David Lesher <wb8foz@mthvax.cs.miami.edu> (07/27/90)
There are several reasons why the Oval Office has retained a Call
Director for many years. Both that set and the Secy of Defense's
terminate many ring down lines. They also provide AutoVON service. Now
many folks have mentioned that AutoVON was an early user of Touch
Tone, with all 16 tone pairs used. The four precedences those provide
serve to knock lesser calls off {FAST!} so as to free the limited
trunkage for more important calls, such as checking on General
Bullmoose's pizza order :-}
What others seem to have ignored is the more important difference
between AutoVON and POTS or FTS: AutoVON is end to end four wire. In
other words, separate {pairs/circuits} carry the audio from X-->Y and
from Y-->X. Now, in general, most calls of any distance end up getting
converted from two wire (your local loop) to four wire either in the
first CO or in the toll switch. That's because {most} amplifiers work
in only one direction {at a time}. There are loop gain extenders of
various types (somewhere I have one Lorain Products made) around that
try to switch directions and keep up with a conversation, but in
general they don't work very well. The problem with hybrids (that
convert two to four wire and verse vica) is that they are not perfect.
If they were, all the energy would go where it should. But since the
hybrid reflects a little of the incoming (4w) signal back out the
outgoing, instead of sending it all down the 2w side, problems ensue.
One of these is "talk echo" where you hear yourself, fractions of a
second later, as the far-end hybrid echos. Another is "singing" or
feedback, where the whole system oscillates. You can minimise these by
keeping the gain low, among other things.
But an important function of AutoVON is the conference call. The Secy
Def or the CIC {or both} may, in a crisis, want to have five to ten
bigwigs all on at one, some of whom are in DC, others in Texas, plus
the guy in Seoul and the other one at Frankfurt. So you need that gain
we just turned down last paragraph. At least when AutoVON was
designed, the only way to do this was to use true, end to end four
wire.
But that's not very hard to do, at least in the set. You see, you and
I are four wire. We speak and listen by different ports. But the
network in your set turns it into two wire so as to keep the local
loop cost low. Thus all you need to do is {basically} NOT USE A
NETWORK. Just connect the handset transmitter to your talk pair, and
your receiver to the listen side. It's really not quite that simple,
but you get the idea. But wait, that's the AutoVON line. What about
all the REST of the lines on the Call Director? They're normal
two-wire, right? Yep, gold star Jamie, they are.
I have seen schematics for, and photographs of, the wiring in the
closet by the Secy Defense's desk. {I am VERY glad that I am not the
one who has to maintain it. I think it has about the same number of
jumpers as a small town CDO :-]} Somewhere in there is one of those
little cute gray can WECO relays labeled "four wire". It in turn is
controlled by the A-control circuits from the set. If The_Man punches
up a four wire line, it disables the network. If not then it is used
normally.
So at least one reason that those phones have been there so long is
that they provide switchable two/four wire service. Another is that
they offer lots of customabilty, mainly cuz there's room in the closet
for more relays. It may be crude, but it IS time-proven.
Plus, do YOU want to be the guy who replaces it with a Whiz-Bang-Boof
X-3060, that works FINE everywhere else in the world? Not me, Mr.
Murphy, not me....
Unexplained is why, after all this trouble, is AutoVON's service so
awful, that even on normal, two port domestic calls, you can't hear a
thing? Danged if I know......
Mention of the photo of JFK's desk brings a piece of trivia to mind.
FTS, the Federal Telephone System, the large disjoint system that {in
theory!} provides intra-government telecommunications, came about
because at the height of the Cuban Missile Crisis, he could not, at a
critical juncture, get a dial tone!
I seem to recall that FTS started out with four underutilized CO's
serving as tandems. DC's is in the middle of Maryland somewhere.
wb8foz@mthvax.cs.miami.edu (305) 255-RTFM
pob 570-335 33257-0335