Each telephone number in the North American Numbering Plan (NANP) consists of three parts. There is a three‑digit area code or NPA, followed by a three‑digit prefix or NXX, followed by a four‑digit line number or XXXX. Each prefix is uniquely assigned to a particular rate center (a town or part of a city), and also to a specific operating company. In most cases, both billing and routing use the NPA‑NXX to determine the cost of the call and route the call to the correct switch. Thus, all 10,000 numbers in a given prefix must be in the same rate center and also served by the same operating company. Until quite recently, there was only one operating company in any particular rate center; however, the advent of local telephone competition means that in some cities, each rate center may have dozens of operating companies. Each of those operating companies must have an entire prefix in that rate center, even before it can sign up its first customer. This method of allocating numbers was quite reasonable when local service was a monopoly, but it is alarmingly inefficient in a competitive environment. Indeed, it is this inefficiency — not cellular phones, faxes, modems, or additional lines for homes and small offices — that is the single largest factor driving the explosion of new area codes in the late 1990's. Clearly, the current allocation method must be changed.
There are three major categories into which most number conservation proposals fall:
Rate-center consolidation is self-explanatory: combine two or more adjacent rate centers into one. The number conservation aspect arises from the simple fact that each new competing carrier needs only a single block of numbers to serve the new rate center, instead of a block for each of the old rate centers. However, there is considerable controversy involved in the process. Small towns often bristle at the suggestion of losing their telephonic geographic identity if, for example, Springfield and Shelbyville are combined to form the SPFLDSHLBY rate center (or perhaps a more cryptic compromise name such as MONTYBURNS). In more ambitious rate‑center consolidations, there may be adjustments required to the local calling areas and nearby toll rates, possibly with an increase in the cost of local service to offset the loss of toll revenues. In any case, rate‑center consolidation has limited applicability. In practice, rate centers can be combined only to a certain point.
Number pooling promises greater number conservation benefits. The most common proposal is to allocate blocks of 1,000 numbers to each carrier in a rate center. The entire prefix is still associated with the rate center, in order to preserve billing by area code and prefix alone. However, routing uses an additional digit of the number. Thus, NXX‑1234 and NXX‑2345 would be numbers in the same rate center, but possibly served by different companies. The benefit is clear: a new entrant ties up only one‑tenth as many unused numbers. However, this level of number pooling is only an interim measure, compared to the best solution.
Local Number Portability, or LNP, at its most basic, allows a subscriber to change local carriers while keeping the same telephone number, so long as the subscriber remains in the same rate center. In areas with LNP, every single call requires a database lookup on the full number to determine the correct carrier to handle the call. Under the old system, this information could be determined with only the NPA‑NXX; with number pooling, the routing requires the first seven digits, NPA‑NXX‑Xxxx. However, some LNP schemes still require that each carrier receive at least one entire prefix in each rate center for assignment to new customers. Only existing customers can take their numbers with them; if a customer simply turns off service on a particular number, the carrier still holds that number for reassignment.
At least for the initial implementation, LNP will not affect wireless carriers (cellular, PCS, paging, and other uses), because a number of technical and policy issues need to be resolved. However, the frequency bands for wireless communications are limited, meaning that there can be only a limited number of carriers in a given area. Thus, the benefits of applying LNP to wireless services are less dramatic than for conventional wireline services.
The LNP model advanced by the New York Public Service Commission for the New York City area codes provides that all unused numbers, both never‑used and "recycled" numbers, will be placed in a common pool. Each carrier that is licensed to do business in New York City can draw numbers from that common pool to fill orders from new customers. No carrier needs to — nor is allowed to — sreserve a large block of unused numbers. Number pooling then becomes a step backwards, since LNP means that numbers are allocated to the carriers in blocks of 1 number, rather than 10,000 or even 1,000. Rate‑center consolidation becomes far less important, since there is no longer the gross inefficiency in number allocation caused by requiring large discrete blocks of numbers for each carrier in each rate center.
If LNP is implemented properly, it is the complete solution to the number conservation problem. LNP must apply not only to existing subscribers changing to different local carriers, but also to unused numbers being assigned to new subscribers.
Unfortunately, that model is not being followed in other areas where LNP is being implemented. The basic form of LNP allows only existing subscribers to change carriers. If Telco X has the 234 prefix allocated to it, then an existing customer can change to Telco Y and take her 234‑XXXX number with her, but a new customer who wants a number in the 234 prefix must get it from Telco X, although that customer can then turn around and switch to Telco Y or Telco Z. Further, each new carrier must still be allocated a block of 10,000 numbers (or 1,000 numbers, with number pooling) in each rate center. Worse yet, if an overlay is implemented, the incumbent carrier has an enormous advantage in being able to assign new numbers in the familiar area code, since all subscribers who simply turn off their service return the number to the incumbent carrier's private pool to be reassigned a few months later.
Part of the problem is that, under the current prevailing model of LNP, each carrier's switch is still assigned a unique NPA‑NXX combination that is used for routing purposes in the LNP database. In other words, the LNP database looks up the dialed number NPA‑NXX‑XXXX and determines which operating company serves that number and from which switch, giving the result in the form of an NPA‑NXX routing ID for the required switch. That means that each area code can accommodate a maximum of 792 rate centers, divided by the number of local carriers. If there are 24 local carriers (a modest assumption, in many large metropolitan areas), there can be no more than 33 rate centers per area code. As new carriers are added to the mix, the area code must split, even though there may be millions of unused numbers in the existing area code.
There is only one respect in which this form of LNP achieves any degree of number conservation. Without LNP, an existing customer who changes carriers can only keep the old telephone number by using some form of transparent call forwarding from the old number to the new, meaning that two telephone numbers are tied up even though only one is in active use. The savings from eliminating that cumbersome procedure are far smaller than the savings that could be realized by using the New York City LNP model.
In its best form, Local Number Portability should include the following elements:
If there is a reasonable possibility of more than 7,920 switches in a particular NPA, then one or more of the following measures could be applied:
Each of these measures should be used in preference to effecting an area code split or overlay, unless the overall demand for numbers dictates area code relief. In other words, don't split or overlay just because you're running out of switch identifiers, if you've got plenty of numbers still available.
The LNP scheme outlined above will cost more to implement than the most basic LNP. The portability database will be required not only to do real‑time lookups to determine the routing for call setup, but also to maintain a list of all unused numbers in each rate center and their assignments to end users. There will also be costs associated with auditing and enforcing compliance with the rules proposed here, as well as ongoing administrative costs for hardware and personnel. Those costs will be substantial. There will also be administrative costs in changing the switch identifier to seven digits (NPA‑NXXX) instead of six, and additional costs if that format requires further extension.
However, the benefits to be gained from this scheme are enormous. Area code 847 in northeastern Illinois is currently slated to overlay in April 1999, even though the Illinois Citizens Utility Board estimates that half of the possible numbers in 847 are not in use. Similar stories can be found in almost every state. Whether area code relief comes in the form of a traditional geographic split or an overlay, there are significant costs to individuals and businesses, as well as to carriers themselves. The formula for LNP shown here will ensure that area code relief will be imposed only when it is truly needed, when the area code is experiencing a genuine shortage of available numbers. The substantial savings from avoiding unnecessary area code splits and overlays will offset much of the added cost of the improved LNP scheme. Most especially, this improved LNP scheme may enable the NANP to prolong the life of the current ten‑digit telephone number format, currently projected to run out of numbers in about the year 2025.
The Federal Communications Commission has mandated LNP, with a schedule for implementation, beginning in the largest metropolitan areas. Presumably the FCC would also be within its authority to specify additional requirements for LNP. If the FCC fails to take such action, it would also be within the purview of the individual state public utilities commissions, since the FCC guidelines provide only a set of minimum requirements.
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