Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!texbell!vector!telecom-gateway From: nvuxr!deej@bellcore.bellcore.com (David Lewis) Newsgroups: comp.dcom.telecom Subject: Re: Cryptic Abbreviations Message-ID: Date: 5 Nov 89 18:03:16 GMT Sender: news@vector.Dallas.TX.US Organization: Bellcore, Livingston, NJ Lines: 94 Approved: telecom-request@vector.dallas.tx.us X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@vector.dallas.tx.us X-TELECOM-Digest: volume 9, issue 500, message 3 of 9 In article , U5434122@ucsvc.ucs. unimelb.edu.au writes: > Could someone please explain some North American abbreviations: > NXX-XXXX (why 'N' ) Warning: I started writing a response to this and ended up trying to provide a shorthand explanation of the North American Numbering Plan. The strict answer to the question provided is three lines down; the rest is further information.... lots further...] As to why "N" and "X", I don't know. "N" is ex-Bell System shorthand for a digit in the range 2-9; X for a digit in the range 0-9. They came into use, as far as I know, at the time that all-number dialing came into use (when exchanges became known by number, instead of NAme...) North American Numbering Plan (NANP) (that which defines the syntax for telephone numbers in North America) syntax for a telephone number is: N(0/1)X-NNX-XXXX OR N(0/1)X-NXX-XXXX. The first has been the syntax since the introduction of all-number dialing until the introduction of "interchangeable codes" -- 3-digit codes which can be either an exchange or an area code (NPA code, or Numbering Plan Area code, in NANP parlance). It made for simpler switches, since the switch can do a three-digit analysis -- if the second digit is 0 or 1, it has to collect seven more digits; if the second digit is 2-9, it has to collect four more digits. Unfortunately, it also restricts the number of available NPA codes and exchange codes. Therefore, the NANP has been modified to permit interchangeable codes. This is first being implemented according to the second syntax above -- the second digit of an exchange code, in areas which have implemented interchangeable codes, can now be 0-9 instead of being limited to 2-9. This adds 152 new available exchanges in each NPA (although practically the number is something less, because the "home NPA" and N00 codes are not recommended for use as exchanges). The second step, interchangeable NPA codes, will result in NPAs being of the format NXX as well, instead of N(0/1)X. That will come about when the available set of NPA codes is exhausted, predicted to happen sometime around 1995. This also gets into a discussion of dialing methods, which came up in another post. (I do seem to digress a bit, don't I...) Given the use of interchangeable exchange codes (I really should use the proper terminology -- "interchangeable CO codes"), switches can no longer simply examine the second digit dialed and determine whether it's an NPA code or CO code. This is the reason Bellcore is recommending the "Prefix method" of dialing -- 7D and 1+10D being allowable dialed numbers, 1+7D and 10D not being allowable. This is a "clean" technical solution -- leading 1 means collect 10 digits, no leading 1 means collect 7 digits (plus special cases like N11). Unfortunately, as has been pointed out, it's not so clean a solution when considering customer toll restriction. What is considered a "toll" call and what is considered a "local" call is not a strictly technical decision, and will become less and less easily mapped to home/foreign NPA considerations (particularly with more and more cases of multiple NPAs in a single city, for example). A leading 1 may or may not be toll; no leading 1 may or may not be toll. CO switches have the appropriate routing and billing tables loaded to keep track of that -- but CPE doesn't. I suppose the paranoid out there could argue that this is all a BOC plot to destroy competition with CPE in the Toll Restriction Service market, but I don't believe that's the case. Far more important an issue is that not using the Prefix method requires a timer after the seventh digit dialed to determine if end of dialing is reached. This adds to call setup time -- after dialing seven digits when you mean to dial seven digits, the CO switch has to wait and see if you intend to dial an eighth -- and increases the probability of error -- you forget the number halfway through, so dial NPA-NXX-X and stop to look up the remainder, and the switch times out and places the call to NPA-NXXX. Granted, that's unlikely given that the syntax is such that most people remember NPA-NXX- then stop to look up the XXXX. The call setup time considerations are most definitely not trivial, though -- consider trying to sell state regulatory agencies on increasing the call setup time by two seconds for *every* phone call placed by every Joe Random in the state, to make it easier for "all them big corporations with all their fancy equipment" to have toll restriction or other dialing plan services in CPE... I'd rather not, myself. Apologies for the digression. David G Lewis ...!bellcore!nvuxr!deej "If this is paradise, I wish I had a lawnmower."