TELECOM Digest OnLine - Sorted: Re: Hypothetical SxS Question

Re: Hypothetical SxS Question

Neal McLain (
Wed, 07 Dec 2005 18:12:00 -0600 wrote:

> Suppose we wanted to install a PABX using step-by-step gear in a condo
> complex. We need 250 stations. My question regards the most
> efficient station number assignments.

> One person says the stations ought to have a four digit number that
> corresponds to the apartment number. There are 19 buildings with
> 10-15 units per building. So unit #103 would get phone number 0103
> and unit #1513 would be phone 1513.

> While the above is easier to remember, wouldn't that be a waste of SxS
> terminals and require more switch units without any gain in
> efficiency? Isn't a four digit code inherently more complex than a
> three digit in an SxS environment? I think the phone ought to be
> numbered strictly sequentially, starting from 111 and going upward.

The most "efficient" numbering plan (least amount of equipment) would be
to assign extension numbers in three 3-digit ranges:


This arrangement provides a total of 300 lines, enough to accommodate
250 apartments with spares for other uses (office, maintenance room,
loading dock, etc.).

Advantage: This plan requires only three sets of switches:
linefinders, first selectors, and connectors. By contrast, the
hypothetical 4-digit plan that "one person" suggests would require
four sets: linefinders, first selectors, second selectors, and
connectors. As you note, a four digit code [is] inherently more
complex than a three digit in an SxS environment."

Advantage: This plan avoids two SxS taboos: subscriber numbers beginning
with 0 or 1:

- Initial 0 is taboo because "experience has shown that in a single-
office system it is not advisable to use subscribers' numbers
commencing with the numeral zero (0) unless absolutely necessary
to secure the desired capacity" [1]. And, of course, zero is
traditionally used to reach the local operator, if any.

- Initial 1 is taboo because an SxS (or any other rotary-dial) switch
can't distinguish between an intentionally-dialed 1 and a false
switchhook depression.

Disadvantage: It's not possible to match subscriber numbers to apartment
numbers (although, as PAT notes, this might not be a good idea anyway).

[1] Kempster B. Miller. "Telephone Theory and Practice" vol. 3
"Automatic Switching and Auxiliary Equipment." New York: McGraw Hill,
1933, p. 129.

Neal McLain

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