North American Numbering Plan
From Hill2dot0
The international numbering plan defined by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) is defined in E.164. It divides the world into nine geographic zones. For example, World Zone 1 includes all of North America except Mexico. Within a zone, the ITU-T assigns country codes that are one, two, or three digits. For our country, the North American Numbering Plan (NANP) defines a nationally significant number (NSN). AT&T administered the NANP beginning in 1947 and Telcordia began administering the plan after divestiture. The current administrator is NeuStar.
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Structure of the NANP Telephone Number
The NANP is a hierarchical scheme that uses ten digits, divided into three basic fields:
- The first three digits define the numbering plan area (NPA), also known as the area code. Prior to 1995, the form of the NPA was N 0/1 X where N was a digit in the range 2–9 and X was a digit in the range 0–9. The middle digit was 0 or 1. This limited the number of NPAs to 8x2x10, or 160. The modern NXX scheme (see below) yields 800 theoretical codes; however only 675 are usable since 125 of the codes (such as 800, 888, 877, and 900) are reserved for special services.
- The next part of the number is the central office code, also known as the exchange code. This also takes the NXX form, where N and X are as defined above. This allows 8x10x10, or 800 central office codes, of which only 792 are usable. The reason is that the N11 codes (e.g., 411) are reserved for services such as directory assistance.
- The last part of the number is the subscriber line code, also known as the line number. It is four digits long and in the range 0000–9999. This means that 10,000 numbers are available within a specific central office code. Some central office switches serve far more than 10,000 subscriber lines and will therefore have several assigned central office codes. Others might serve only a few hundred subscribers. In the latter case, an entire central office code is usually allocated to the switch even though only a few line numbers will be used. Historically, it was rare to split a central office code over two switches. By avoiding such splits, routing of calls to the proper switch can be performed using only the NPA or NPA-NXX codes. since the Telecommunications Act of 1996 (TA96), it is possible for several LECs to provide services out of the same switch. The individual LECs are often given individual central office codes. To prevent a significant waste of subscriber lines for CLECs that have only a few hundred subscribers, number pooling makes it possible to allocate subscriber line numbers in blocks of 1,000 numbers instead of 10,000 numbers (see below).
Changes in the Area Code
Explosive growth in telephones for cellular and fax use, and for other business purposes, caused the last available NPA codes to be assigned well before the intended year 2000, as originally anticipated. Consequently, as of January 1, 1995, the form of the NPA code changed to NXX; thus, instead of only 160 area codes, there are now 800. The first of the area codes with the new structure were used in early 1995 in Alabama, Southern California, and Chicago. One problem with the assignment of new codes is that if a geographic split is made, many customers must be assigned the new area code. While this is a nuisance for residential subscribers, it can be costly for businesses both in terms of changed stationery and business cards, and in terms of lost revenue. Therefore, some states have required that new area codes be assigned on an overlay basis. That is, a new area code will coexist in an area with an existing code. Providers of new services do not like this solution because they believe it discriminates against a particular carrier by forcing only the new carrier’s customers to have the new code.
A significant effect of the new NPA format is that dialing of 1+7 digits for home NPA toll calls is no longer permissible. States decide whether calls within a local calling area are dialed as 7 or 10 digits without the 1+ prefix, or as 1+10 digits for toll calls. In other words, 1+ indicates that the call is a toll call. This is a transitional plan until a suitable alternative toll indicator is found. In other states they have decided to use the prefix “1” to indicate a 10-digit number follows. So home NPA toll calls are dialed using just the seven-digit number, and foreign NPA local calls are dialed as “1” followed by the ten-digit number.
Special Numbers
We are all familiar with special service codes such as 411 for local directory assistance and 611 for repair service. 911 is defined as the universal emergency number. These numbers are generically referred to as N11. They include the following:
- 211 Community information and referral services
- 311 Non-emergency police and other governmental services (US)
- 411 Local directory assistance
- 511 Traffic and transportation information (US)
- 611 Repair service
- 711 Telecommunications relay service (TRS)
- 811 Business office
- 911 Emergency
In addition to these there are also a number of N00 codes called service access codes. The most familiar of these is the 800 code used to denote toll-free calling, where the called party, rather than the calling party, is charged for the call. This code is an important marketing tool for businesses that allows customers to call toll-free to place orders, obtain customer assistance, etc. Within an NPA there can only be 800 NXX codes and within an NXX there are only 10,000 line numbers. However the 800-855 line numbers, in the format 800-855-XXXX, are used to access telephone network services for the deaf, hard of hearing, or speech impaired. The explosive growth of 800 service brought about the imminent depletion of 800 numbers in the late 1990s. Consequently, Telcordia identified the additional codes for use with toll-free services upon the exhaustion of 800 numbers. The 888 code for toll-free service was introduced in March 1996 and the 877 code was introduced in January 1997. A fourth toll-free calling code (866) went into effect July 29, 2000. The next toll-free code scheduled for service is 855, which was scheduled for November 18, 2000, but has been delayed. The 844, 833, and 822 codes have been assigned for use as toll free codes, and will be introduced as needed.
The introduction of personal communications services (PCS) opens up the opportunity for people to have a single number for telephone, fax, paging, and other services. This number will follow the person everywhere just as a cellular customer can be reached while roaming. Telcordia has already begun assigning blocks of numbers to carriers for PCS use from the 500 Service Access Code. Although, based upon the current assignment rate, the North American Numbering Plan Administration estimates the supply of 500-NXX codes should last for the next 20 years.
900 numbers are used for premium services. The cost of each 900 call is billed to the calling party.
Running Out of Numbers
While it may be hard to believe, the fact is that we are running out of available numbers in the NANP. The conversion to interchangeable area codes and central office codes has increased the supply of both however, the rate of consumption of codes has increased significantly and threatens the scheme with exhaust in the foreseeable future.
As of the end of 2006, 322 area codes were in use, which is only a minor increase from the 314 area codes in use at the end of 2003. With that many assigned from a pool of 675, one would think they would last indefinitely. Indeed, examination of recent history shows that only nine new Area Codes were assigned between the end of 1984 and 1994. However, in 2003 alone three new area codes were assigned, and assignment is again on the ascent. This increase in the rate of consumption sets off alarm bells. In fact the North American Numbering Plan Administration (NANPA) estimates that at the current rate of telephone number assignment, roughly 7000 per year, the projected exhaustion date will be around 2037.
The obvious question is what causes such an increase in the need for telephone numbers? Historically the answer lay in the increased use of fax machines, [[cellular telephone]s, and second lines at home for Internet access. However, the demand for fax machines and second lines has dropped of significantly with the increased penetration of broadband Internet services like DSL and cable modem-based services. Replacing them, however, is an increased set of companies providing voice services over the Internet, the so-called Internet telephone service providers (ITSP). Another factor is the need for numbers by competitive local exchange carriers or CLECs.
Number Pooling
We have historically assigned numbers in blocks of 10,000. That is, we have assigned a central office code for a new switch and that code is capable of supporting 10,000 line numbers even though the switch to which it is assigned may serve a small area with few subscribers. The effect is to have considerable waste in the numbering plan. However, recall we did not want to split a central office code over more than one switch because we use the code for routing to a particular switch. Add to that the fact that a CLEC that wishes to offer service throughout an NPA with fifty rate centers typically wants at least one central office code for each rate center even though all those codes will not be used initially. With a maximum of 792 central office codes in an NPA, as few as 16 CLECs could exhaust the supply of central office codes. When this happens, then the NPA must be split or overlaid with a new area code.
Clearly this situation forces us to seek a solution that delays the exhaust of the supply of area codes. One such solution that is number pooling. This is the term applied to the assignment of numbers in blocks of 1,000 rather than 10,000. This effectively means that the same central office code could be assigned to multiple switches owned by multiple carriers. What makes this possible is the implementation of local number portability (LNP). LNP forces the implementation of an infrastructure that can look up individual line numbers and route to a specific switch based on the carrier serving that customer. This same infrastructure could be used to support number pooling. If we eventually deplete all of our usable telephone numbers under the existing numbering plan then a new plan would need to be created by adding one or more digits to the current ten-digit dialing pattern. The FCC estimates that this could cost between $50 and $150 billion to implement and approximately ten years to complete the expansion.
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