PRINCIPLES OF THE MULTIPLE SWITCHBOARD
=Field of Utility.= The multiple switchboard, unlike the transfer board,
provides means for each operator to complete, without assistance, a
connection with any subscriber's line terminating in the switchboard no
matter how great the number of lines may be. It is used only where the
simple switchboard will not suffice; that is, where the number of lines
and the consequent traffic is so great as to require so many operators
and, therefore, so great a length of board as to make it impossible for
any one operator to reach all over the face of the board without moving
from her position.
=The Multiple Feature.= The fundamental feature of the multiple
switchboard is the placing of a jack for every line served by the
switchboard within the reach of every operator. This idea underlying the
multiple switchboard may be best grasped by merely considering the
mechanical arrangement and grouping of parts without regard to their
details of operation. The idea is sometimes elusive, but it is really
very simple. If the student at the outset will not be frightened by the
very large number of parts that are sometimes involved in multiple
switchboards, and by the great complexity which is apparent in the
wiring and in the action of these parts; and will remember that this
apparent complexity results from the great number of repetitions of the
same comparatively simple group of apparatus and circuits, much will be
done toward a mastery of the subject.
The multiple switchboard is divided into sections, each section being
about the width and height that will permit an ordinary operator to
reach conveniently all over its face. The usual width of a section
brought about by this limitation is from five and one-half to six feet.
Such a section affords room for three operators to sit side by side
before it. Now each line, instead of having a single jack as in the
simple switchboard, is provided with a number of jacks and one of these
is placed on each of the sections, so that each one of the operators may
have within her reach a jack for each line. It is from the fact that
each line has a multiplicity of jacks, that the term multiple
switchboard arises.
_Number of Sections._ Since there is a jack for each line on each
section of the switchboard, it follows that on each section there are as
many jacks as there are lines; that is, if the board were serving 5,000
lines there would be 5,000 jacks. Let us see now what it is that
determines the number of sections in a multiple switchboard. In the
final analysis, it is the amount of traffic that arises in the busiest
period of the day. Assume that in a particular office serving 5,000
lines, the subscribers call at such a very low rate that even at the
busiest time of the day only enough calls are made to keep, say, three
operators busy. In this case there would be no need for the multiple
switchboard, for a single section would suffice. The three operators
seated before that section would be able to answer and complete the
connections for all of the calls that arose. But subscribers do not call
at this exceedingly low rate. A great many more calls would arise on
5,000 lines during the busiest hour than could be handled by three
operators and, therefore, a great many more operators would be required.
Space has to be provided for these operators to work in, and as each
section accommodates three operators the total number of sections must
be at least equal to the total number of required operators divided by
three.
Let us assume, for instance, that each operator can handle 200 calls
during the busy hour. Assume further that during the busy hour the
average number of calls made by each subscriber is two. One hundred
subscribers would, therefore, originate 200 calls within this busy hour
and this would be just sufficient to keep one operator busy. Since one
operator can handle only the calls of one hundred subscribers during the
busy hour, it follows that as many operators must be employed as there
are hundreds of subscribers whose lines are served in a switchboard, and
this means that in an exchange of 5,000 subscribers, 50 operators'
positions would be required, or 16-2/3 sections. Each of these sections
would be equipped with the full 5,000 jacks, so that each operator could
have a connection terminal for each line.
_The Multiple._ These groups of 5,000 jacks, repeated on each of the
sections are termed multiple jacks, and the entire equipment of these
multiple jacks and their wiring is referred to as the multiple. It will
be shown presently that the multiple jacks are only used for enabling
the operator to connect with the called subscriber. In other words these
jacks are for the purpose of enabling each operator to have within her
reach any line that may be called for regardless of what line originates
the call. We will now consider what arrangements are provided for
enabling the operator to receive the signal indicating a call and what
provisions are made for her to answer the call in response to such a
signal.
=Line Signals.= Obviously it is not necessary to have the line signals
repeated on each section of the board as are the multiple jacks. If a
line has one definite place on the switchboard where its signal may be
received and its call may be answered, that suffices. Each line,
therefore, in addition to having its multiple jacks distributed one on
each section of the switchboard, has a line signal and an individual
jack immediately associated with it, located on one only of the
sections. This signal usually is in the form of a lamp and is termed the
line signal, and this jack is termed the answering jack since it is by
means of it that the operator always answers a call in response to the
line signal.
_Distribution of Line Signals._ It is evident that it would not do to
have all of these line signals and answering jacks located at one
section of the board for then they would not be available to all of the
operators. They are, therefore, distributed along the board in such a
way that one group of them will be available to one operator, another
group to another operator, and so on; the number of answering jacks and
signals in any one group being so proportioned with respect to the
number of calls that come in over them during the busy hour that it will
afford just about enough calls to keep the operator at that position
busy.
We may summarize these conditions with respect to the jack and
line-signal equipment of the multiple switchboard by saying that each
line has a multiple jack on each section of the board and in addition to
this has on one section of the board an answering jack and a line
signal. These answering jacks and line signals are distributed in groups
along the face of the board so that each operator will receive her
proper quota of the originating calls which she will answer and, by
virtue of the multiple jack, be able to complete the connections with
the desired subscribers without moving from her position.
=Cord Circuits.= Each operator is also provided with a number of pairs
of cords and plugs with proper supervisory or clearing-out signals and
ringing and listening keys, the arrangement in this respect being
similar to that already described in connection with the simple
switchboard.
=Guarding against Double Connections.= From what has been said it is
seen that a call originating on a given line may be answered at one
place only, but an outgoing connection with that line may be made at any
position. This fact that a line may be connected with when called for at
any one of the sections of the switchboard makes necessary the provision
that two or more connections will not be made with the same line at the
same time. For instance, if a call came in over a line whose signal was
located on the first position of the switchboard for a connection with
line No. 1,000, the operator at the first position would connect this
calling line with No. 1,000 through the multiple jack on the first
section of the switchboard. Assume now that some line, whose signal was
located on the 39th position of the switchboard, should call also for
line No. 1,000 while that line was still connected with the first
calling subscriber. Obviously confusion would result if the operator at
the 39th position, not knowing that line No. 1,000 was already busy,
should connect this second line with it, thereby leaving both of the
calling subscribers connected with line No. 1,000, and as a result all
of these three subscribers connected together.
The provisions for suitable means for preventing the making of a
connection with a line that is already switched at some other section of
the switchboard, has offered one of the most fertile fields for
invention in the whole telephone art. The ways that have been proposed
for accomplishing this are legion. Fortunately common practice has
settled on one general plan of action and that is to so arrange the
circuits that whenever a line is switched at one section, such an
electrical condition will be established on the forward contacts of all
of its multiple jacks that any operator at any other section in
attempting to make a connection with that line will be notified of the
fact that it is already switched by an audible signal, which she will
receive in her head receiver. On the other hand the arrangement is such
that when a line is not busy, _i. e._, it is not switched at any of the
positions of the switchboard, the operator on attempting to make a
connection with such a line will receive no such guarding signal and
will, therefore, proceed with the connection.
We may liken a line in a multiple switchboard to a lane having a number
of gates giving access to it. One of these gates--the answering jack--is
for the exclusive use of the proprietor of that lane. All of the other
gates to the lane--the multiple jacks--are for affording means for the
public to enter. But whenever any person enters one of these gates, a
signal is automatically put up at all of the other gates forbidding any
other person to enter the lane as long as the first person is still
within.
[Illustration: Fig. 336. Principle of Multiple Switchboard]
=Diagram Showing Multiple Board Principle.= For those to whom the
foregoing description of the multiple board is not altogether clear, the
diagram of Fig. 336 may offer some assistance. Five subscribers' lines
are shown running through four sections of a switchboard. Each of these
lines is provided with a multiple jack on each section of the board.
Each line is also provided with an answering jack and a line signal on
one of the sections of the board. Thus the answering jacks and the line
signals of lines _1_ and _2_ are shown in Section I, that of line _4_ is
shown in Section II, that of line _3_ in Section III, and that of line
_5_ in Section IV. At Section I, line _1_ is shown in the condition of
having made a call and having had this call answered by the operator
inserting one of her plugs into its answering jack. In response to the
instructions given by the subscriber, the operator has inserted the
other plug of this same pair in the multiple jack of line _2_, thus
connecting these two lines for conversation. At Section III, line _3_ is
shown as having made a call, and the operator as having answered by
inserting one of her plugs into the answering jack. It happens that the
subscriber on line _3_ requests a connection with line _1_, and the
condition at Section III is that where the operator is about to apply
the tip of the calling plug to the jack of line _1_ to ascertain whether
or not that line is busy. As before stated, when the contact is made
between the tip of the calling plug and the forward contact of the
multiple jack, the operator will receive a click in the ear (by means
that will be more fully discussed in later chapters), this click
indicating to her that line _1_ is not available for connection because
it is already switched at some other section of the switchboard.
=Busy Test.= The busy signal, by which an operator in attempting to make
a connection is informed that the line is already busy, has assumed a
great variety of forms and has brought forth many inventions. It has
been proposed by some that the insertion of a plug into any one of the
jacks of a line would automatically close a little door in front of each
of the other jacks of the line, therefore making it impossible for any
other operator to insert a plug as long as the line is in use. It has
been proposed by others to ring bells or to operate buzzers whenever the
attempt was made by an operator to plug into a line that was already in
use. Still others have proposed to so arrange the circuits that the
operator would get an electric shock whenever she attempted to plug into
a busy line. The scheme that has met with universal adoption, however,
is that the operator shall, when the tip of her calling plug touches the
forward contact of the jack of a line that is already switched, receive
a click in her telephone which will forbid her to insert the plug. The
absence of this click, or silence in her telephone, informs her that she
may safely make the connection.
_Principle._ The means by which the operator receives or fails to
receive this click, according to whether the line is busy or idle, vary
widely, but so far as the writers are aware they all have one
fundamental feature in common. The tip of the calling plug and the test
contact of all of the multiple jacks of an idle line must be absolutely
at the same potential before the test, so that no current will flow
through the test circuit when the test is actually made. The test
thimbles of all the jacks of a busy line must be at a different
potential from the tip of the test plug so that a current will flow and
a click result when the test is made.
_Potential of Test Thimbles._ It has been found an easy matter to so
arrange the contacts in the jacks of a multiple switchboard that
whenever the line is idle the test thimbles of that line will be a
certain potential, the same as that of all the unused calling plug tips.
It has also been easy to so arrange these contacts that the insertion of
a plug into any one of the jacks will, by virtue of the contacts
established, change the potential of all the test thimbles of that line
so that they will be at a different potential from that of the tips of
the calling plugs. It has not been so easy, however, to provide that
these conditions shall exist under all conditions of practice. A great
many busy tests that looked well on paper have been found faulty in
practice. As is always the case in such instances, this has been true
because the people who considered the scheme on paper did not foresee
all of the conditions that would arise in practice. Many busy-test
systems will operate properly while everything connected with the
switchboard and the lines served by it remains in proper order. But no
such condition as this can be depended on in practice. Switchboards, no
matter how perfectly made and no matter with how great care they may be
installed and maintained, will get out of order. Telephone lines will
become grounded or short-circuited or crossed or opened. Such
conditions, in a faulty busy-test system, may result in a line that is
really idle presenting a busy test, and thus barring the subscriber on
that line from receiving calls from other lines just as completely as if
his line were broken. On the other hand, faulty conditions either in the
switchboard or in the line may make a line that is really busy, test
idle, and thus result in the confusion of having two or more subscribers
connected to the same line at the same time.
_Busy-Test Faults._ To show how elusive some of the faults of a busy
test may be, when considered on paper, it has come within the
observation of the writers that a new busy-test system was thought well
enough of by a group of experienced engineers to warrant its
installation in a group of very large multiple switchboards, the cost of
which amounted to hundreds of thousands of dollars, and yet when so
installed it developed that a single short-circuited cord in a position
would make the test inoperative on all the cords of that
position--obviously an intolerable condition. Luckily the remedy was
simple and easily applied.
In a well-designed busy-test system there should be complete silence
when the test is made of an idle line, and always a well-defined click
when the test is made of a busy line. The test on busy lines should
result in a uniform click regardless of length of lines or the condition
of the apparatus. It does not suffice to have a little click for an idle
line and a big click for a busy line, as practice has shown that this
results in frequent errors on the part of the operators.
Good operating requires that the tip of the calling plug be tapped
against the test thimble several times in order to make sure of the
state of the called line.
In some multiple switchboards the arrangement has been such that the
jacks of a line would test busy as soon as the subscriber on that line
removed his receiver from its hook to make a call, as well as while any
plug was in any jack of that line. The advocates of this added feature,
in connection with the busy test, have claimed that the receiver, when
removed from its hook in making a call, should make the line test busy
and that a line should not be connected with when the subscriber's
receiver was off its hook any more than it should be when it was already
connected with at some other section of the switchboard. While it is
true that a line may be properly termed busy when the subscriber has
removed his receiver in order to make a call, it is not true that there
is any real necessity for guarding against a connection with it while he
is waiting for the operator to answer. Leaving the line unguarded for
this brief period may result in the subscriber, who intended to make the
call, having to defer his call until he has conversed with the party who
is trying to reach him. This cannot be said to be a detriment to the
service, however, since the second party gets the connection he desires
much sooner than he otherwise would, and the first party may still make
his first intended call as soon as he has disposed of the party who
reached him while he was waiting for his own operator to answer. It may
be said, therefore, in connection with this matter of making the line
test busy as soon as a subscriber has removed his receiver from the
hook, that it is not considered an essential, and in case of those
switchboard systems which naturally work out that way it is not
considered a disadvantage.
=Field of Each Operator.= It was stated earlier in this chapter that as
each section accommodated three operators, the total number of sections
in a switchboard will be at least one-third the total number of required
operators. This thought needs further development, for to stop at that
statement is to arrive somewhat short of the truth. In order to do this
it is necessary to consider the field in the multiple, reached by each
operator. The section is of such size, or should be, that an operator
seated in the center position of it may, without undue effort, reach all
over the multiple. But the operator at the right-hand position cannot
reach the extreme left portion of the multiple of that section, nor can
the operator at the left reach the extreme right. How then may each
operator reach a jack for every line? Remembering that the multiple
jacks are arranged exactly the same in each section, each jack always
occupying the same relative position, it is easy to see that while the
operator at a right-hand position of a section cannot reach the
left-hand third of the multiple in her own section, she may reach the
left-hand third of the multiple in the section at her right, and this,
together with the center and right-hand thirds of her own section,
represents the entire number of lines. So it is with the left-hand
operator at any section, she reaches two-thirds of all the lines in the
multiple of her own section and one-third in that of the section at her
left.
_End Positions._ This makes it necessary to inquire about the operators
at the end positions of the entire board. To provide for these the
multiple is extended one-third of a section beyond them, so as to supply
at the ends of the switchboard jacks for those lines which the end
operators cannot reach on their own sections. Sometimes instead of
adding these end sections to the multiple for the end operators, the
same result is accomplished by using only the full and regular sections
of the multiple, and leaving the end positions without operators'
equipment, as well as without answering jacks, line signals, and cords
and plugs, so that in reality the end operator is at the middle position
of the end section. This, in our opinion, is the better practice, since
it leaves the sections standard, and makes it easier to extend the
switchboard in length, as it grows, by the mere addition of new sections
without disturbing any of the old multiple.
=Influence of Traffic.= We wish again to emphasize the fact that it is
the traffic during the busiest time of day and not the number of lines
that determine the size of a multiple switchboard so far as its length
is concerned. The number of lines determines the size of the multiple in
any one section, but it is the amount of traffic, the number of calls
that are made in the busiest period, that determines the number of
operators required, and thus the number of positions. Had this now very
obvious fact been more fully realized in the past, some companies would
be operating at less expense, and some manufacturers would have sold
less expensive switchboards.
The whole question as to the number of positions boils down to how many
answering jacks and line signals may be placed at each operator's
position without overburdening the operator with incoming traffic at the
busy time of day. Obviously, some lines will call more frequently than
others, and hence the proper number of answering jacks at the different
positions will vary. Obviously, also, due to changes in the personnel of
the subscribers, the rates of calling of different groups of lines will
change from time to time, and this may necessitate a regrouping of the
line signals and answering jacks on the positions; and changes in the
personnel of the operators or in their skill also demand such
regrouping.
_Intermediate Frame._ The intermediate distributing frame is provided
for this purpose, and will be more fully discussed in subsequent
chapters. Suffice it to say here that the intermediate distributing
frame permits the answering jacks and line signals to be shifted about
among the operators' positions, so that each position will have just
enough originating traffic to keep each of the operators economically
busy during the busiest time of the day.