# Re: Hypothetical SxS Question

• posted

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