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:
211-200 311-300 411-400
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