PRIVATE BRANCH EXCHANGES
=Definitions.= A telephone exchange devoted to the purely local uses of
a private establishment such as a store, factory, or business office, is
a private exchange. If, in addition to being used for such local
communication, it serves also for communication with the subscribers of
a city exchange, it becomes in effect a branch of the city exchange and,
therefore, a private branch exchange. The term "P. B. X." has become a
part of the telephone man's vocabulary as an abbreviation for private
branch exchange.
Private exchanges for purely local use require no separate treatment as
any of the types of switching equipments for interconnecting the lines
for communication, that have been or that will be described herein, may
be used. The problem becomes a special one, however, when communication
must also be had with the subscribers of a public exchange, since then
trunking is involved in which the conditions differ materially from
those encountered in trunking between the several offices in a
multi-office exchange.
For such communication one or more trunk lines are led from the private
branch office usually to the nearest central office of the public
exchange and such trunks are called private branch-exchange trunks. They
are the paths for communication between the private exchange and the
public exchange. For establishing the connections either between the
local lines themselves or between the local lines and the trunks, and
for performing other duties that will be referred to, one or more
private branch-exchange operators are employed at the switchboard of the
private establishment.
The private branch exchange may operate in conjunction with a manual or
an automatic public exchange, but whether manual or automatic, the
private exchange is usually manually operated, although it is quite
possible to make a private branch exchange that is wholly automatic and
will, therefore, involve no operator at all.
=Functions of the Private Branch-Exchange Operator.= It is possible, as
just stated, entirely to dispense with the private branch-exchange
operator so far as the mere connection and disconnection of the lines is
concerned. But the real function of the private branch-exchange operator
is a broader one than this and it is for this reason that even in
connection with automatic public exchanges, operators are desirable at
the private branches. The private branch-exchange operator is, as it
were, the doorkeeper of the telephone entrance to the private
establishment. She is the person first met by the public in entering
this telephone door. There is the same reason, therefore, why she should
be intelligent, courteous, and obliging as that the ordinary doorkeeper
should possess these characteristics.
As to incoming traffic to a private branch exchange, an intelligent
operator may do much toward directing the calls to the proper department
or person, even though the person calling may have little idea as to
whom he desires to reach. This saves the time of the person who makes
the call as well as that of the people at the private branch stations,
since it prevents their being unnecessarily called.
The functions of the private branch-exchange operator are no less
important with respect to outgoing calls. It is the duty of the operator
to obtain connections through the city exchange for the private branch
subscriber, who merely asks for a certain connection and hangs up his
receiver to await her call when she shall have obtained it. This saving
of time of busy people by having the branch-exchange operator make their
calls for them has one attending disadvantage, which is that the person
in the city exchange who is called does not, when he answers his
telephone, find the real party with whom he is to converse, but has to
wait until that party responds to the private branch operator's call.
This is akin to asking a person to call at one's office and then being
out when he gets there. This drawback is greatly accentuated where both
the parties that are to be involved in the connection are people high in
authority in certain establishments at private branch exchanges. Some
business houses have made the rule that the private branch operator
shall not connect with their lines until she has actually heard the
voice of the proper party at the other end. When two subscribers in two
different private branch exchanges where this rule is enforced, attempt
to get into communication with each other, the possibilities of trouble
are obvious.
All that may be said on this matter is that the person who calls another
by telephone should extend that person the same courtesies that he would
had he called him in person to his office; and that a person who is
called by telephone by another should meet him with the same
consideration as if he had received a personal call at his office or
home. The arbitrary ruling made by some corporations and persons, which
results always in the "other fellow's" doing the waiting, is not
ethically correct nor is it good policy.
=Private Branch Switchboards.= Private branch switchboards may be of
common-battery or magneto types regardless of whether they work in
conjunction with main office equipments having common-battery or magneto
equipments. Usually a magneto private branch exchange works in
conjunction with a magneto main office, but this is not always true.
There are cases where the private branch equipment of modern
common-battery type works in conjunction with main office equipment of
the magneto type; and in some of these cases the private branch exchange
has a much larger number of subscribers than the main office. This is
likely to be true in large summer resort hotels located in small and
otherwise unimportant rural districts. In one such case within our
knowledge the private branch exchange has a larger number of stations
than the total census population of the town, resulting in an apparent
telephone development considerably greater than one hundred per cent.
_Magneto Type._ Where both the private branch and the main office
equipments are of the magneto type, the private branch requirements are
met by a simple magneto switchboard of the requisite size, and the
trunking conditions are met by ring-down trunks extending to the main
office. In this case the supervision is that of the ordinary
clearing-out drop type, the operators working together as best they may.
_Common-Battery Type._ The cases where the private branch board is of
common-battery type and the main office of magneto type are
comparatively so few that they need not be treated here. Where they do
occur they demand special treatment because the main portion of the
traffic over the trunk lines to the city or town central office is
likely to be toll traffic through that office over long-distance lines.
The principal reason why the equipment of the town offices under such
conditions is magneto rather than common battery is that the traffic
conditions are those of short season and heavy toll, and common-battery
switching equipment at the main office has no especial advantages for
toll work.
[Illustration: Fig. 438. Desk Type, Private Branch Board]
For small private branch exchanges the desk type of switch board, shown
in Fig. 438, is largely used. The operator frequently has other work to
do and the desk is, therefore, a convenience. In larger private
exchanges, such as those requiring more than one operator, some form of
upright cabinet is employed, and if, as sometimes occurs, the branch
exchange is of such size as to demand a multiple board, then the general
form of the board does not differ materially from the standard types of
multiple board employed in regular central office work. The most common
private branch-exchange condition is that of a common-battery branch
working into a common-battery main office. In such the main point to be
considered is that of supervision of trunk-line connections.
_Cord Type._ For the larger sizes of branch exchange switchboards, the
switching apparatus is practically the same as that of ordinary manual
switchboards wherein the connections are made between the various lines
by means of pairs of cords and plugs. The private branch-exchange trunk
lines usually terminate on the private branch board in jacks but in some
cases plug-ended trunks are used.
[Illustration: Fig. 439. Key Type, Private Branch Board]
The line signals may consist in mechanical visual signals or in lamps,
the choice between these depending largely on the source of battery
supply at the branch exchange, a matter which will be considered later.
The trunk-line signals at the private branch board are usually ordinary
drops which are thrown when the main-exchange operator rings on the line
as she would on an ordinary subscriber's line. Frequently, however, lamp
signals are used for this purpose, being operated by locking relays
energized when the main-office operator rings or, in some cases,
operated at the time when the main-office operator plugs into the
trunk-line jack.
[Illustration: Fig. 440. Circuits, Key-Type Board]
_Key Type._ For small private branch-exchange switchboards, a type
employing no cords and plugs has come into great favor during recent
years. Instead of connecting the lines by jacks and plugs, they are
connected by means of keys closely resembling ordinary ringing and
listening keys. Such a switchboard is shown in Fig. 439, this having a
capacity of three trunks, seven local lines, and the equivalent of five
cord circuits. The drops associated with the three trunks may be seen in
the upper left-hand side of the face of the switchboard. Immediately
below these in three vertical rows are the keys which are used in
connecting the trunks with the "cord circuits" or connecting bus wires.
At the right of the drop associated with the trunks are seven visual
signals, these being the calling signals of the local lines. The seven
vertical rows of keys, immediately to the right of the three trunk-line
rows, are the line keys. The throwing of any one of these keys and of a
trunk-line key in the same horizontal row in the same direction will
connect a line with a trunk through the corresponding bus wires, leaving
one of the supervisory visual signals, shown at the extreme top of the
board, connected with the circuit. The keys in a single row at the right
are those by means of which the operator may bridge her talking set
across any of the "cord circuits." The circuits of this particular board
are shown in Fig. 440. This is equipped for common-battery working, the
battery feed wires being shown at the left.
=Supervision of Private Branch Connections.= At the main office where
common-battery equipment is used, the private branch trunks terminate
before the _A_-operators exactly in the same way as ordinary
subscribers' lines, _i. e._, each in an answering jack and lamp at one
position and in a multiple jack on each section. It goes without saying,
therefore, that the handling of a private branch call, either incoming
or outgoing, should be done by the _A_-operator in the same manner as a
call on an ordinary subscriber's line, and that the supervision of the
connection should impose no special duties on the _A_-operator.
There has been much discussion, and no final agreement, as to the proper
method of controlling the supervisory lamp at the main office of a cord
that is, at the time, connected to a private branch trunk. Three general
methods have been practiced:
The first method is to have the private branch subscriber directly
control the supervisory lamp at the main office without producing any
effect upon the private branch supervisory signal; this latter signal
being displayed only after the connection has been taken down at the
main office and in response to the withdrawal of the main office plug
from the private branch jack. This is good practice so far as the
main-office discipline is concerned but it results in a considerable
disadvantage to both the city and private branch subscribers in that it
is impossible for the private branch subscriber, when connected to the
other, to re-signal the private branch operator without the connection
being first taken down.
The second method is to have the private branch subscriber control both
the supervisory signal at the private branch board and at the main
board. This has the disadvantage of bringing both operators in on the
circuit when the private branch subscriber signals.
The third method, and one that seems best, is to place the supervisory
lamp of the private branch board alone under the control of the private
branch subscriber, so that he may attract the attention of the private
branch operator without disturbing the supervisory signal at the main
office. The supervisory signal at the main office in this case is
displayed only when the private branch operator takes down the
connection. This practice results in a method of operation at the main
office that involves no special action on the part of the _A_-operator.
She takes down the connection only when the main-office subscriber has
hung up his telephone and the private branch subscriber has disconnected
from the trunk.
Whatever method is employed, private branch disconnection is usually
slow, and for this reason many operating companies instruct the
_A_-operators to disconnect on the lighting of the supervisory lamp of
the city subscriber.
=With Automatic Offices.= Private branch exchanges most used in
connection with automatic offices employ manual switchboards, with the
cord circuits of which is associated a signal transmitting device by
which the operator instead of the subscriber may manipulate the
automatic apparatus of the public exchange by impulses sent over the
private branch-exchange trunk lines. The subscriber's equipment at the
private branch stations may be either automatic or manual. Frequently
the same private branch exchange will contain both kinds. With the
manual sub-station equipment the operation is exactly the same as in a
private branch of a manual exchange, except that the private branch
operator by means of her dial makes the central-office connection
instead of telling the main-office operator to do so for her. With
automatic sub-station equipment at the private branch the subscribers,
by removing their receivers from their hooks, call the attention of the
private branch operator, who may receive their orders and make the
desired central-office connection for them, or who may plug their lines
through to the central office and allow the subscribers to make the
connection themselves with their own dials.
In automatic equipment of the common-battery type, some change always
takes place in the calling line at the time the called subscriber
answers. In the three-wire system during the time of calling, both wires
are of the same polarity with respect to earth. At the time of the
answering of the called subscriber, the two wires assume different
polarities, one being positive to the other. Such a change is sufficient
for the actuation of devices local to the private exchange switchboard
and may be interpreted through the calling supervisory signal in such a
way as to allow it to glow during calling and not to glow after the
called subscriber has answered. In the two-wire automatic system a
similar change can be arranged for, with similar advantageous results.
_Secrecy._ In private exchanges operating in connection with automatic
central offices, the secret feature of individual lines may or may not
be carried into the private exchange equipment. Some patrons of
automatic exchanges set a high value on the absence of any operator in a
connection and transact business over such lines which they would not
transact at all over manual lines or would not transact in the same way
over manual lines. To some such patrons, the presence of a private
exchange operator, even though employed and supervised by themselves,
seems to be a disadvantage. To meet such a feeling, it is not difficult
to arrange the circuits of a private exchange switchboard so that the
operator may listen in upon a cord circuit at any time and overhear what
is being said upon it _so long as two subscribers are not in
communication on that cord circuit_. That is, she may answer a call and
may speak to the calling person at any time she wishes until the called
person answers. When he does answer and conversation can take place,
some device operates to disconnect her listening circuit from the cord
circuit, not to be connected again until at least one of the subscribers
has hung up his receiver. With private exchange apparatus so arranged,
the secrecy of the system is complete.
=Battery Supply.= There are three available methods of supplying direct
current for talking and signaling purposes to private branch exchanges,
each of which represents good practice under certain conditions. First,
by means of pairs of wires extended from the central-office battery;
second, by means of a local storage battery at the private branch
exchange charged over wires from the central office; and third, by means
of a local storage battery at the private exchange charged from a local
source.
The choice of these three methods depends always on the local conditions
and it is a desirable feature, to be employed by large operating
companies, to have all private branch-exchange switchboards provided
with simple convertible features contained within the switchboard for
adapting it to any one of these methods of supplying current.
If a direct-current power circuit is available at the private branch
exchange, it may be used for charging the local storage battery by
inserting mere resistance devices in the charging leads. If the local
power circuit carries alternating current, a converting device of some
sort must be used and for this purpose, if the exchange is large enough
to warrant it, a mercury rectifier is an economical and simple device.
The supply of current to private branch exchanges over wires leading to
the central-office battery has the disadvantage of requiring one or
several pairs of wires in the cables carrying the trunk wires. No
special wires are run, regular pairs in the paper insulated line or
trunk cables being admirably suited for the purpose. Sufficient
conductivity may be provided by placing several such pairs in multiple.
If the amount of current required by the private exchange warrants it,
pairs of charging wires from the central office may be fewer if a
battery is charged over them than if they are used direct to the bus
bars of the private exchange switchboard. If they are used in the latter
way, and this is simpler for reasons of maintenance, some means must be
provided to prevent the considerable resistance of the supply wires from
introducing cross-talk into the circuit of the private exchange. This is
accomplished by bridging a considerable capacity across the supply pairs
at the private exchange--ten to twelve microfarads usually suffice. This
point has already been referred to and illustrated in connection with
Fig. 141.
The number of pairs of wires, or, in other words, the amount of copper
in the battery lead between the central office and the private
branch-exchange switchboard needs to be properly determined not only to
eliminate cross-talk when the proper condensers are used with them, but
to furnish the proper difference of potential at the private exchange
bus bars, so that the line and supervisory signals will receive the
proper current. It is a convenience in installing and maintaining
private exchange switchboards of this kind to prepare tables showing the
number of pairs of No. 19 gauge and No. 22 gauge wires required for a
private exchange at a given distance from its central office and of a
probable amount of traffic. The traffic may be expressed in the maximum
number of pairs of cords which will be in use at one time. With this
fact and the distance, the number of pairs of wires required may be
determined.
=Ringing Current.= The ringing current may be provided in two ways: over
pairs of wires from the city-office ringing machines or by means of a
local hand generator, or both. A key should enable either of these
sources of ringing current to be chosen at will.
=Marking of Apparatus.= All apparatus should be marked with permanent
and clear labels. That private exchange switchboard is best at which an
almost uninformed operator could sit and operate it at once. It is not
difficult to lay out a scheme of labels which will enable such a board
to be operated without any detailed instructions being given.
=Desirable Features.= The board should contain means of connecting
certain of the local private exchange lines to the central-office trunks
when the board is unattended. Also, it is desirable that it should
contain means whereby any local private exchange line may be connected
to the trunk so that its station will act as an ordinary subscriber's
station. Whether the trunks of the private exchange lead to a manual or
an automatic equipment, it often is desired to connect a local line
through in that way, either so that the calling person may make his
calls without the knowledge of the private exchange operator, because he
wishes to make a large number of calls in succession, or because for
some other reason he prefers to transact his business directly with or
through the exchange than to entrust it to his operator.