also need to involve a check with the NP database.

Obviously, SIP routing works differently from SS7 routing, since each mechanism is intrinsically distinct, but NP is another example of the flexibility of the SIP/IMS approach than the legacy IN/SS7 modus operandi. That said in the IMS domain subscribers usually have more choice as to which provider they connect to.

Key Takeaways
NP is about being reachable at both a new and previous operator(s) via a given number. Today, interconnect with other operators is typically still carried out in the “old fashioned” way by relying on SS7, which essentially means that NP is realized through IN databases. ENUM for NP only comes into play when interconnection becomes IP-based. An IMS deployment will have an ENUM database, but this would typically only include “on-net” destinations – i.e., numbers on the same IMS network, all handled by the same operator.

While many expect that with the ongoing NGN migration, NP should become easier in the future (since SIP is designed around domain names), the practical realities suggest otherwise. IMS was still devised from the ground up to preserve some classic business models where the TNs (telephone numbers) were the key element for billing and routing purposes. As a result, SIP implementations of IMS network elements have to be tweaked in order to follow these older models. For instance, we still will not find operators wishing to charge yearly domain registration fees rather than monthly minute termination fees. Since voice is usually the initial “IMS app” deployed and users interact with voice services via the legacy 12 button numeric keypad interface to dial numbers, it will be practically impossible to avoid the preservation of telephone numbers within the IMS architecture, at least for the foreseeable future. Telephone numbers are not only imperative for billing and routing purposes, but also an integral part of the end-user experience, and it will take a long time before the status quo changes.

SIP is a building foundation of IMS, but the fact remains that IMS limits some of the inherent flexibility of native SIP, including full support of domain names, URI schemes and topology flexibility. Reasons for this limitation include:

1. Standardization approach was wrong from the start, as it was conceived from a “walled garden” or “gated community” perspective (while SIP was built from the ground up as a fully “open environment”). IMS also attempted to predefine services and service interactions, which runs contrary to the Web 2.0 mashup service creation philosophy.

2. Emergence of proprietary SIP implementations: Network Equipment Vendors (NEVs) started creating their own SIP extensions and other proprietary features in order to better differentiate themselves from their competitors. This is almost a repeat of the situation in the 90’s with various IN standards and implementations.

The bottom line is that a whole array of operational management issues has been typically ignored by the NEVs. The above issues will obviously impact the management of services such as NP, and the arsenal of options available to operators for such offerings. The situation will be dramatically improved if NEVs stick more closely with the original concept of SIP and focus more on helping service providers to manage the service and network.

Ronald Gruia is Program Leader and Senior Strategic Analyst at Frost & Sullivan covering Emerging Communications Solutions. Reach him at [email protected].

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