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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 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. 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 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. Reluctant solution

To alleviate complaints about such changes in 1992 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. New York was the first area to employ this solution using the 917 area code in addition to the older 212. 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. 10-digit dialing is not a technical requirement but a U.S. Federal Communications Commission (FCC) mandate to promote fairness among telephone companies.1 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 Maryland and Texas have also used overlays heavily and are (or soon will be) completely or mostly overlaid. In California on the other hand only four of 27 areas have overlays and all were very recently implemented.

The first example of an entire state previously only served by a single code being overlaid was in West Virginia.

Overlays have also become more popular than split plans in Canada during the previous decade. All of British Columbia and Alberta and large parts of Ontario and Quebec are currently overlaid. By 2013 all of Quebec all of Manitoba and almost all of Ontario will be overlaid as well. Types of overlays The North American Numbering Plan Administration recognizes different forms of overlays: Distributed overlay (or all-services overlay): an entire existing area gains another area code serving the entire area. Most overlays are of this kind. Single concentrated overlay: only the high-growth portion of an existing area gains a second area code. Multiple concentrated overlay: the entire existing area gains multiple additional area codes each of which serves a different subsection of the original. There are no known examples of such being implemented in the NANPA. Multiple-area distributed overlay: two or more area codes gain a single new area code covering such an area. Examples include 872 in Chicago Illinois (over 312 and 773) and 587 in Alberta (over 403 and 780). Boundary-extension overlay: a neighboring area code (either an overlay code or single primary area code) is expanded to serve the area as well. Examples include 321 over 407 in central Florida and 778 over 250 in British Columbia. 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. See also List of area code overlays Interexchange carrier Telephone exchange Telephone numbering plan North American Numbering Plan Exhaust date References FCC. "Area Codes: Frequently Asked Questions". Archived from the original on May 11 2009. http://web.archive.org/web/20060615015544/http://www.fcc.gov/Bureaus/CommonCarrier/Factsheets/areacode.html. Retrieved 2006-06-22.  External links FCC FAQ concerning area codes at the Wayback Machine. FCC FAQ - Carrier Identification Codes (CICs) and Seven-Digit Carrier Access Code (CAC) Dialing