Local Number Portability

Posted: 01/1998

By Patt Rice

Who said you can’t take it with you? You can now.

Well, at least your telephone number. For the last several months–in the face of
emerging local competition–the telecommunications industry has been devising an automated
means of allowing local subscribers to change carriers while keeping their telephone
number. This service is referred to as local number portability (LNP).

Although great for competition, LNP introduces significant technical challenges. The
technological impact is felt in two ways: the administration of numbers from one carrier
to another and call processing from one switching center to another. In either case, these
challenges represent a major investment by the carrier.

First, let’s look at the administrative changes. Today, carriers are assigned numbers
in 10,000-number blocks by the North American Numbering Council (NANC). The numbers then
are distributed across various switching centers in a calling area. The carriers typically
have a service order administration center in these calling areas which, through the use
of coordinated computer operational support systems (OSS), assign numbers to their
customers’ equipment and activate their service. These OSS typically interact with other
billing, plant record and engineering systems that require notification of a service
event.

Historically, these computer systems are proprietary "closed systems." This
is understandable because, until LNP, the OSS have assumed that all the numbers in the
assigned blocks belong to their network. Now these systems must determine and account for
numbers within the block that have been moved to another network. Also prior to LNP,
hardly ever was there a need to coordinate subscriber administration with other carriers.
So, the proprietary nature of the deployed OSS makes communications with other carrier OSS
virtually impossible. Even if there is no communication issue, the administrative
processes that vary from company to company offer another level of complexity.

To overcome these communications and coordination obstacles and to introduce a fair and
equitable solution, the industry has adopted regional neutral administrators, called
Number Portability Administration Centers (NPACs). There are seven NPACs in the United
States, one per regional Bell operating company (RBOC) area. The NPACs are managed under
contract by an independent company that is not in business as a telecommunications
carrier. The independent company is selected for a contract period by a limited liability
corporation made up of interexchange carriers (IXCs), competitive local exchange carriers
(CLECs) and incumbent LECs in each region. Currently, Lockheed-Martin has been selected to
provide service and management for the Ameritech, Bell Atlantic, NYNEX and US WEST
regions, while Perot Systems has been selected to provide service and management for the
BellSouth, Pacific Telesis and SBC regions.

The role of the NPAC is to receive number porting requests from various carriers
through a standard formatted interface. NPAC then coordinates the number exchange in an
automated and timely fashion between the two carriers. The NPAC also has the
responsibility to maintain a master database for the assigned region.

To perform this function, two new OSS have been introduced. The local service order
administration (LSOA) system and the local service management system (LSMS) now are
required by carriers. The LSOA provides an industry-standard computer interface from the
existing service order OSS to the NPAC regional service order administration (RSOA). The
LSOA also maintains a record of changes and the status of each port request. The LSMS
communicates to the NPAC using the same industry-standard interface as the LSOA system.
This provides the ability to download ported numbers from the NPAC system to the LNP
database deployed in each carrier’s network. The LNP database can reside in a service
control point (SCP), or it may be integrated into a signaling transfer point (STP). Both
methods utilize signaling system 7 (SS7) STPs. The LSMS and the LNP database typically are
purchased from the same supplier because the interface from the LSMS to the LNP database
has not yet been standardized.

With the LSOA and LSMS in place, a request for a number to be ported comes in from a
competing carrier’s LSOA to the NPAC. The NPAC then communicates to the other carrier,
identifying the requested number. Once the NPAC coordinates between the LSOA systems, it
records the port information in the master database and downloads the ported number and
associated information to both carriers’ LSMS. Each LSMS then updates its respective LNP
database. To maintain LNP data synchronization throughout the network, each carrier has
the ability to audit its database contents with the NPAC’s master database through the
LSMS (Figure 1).

Not only does LNP add complexity to the administration of numbers, it also adds a
significant burden to the existing SS7 networks. The implementation of LNP requires that
an NPA-NXX is considered portable when at least one number is ported within that
10,000-number block of numbers. Once the NPA-NXX is ported, every interswitch call
terminating on that NPA-NXX will require access to the LNP database to determine if the
number is ported. Access to the LNP database is performed by the next-to-last carrier
(referred to as the "N-1" carrier) to complete the call.

To gain an appreciation of these myriad changes, following is a high-level review of
the steps involved in basic call setup. This example consists of a subscriber who
originates a call that terminates in another switching center (i.e., an interswitch call)
where the NPA-NXX is considered to be ported:

1. When the calling party’s switch determines that the NPA-NXX dialed is ported, a
database query is sent to the N-1 carrier’s database to determine if the dialed number has
been ported.

2. If the number has been ported, the database returns an LRN to the calling party’s
switch. The LRN is an address representation of the switch to which the number has been
ported. The switch then will complete translations and route the call to the switch
serving the ported customer.

3. If the number has not been ported, the database will return the information request,
and the calling party’s switch will translate and route the calling party’s dialed digits
(Figure 2).

Under these new procedures, when LNP is deployed in the top 100 metropolitan service
areas (MSAs), and when porting is fully underway, it is conceivable that the majority of
all interswitch calls will require an LNP database hookup. This will have a tremendous
impact–in terms of capacity–on the existing SS7 networks for both signaling facilities
and network elements. Carriers will have to monitor these developments on their own
signaling networks to ensure they maintain the integrity of their service offerings when
they implement LNP.

Patt Rice is senior manager, national accounts for Morrisville, N.C.-based Tekelec,
a supplier of diagnostic systems and switching solutions for the telecommunications
industry.