In telephony, especially in North America, an overlay plan is the practice of introducing a new area code by applying it onto a geographic area that is already occupied by one or more existing area codes, resulting in two (or more) area codes serving the same area.

Contents 1 Methodology 1.1 Rapid growth 1.2 Reluctant solution 1.3 Compromise 1.4 Popularity 2 Types of overlays 3 Number pool management 4 See also 5 References 6 External links

[edit] Methodology

Prior to the introduction of overlay plans, the method of introducing new area codes (also known as numbering plan areas or NPAs) in a region was to divide the existing territory of one area code into two (or more) pieces, allowing the more established or developed section to retain the original area code, and changing the numbering space of the other section(s) to have a new area code. For example, the original area code for the entire state of Washington was 206; today 206 applies to only the city of Seattle and immediate vicinity. This practice became known as a split plan.

[edit] Rapid growth

Urban sprawl accelerated the rate of expansion of metropolitan areas, and multiple split plans have caused the geographical area of a given area code in those regions to shrink. Also, the rapid growth in popularity of mobile phones, in addition to regular land line growth, has increased demand for new phone numbers even more. The practice of instituting an area code split has become much more costly because of various business or administrative inconveniences to customers, such as changes in stationery, advertising, and otherwise communicating and remembering the number change among friends and customers.

The rise in popularity of mobile phones has added to the pressure against split plans, as an area code change affecting the exchange in which a cell phone is based requires the wireless carrier to reassign the number of every device based in those areas.

[edit] Reluctant solution

To alleviate complaints about such changes, in the late 1990s, the telecommunication industry began to introduce "overlay plans" as a means to introduce new area codes. In this model, one fixed geographic area would concurrently have multiple valid area codes throughout. This plan's main benefit, which addressed many of the issues causing resistance to split plans, was that all existing phone numbers remained unaffected by the new area code. Newly-assigned numbers in the overlay plan areas would have the new area code. As a result, two telephones, located next to each other in the same building or house, could have different area codes.

[edit] Compromise

However, overlay plans introduced a new inconvenience: mandatory 10-digit dialing (i.e. the area code must be included), even for local calls, in the affected area. This and the difficulty of remembering all the area codes in a geographic area made overlay plans only marginally less unpopular than split plans. It should be noted that 10-digit dialing is not a technical requirement but an FCC mandate to promote fairness among telephone companies.^[1]

[edit] Popularity

Overlay plans have been used much more widely in some areas than others. For example, the northern third of Ohio is covered by two large overlay complexes, as is northern Georgia. Connecticut, Illinois, Oregon, New Jersey, Massachusetts, and Maryland have also used overlays heavily, and are (or soon will be) completely or mostly overlaid. In California, about one third of the state, mostly in the southern California is overlaid, despite the many splits that have resulted in tiny divisions. The first example of an entire state previously only served by a single code being overlaid was in West Virginia.

Overlays have also generally been more popular in Canada, with all of British Columbia and Alberta and large portions of Ontario and Quebec overlaid by the end of 2008.

[edit] Types of overlays

The North American Numbering Plan Administration recognizes different forms of overlays:

• Distributed overlay - in which an entire existing area gains another area code serving the entire area.
• Single concentrated overlay - in which only the high-growth portion of an existing area gains a second area code.
• Multiple concentrated overlay - in which the entire existing area gains multiple additional area codes, each of which serves a different subsection of the original.

Some overlay plans not fitting the above forms have been noted:^[2]

• Two or more existing area codes collectively gain a single overlay area covering the entire area. Washington's planned 564 would have fit this description, as would the plans to introduce 587 in Alberta and expand 778 in British Columbia from a concentrated overlay of 604 to simultaneously overlay 604 and 250
• A area code is extended to overlay on top of a neighboring area code. Florida's 321 was overlaid in this way onto 407, from which it had originally split.

[edit] Number pool management

The persistent unpopularity of new area code creation, whether by split or overlay plans, led to a change in the rules of number block allocation, in order to conserve the pool of available phone numbers. This change, which allowed for the assignment of smaller number blocks, is commonly known as number pooling. This has noticeably slowed the need for area code growth, but not completely. For example, the Western Washington area narrowly avoided needing an overlay in 2001. Area code 564, originally planned for introduction in October 2001, was canceled in August 2001 after state regulators determined that the existing number pool had begun to be used more efficiently.

[edit] See also

• List of area code overlays
• Interexchange carrier
• Telephone exchange
• Telephone numbering plan
• North American Numbering Plan
• Exhaust date

[edit] References

[edit] External links

• FCC FAQ concerning area codes at the Internet Archive
• FCC FAQ - Carrier Identification Codes (CICs) and Seven-Digit Carrier Access Code (CAC) Dialing