Overlay plan

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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.

1 Methodology
2 Types of overlays
3 Number pool management
4 References
5 See also
6 External links

[edit] Methodology

Prior to the introduction of overlay plans, the method of introducing new NPAs (area codes) in an area was to divide the existing area of one NPA into two (or more) pieces, allowing the more established or developed section to retain the original NPA, and changing the numbering space of the other section(s) to have a new NPA. For example, the original NPA for the entire state of Washington was 206; today 206 only applies to 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 NPAs in those areas 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 NPA 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 NPAs. In this model, one fixed geographic area would concurrently have multiple valid NPAs throughout. This plan's main benefit, which addressed much of the issues causing resistance to split plans, was that all existing phone numbers remained unaffected by the new NPA. 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] Types of overlays

The NANPA recognizes different forms of overlays:

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

[edit] Number pool management

The persistent unpopularity of new NPA 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 NPA growth, but not completely. For example, the Western Washington area narrowly avoided needing an overlay NPA in 2001. Area code 564, originally planned for introduction in October 2001, was cancelled in August 2001 after state regulators determined that the existing number pool had begun to be used more efficiently.