Mike's PBX Cookbook

BARS programming course
- A short course for a long topic -

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A short history of Direct Distance Dialing (DDD)

Back when I was a kid and most telephone numbers were six digits long, Ma Bell decided that it would be cheaper to let people dial there own long distance calls than it would be to hire enough operators to continue processing all LD calls manually.  Thus, the North American Plan Area (NAPA) was created.  Local numbers all became seven digits long.  Yes, I know that some Community Dial Offices (CDO) would work with only five or six digits.  This is because they would absorb one or more leading digits.  It was a common trick of the number 1-SXS office.  If say 4 was to be adsorbed, then each time a 4 was dialed it would reset.  Only when it received something other than a 4 would it start to process the call.  This is no longer a common practice in the US, but I know of one office in Florida that was still working this way in the mid 1980’s.

This is also about the time when exchanges started to acquire names like “PEnsylvania 6-5000”.  The “metropolitan dial” (letters as well as numbers) came into use at this time to accommodate the dialing of these exchanges.  The other invention of the NAPA was the Numbering Plan Area (NPA) or Area Code (AC).  The dialing pattern was the toll access digit (1), the NPA (NAX), the office code (NNX), and the terminal digits or directory number (XXXX).  This is the source of the NPA and NXX type designations when programming BARS in load 90.  This is also why the metropolitan dial has letters only in the 2 through 9 range.

A= 0 or 1
N= 2 through 9
X= 0 through 9

Telephone company employees lettered the digit positions after the access code for there own convenience.  Therefore, by examining the “B” digit position, one can ascertain if it was an area code or an office code.  Many PBX venders used to block toll calls by checking for a 0 or 1 in the Toll Access digit position or in the “B” digit position.  Two checks were used because some trunks do not require a Toll Access digit.  In most cases, all calls on the trunk are toll, but there were exceptions so the manufacturers covered all of the bases.

This scheme worked for a few decades until they started to have trouble dreaming up names for all of the possible combinations.  What can you do for an exchange name for 552 (JKL, JKL, 2)?  Remember the exchange name was always the first two letters in the name.  So when people started to buy lines for FAX machines and modems, they scrapped the names and went just with the digits.  This bought them just enough time until cell phones became popular and that totally blew it for the old pattern.

Since the maximum number of digits in a phone number is limited by international agreement, the best answer was to improve the efficiency of the usage of the existing available digits.  Thus, the Extended North American Plan Area (ENAPA) was created.  The first thing they did was to change the office code from NNX (640 possible combinations) to NXX (800 possible combinations).  The Next was to change the NPA from NAX (160 possible codes) also to NXX.  This change forced the retirement of eight digit dialing for Interlata calls (toll calls within an area code).  Intralata calls (local calls) are now always either 7 or 10 digits and all long distance calls are 11 digits.

0-Local Operator
00-Toll Operator
101XXXX-Equal access for LD carriers
211-Community Information and Referral service (US)
311-Non-Emergency Police and other governmental Services (US)
411-Local directory assistance
511-Traffic and transportation information (US); Reserved
611-Repair Service
711-Telecommunications Relay Service (TRS)
811-Business Office
911-Emergency Assistance

The only other thing cluttering up the dialing plan are the special service codes.  In the U.S., the FCC administers N11 codes, and recognizes only 211, 311, 511, and 711 as nationally assigned.  Other codes have traditional uses indicated in the table to the left.  This table reflects the information available on the NANPA web site.  Updated on 7/24/2000, it reflects the assignment of 211 and 511 in the U.S.  It does not reflect the practice of charging for directory assistance by dialing 1411, and long distance directory assistance by dialing 1(NPA) 555-1212.  This was once the only function in the 555 exchange.  That is no longer true and other assignments are showing up around the country.

Since most systems tend to be installed anyplace but the U.S., the above is provided as background only.  BARS is intended for use within the North American Dialing Plan Area.  Local international dialing patterns can be programmed (special numbers, SPN's), but it will have to be done without the aid of internal number length checking.  For our purposes, the greatest drawback of using BARS is that it does not support wild card characters.  That means that every possible number pattern that can be dialed, and many that can’t, must be explicitly programmed in the system.  I will go into much more detail about this in another lesson.

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