TELECOM Digest OnLine - Sorted: Re: Hypothetical SxS Question

Re: Hypothetical SxS Question

Robert Bonomi (
Fri, 09 Dec 2005 15:50:42 -0000

In article <>,
<> wrote:

> This question is purely speculative only, not for real use.

> 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.

A 'subset' 4-digit system (where all digits are in the range 1-4) would
likely be the 'most efficient' hardware construct. this gives you 256
'numbers', which is a close match to your 250 station requirement.

> The Bell System history talks about "graded multiples" to more
> efficiently use trunks and switchgear in central offices, but I don't
> think that would apply in this application.

> Now if we wanted to implement the above using modern technology, would
> only a PC be required with appropriate software and cards? I guess
> we'd need capacity for about five conversations at once.

Correct, one approach would be a _little_ PC (e.g. a 386 box),
Asterisk software, and an appropriate number of line cards.

And, of course, with computer-based system, you can assign 'dialable'
numbers any way you want. You don't have any additional 'mechanical
complexity' based on the length of the numbering plan.

The problem is going to be in those 'line cards'. 250 'stations'
requires 250 POTS ports on the 'switch'. Now, you _can_ get that many
ports on the PC, no problem. A mere 3 quad-port T-1 cards does it.
But then you need 11 "D-4" channel banks, or equivalent, to break
those T-1s out to the individual POTS circuits.

Then, if this is going to be more than an 'intercom' system, you need
a bunch of 'trunk' lines to the C.O. '5 conversations' may be a
reasonable 'average' usage (although it strikes me as _low_), but
you'll need *several* times more than that to handle reasonable
variability in call volumes. probably more like 50 (at least!) to
handle 'reasonable' peak calling without having to give fast-busy, due
to lack of 'trunk' circuits.

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