TRUNKING IN MULTI-OFFICE SYSTEMS
It has been stated that a single exchange may involve a number of
offices, in which case it is termed a multi-office exchange. In a
multi-office exchange, switchboards are necessary at each office in
which the subscribers' lines of the corresponding office district
terminate. Means for intercommunication between the subscribers in one
office and those in any other office are afforded by inter-office trunks
extended between each office and each of the other offices.
If the character of the community is such that each of the offices has
so few lines as to make the simple switchboard suffice for its local
connections, then the trunking between the offices may be carried out in
exactly the same way as explained between the various simple
switchboards in a transfer system, the only difference being that the
trunks are long enough to reach from one office to another instead of
being short and entirely local to a single office. Such a condition of
affairs would only be found in cases where several small communities
were grouped closely enough together to make them operate as a single
exchange district, and that is rather unusual.
The subject of inter-office trunking so far as manual switchboards are
concerned is, therefore, confined mainly to trunking between a number of
offices each equipped with a manual multiple switchboard.
=Necessity for Multi-Office Exchanges.= Before taking up the details of
the methods and circuits employed in trunking in multi-office systems,
it may be well to discuss briefly why the multi-office exchange is a
necessity, and why it would not be just as well to serve all of the
subscribers in a large city from a single huge switchboard in which all
of the subscribers' lines would terminate. It cannot be denied, when
other things are equal, that it is better to have only one operator
involved in any connection which means less labor and less liability of
error.
The reasons, however, why this is not feasible in really large
exchanges are several. The main one is that of the larger investment
required. Considering the investment first from the standpoint of the
subscriber's line, it is quite clear that the average length of
subscriber's line will be very much greater in a given community if all
of the lines are run to a single office, than will be the case if the
exchange district is divided into smaller office districts and the lines
run merely from the subscribers to the nearest office. There is a direct
and very large gain in this respect, in the multi-office system over the
single office system in large cities, but this is not a net gain, since
there is an offsetting investment necessary in the trunk lines between
the offices, which of course are separate from the subscribers' lines.
Approaching the matter from the standpoint of switchboard construction
and operation, another strong reason becomes apparent for the employment
of more than one office in large exchange districts. Both the
difficulties of operation and the expense of construction and
maintenance increase very rapidly when switchboards grow beyond a
certain rather well-defined limit. Obviously, the limitation of the
multiple switchboard as to size involves the number of multiple jacks
that it is feasible to place on a section. Multiple switchboards have
been constructed in this country in which the sections had a capacity of
18,000 jacks. Schemes have been proposed and put into effect with
varying success, for doubling and quadrupling the capacity of multiple
switchboards, one of these being the so-called divided multiple board
devised by the late Milo G. Kellogg, and once used in Cleveland, Ohio,
and St. Louis, Missouri. Each of these boards had an ultimate capacity
of 24,000 lines, and each has been replaced by a "straight" multiple
board of smaller capacity. In general, the present practice in America
does not sanction the building of multiple boards of more than about
10,000 lines capacity, and as an example of this it may be cited that
the largest standard section manufactured for the Bell companies has an
ultimate capacity of 9,600 lines.
European engineers have shown a tendency towards the opposite practice,
and an example of the extreme in this case is the multiple switchboard
manufactured by the Ericsson Company, and installed in Stockholm, in
which the jacks have been reduced to such small dimensions as to permit
an ultimate capacity of 60,000 lines.
The reasons governing the decision of American engineers in
establishing the practice of employing no multiple switchboards of
greater capacity than about 10,000 lines, briefly outlined, are as
follows: The building of switchboards with larger capacity, while
perfectly possible, makes necessary either a very small jack or some
added complexity, such as that of the divided multiple switchboard,
either of which is considered objectionable. Extremely small jacks and
large multiples introduce difficulties as to the durability of the jacks
and the plugs, and also they tend to slow down the work of operators and
to introduce errors. They also introduce the necessity of a smaller
gauge of wire through the multiple than it has been found desirable to
employ. Considered from the standpoint of expense, it is evident that as
a multiple switchboard increases in number of lines, its size increases
in two dimensions, _i. e._, in length of board and height of section,
and this element of expense, therefore, is a function of the square of
the number of lines.
The matter of insurance, both with respect to the risk as to property
loss and the risk as to breakdown of the service, also points distinctly
in the direction of a plurality of offices rather than one. Both from
the standpoint of risk against fire and other hazards, which might
damage the physical property, and of risk against interruption to
service due to a breakdown of the switchboard itself, or a failure of
its sources of current, or an accident to the cable approaches, the
single office practice is like putting all one's eggs in one basket.
Another factor that has contributed to the adoption of smaller
switchboard capacities is the fact that in the very large cities even a
40,000 line multiple switchboard would still not remove the necessity of
multi-office exchanges with the consequent certainty that a large
proportion of the calls would have to be trunked anyway.
Undoubtedly, one of the reasons for the difference between American and
European practice is the better results that American operating
companies have been able to secure in the handling of calls at the
incoming end of trunks. This is due, no doubt, in part to the
differences in social and economic conditions under which exchanges are
operated in this country and abroad, and also in part to the
characteristics of the English tongue when compared to some of the other
tongues in the matter of ease with which numbers may be spoken. In
America it has been found possible to so perfect the operation of
trunking under proper operating conditions and with good equipment as to
relieve multi-office practice of many of the disadvantages which have
been urged against it.
=Classification.= Broadly speaking there are two general methods that
may be employed in trunking between exchanges. The first and simplest of
these methods is to employ so-called _two-way trunks_. These, as their
name indicates, may be used for completing connections between offices
in either direction, that is, whether the call originates at one end or
the other. The other way is by the use of _one-way trunks_, wherein each
trunk carries traffic in one direction only. Where such is the case, one
end of the trunk is always used for connecting with the calling
subscriber's line and is termed the _outgoing_ end, and the other end is
always used in completing the connection with the called subscriber's
line, and is referred to as the _incoming_ end. Traffic in the other
direction is handled by another set of trunks differing from the first
set only in that their outgoing and incoming ends are reversed.
As has already been pointed out, a system of trunks employing two-way
trunks is called a _single-track system_, and a system involving two
sets of one-way trunks is called a _double-track system_. It is to be
noted that the terms outgoing and incoming, as applied to the ends of
trunks and also as applied to traffic, always refer to the direction in
which the trunk handles traffic or the direction in which the traffic is
flowing with respect to the particular office under consideration at the
time. Thus an _incoming trunk_ at one office is an _outgoing trunk_ at
the other.
_Two-Way Trunks._ Two-way trunks are nearly always employed where the
traffic is very small and they are nearly always operated by having the
_A_-operator plug directly into the jack at her end of the trunk and
displaying a signal at the other end by ringing over the trunk as she
would over an ordinary subscriber's line. The operator at the distant
exchange answers as she would on an ordinary line, by plugging into the
jack of that trunk, and receives her orders over the trunk either from
the originating operator or from the subscriber, and then completes the
connection with the called subscriber. Such trunks are often referred to
as "ring-down" trunks, and their equipment consists in a drop and jack
at each end. In case there is a multiple board at either or both of the
offices, then the equipment at each end of the trunk would consist of a
drop and answering jack, together with the full quota of multiple jacks.
It is readily seen that this mode of operation is slow, as the work that
each operator has to do is the same as that in connecting two local
subscribers, plus the time that it takes for the operators to
communicate with each other over the trunk.
_One-Way Trunks._ Where one-way trunks are employed in the double-track
system, the trunks, assuming that they connect multiple boards, are
provided with multiple jacks only at their outgoing ends, so that any
operator may reach them for an outgoing connection, and at their
incoming ends they terminate each in a single plug and in suitable
signals and ringing keys, the purpose of which will be explained later.
Over such trunks there is no verbal communication between the operators,
the instructions passing between the operators over separate order-wire
circuits. This is done in order that the trunk may be available as much
as possible for actual conversation between the subscribers. It may be
stated at this point that the duration of the period from the time when
a trunk is appropriated by the operators for the making of a certain
connection until the time when the trunk is finally released and made
available for another connection is called the _holding time_, and this
holding time includes not only the period while the subscribers are in
actual conversation over it, but also the periods while the operators
are making the connection and afterwards while they are taking it down.
It may be said, therefore, that the purpose of employing separate order
wires for communication between the operators is to make the holding
time on the trunks as small as possible and, therefore, for the purpose
of enabling a given trunk to take part in as many connections in a given
time as possible.
In outline the operation of a one-way trunk between common-battery,
manual, multiple switchboards is, with modifications that will be
pointed out afterwards, as follows: When a subscriber's line signal is
displayed at one office, the operator in attendance at that position
answers and finding that the call is for a subscriber in another office,
she presses an order-wire key and thereby connects her telephone set
directly with that of a _B_-operator at the proper other office. Unless
she finds that other operators are talking over the order wire, she
merely states the number of the called subscriber, and the _B_-operator
whose telephone set is permanently connected with that order wire merely
repeats the number of the called subscriber and follows this by
designating the number of the trunk which the _A_-operator is to employ
in making the connection. The _A_-operator, thereupon, immediately and
without testing, inserts the calling plug of the pair used in answering
the call into the trunk jack designated by the _B_-operator; the
_B_-operator simultaneously tests the multiple jack of the called
subscriber and, if she finds it not busy, inserts the plug of the
designated trunk into the multiple jack of the called subscriber and
rings his bell by pressing the ringing key associated with the trunk
cord used. The work on the part of the _A_-operator in connecting with
the outgoing end of the trunk and on the part of the _B_-operator in
connecting the incoming end of the trunk with the line goes on
simultaneously, and it makes no difference which of these operators
completes the connection first.
It is the common practice of the Bell operating companies in this
country to employ what is called automatic or machine ringing in
connection with the _B_-operator's work. When the _B_-operator presses
the ringing key associated with the incoming trunk cord, she pays no
further attention to it, and she has no supervisory lamp to inform her
as to whether or not the subscriber has answered. The ringing key is
held down, after its depression by the operator, either by an
electromagnet or by a magnet-controlled latch, and the ringing of the
subscriber's bell continues at periodic intervals as controlled by the
ringing commutator associated with the ringing machine. When the
subscriber answers, however, the closure of his line circuit results in
such an operation of the magnet associated with the ringing key as to
release the ringing key and thus to automatically discontinue the
ringing current.
When a connection is established between two subscribers through such a
trunk the supervision of the connection falls entirely upon the
_A_-operator who established it. This means that the calling supervisory
lamp at the _A_-operator's position is controlled over the trunk from
the station of the called subscriber, the answering supervisory lamp
being, of course, under the control of the calling subscriber as in the
case of a local connection. It is, therefore, the _A_-operator who
always initiates the taking down of a trunk connection, and when, in
response to the lighting of the two lamps, she withdraws her calling
plug from the trunk jack, the supervisory lamp associated with the
incoming end of the trunk at the other office is lighted, and the
_B_-operator obeys it by pulling down the plug.
If, upon testing the multiple jack of the called subscriber's line, the
_B_-operator finds the line to be busy, she at once inserts the trunk
plug into a so-called "busy-back" jack, which is merely a jack whose
terminals are permanently connected to a circuit that is intermittently
opened and closed, and which also has impressed upon it an alternating
current of such a nature as to produce the familiar "buzz-buzz" in a
telephone receiver. The opening and closing of this circuit causes the
calling supervisory lamp of the _A_-operator to flash at periodic
intervals just as if the called subscriber had raised and lowered his
receiver, but more regularly. This is the indication to the _A_-operator
that the line called for is busy. The buzzing sound is repeated back
through the cord circuit of the _A_-operator to the calling subscriber
and is a notification to him that the line is busy.
Sometimes, as is practiced in New York City, for instance, the buzzing
feature is omitted, and the only indication that the calling subscriber
receives that the called-for line is busy is being told so by the
_A_-operator. This may be considered a special feature and it is
employed in New York because there the custom exists of telling a
calling subscriber, when the line he has called for has been found busy,
that the party will be secured for him and that he, the calling
subscriber, will be called, if he desires.
A modification of this busy-back feature that has been employed in
Boston, and perhaps in other places, is to associate with the busy-back
jack at the _B_-operator's position a phonograph which, like a parrot,
keeps repeating "Line busy--please call again." Where this is done the
calling subscriber, _if he understands what the phonograph says_, is
supposed to hang up his receiver, at which time the _A_-operator takes
down the connection and the _B_-operator follows in response to the
notification of her supervisory lamp. The phonograph busy-back scheme,
while ingenious, has not been a success and has generally been
abandoned.
As a rule the independent operating companies in this country have not
employed automatic ringing, and in this case the _B_-operators have
been required to operate their ringing keys and to watch for the
response of the called subscriber. In order to arrange for this, another
supervisory lamp, termed the _ringing lamp_, is associated with each
incoming trunk plug, the going out of this lamp being a notification to
the _B_-operator to discontinue ringing.
=Western Electric Trunk Circuits.= The principles involved in
inter-office trunking with automatic ringing, are well illustrated in
the trunk circuit employed by the Western Electric Company in connection
with its No. 1 relay boards. The dotted dividing line through the center
of Fig. 371 represents the separating space between two offices. The
calling subscriber's line in the first office is shown at the extreme
left and the called subscriber's line in the second office is shown at
the extreme right. Both of these lines are standard multiple switchboard
lines of the form already discussed. The equipment illustrated in the
first office is that of an _A_-board, the cord circuit shown being that
of the regular _A_-operator. The outgoing trunk jacks connecting with
the trunk leading to the other office are, it will be understood,
multipled through the _A_-sections of the board and contain no relay
equipment, but the test rings are connected to ground through a
resistance coil _1_, which takes the place of the cut-off relay winding
of a regular line so far as test conditions and supervisory relay
operation are concerned. The equipment illustrated in the second office
is that of a _B_-board, it being understood that the called subscriber's
line is multipled through both the _A_- and _B_-boards at that office.
The part of the equipment that is at this point unfamiliar to the reader
is, therefore, the cord circuit at the _B_-operator's board. This
includes, broadly speaking, the means: (1) for furnishing battery
current to the called subscriber; (2) for accomplishing the ringing of
the called subscriber and for automatically stopping the ringing when he
shall respond; (3) for performing the ordinary switching functions in
connection with the relays of the called subscriber's line in just the
same way that an _A_-operator's cord carries out these functions; and
(4) for causing the operation of the calling supervisory relay of the
_A_-operator's cord circuit in just the same manner, under control of
the connected called subscriber, as if that subscriber's line had been
connected directly to the _A_-operator's cord circuit.
[Illustration: Fig. 371. Inter-Office Connection--Western Electric
System]
The operation of these devices in the _B_-operator's cord circuit may
be best understood by following the establishment of the connection.
Assuming that the calling subscriber in the first office desires a
connection with the subscriber's line shown in the second office, and
that the _A_-operator at the first office has answered the call, she
will then communicate by order wire with the _B_-operator at the second
office, stating the number of the called subscriber and receiving from
that operator in return the number of the trunk to be employed. The two
operators will then proceed simultaneously to establish the connection,
the _A_-operator inserting the calling plug into the outgoing trunk
jack, and the _B_-operator inserting the trunk plug into the multiple
jack of the called subscriber's line after testing. We will assume at
first that the called subscriber's line is found idle and that both of
the operators complete their respective portions of the work at the same
time and we will consider first the condition of the calling supervisory
relay at the _A_-operator's position.
The circuit of the calling supervisory lamp will have been closed
through the resistance coil _1_ connected with the outgoing trunk jacks
and the lamp will be lighted because, as will be shown, it is not yet
shunted out by the operation of its associated supervisory relay.
Tracing the circuit of the calling supervisory relay of the
_A_-operator's circuit, it will be found to pass from the live side of
the battery to the ring side of the trunk circuit through one winding of
the repeating coil of the _B_-operator's cord; beyond this the circuit
is open, since no path exists through the condenser _2_ bridged across
the trunk circuit or through the normally open contacts of the relay _3_
connected in the talking circuit of the trunk. The association of this
relay _3_ with the repeating coil and the battery of the trunk is seen
to be just the same as that of a supervisory relay in the _A_-operator's
cord, and it is clear, therefore, that this relay _3_ will not be
energized until the called subscriber has responded. When it is
energized it will complete the path to ground through the _A_-operator's
calling supervisory relay and operate to shunt out the _A_-operator's
calling supervisory lamp in just the same manner as if the
_A_-operator's calling plug had been connected directly with the line of
the calling subscriber. In other words, the called subscriber in the
second office controls the relay _3_, which, in turn, controls the
calling supervisory relay of the _A_-operator, which, in turn, shunts
out its lamp.
The connection being completed between the two subscribers, the
_B_-operator depresses one or the other of the ringing keys _5_ or _6_,
according to which party on the line is called, assuming that it is a
two-party line. It will be noticed that the springs of these ringing
keys are not serially arranged in the talking circuit, but the cutting
off of the trunk circuit back of the ringing keys is accomplished by the
set of springs shown just at the left of the ringing keys, which set of
springs _7_ is operated whenever either one of the ringing keys is
depressed. An auxiliary pair of contacts, shown just below the group of
springs _7_, is also operated mechanically whenever either one of the
ringing keys is depressed, and this serves to close one of two normally
open points in the circuit of the ringing-key holding magnet _8_. This
holding magnet _8_ is so arranged with respect to the contacts of the
ringing key that whenever any one of them is depressed by the operator,
it will be held depressed as long as the magnet is energized just the
same as if the operator kept her finger on the key. The other normally
open point in the circuit of the holding magnet _8_ is at the lower pair
of contacts of the test and holding relay _9_. This relay is operated
whenever the trunk plug is inserted in the jack of a called line,
regardless of the position of the subscriber's equipment on that line.
The circuit may be traced from the live side of the battery through the
trunk disconnect lamp _4_, coil _9_, sleeve strand of cord, and to
ground through the cut-off relay of the line. The insertion of the trunk
plug into the jack thus leaves the completion of the holding-magnet
circuit dependent only upon the auxiliary contact on the ringing key,
and, therefore, as soon as the operator presses either one of these
keys, the clutch magnet is energized and the key is held down, so that
ringing current continues to flow at regular intervals to the called
subscriber's station.
The ringing current issues from the generator _10_, but the supply
circuit from it is periodically interrupted by the commutator _11_
geared to the ringing-machine shaft. This periodically interrupted
ringing current passes to the ringing-key contacts through the coil of
the ringing cut-off relay _12_, and thence to the subscriber's line. The
ringing current is, however, insufficient to cause the operation of this
relay _12_ as long as the high resistance and impedance of the
subscriber's bell and condenser are in the circuit. It is, however,
sufficiently sensitive to be operated by this ringing current when the
subscriber responds and thus substitutes the comparatively low
resistance and impedance path of his talking apparatus for the previous
path through his bell. The pulling up of the ringing cut-off relay _12_
breaks a third normally closed contact in the circuit of the holding
coil _8_, de-energizing that coil and releasing the ringing key, thus
cutting off ringing current. There is a third brush on the commutator
_11_ connected with the live side of the central battery, and this is
merely for the purpose of assuring the energizing of the ringing cut-off
relay _12_, should the subscriber respond during the interval while the
commutator _11_ held the ringing current cut off. The relay _12_ may
thus be energized either from the battery, if the subscriber responds
during a period of silence of his ringer, or from the generator _10_, if
the subscriber responds during a period while his bell is sounding; in
either case the ringing current will be promptly cut off by the release
of the ringing key.
The trunk operator's "disconnect lamp" is shown at _4_, and it is to be
remembered that this lamp is lighted only when the _A_-operator takes
down the connection at her end, and also that this lamp is entirely out
of the control of the subscribers, the conditions which determine its
illumination being dependent on the positions of the operators' plugs at
the two ends of the trunk. With both plugs up, the lamp _4_ will receive
current, but will be shunted to prevent its illumination. The path over
which it receives this current may be traced from battery through the
lamp _4_, thence through the coil of the relay _9_ and the cut-off relay
of the called subscriber's line. This current would be sufficient to
illuminate the lamp, but the lamp is shunted by a circuit which may be
traced from the live side of battery through the contact of the relay
_13_, closed at the time, and through the coil of the trunk cut-off
relay coil _14_. The resistance of this coil is so proportioned to the
other parts of the circuit as to prevent the illumination of the lamp
just exactly as in the case of the shunting resistances of the lamps in
the _A_-operator's cord. It will be seen, therefore, that the supply of
current to the trunk disconnect lamp is dependent on the trunk plug
being inserted into the jack of the subscriber's line and that the
shunting out of this lamp is dependent on the energization of the relay
_13_. This relay _13_ is energized as long as the _A_-operator's plug is
inserted into the outgoing trunk jack, the path of the energizing
circuit being traced from the live side of the battery at the second
office through the right-hand winding of this relay, thence over the tip
side of the trunk to ground at the first office. From this it follows
that as long as both plugs are up, the disconnect lamp will receive
current but will be shunted out, and as soon as the _A_-operator pulls
down the connection, the relay _13_ will be de-energized and will thus
remove the shunt from about the lamp, allowing its illumination. The
left-hand winding of the relay _13_ performs no operating function, but
is merely to maintain the balance of the talking circuit, it being
bridged during the connection from the ring side of the trunk to ground
in order to balance the bridge connection of the right-hand coil from
the live side of battery to the tip side of the trunk circuit.
The relay _14_, already referred to as forming a shunt for the trunk
disconnect lamp, has for its function the keeping of the talking circuit
through the trunk open until such time as the relay _13_ operates, this
being purely an insurance against unnecessary ringing of a subscriber in
case the _A_-operator should by mistake plug into the wrong trunk. It is
not, therefore, until the _A_-operator has plugged into the trunk and
the relay _13_ has been operated to cause the energization of the relay
_14_ that the ringing of the called subscriber can occur, regardless of
what the _B_-operator may have done.
The relay _9_ has an additional function to that of helping to control
the circuit of the ringing-key holding magnet. This is the holding of
the test circuit complete until the operator has tested and made a
connection and then automatically opening it. The test circuit of the
_B_-operator's trunk may be traced, at the time of testing, from the
thimble of the multiple jack under test, through the tip of the cord,
thence through the uppermost pair of contacts of the relay _9_ to ground
through a winding of the _B_-operator's induction coil. After the test
has been made and the plug inserted, the relay _9_, which is operated by
the insertion of the plug, acts to open this test circuit and at the
same time complete the tip side of the cord circuit.
In the upper portion of Fig. 371 the order-wire connections, by which
the _A_-operator and the _B_-operator communicate, are indicated. It
must be remembered in connection with these that the _A_-operator only
has control of this connection, the _B_-operator being compelled
necessarily to hear whatever the _A_-operators have to say when the
_A_-operators come in on the circuit.
[Illustration: Fig. 372. Incoming Trunk Circuit]
The incoming trunk circuit employed by the Western Electric Company for
four-party line ringing is shown in Fig. 372, it being necessarily
somewhat modified from that shown in Fig. 371, which is adapted for
two-party line ringing only. In addition to the provision of the
four-party line ringing keys, by which positive or negative pulsating
current is received over either limb of the line, and to the provision
of the regular alternating current ringing key for ringing on single
party lines, it is necessary in the ringing cut-off relay to provide for
keeping the alternating and the pulsating ringing currents entirely
separate. For this reason, the ringing cut-off relay _12_ is provided
with two windings, that at the right being in the path of the
alternating ringing currents that are supplied to the alternating
current key, and that at the left being in the ground return path for
all of the pulsating ringing currents supplied to the pulsating keys.
With this explanation it is believed that this circuit will be
understood from what has been said in connection with Fig. 371. The
operation of the holding coil _8_ is the same in each case, the holding
magnet in Fig. 372 serving to hold depressed any one of the five ringing
keys that may have been used in calling the subscriber.
[Illustration: AUTOMATIC EQUIPMENT, MAIN OFFICE, BERKELEY, CALIFORNIA A
Feature of Interest Here is That the Cement Floor is Treated with a
Filler and Painted, with No Other Covering.]
[Illustration: Fig. 373. Western Electric Trunk Ringing Key]
The standard four-party line, trunk ringing key of the Western Electric
Company is shown in Fig. 373. In this the various keys operate not by
pressure but rather by being pulled by the finger of the operator in
such a way as to subject the key shaft to a twisting movement. The
holding magnet lies on the side opposite to that shown in the figure and
extends along the full length of the set of keys, each key shaft being
provided with an armature which is held by this magnet until the magnet
is de-energized by the action of the ringing cut-off relay.
[Illustration: Fig. 374. Trunk Relay]
[Illustration: Fig. 375. Trunk Relay]
The standard trunk relays employed by the Western Electric Company in
connection with the circuits just described are shown in Figs. 374 and
375. In each case the dust-cap or shield is also shown. The relay of
Fig. 374 is similar to the regular cut-off relay and is the one used for
relays _9_ and _14_ of Figs. 371 and 372. The relay of Fig. 375 is
somewhat similar to the subscriber's line relay in that it has a tilting
armature, and is the one used at _13_ in Figs. 371 and 372. The trunk
relay _3_ in Figs. 371 and 372 is the same as the _A_-operator's
supervisory relays already discussed.
It has been stated that under certain circumstances _B_-operator's trunk
circuits devoid of ringing keys, and consequently of all keys, may be
employed. This, so far as the practice of the Bell companies is
concerned, is true only in offices where there are no party lines, or
where, as in many of the Chicago offices, the party lines are worked on
the "jack per station" basis. In "jack per station" working, the
selection of the station on a party line is determined by the jack on
which the plug is put, rather than by a ringing key, and hence the
keyless trunk may be employed.
[Illustration: Fig. 376. Keyless Trunk]
A keyless trunk as used in New York is shown in Fig. 376. This has no
manually operated keys whatever, and the relay _17_, when it is
operated, establishes connection between the ringing generator and the
conductors of the trunk plug. The relays _3_, _13_, and _12_ operate in
a manner identical with those bearing corresponding numbers in Fig.
371. As soon as the trunk operator plugs into the multiple jack of the
called subscriber, the relay _16_ will operate for the same reason that
the relay _9_ operated in connection with Fig. 371. The trunk disconnect
lamp will receive current, but if the operator has already established
connection with the other end of the trunk, this lamp will not be
lighted because shunted by the relay _17_, due to the pulling up of the
armature of the relay _13_. The relay _15_ plays no part in the
operation so far described, because of the fact that its winding is
short-circuited by its own contacts and those of relay _12_, when the
latter is not energized. As a result of the operation of the relay _17_,
ringing current is sent to line, the supply circuit including the coil
of the relay _12_. As soon as the subscriber responds to this ringing
current, the armature of the relay _12_ is pulled up, thus breaking the
shunt about the relay _15_, which, therefore, starts to operate in
series with the relay _17_, but as its armatures assume their attracted
position, the relay _17_ is cut out of the circuit, the coil of the
relay _15_ being substituted for that of the relay _17_ in the shunt
path around the lamp _4_. The relay _17_ falls back and cuts off the
ringing current. The relay _15_ now occupies the place with respect to
the shunt around the lamp _4_ that the relay _17_ formerly did, the
continuity of this shunt being determined by the energization of the
relay _13_. When the _A_-operator at the distant exchange withdraws the
calling plug from the trunk jack, this relay _13_ becomes de-energized,
breaking the shunt about the lamp _4_ and permitting the display of that
lamp as a signal to the operator to take down the connection. It may be
asked why the falling back of relay _15_ will not again energize relay
_17_ and thus cause a false ring on the called subscriber. This will not
occur because both the relays _15_ and _17_ depend for their
energization on the closure of the contacts of the relay _13_, and when
this falls back the relay _17_ cannot again be energized even though the
relay _15_ assumes its normal position.
=Kellogg Trunk Circuits.= The provision for proper working of trunk
circuits in connection with the two-wire multiple switchboards is not an
altogether easy matter, owing particularly to the smaller number of
wires available in the plug circuits. It has been worked out in a highly
ingenious way, however, by the Kellogg Company, and a diagram of their
incoming trunk circuit, together with the associated circuits involved
in an inter-office connection, is shown in Fig. 377.
[Illustration: Fig. 377. Inter-Office Connection--Kellogg System]
This figure illustrates a connection from a regular two-wire multiple
subscriber's line in one office, through an _A_-operator's cord circuit
there, to the outgoing trunk jacks at that office, thence through the
incoming trunk circuit at the other office to the regular two-wire
multiple subscriber's line at that second office. The portion of this
diagram to be particularly considered is that of the _B_-operator's cord
circuit. The trunk circuit terminates in the multipled outgoing trunk
jacks at the first office, the trunk extending between offices
consisting, of course, of but two wires. We will first consider the
control of the calling supervisory lamp in the _A_-operator's cord
circuit, it being remembered that this control must be from the called
subscriber's station. It will be noticed that the left-hand armature of
the relay _1_ serves normally to bridge the winding of relay _2_ across
the cord circuit around the condenser _3_. When, however, the relay _1_
pulls up, the coil of relay _4_ is substituted in this bridge connection
across the trunk. The relay _2_ has a very high resistance
winding--about 15,000 ohms--and this resistance is so great that the tip
supervisory relay of the _A_-operator's cord will not pull up through
it. As a result, when this relay is bridged across the trunk circuit,
the tip relay on the calling side of the _A_-operator's cord circuit is
de-energized, just as if the trunk circuit were open, and this results
in the lighting of the _A_-operator's calling supervisory lamp. The
winding of the relay _4_, however, is of low resistance--about 50
ohms--and when this is substituted for the high-resistance winding of
the relay _2_, the tip relay on the calling side of the _A_-operator's
cord is energized, resulting in the extinguishing of the calling
supervisory lamp. The illumination of the _A_-operator's calling
supervisory lamp depends, therefore, on whether the high-resistance
relay _2_, or the low-resistance relay _4_, is bridged across the trunk,
and this depends on whether the relay _1_ is energized or not. The relay
_1_, being bridged from the tip side of the trunk circuit to ground and
serving as the means of supply of battery current to the called
subscriber, is operated whenever the called subscriber's receiver is
removed from its hook. Therefore, the called subscriber's hook controls
the operation of this relay _1_, which, in turn, controls the conditions
which cause the illumination or darkness of the calling supervisory lamp
at the distant office.
Assuming that the _A_-operator has received and answered a call, and has
communicated with the _B_-operator, telling her the number of the
called subscriber, and has received, in turn, the number of the trunk to
be used, and that both operators have put up the connection, then it
will be clear from what has been said that the calling supervisory lamp
of the _A_-operator will be lighted until the called subscriber removes
his receiver from its hook, because the tip relay in the _A_-operator's
cord circuit will not pull up through the 15,000-ohm resistance winding
of the relay _2_. As soon as the subscriber responds, however, the relay
_1_ will be operated by the current which supplies his transmitter. This
will substitute the low-resistance winding of the relay _4_ for the
high-resistance winding of the relay _2_, and this will permit the
energizing of the tip supervisory relay of the _A_-operator and put out
the calling supervisory lamp at her position. As in the Western Electric
circuit, therefore, the control of the _A_-operator's calling
supervisory lamp is from the called subscriber's station and is relayed
back over the trunk to the originating office.
In this circuit, manual instead of automatic ringing is employed,
therefore, unlike the Western Electric circuit, means must be provided
for notifying the B-operator when the calling subscriber has answered.
This is done by placing at the _B_-operator's position a ringing lamp
associated with each trunk cord, which is illuminated when the
_B_-operator places the plug of the incoming trunk into the multiple
jack of the subscriber's line, and remains illuminated until the
subscriber has answered. This is accomplished in the following manner:
when the operator plugs into the jack of the line called, relay _5_ is
energized but is immediately de-energized by the disconnecting of the
circuit of this relay from the sleeve conductor of the cord when the
ringing key is depressed, the selection of the ringing key being
determined by the particular party on the line desired. These ringing
keys have associated with them a set of springs _9_, which springs are
operated when any one of the ringing keys is depressed. Thus, with a
ringing key depressed and the relay _5_ de-energized, the ringing lamp
will be illuminated by means of a circuit as follows: from the live side
of the battery, through the ringing lamp _12_, through the back contact
and armature of the relay _6_, through the armature and contact of relay
_4_, then through the armature and front contact of relay _2_--which at
this time is the relay bridged across the trunk and, therefore,
energized--and thence through the back contact and armature of relay
_5_ to ground. When the subscriber removes his receiver from the hook,
the relay _1_ will become energized as previously described, and will,
therefore, operate relay _6_ to break the circuit of the ringing lamp.
The circuit thus established by the operation of relay _1_ is as
follows: from the live side of battery, through the winding of relay
_6_, through the armature and contact of relay _1_, through the armature
and contact of relay _4_, through the armature and front contact of
relay _2_, thence through the armature and back contact of relay _5_ to
ground. As soon as the _B_-operator notes that the ringing lamp has gone
out, she knows that no further ringing is required on that line, thus
allowing the operation of relay _5_ and accomplishing the locking out of
the ringing lamp during the remainder of that connection. The relay _6_,
after having once pulled up, remains locked up through the rear contact
of the left-hand armature of relay _5_ and ground, until the plug is
removed from the jack.
At the end of the conversation, when the _A_-operator has disconnected
her cord circuit on the illumination of the supervisory signals, both
relays _2_ and _4_ will be in an unoperated condition and will provide a
circuit for illuminating the disconnect lamp associated with the
_B_-operator's cord. This circuit may be traced as follows: from battery
through the disconnect lamp, through the armatures and contacts of
relays _2_ and _4_, thence through the front contact and armature of
relay _5_ to ground, thus illuminating the disconnect lamp. The ringing
lamp will not be re-illuminated at this time, due to the fact that it
has been previously locked out by relay _6_. The operator then removes
the plug from the jack of the line called, and the apparatus in the
trunk circuit is restored to normal condition.
In the circuit shown only keys are provided for ringing two parties.
This circuit, however, is not confined to the use of two-party lines,
but may be extended to four parties by simply duplicating the ringing
keys and by connecting them with the proper current for selectively
ringing the other stations.
The method of determining as to whether the called line is free or busy
is similar to that previously described for the _A_-operator's cord
circuit when making a local connection, and differs only in the fact
that in the case of the trunk cord the test circuit is controlled
through the contacts of a relay, whereas in the case of the
_A_-operator's cord, the test circuit was controlled through the
contacts of the listening key. The function of the resistance _10_ and
the battery connected thereto is the same as has been previously
described.
The general make-up of trunking switchboard sections is not greatly
different from that of the ordinary switchboard sections where no
trunking is involved. In small exchanges where ring-down trunks are
employed, the trunk line equipment is merely added to the regular jack
and drop equipment of the switchboard. In common-battery multiple
switchboards the _A_-boards differ in no respect from the standard
single office multiple boards, except that immediately above the
answering jacks and below the multiple there are arranged in suitable
numbers the jacks of the outgoing trunks.
Where the offices are comparatively small, the incoming trunk portions
of the _B_-boards are usually merely a continuance of the _A_-boards,
the subscriber's multiple being continuous with and differing in no
respect from that on the _A_-sections. Instead of the usual pairs of
_A_-operators' plugs, cords, and supervisory equipment, there are on the
key and plug shelves of these _B_-sections the incoming trunk plugs and
their associated equipment.
In large offices it is customary to make the _B_-board entirely separate
from the _A_-board, although the general characteristics of construction
remain the same. The reason for separate _A_- and _B_-switchboards in
large exchanges is to provide for independent growth of each without the
growth of either interfering with the other.
A portion of an incoming trunk, or _B_-board, is shown in Fig. 378. The
multiple is as usual, and, of course, there are no outgoing trunk jacks
nor regular cord pairs. Instead the key and plug shelves are provided
with the incoming-trunk plug equipments, thirty of these being about the
usual quota assigned to each operator's position.
In multi-office exchanges, employing many central offices, such, for
instance, as those in New York or Chicago, it is frequently found that
nearly all of the calls that originate in one office are for subscribers
whose lines terminate in some other office. In other words, the number
of calls that have to be trunked to other offices is greatly in excess
of the number of calls that may be handled through the multiple of the
_A_-board in which they originate. It is not infrequent to have the
percentage of trunked calls run as high as 75 per cent of the total
number of calls originating in any one office, and in some of the
offices in the larger cities this percentage runs higher than 90 per
cent.
[Illustration: Fig. 378. Section of Trunk Switchboard]
[Illustration: Fig. 379. Section of Partial Multiple Switchboard]
This fact has brought up for consideration the problem as to whether,
when the nature of the traffic is such that only a very small portion of
the calls can be handled in the office where they originate, it is worth
while to employ the multiple terminals for the subscribers' lines on the
_A_-boards. In other words, if so great a proportion as 90 per cent of
the calls have to be trunked any way, is it worth while to provide the
great expense of a full multiple on all the sections of the _A_-board in
order to make it possible to handle the remaining 10 per cent of the
calls directly by the _A_-operators?
As a result of this consideration it has been generally conceded that
where such a very great percentage of trunking was necessary, the full
multiple of the subscribers' lines on each section was not warranted,
and what is known as the partial multiple board has come into existence
in large manual exchanges. In these the regular subscribers' multiple is
entirely omitted from the _A_-board, all subscribers' calls being
handled through outgoing trunk jacks connected by trunks to _B_-boards
in the same as well as other offices. In these partial multiple
_A_-boards, the answering jacks are multipled a few times, usually
twice, so that calls on each line may be answered from more than one
position. This multipling of answering jacks does not in any way take
the place of the regular multipling in full multiple boards, since in no
case are the calls completed through the multiple jacks. It is done
merely for the purpose of contributing to team work between the
operators.
A portion of such a partial multiple _A_-board is shown in Fig. 379.
This view shows slightly more than one section, and the regular
answering jacks and lamps may be seen at the bottom of the jack space
just above the plugs. Above these are placed the outgoing trunk jacks,
those that are in use being indicated with white designation strips.
Above the outgoing trunk jacks are placed the multiples of the answering
jacks, these not being provided with lamps.
The partial multiple _A_-section of Fig. 379 is a portion of the
switchboard equipment of the same office to which the trunking section
shown in Fig. 378 belongs. That this is a large multiple board may be
gathered from the number of multiple jacks in the trunking section,
8,400 being installed with room for 10,500. That the board is a portion
of an equipment belonging to an exchange of enormous proportions may be
gathered from the number of outgoing trunk jacks shown in the _A_-board,
and in the great number of order-wire keys shown between each of the
sets of regular cord-circuit keys. The switchboards illustrated in these
two figures are those of one of the large offices of the New York
Telephone Company on Manhattan Island, and the photographs were taken
especially for this work by the Western Electric Company.
=Cable Color Code.= A great part of the wiring of switchboards
requires to be done with insulated wires grouped into cables.
In the wiring of manual switchboards as described in the seven
preceding chapters, and of automatic and automanual systems and
of private branch-exchange and intercommunicating systems
described in succeeding chapters, cables formed as follows are
widely used:
Tinned soft copper wires, usually of No. 22 or No. 24 B. & S.
gauge, are insulated, first with two coverings of silk, then
with one covering of cotton. The outer (cotton) insulation of
each wire is made of white or of dyed threads. If dyed, the
color either is solid red, black, blue, orange, green, brown,
or slate, or it is striped, by combining one of those colors
with white or a remaining color. The object of coloring the
wires is to enable them to be identified by sight instead of by
electrical testing.
Wires so insulated are twisted into pairs, choosing the colors
of the "line" and "mate" according to a predetermined plan. An
assortment of these pairs then is laid up spirally to form the
cable core, over which are placed certain wrappings and an
outer braid. A widely used form of switchboard cable has paper
and lead foil wrappings over the core, and the outer cotton
braid finally is treated with a fire-resisting paint.
STANDARD COLOR CODE FOR CABLES
+---------------+-------------------------------------------------+
| | MATE |
| LINE WIRE +-------+-------+-------+-----------+-------------+
| | White | Red | Black | Red-White | Black-White |
+---------------+-------+-------+-------+-----------+-------------+
| Blue | 1 | 21 | 41 | 61 | 81 |
| Orange | 2 | 22 | 42 | 62 | 82 |
| Green | 3 | 23 | 43 | 63 | 83 |
| Brown | 4 | 24 | 44 | 64 | 84 |
| Slate | 5 | 25 | 45 | 65 | 85 |
| Blue-White | 6 | 26 | 46 | 66 | 86 |
| Blue-Orange | 7 | 27 | 47 | 67 | 87 |
| Blue-Green | 8 | 28 | 48 | 68 | 88 |
| Blue-Brown | 9 | 29 | 49 | 69 | 89 |
| Blue-Slate | 10 | 30 | 50 | 70 | 90 |
| Orange-White | 11 | 31 | 51 | 71 | 91 |
| Orange-Green | 12 | 32 | 52 | 72 | 92 |
| Orange-Brown | 13 | 33 | 53 | 73 | 93 |
| Orange-Slate | 14 | 34 | 54 | 74 | 94 |
| Green-White | 15 | 35 | 55 | 75 | 95 |
| Green-Brown | 16 | 36 | 56 | 76 | 96 |
| Green-Slate | 17 | 37 | 57 | 77 | 97 |
| Brown-White | 18 | 38 | 58 | 78 | 98 |
| Brown-Slate | 19 | 39 | 59 | 79 | 99 |
| Slate-White | 20 | 40 | 60 | 80 | 100 |
+---------------+-------+-------+-------+-----------+-------------+
The numerals represent the pair numbers in the cable.
The wires of spare pairs usually are designated by solid red
with white mate for first spare pair, and solid black with
white mate for second spare pair. Individual spare wires
usually are colored red-white for first individual spare, and
black-white for second individual spare.