Overlay complex

Intelecommunications,an area codeoverlay complexis atelephone numbering planthat assigns multiplearea codesto the same geographicnumbering plan area(NPA). Area code overlays are implemented in territories of theNorth American Numbering Plan(NANP) to mitigate exhaustion of central office codes in growth areas. The method has been in use since 1992, and has been the exclusive method of area code relief since 2007.

History

From the inception of the North American Numbering Plan in 1947 until 1992, the only method of introducing new area codes was thearea code split.It divided an existing numbering plan area (NPA) into multiple regions, one of which retained the existing area code. The other parts were assigned a new NPA code each. The retaining area was usually the historically more established or developed place, and required no numbering changes. It gained numbering capacity by acquiring thecentral office codesof the other areas, because those were delegated to a new area code in the process.The existing central office codes in the separated areas are maintained in the new numbering plan area, so that the only change for existing subscribers is the change of area code. Thus, only area code references, where present for long-distance calling, required updating in directories, letterheads and business cards.

With the proliferation ofelectronic central office switchingstarting in the 1980s, it became possible to implement another method for expanding numbering plan resources, the area codeoverlay.In an overlay numbering plan, also calledoverlay complex,an additional area code is assigned to an existing numbering plan area, thereby avoiding the need for existing customers in the reassigned regions to change telephone numbers. The first use of that solution was inNew York Cityin 1992, whenarea code 917was overlaid to two numbering plan areas,Manhattan'sarea code 212,andarea code 718in the outer boroughs.

Methodology

Most implementations of the overlay method have required ten-digit dialing, by including the area code, for all subscribers for all calling destinations, local or long-distance.

Exceptions

Near numbering plan area boundaries, special situations arose at times when established communities extended on both sides of the boundary. To preserve local community integrity, identity, and convenience, a party could dial an office prefix for a local call with only seven-digits, but the destination was technically located in the adjacent number plan area. If a subscriber called a distant office prefix with the same area code, the call would require the dialing prefix1before the seven- or ten-digit number.

A similar practice was implemented on a large scale in the metropolitan area ofWashington, DC,and its suburbs inMarylandandVirginia.Most of the Washington region's inner ring is a single local calling area, even though it is split between three area codes–the District'sarea code 202,Maryland'sarea code 301andNorthern Virginia'sarea code 703.For most of the second half of the 20th century, it was possible to dial any number in the metro area with just seven digits. The entire area implemented a system ofcentral office code protectionin which no central office code was duplicated in the entire area. Each existing central office code was properly routed with each area code in the region so that each telephone number in the region could be dialed with any of the regional area codes, giving the appearance of an overlay plan. One side effect of this scheme was that if a number was in use in any portion of the Washington area, the corresponding number in Maryland or Virginia could only be used in an area considered a safe distance from the metro area, such as western Virginia or theEastern Shore of Maryland.This system was ended in 1991 due to an impending number shortage on both sides of thePotomac River.

A similar method was employed inCanada's capital,Ottawa.It shared the same calling area asHull,its twin city inQuebec,even though Ottawa was inarea code 613and Hull was inarea code 819.Until 2006, it was possible to place a call between Ottawa and Hull with only seven digits. That continued even after Hull was merged into the larger city ofGatineauin 2002. That was implemented in a way that the same number could not be duplicated anywhere in easternOntarioor western Quebec, even in areas a safe distance from the Ottawa area. However, Canada's inefficient number allocation system and the proliferation of cellphones brought 819 to the brink of exhaustion by the turn of the century. The only available numbers in 819 could have theoretically been used in the former Hull, but they could not be issued without ending seven-digit dialing between Ottawa and Hull. As was the case in Washington, the only way to free up numbers was to end seven-digit dialing in the area, which was done in 2006. The sole legacy of the old system is a "dual dialability" system for federal government numbers on both sides of the provincial border; all federal government offices on the Quebec side duplicated several exchanges worth of their counterparts on the Ontario side.

Rapid growth

Urban sprawlaccelerated the rate of expansion ofmetropolitan areas,and multiple split plans have caused the geographical area of a given area code in those regions to shrink, except in countries that assign shorter area codes and longer local numbers in areas with highpopulation densities.The rapid growth in popularity of electronic devices (pagersand thenmobile phones), in addition to regularlandlinegrowth, increased demand for new phone numbers even more. Although landline growth has sharply dropped and even decreased, largely by the elimination of residential landlines in favor of personal mobile phones, it has been replaced by the even worse problem ofdata-only devices (hotspots,modems,netbooks,and especially populartablets), which still require a telephone number for use oncellular networkseven if they are unable to make or receive regularcalls.

The rise in popularity of mobile devices has added to the pressure against split plans, as an area-code change affecting the exchange in which a cellphone is based not only forces customers to reprogram their phones but also requires thewireless carrierto reassign the number of every device based in those areas. While phones with aSIM cardhave their ownmobile device numberupdated automatically,there is still the inconvenience of users having to update theircontact listsfor local family members or others who have had their numbers changed too.

Compromise

Most overlay plans introduced the inconvenience of mandatoryten-digit dialingin which the area code must always be included even forlocal calls.Ten-digit dialing is not a technically necessary requirement, and 917 was initially deployed without it, but a USFederal Communications Commission(FCC) mandate instituted it at the demand ofmajor telephone companies,to whom an overlay is considered adisadvantagetocompetitors.In Canada, ten-digit dialing is also a requirement of theCanadian Radio-television and Telecommunications Commissionin overlay area codes.

Popularity

Overlays were initially met with resistance, as they result in multiple area codes in the same geographic area, which requires ten-digit dialing. In many cases, such as847inChicago's northwestern suburbs and212inNew York City,an overlay was an additional disruption to a community that had already been subject to one or multiple code splits, encountering pushback from state regulators or consumer groups. However, overlay plans were eventually used much more widely in some areas than others. For example, much ofOhiohas large overlay complexes, as has northernGeorgiaand theNorth CarolinaPiedmont.Connecticut,Illinois,Oregon,New Jersey,Massachusetts,Maryland,Pennsylvania,Texas,Virginiaand, more recently,Californiahave also implemented many overlays. No area code splits have been performed since 2007, whenarea code 575split off505inNew Mexico,and no splits are proposed.

Telecommunications companies have increasingly favored overlays even in sparsely-populated rural areas in which ten-digit local numbers are unnecessary, as split plans force cellular providers to reprogram millions of client handsets. Customers also incur costs to publish new letterheads and to reprogram stored address book data on individual devices. Overlays have become more expedient as the proliferation of cell phones has accelerated area codes exhaustion. In most relief planning projects, overlays have become the sole option considered in the planning stage.

This is especially practiced for area codes that have been pushed back to the brink of exhaustion after being recently split, as carriers want to minimize additional customer inconveniences. For example,Marylandhad been served solely byarea code 301from the implementation of the area code system until 1990, when everything fromBaltimoreeastward was split off asarea code 410.Within only five years, 410 was already on the brink of exhaustion due to Baltimore's rapid growth and the proliferation of cell phones, pagers, fax machines and dial-up Internet lines. However, the area's dominant telephone provider, Bell Atlantic (now part ofVerizon) realized that a split would have forced residents of either Baltimore or the Eastern Shore to change their numbers for the second time in the 1990s. Accordingly, area code 443 was created as an overlay for all of the eastern portion of Maryland in 1996.

Overlay plans also favorincumbent wireline carriersover new entrants, as the established firm already has large allocations of numbers in the more desirable existing code, and subscribers of new or growingcompeting carriersare relegated to unfamiliar, new codes.

The first example of an area code overlaid on an entire U.S. jurisdiction wasPuerto Rico,with area code 939 added to the numbering plan area of area code 787, in 2001. Puerto Rico had just been split fromarea code 809,the area code for much of the Caribbean, in 1996. A split was deemed unfeasible due to the island's extremely dense population and the lack of a suitable boundary for a split.Additionally, a split would have forced half of Puerto Ricans to change telephone numbers for the second time in a decade.

The first instance of astatewideoverlay wasWest Virginia'sarea code 304,in 2009. West Virginia had been served by 304 since the inception of theNorth American Numbering Plan,but it was obvious by 2007 that the state needed a new area code. At first, state officials voted to split off northern West Virginia with area code 681 and to leave southern West Virginia in 304. However, lobbying by the telecommunications industry led the state to reverse the decision and turn 681 into an overlay.Idahofollowed that precedent in 2017 by overlayingarea code 208with 986.

The area codes for four of the five largest Canadian markets (416inToronto,514inMontreal,604inVancouver,and403inCalgary) were split in the 1990s. Overlays gained popularity in the Canadian telephone network in the early 2000s. The proliferation of cellphones, fax machines, pagers, and dial-up Internet connections, particularly in larger markets threatened central office code exhaustion in many areas.

Particularly severe allocation problems manifested themselves at the turn of the century inMetro Torontoand theGolden Horseshoe,the center of Canada's largest local calling area. This area had been served by 416 since the institution of the area code system. Despite the Golden Horseshoe's explosive growth in the second half of the 20th century, numbers in the region were used up fairly quickly. As a result, the number allocation problem was not as severe in 416 as it was in the rest of Canada. Nevertheless, it was severe enough to force Toronto's suburban ring intoarea code 905in 1993. While the Golden Horseshoe would have likely needed another area code due to its continued growth, it is very likely that the immediate need for relief would have been staved off had it been possible to reallocate numbers from the smaller rate centers in the region to Metro Toronto. Within two years of the 905 split, 416 was back on the verge of exhaustion. American cities such as New York City, Chicago, andLos Angeleshad been split between two area codes, but that solution was quickly ruled out for Metro Toronto because of the area's extremely dense population and the lack of a suitable boundary for a split. It was ultimately decided to overlay 416 with area code 647 in 2001, two years after Metro Toronto was merged into the "megacity" of Toronto.

The successful implementation of 647 triggered the rapid adoption of overlays across Canada. By 2023, seven-digit dialing had been effectively eliminated nationwide in Canada (as well as in most of the United States), due to both overlays and other regulatory requirements. Overlays are now the preferred method of expansion in Canada, and no numbering plan areas have been split in the country since 1999.

British Columbiaand thePrairie Provinceshave followed Puerto Rico and West Virginia's lead with province-wide overlays. In those cases, a factor behind the decision was the desire to spare rural areas the expense and burden of changing their numbers.

Types

TheNorth American Numbering Plan Administratorrecognizes several types of overlays:

In adistributed overlay,orall-services overlay,an entire existing numbering plan area (NPA) gains another area code serving the entire area. Most area code overlays are of this kind.

In asingle concentrated overlay,only the high-growth portion of an existing area gains a second area code. This was previously implemented in several numbering plan areas, but all of them have since covered entire NPAs.

In amultiple concentrated overlay,the entire existing numbering plan area gains multiple additional area codes, each of which serves a different subsection of the original. This has never been implemented.

In amultiple-area distributed overlay,two or more numbering plan areas gain a single new area code covering the entire area, either concurrently or in phases. Examples include872inChicago,Illinois(over312and773) as well as587and825inAlberta(over403and780). In a phased example,area code 564was implemented initially only onarea code 360in 2017, but will expand to three other codes when needed, in westernWashington.

Aboundary-extension overlayis a type in which one or more neighboring numbering plan areas (with an overlay code or a primary area code) is expanded. Examples include321over407in centralFloridaand778over250inBritish Columbia.In 1999,281was the first of this type, merging with 713 inHouston.Later that year,972merged with 214, followed by323merging with 213 inLos Angelesin 2017, and858,in San Diego, with 619 a year later. In 2023, the first multiple boundary-extension overlay was implemented with the folding of area codes602,480,and623into a single overlay complex for thePhoenix metropolitan area.

Service-specific overlaysare not practiced in the NANP. Only one case, the first overlay area code in the NANPA,917,was an example of this. It was established originally as an area code specifically for cellphones and pagers inNew York City,but soon afterward, the FCC specified that area codes must not be service-specific, but it grandfathered this use for 917. Later, 917 became available for landline assignments in New York City.

Number pool management

The persistent unpopularity of new area code creation by splits or overlays has contributed to a change in the rules of number block allocation to conserve the pool of available telephone numbers. The change, which allowed for the assignment of smaller number blocks, is commonly known astelephone number pooling,and has slowed the need for new area codes. For example, the western half of Washington, including Seattle, narrowly avoided the need for an overlay in 2001.Area code 564,originally planned for introduction in October 2001, was delayed indefinitely in August 2001 after state regulators determined that the use of number pooling had resulted in more efficiency in the use of telephone numbers in western Washington. The 564 overlay complex was delayed until 2017.

Number pooling is not practiced in Canada. Every competing carrier is issued blocks of 10,000 numbers for every rate center in which it offers service, regardless of its actual subscriber count. Individual rate centers exist even in the smallest hamlets, and even tiny unincorporated villages receive multiple blocks of 10,000 numbers. Larger cities, particularly "megacities" that were created by amalgamations in the 1990s and early 2000s, have multiple rate centers, which were not combined for years, if at all. For example,Mississauga,Canada's sixth-largest city, is split between five rate centers even though it has been a single municipality since 1974.Hamilton,the nation's tenth-largest city, is split between nine rate centers.Surrey,the nation's twelfth-largest city, is split between three rate centers and part of a fourth, while Ottawa is split between 11. Overlays became the preferred relief solution in Canada, partly because it is not possible to reassign a number from a smaller rate center to a larger one, even if the smaller rate center has enough numbers to serve it. By 2013, as mentioned above, seven-digit dialling had been eliminated in all eightof Canada's original numbering plan areas.

See also

• List of area code overlays
• Interexchange carrier
• Telephone exchange
• Exhaust date

External links

• FCC FAQ concerning area codesat theWayback Machine(archive index)
• FCC FAQ - Carrier Identification Codes (CICs) and Seven-Digit Carrier Access Code (CAC) Dialing