Local number portability

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Local number portability (LNP) is both a regulatory and technical issue concerning the flexibility of phone number assignments. It specifically refers to the ability for a subscriber to take a telephone number assigned to them by a particular local exchange carrier (LEC) and transfer it to a different LEC when they change service providers. A mobile variation of LNP is called full mobile number portability (FMNP).

The Telecommunications Act of 1996 (TA96) did much more than allow LECs and IECs into each others’ markets. By requiring incumbent LECs (ILEC) to show that local competition exists before offering long distance services in a given market, TA96 has indirectly required LNP. Local number portability will allow local subscribers to change from an ILEC to a competitive LEC (CLEC) and keep their telephone numbers.

This has several benefits to the customer and network alike. The customer does not have to worry about changing forms and other paperwork. The network directories keep the same information as before. Unfortunately, a subscriber’s telephone number does more than just identify the customer. The telephone number locates the subscriber within the public switched telephone network (PSTN) for call routing purposes. The entire North American Numbering Council (NANC) system operates on the numbering plan area (NPA), NXX (end office code), XXXX (access line code) structure. The combination of NPA and NXX codes locates any subscriber in the country within a few miles (e.g., 802-555 is in Vermont).

Local Number Portability

Without local number portability, all of the software in switching offices, ILECs, CLECs, and IECs alike, would have to be changed to route the call properly. The FCC has specified that the implementation of LNP will be the local routing number (LRN) technique. The regional Bell operating companies (RBOC) favored a method called Query on Release in which calls would be routed to the switch first assigned an NXX code and then interrogate a database for routing instructions only if the call could not be completed.

LRN, however, requires that once a switch has been LNP-enabled and at least one telephone number in an NXX has been ported to another carrier, a call to any number in that NXX causes a “database dip” to determine how the call should be routed. The visual illustrates the method. A caller dials the directory number (DN) of the customer they wish to reach. If the network determines that the call is to an NXX that is LNP-enabled, a packet is sent to an SCP database in the SS7 network to determine proper routing. The database returns the address of the central office that now hosts the number and a call setup sequence is initiated to complete the call.

Most ILECs have implemented LNP such that once an office is enabled, calls to all numbers in the office undergo the database dip regardless of whether any numbers have actually been ported.

In 2003, LNP was extended to include wireless telephones. This change allows wireless customers to change carriers and keep their cell phone number. In addition, landline customers can port their numbers to wireless phones, an option many people have elected to take. These factors add to the growth in wireless and the continued decline in landline subscribers.


<mp3>http://podcast.hill-vt.com/podsnacks/2007q2/lnp.mp3%7Cdownload</mp3> | Local number portability (LNP)