Voice
Communication in Business Volume 1
Essays on telecommunications,
1969-1980
Part V
On To The Digital Future
During most of the
1970s, I was so busy with PBXs and business communication that I
seldom had time to keep up with the larger issues. However, PBX
design was enmeshed in the digital vs. analog argument, and the
subject could not be avoided. It is, of course, the most important
single issue facing everyone involved with telecommunications today.
There are many articles
and books in print that insist that digital PBXs "have more
features," and that digital PBXs "are compatible with the office of
the future." Both of these statements require very careful
understanding of the actual facts and issues if they are to make
sense.
"Features," in general,
are a function of a PBX system's computer control. A computer can be
programmed to do many things and, if provided with a reasonable
array of peripheral equipment, can cause quite surprising results.
Computers of this sort are "digital," and many people want to define
a PBX or anything else as digital if it is controlled by such a
device. This, in my opinion, is like defining a furnace as "digital"
because it is controlled by a digital thermostat, or a toaster and
coffee pot as "digital" when a home computer freak has his favorite
toy activate them before he gets up in the morning.
On the other hand,
another school of thought says any system that handles digital
signals is "digital." This of course, makes a wheelbarrow full of
reels of computer tape a digital wheelbarrow. So we have a problem.
Just what IS a digital PBX, and what, if any, are its advantages? In
a nut-shell, a digital PBX (or CO switch) is one that uses digital
techniques to handle digital signals. What some of the implications
are we will explore in the following chapters.
One must keep in mind
that there are two main variations in digital signal encoding: delta
modulation and pulse code modulation. They are not directly
compatible, of course, but that is not the whole story. There are at
least three versions of delta mod on the market, one in the PBX from
Digital Telephone Systems, another in the Focus PBX from American
Telecom, and a third used by SBS in its proposed business
transmission system. None is compatible with either of the others on
a digital basis. Further, the Bell System is using a version of
delta mod in a station carrier system and in their Voice Storage
System or VSS (a digital automatic telephone answering machine). I'm
not sure if these two are compatible with each other or with
anything else, but I am relatively pessimistic.
Turning to pulse code
modulation, there are two standard formats, European and American.
These are almost compatible, but not quite. One uses 32 channels and
the other 24 as a basic building block, and the coding and
supervisory techniques vary. Rolm, of course, differs from both of
the above. Not only does Rolm use a different coding, it also uses a
different rate of sampling and digitizing the signal itself.
Clearly, all digitals
are not created equal.
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