Exchange code protection

In telephony, central office code protection or exchange code protection is a numbering policy intended to ensure that the same local telephone number is not assigned in both of a pair of adjacent communities on opposite sides of a telephone area code boundary. It is most commonly used to preserve seven-digit dialling in border communities in the North American Numbering Plan.

Central office (CO code) protection was once common in communities such as Ottawa-Hull[1] or Washington, D.C. which were constructed on provincial or state boundary lines. It is declining in use as inefficient allocation of numbering resources to the growing number of competitive local exchange carriers has caused depletion of available number prefixes, often requiring ten-digit local calls or overlay plans where multiple area codes serve the same geographic location.

Operation

In the North American Numbering Plan, telephone numbers are allocated to individual communities as blocks of 10000 consecutive numbers; each block is identified by +1 - area code (3 digits) - exchange code (3 digits). (The remaining four digits identify a specific local number within the exchange.) These blocks may then be split between multiple local providers or assigned directly to individual local telephone companies.

As an example, the entire +1-506-752- prefix is assigned to Aliant Telecom (NB) and serves Campobello Island, New Brunswick.[2] Campobello is a border community[3] and is a local call to Lubec, Maine[4] in area code 207.

The +1-207-733- prefix serves Lubec, split between FairPoint Communications and a pair of competitors using number pooling.[5] Both area code 506 and 207 use seven digits for local calls.

A seven-digit local call from Campobello to Lubec is possible, provided that 733-XXXX is not assigned to anything in the 506 area code which is local anywhere near Campobello. This may be done in one of two ways:

Similarly, Edmundston is local to Madawaska, Maine exchanges +1-207-316, +1-207-436 and +1-207-728.[7] The corresponding +1-506-316, +1-506-436 and +1-506-728 prefixes (as of 2014) are not issued by the Canadian Numbering Administrator (CNAC):

506,316,,,Available,,Available outside Madawaska: Maine EAS
506,436,,,Available,,Available outside Madawaska: Maine EAS
506,728,,,Available,,Available outside Madawaska: Maine EAS[8]

A few small towns on US state boundaries have one central office which serves both sides, using different prefixes in different area codes.[9] All South Coffeyville, Oklahoma exchange prefixes are served from Coffeyville, Kansas, population 10000;[10][11] the two are local to each other but long distance to everything else.

Limitations

If code protection is implemented by reserving every seven-digit number for a border town in both (or all) of the affected area codes, that community and points in its local calling area occupy numbering resources at double the otherwise-expected rate. This is a minor drawback in small cities, such as Lloydminster, but can consume numbers rapidly in larger centres such as St. Louis or Kansas City.

In large cities located directly on area code boundaries (such as Ottawa-Hull in 613/819, the fourth-largest Canadian metropolitan area), this can lead to a situation where none of the many vacant prefixes are assignable to a city without breaking seven-digit calling across the area code boundary. Code protection in Ottawa-Hull broke down in 2006 for this reason. The 1-819 versions of Ottawa 1-613 numbers were reserved and made unavailable to anyone, instead of being assigned out-of-region, so eventually the few remaining 1-819 prefixes could not be assigned without requiring ten-digit dialling from Ottawa to Hull. Similarly, seven-digit calling between Washington, D.C. and its suburbs broke down in 1991.

A local calling area spanning three US states (such as Sioux City, Iowa extending into North Sioux City, South Dakota and South Sioux City, Nebraska) would require any number local to any part of the town to be reserved in all three area codes.

Even if a city is not directly on an area code boundary, it may require code protection if a suburb in its local calling area can call another area code locally. Chicago is long distance to Indiana, but Calumet City, Illinois is local to both Chicago, Illinois and East Chicago, Indiana.[12] A dial plan which uses purely seven-digit local calling from Calumet City would protect a massive number of Chicago 312 numbers from assignment in Indiana's area code 219, forcing that code to split from central Indiana's 317 in the first year (1948) after the original area codes were assigned (1947).

Severing the border community from its exhausted home area code using a split plan may prolong the life of a seven-digit exchange code protection scheme if the piece being split off is outside the extended local calling area. Code protection is most often removed entirely in large cities if an overlay plan is implemented, as these plans break the seven-digit dialling that code protection was devised to preserve. The plan also can be expected to break down if new split-plan codes are added within a Chicago-sized city, as the city itself no longer fits into one seven-digit area.

References

  1. ↑ http://www.crtc.gc.ca/eng/archive/2001/PT2001-71a.htm
  2. ↑ http://cnac.ca/data/COCodeStatus_NPA506.txt
  3. ↑ "Campobello's sea change". Portland Press Herald.
  4. ↑ "Local calling guide: Rate centre information, Campobello".
  5. ↑ "Local calling guide: NPA-NXX search, 207-733".
  6. ↑ "Local calling guide: NPA-NXX search, 506-733".
  7. ↑ "Local calling guide: Rate centre information, Madawaska".
  8. ↑ http://cnac.ca/data/COCodeStatus_NPA506.txt
  9. ↑ http://massis.lcs.mit.edu/telecom-archives/TELECOM_Digest_Online2006-1/1065.html
  10. ↑ http://www.localcallingguide.com/lca_prefix.php?switch=CFVLKS10DS0
  11. ↑ http://www.localcallingguide.com/lca_prefix.php?switch=PRSSKSBL6MD
  12. ↑ http://massis.lcs.mit.edu/telecom-archives/TELECOM_Digest_Online2006-1/1082.html

See also