LONG-DISTANCE SWITCHING
=Definitions.= Telephone messages between communities are called
long-distance messages. They are also called toll messages. Almost all
long-distance traffic is handled by message-rate (measured-service)
methods of charge. All measured-service messages are toll messages,
whether they are completed within a given community or between
communities. The term "long-distance," therefore, is more descriptive
than the term "toll." The subject of local and long-distance measured
service is treated exhaustively in a chapter of its own.
Some telephone-exchange operating companies call their own inter-city
business "toll," and use the term "long-distance" for business carried
between exchanges for them by another company. The distinction seems to
be unwarranted.
=Use of Repeating Coil.= Most long-distance lines are magneto circuits.
If they are switched to grounded circuits, repeating coils need to be
inserted. Toll switching equipments contain means of inserting repeating
coils in the connecting cords when required. Their use reduces the
volume of transmitted speech, but often is essential even in connecting
metallic circuit lines, as a quiet local metallic circuit may have a
ground upon it which will cause excessive noises when a quiet
long-distance line is connected to it.
=Switching through Local Board.= In the simplest form of long-distance
switching, the lines terminate in switchboards with local lines and may
be connected with each other and with the local lines through the
regular cord circuits, if the equipment be of the magneto type. The
waystations on such a line are equipped with magneto generators. These
waystations may signal each other by bell ringing; the central office
may call any waystation by ringing the proper signal and may supervise
in a way all traffic on such lines by noting the calls for other
stations than the supervising exchange.
=Operators' Orders.= _By Call Circuits._ Where the long-distance traffic
between two communities is large, economy requires that the sending of
signals by ringing over the line, waiting for an answer, and then
reciting the details of the call, be improved upon. If the traffic is
large and the distance between communities small, call circuits are
established in the same way as between the switchboards in several
manual central offices of an exchange. The long-distance operator
handling the originating call passes the necessary details to the
distant operator by telephone over the call circuit. Such circuits also
are known as order circuits. They are accessible to originating
operators at keys and are connected directly and permanently to the
telephone sets of receiving operators. One call circuit can handle the
orders for a large number of actual conversation circuits. The operator
at the receiving end designates the conversation circuit which shall be
used, the originating operator following that instruction.
_By Telegraph._ Where traffic and distance are large, conversation lines
cost more than in the case last assumed. It then is of greater
importance to use all the possible talking circuits for actual
conversations in order that the revenue may be as high as possible. A
phantom circuit good enough for call circuit purposes would be good
enough for actual commercial messages, therefore, it is customary to
furnish such originating and receiving operators with Morse telegraph
sets. The lines are obtained by applying composite apparatus to the
conversation circuits. Two Morse circuits can be had from each
long-distance line without impairing any quality of that line except the
ability to ring over it. As one Morse circuit can carry information
enough between two operators to enable them to keep many telephone
circuits busy, they do not need to ring upon the composited lines, so
that nothing is lost while revenue is gained.
=Two-Number Calls.= In cases where the traffic between communities is
large, where the rate is small, and where the conversations are short
and more on the general order of local calls, it is usual to handle the
switches exactly as local calls are trunked between central offices of
the same exchange. That is, the subscriber's operator who answers the
call trunks it, by the assistance of a call circuit and an incoming
trunk operator. The subscriber's operator records only the numbers of
the calling and called subscribers. No long-distance operators at all
assist in these connections. They are known as "two-number calls." The
calling subscriber remains at his telephone until the conversation is
finished.
=Particular-Party-Calls.= In cases where the traffic is smaller, and
where the rate is large, it is customary to handle the calls through
long-distance operators. The ticket records the particular party wished,
and the calls are named "particular party" calls. In such connections
the calling patron is allowed to hang up his receiver, after his call is
recorded, and is called again when his correspondent is found and is
ready to talk. This makes _all calls for conversations_ outgoing ones.
Only recording operators receive calls _from_ patrons. Line operators
make calls _to_ patrons.
=Trunking.= Long-distance lines entering a city usually terminate in one
office only, no matter how many offices the local exchange may have. It
is possible to terminate these long-distance lines on a position of the
multiple switchboard for local lines. For a variety of reasons this is
not practiced except in special cases. The usual method is to terminate
them in a special long-distance board and to provide trunk lines from
this board to the one or more local switchboards of the exchange. In
common-battery systems these toll trunks are so arranged that the called
local subscriber receives transmitter current from the office nearest to
him, yet is able to show the long-distance operator the position of his
switch hook and is able to be called by the long-distance operator
without the intervention of the switching operator in the local office,
even though two repeating coils may be in the trunk circuit.
_Through Ringing._ There is a distinct traffic advantage in having the
ringing of the subscriber under the control of the long-distance
operator. The latter may call for the subscriber by stating her wish
over the call circuit associated with the long-distance trunk. The
connection having been made by the switching operator, the long-distance
operator may withhold ringing the subscriber's bell until all is in
readiness for the conversation.
_High-Voltage Toll Trunks._ In some systems, the long-distance trunks
are further specialized by being enabled to furnish transmitter current
to subscribers at a higher voltage than is used in local conversations.
With a given construction of transmitters there is a critical maximum
current which can be carried by the granular carbon of the instrument
without excessive heating, consequent noises, and permanent damage. The
shortest lines and the longest lines of an exchange district being
served by a source of current common to all, the standard potential of
this source must be such as to give the longest lines current enough
without giving the shortest lines too much. The very longest local
lines, however, do not receive current enough from the standard
potential to give maximum efficiency when talking over long distances,
though they get enough for local conversations. By providing a battery
with a voltage twice that used for local conversations and connecting it
into the current supply element of the toll trunk through non-inductive
resistances, not too much current may be given to the shortest lines and
considerably more than normal current to the longest lines.
=Ticket Passing.= When only one operator is necessary in a town, her
duty being to switch both local and long-distance lines, she may write
her own tickets and execute them entire. In larger communities with
larger long-distance traffic, the duties need to be specialized. The
subscribers' wants as to long-distance connections are given by
themselves to recording long-distance operators, who write them on
tickets and pass these to operators who get the parties together. The
problem of ticket-passing becomes important and many mechanical carriers
have been tried, culminating in the system which utilizes vacuum tubes.
This is in some ways similar to vacuum or compressed-air tube systems
for carrying cash in retail stores. The ticket is carried, however,
without any enclosing case and the tubes are flat instead of round, _i.
e._, they are rectangular in section. By suitable means a vacuum is
maintained in a large common tube having a tap to a box-like valve at
each line operator's position. A ticket tube connects this valve with a
distributing table at or near which the tickets are written. The tickets
are of uniform size and are so made as to enable a flap to be bent up
easily along one edge. The distributing operator has merely to insert
the ticket, bent edge foremost, in the open end of the tube, whereupon
the air pressure behind it will drive it through to its destination,
near by or far away. The tickets travel thirty feet a second. The tube
may be bent into almost any required form. The ticket, on arriving at a
line operator's position, slides between two springs, breaking a shunt
around a relay and allowing the latter to light the lamp.
=Waystations.= Waystations on long-distance lines may be equipped in
several ways. Most of them have magneto sets and can ring each other.
Some are equipped with common-battery sets and get all current for
signaling and transmission from a terminal central office. In the latter
case, there is the advantage that the ringers are in series with
condensers, assisting greatly in tests for fault locations. Such tests
are hindered by the presence of ringer bridges across the line, as in
magneto practice. Condensers can be inserted in series with ringers of
magneto sets if the testing advantage is valued highly enough. A
disadvantage of the use of common-battery sets in waystations on
long-distance lines is the lessened transmission volume of the stations
farthest from the current source.
_Center Checking._ An operating advantage of common-battery sets on
long-distance lines is that all calls are forced to be answered by the
terminal station. Waystations can not call each other, as they have no
calling means. With magneto sets, waystation agents sometimes call each
other direct and neglect to record the call and to remit its price. When
they can not call each other direct, the revenues of the company
increase.
A traffic method which requires all calls from waystations to be made to
a central switching office is called a center-checking system. It is so
called because all checking for stations so switched is done at the
central point instead of each waystation keeping its own records of
calls sent and received. In such practice it is usual to bill each
station once a month for the messages it sent. Where center checking is
not practiced, the agent makes a report and sends a remittance. Center
checking comes about naturally for waystations having no ringing
equipment.
Center checking originated long before the invention of common-battery
systems. It requires merely that no waystation shall have a generator
which can ring a bell. The method most widely used is to equip the
waystations with magneto generators which produce direct currents only;
such a generator cannot operate a polarized ringer. It is not usual to
produce the direct current by actually rectifying the alternating
current, but merely by omitting half the impulses, sending to the line
only alternate half-cycles of the current generated. Any drop or relay
adapted to respond to regular ringing current will respond to this
modified form of generator.