There are three phases to Local Number Portability. The first phase, service provider portability, is being implemented now. This allows a subscriber to select a new local service provider while keeping their existing telephone number. It also allows a subscriber to move within their rate center while maintaining the same telephone number.

The next phase of Local Number Portability is service portability. This allows subscribers to change the type of service they have while keeping their telephone numbers. For example, if a subscriber changes from a POTS line to an ISDN service, he or she has to obtain a new telephone number, because the switching equipment used to provide the ISDN service supports a different block of numbers. With LNP, the subscriber does not have to give up the telephone number when changing the type of service.

The most difficult phase of Local Number Portability is location portability. This will allow a subscriber to move from city to city, or even state to state, while maintaining the same telephone number. This has a much more global impact. Even subscribers are accustomed to associating geographical areas with telephone numbers. It will be difficult for anyone to determine where he or she is calling once location portability is implemented.

Currently, porting is only supported within a rate center. A rate center is a geographic area, usually within a LATA, which is used for determining the time and distance used in billing of phone calls. Rate centers are determined by using (V)ertical and (H)orizontal coordinates. Porting a number outside of a rate center will present many technological challenges in itself, and is being addressed in later implementations of number portability.

The Telecommunications Act of 1996 and the FCC mandate (docket 95-116) do not specify exactly how LNP is to be implemented. They simply outline the ground rules to be used when implement it in the network. The first trial of LNP took place in Illinois, under the direction of the Illinois Commerce Commission (ICC). The ICC specified some rules regarding the actual implementation, such as using a Location Routing Number (LRN). These have been published in the AT&#amp;T specification FSD 30-12-0001. These implementation requirements have become the de facto standard for LNP and have been widely accepted throughout the industry.

Copied with permission,McGraw-Hill Telecommunications from the book Signaling System #7, Second Edition by Travis Russell, 1998 McGraw-Hill Telecommunications, pages 466-467.