Aswath Weblog

On Number Portability

In the aftermath of the devastation caused by Hurricane Katrina, Stuart Henshall proposed that displaced citizens of the affected area can be reconnected to the communications network, if their telephone numbers are “virtualized” and rerouted to them using VoIP technology. Being a splendid idea, it was immediately seconded by many influential bloggers like Jeff Pulver, Tom Evslin and others. The purpose of this note is to analyze what it takes to achieve this objective and to point out potential long term implications if we take this proposal to the limit.

I suspect that “virtualizing” a number is derived from “virtual number” service offered by many VoIP service providers. With this service, a VoIP subscriber residing in a geographical area can be reached via a number normally allocated in a different geographical area. So, in this case, the residents of Gulf coast can receive calls directed to their old number will be handed over to their number in the area where they have relocated to. Most people had tacitly assumed that this will be done by a VoIP service provider. But if you look at the technical details, it could be done by PSTN operators as well, because the fundamental technology is Number Portability.

With Number Portability, calls terminating at a switch can be redirected to another switch. Due to PSTN’s peculiar rate structure, the application of Number Portability has been restricted to only those cases where both the switches are in the same rate center. (This is called Local Number Portability.) In the case of Katrina, Stuart’s proposal would not have helped because it is very likely that all the switches in the affected area would have been in disrepair. Accordingly, FCC in its emergency ruling suspended the “local” restriction and said the number could be ported to any location. So conceivably, BellSouth could have assigned an alternate switch for all its customers. This is my first observation, but not the main one.

Due to the “local” restriction, the needed Number Portability query is postponed to the last possible switch and it is called “N-1 query”. But if we remove the “local” restriction, then it is better to perform the query as early as possible, what I call “1+ query”. On top of that if we add a further enhancement, that of the response to a query to be dependent on the source of the query. Then there is no more need get a virtual number. Instead the VoIP provider just needs to route the call to the nearest gateway. Users can derive more features along this line. If only …