History--computer based information operator terminal system [telecom]

Yes, some Bell places in Pennsylvania and NJ also used 11n.

I think it was that step-by-step used 11n and panel/crossbar used n11.

I think 11n today is used for test codes. Anyone have a list?

As to toll dialing prefixes, the Bell Labs history mentions there were various ways to tie SxS into the toll network, depending on the traffic and the toll switch. Sometimes intermediate registers were used to store dial digits until a trunk and receiver became available.

As time went on, some SxS were 'senderized' in various ways to get more efficiency and extend their life.

In the 1970s, I recall reading in some small towns a more elaborate toll dialing prefix was required and sometimes a wait for a second dial tone.

SxS switches were also common in Southern California, especially in L.A. and environs.

Wes Leatherock snipped-for-privacy@aol.com snipped-for-privacy@yahoo.com

As John Levine noted, 11N codes are long gone. The 11N numbering space has been reassigned to Vertical Service Codes (VSC) in format 11NN or

112NN (also dialable as *NN or *2NN). List at: There are eight N11 codes. List at: The N11 list doesn't include 011 (international dialing prefix) or 111 (error tone).

They don't exist. See comment about VSCs above.

At this point, I rise to defend my argument that digit 1 was not (and, insofar as possible, still is not) used as the first digit of a subscriber telephone number because "digit 1 cannot be distinguished from an accidental preliminary depression of the switchhook" (Miller,

1933), Wes's argument to the contrary notwithstanding.

Herewith is a list of hypothetical situations in which |1| represents a false switchhook depression and [N] is a footnote reference:

|1|+N11 = N11 code in some switches; may complete to the N11 code. Otherwise fails. |1|+NXX-XXXX = first seven digits of 10D a number. After timeout, call fails. |1|+NXX-NXX-XXXX = possible valid subscriber number. Call may complete. [1] |1|+1+1+anything = 111. Call fails. |1|+1+0+anything = 110X = VSC. Call reaches error tone or VSC. [2] |1|+1+N+anything = 11XX or 112XX = VSC. Call reaches error tone or VSC. [2] |1|+0+1+XXXX+0+NXX-NXX-XXXX = "Dial around" credit-call call. May complete. [3] |1|+0+1+XXXX+1+NXX-NXX-XXXX = "Dial around" paid call. May complete. [4] |1|+0+1+XXXX+anything else = unpredictable. [1] This call would complete in areas with 10D numbers where 1+10D is optional, in which case the caller would reach the desired subscriber number anyway.

[2] This call might complete to a wrong number if caller continues to dial another ten digits. [3] This call could reach a valid subscriber number, but caller would probably abort when asked for a credit card number. [4] This call could reach a valid subscriber number if the caller dials a total of 17 digits, but caller would probably abort before dialing that many digits.

In each of these cases, the call either fails, reaches the correct number, or reaches a situation in which the caller would have to dial more that the expected number of digits. In no case does it reach a wrong number when a subscriber dials the expected number of digits after the false 1.

Q.E.D.

Neal McLain

Thanks for the link references.

What I was wondering about are test codes. For example, I believe "113" initiates a ringback test.

I'm a bit confused by your statement. According to the Bell Labs history, the concern over accidental switchhook depression concerned only deskstand (candle stick) phones. Once the number of such phones declined in service (around the 1950s) it ceased to be an issue.

Actually, I don't understand how an accidental 'one' impulse that could happen. Perhaps when the user held the phone in one hand and lifted the receiver with the other the hookswitch springs allowed a "bounce" and thus a false one. Perhaps users 'flashed' the hookswitch while waiting for a delayed dial tone and that sent out a false 'one' when the dial tone finally came. Note that in old movies users often flash the hookswitch "Operator! Operator!" even on dial phones. Perhaps in manual days hookswitch flashing was routine. (On calls served by cord PBXs or toll calls placed by an operator users were instructed to flash to recall the operator well into the 1970s.) Or perhaps it wasn't a problem at all but they thought it was at the time. (When 7 digit dialing came out they introduced exchange names because they said 7 digits were too many for people to remember, but later they said that was wrong.)

That may have been the case with SxS switches. Today, 11X (and *X) indicate vertical service codes. There are many unassigned VSCs that default to error (or busy) tone. As it happens, the entire 113/*3 block is presently unassigned, "Reserved for expansion to 3-digit VSCs." See

Perhaps it's no longer an issue. Perhaps Wes is right: it never happens in a million cases. I'm not saying it happens; I'm saying that the numbering assignments in use in the NANP today effectively prevent such a call from inadvertently reaching any other valid subscriber number.

I'll be the first to admit that it wasn't originally designed this way. The entire North American Numbering Plan is a patchwork of workarounds designed to accommodate hundreds of existing incompatible numbering plans into something workable. By happy coincidence, the very fact that 1 and 0 weren't used in the candlestick days made them available for all the services added since. Thus:

1+1+ = Vertical Service Code 1+0+ = Carrier Access Code 1+N11 = N11 Code (some switches) 1+NXX+ = Intra-NANP dialed call

0+1+ = International dialing access

0+0+ = IXC operator from some coin phones 0+NXX+ = Intra-NANP credit-card/operator-assisted call

And possibly others that I may have missed.

Neal McLain

The bell 500 telephone, at least (I'm not sure about earlier ones) in- corporated design features specifically to minimize/eliminate 'phantom' digit generation when going off-hook. Something the old-style candlestick phones lacked. Excessive contact 'bounce', producing several make/break contact cycles in a short interval could give rise (pun intended) to the C.O. 'seeing' an unintended dial 'pulse'. By including _mechanical_ damping in the switch mechanism, thus providing "de-bouncing" for the contacts, the potential for phantom digit generation was virtually eliminated.

Of course it got into production. I know the AT&T/Lucent/Avaya Definity and Prologix all have dial by directory available.

I just remembered the number to reach the Southwesteern Bell "Master Employee Locator--1-800-667-MEL. The last time I tried it, mayber a couple of year ago, it had so many features added and so much security added it was pretty hard to use. Wes Leatherock snipped-for-privacy@aol.com snipped-for-privacy@yahoo.com

I think you're missing a digit.